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General Introduction to Organic Compounds
Most of the foodstuffs that we consume every day such as sugar, fats,
starch, vinegar, etc are basically organic compounds. Even though the
organic compounds have been known to man since prehistoric times,
their study practically began from the eighteenth century! The term
“organic compound” was coined by Berzelius in 1807. Let’s explore
more about these compounds
Introduction to Organic Compounds
Earlier people thought that compounds which are obtained from plants
and animals are organic compounds and compounds which are
obtained from minerals, non-living sources are termed as inorganic
compounds. However, the modern definition of organic compounds is
a bit different to this.
An organic compound is defined as any compound whose molecules
contain carbon and hydrogen ( also known as ” hydrocarbons” ) or
compound that is the derivative of it. The branch of science which
deals with the scientific study of structure, properties and reactions of
hydrocarbons and their derivatives is known as organic chemistry.
Browse more Topics under Organic Chemistry
● Classification of Organic Compounds
● Isomerism
● Nomenclature of Organic Compounds
● Purification of Organic Compounds
● Qualitative Analysis of Organic Compounds
● Quantitative Analysis of Organic Compounds
● Structural Representations of Organic Compounds
● Types of Organic Reactions
● Fundamental Concepts of Organic Reaction Mechanism
Characteristics of Organic Compounds
The general characteristics of Organic Compounds include:
● Can be isolated as well as prepared in laboratory
● Comprise almost 90% of all known compounds.
● Mostly built up of only three elements- carbon, hydrogen and
oxygen. Other elements like halogen, nitrogen as well as
phosphorous are also present but to a lesser extent.
● Possess complex structures and high molecular weights
● Their properties are decided by certain active atom or group of
atoms known as the functional group.
● They are mostly insoluble in water but soluble in organic
solvents.
● They are combustible in nature
● Chemical reactions involving organic compounds proceed at
slower rates.
Characteristics due to Presence of Covalent Bonds
A covalent bond is a chemical bond that involves the sharing of
electron pairs between atoms that in turn results in a balance of
attractive and repulsive forces between the atoms. The presence of a
covalent bond renders certain characteristics to the organic
compounds. These include:
● Low melting points and boiling points in comparison to the
inorganic compounds.
● Organic acids and bases are less stronger and thus they have a
limited dissociation in an aqueous medium.
● They exhibit the phenomenon of isomerism in which a single
molecular formula represents several organic compounds
differing in physical and chemical properties.
● They are volatile in nature.
General Characteristics of Members of Homologous Series
A Homologous Series is a group of organic chemical compounds,
usually listed in the order of increasing size, that have a similar
structure (and hence, also similar properties) and whose structures
differ only by the number of CH2– CH2 units in the main carbon
chain. They possess the following general characteristics:
● A general formula describes the members of the homologous
series
● Successive members differ from each other by CH2CH2
● Physical properties change regularly with increasing number of
carbon atoms.
● Members have similar chemical properties because they have
same functional group.
● Members of the homologous series can be prepared using the
same method.
Importance of Organic Compounds
● Organic compounds are important because all living organisms
contain carbon.
● While carbohydrates, proteins and fats, the basic structures of
life, are organic compounds
● They are the basic components of many of the cycles that drive
the earth. For example, the carbon cycle that includes the
exchange of carbon between plants and animals in
photosynthesis and cellular respiration.
● Organic compounds combine with metals to form
organometallic compounds. These compounds are important
industrially. They are used as catalysts, promoters, analysers as
well as stabilizers.
Learn more about Types of Organic Reactions here.
Solved Questions For You
Q 1. Which of these is not a property of organic compounds?
a. They possess complex structures and high molecular weights
b. They are combustible
c. They have high melting as well as boiling points
d. They have low melting as well as boiling points
Ans: The correct answer is c. They have high melting as well as
boiling points.
Classification of Organic Compounds
Organic compounds constitute about 90% of all compounds. That’s
quite a lot, isn’t it? In order to study such a vast number of
compounds, it is necessary to classify them into categories. Let us
know more about the Classification of Organic Compounds as well as
the general categories into which organic compounds are divided.
Classification of Organic Compounds
Depending upon the arrangement of carbon atoms in their structure,
organic compounds are broadly categorized into
● Acyclic or Open Chain compounds
● Cyclic or Closed Chain compounds
The following diagram will give you a clear idea about the
classification of organic compounds:
Browse more Topics under Organic Chemistry
● General Introduction to Organic Compounds
● Isomerism
● Nomenclature of Organic Compounds
● Purification of Organic Compounds
● Qualitative Analysis of Organic Compounds
● Quantitative Analysis of Organic Compounds
● Structural Representations of Organic Compounds
● Types of Organic Reactions
● Fundamental Concepts of Organic Reaction Mechanism
Acyclic or Open Chain Compounds
The carbon atoms are present in the form of an open chain.This chain
may either be a straight chain or a branched chain. These were initially
known as Aliphatic compounds because the compounds of this class
were derived from either animal or vegetable fats
● Straight Chain Compounds: The carbon skeleton is in the form
of a straight chain. Examples:
n-Propane CH3-CH2-CH3
Propene CH2=CH-CH3
● Branched Chain Compounds: The carbon skeleton is in the
form of a branched chain. Examples: Isobutylene
Cyclic or Closed Chain Compounds
They are marked by the presence of one or more closed chains or ring
of atoms in their structure. Depending on whether there is a presence
of any other atom apart from carbon in the constitution of the ring,
they are further classified as:
● Homocyclic or Carbocyclic Compounds
● Heterocyclic Compounds
Homocyclic or Carbocyclic Compounds
The rings in these compounds are entirely made up of carbon atoms.
No other atom is present in the ring skeleton.These can be further
divided into two sub-classes:
● Alicyclic Compounds
● Aromatic Compounds
Alicyclic Compounds
Their name is attributed to their resemblance to Aliphatic compounds
in their properties. The examples of this category include
cyclopropane, cyclobutane, cyclopentane, cyclohexane, etc.
Aromatic Compounds
These are cyclic unsaturated compounds. They derive their name from
the Greek word Aroma which means “fragrant smell” since most of
these compounds bear a pleasant smell. These are further classified
into two types:
● Benzenoid Aromatic Compounds: They are characterized by
the presence of one or more fused or isolated benzene rings as
well as their derivatives in their structure. Depending upon the
number of benzene rings that are fused together in their
structure, they can be further classified as Monocyclic,
Bicyclic, Tricyclic.
● Non-Benzenoid aromatic Compounds: They are characterized
by the presence of a single benzene ring to which other groups
are attached.
Aniline
Bicyclic and Tricyclic Compounds
These are characterized by the presence of two or more rings in their
structure.Examples include Naphthalene, Phenanthrene as well as
Anthracene.
Naphthalene
Phenanthrene
1.
Anthracene
Non-Benzenoid Aromatic Compounds
Aromatic compounds that contain other highly unsaturated rings in
place of the benzene ring are called non-benzenoid aromatic
compounds. Examples include
Azulene
Tropolone
Heterocyclic Compounds
When one or more heteroatoms such as oxygen, nitrogen, sulphur,
boron, silicon etc, are present in the ring such compounds are known
as heterocyclic compounds.
● Alicyclic heterocyclic compounds: Aliphatic heterocyclic
compounds that contain one or more heteroatoms in their rings
are called alicyclic heterocyclic compounds.
● Aromatic heterocyclic compounds Aromatic heterocyclic
compounds that contain one or more heteroatoms in their ring
skeleton are called aromatic heterocyclic compounds.
Solved Questions For You
Que: Which of these is not an aromatic compound?
a.
b.
c.
d.
Ans: The correct answer is option b. It is the structure of cyclohexane
which is an alicyclic compound.
Isomerism
You have probably come across the words ‘isomers and isomerism’ in
your previous classes. But, do you know all about them? Isn’t it funny
that with the same chemical formula, two compounds exhibit different
properties? Are they twins or something? Well, no! In this chapter, let
us study about isomerism in greater detail. We will look at its types
and examples as well.
What is Isomerism?
It is a phenomenon where two or more compounds have the same
chemical formula but possesses different structural formulas, that is,
different properties. This is mainly because of different structural or
spatial arrangements. Isomers are the compounds exhibiting
isomerism.
Types of Isomerism
Basically, there are two types. They are:
● Structural Isomerism
● Stereoisomerism
However, these are again of many subtypes as shown in the figure:
Structural Isomerism
Isomers are structural isomers when they have the same molecular
formula but different structures, as in how they are linked to each
other. Structural isomerism is further of the following types. Let’s
learn about these types one-by-one.
Learn the different Characteristics of Organic Compounds here.
1) Chain Isomerism
Isomers are chain isomers when two or more compounds have the
same molecular formula but differ in the branching of carbon atoms.
For example, we can represent C5H12 as three compounds:
CH3CH2CH2CH2CH3– pentane
2) Position Isomerism
Isomers are position isomers when the two or more compounds differ
in the position of the functional group or substituent atoms. For
example, we can represent C3H7OH in two arrangements:
CH3CH2CH2OH -Propan-1-ol
3) Functional Isomerism
Isomers are functional isomers when the two or more compounds have
an identical molecular formula but differ in the functional group
present. These isomers are functional isomers. For example, we can
represent C3H6O as a ketone and as an aldehyde.
4) Metamerism
This is exhibited by compounds due to the presence of different alkyl
chains on either side of the functional group. For example, we can
represent C4H10O as ethoxyethane (C2H5OC2H5) and
methoxypropane (CH3OC3H7).
Read more about Nomenclature of Organic Compounds here in detail.
Stereo Isomerism
Stereoisomerism is a phenomenon in which compounds have the same
molecular formula but differ in the relative positioning or orientation
of atoms in space. Stereoisomers are the compounds exhibiting
stereoisomerism. We can further classify stereoisomerism into:
● Geometric Isomerism: it is shown by molecules in which their
spatial positions are locked to each other due to the presence of
a ring structure or a double bond.
● Optical Isomerism: Two or more compounds that have the
same molecular arrangement but differ in the optical activity
are optical isomers.
Learn more about different types of Organic Reaction here.
Solved Examples for You
Q1: What is a chiral carbon?
Ans: If all the four valencies of carbon are satisfied by four different
atoms or four different groups of atoms, then carbon is known as
chiral carbon.
Q2: What is tautomerism?
Ans: It is the type of isomerism in which two functional isomers exist
together in equilibrium. The two forms existing in equilibrium are
called as tautomers.
Nomenclature of Organic Compounds
How did you get your name? Easy. Maybe your parents or your
relatives decided. But, who gave names to the organic compounds?
Why are they called what they are called? Have you ever wondered?
In this chapter, we will look at the concept of nomenclature of organic
compounds. We will see how these compounds get their names. Let’s
begin.
Nomenclature of Organic Compounds
In earlier days, people knew organic compounds by their common
names. For example, methane was ‘marsh gas’. This is because we
found it in marshy places. With the evolution of so many organic
compounds and continuous addition of new compounds, dealing with
trivial names became a difficulty.
Therefore, scientists introduced a proper method in order to name the
organic compounds. This uniform system for naming the compounds
is the IUPAC system, which is the International Union of Pure and
Applied Chemistry.
Features of the Trivial System
The name of an organic compound, when in a non-systematic manner
or vernacular name is what is known as a trivial system. There are no
particular set of rules for the trivial name of the compound. In this
system, names are usually simple like acetic acid, toluene, and phenol
etc. For example, tartaric acid is a carbolic acid that we usually find in
tamarind. But in IUPAC, it is 2,3-dihydroxy-1,4-Butanedioic acid.
Browse more Topics under Organic Chemistry
● General Introduction to Organic Compounds
● Classification of Organic Compounds
● Isomerism
● Purification of Organic Compounds
● Qualitative Analysis of Organic Compounds
● Quantitative Analysis of Organic Compounds
● Structural Representations of Organic Compounds
● Types of Organic Reactions
● Fundamental Concepts of Organic Reaction Mechanism
Drawbacks of this System
● Many trivial names are present for a single compound. For
example, Phenol has different names like hydroxybenzene,
carbolic acid, and phenol.
● This system is limited to few compounds in each group. For
example, the first two members of the carbolic acid family
have trivial names, formic acid, and acetic acid respectively but
carbolic acid with more atoms does not have any trivial names.
● There are no particular guidelines for naming complex
compounds.
Chemical Nomenclature: IUPAC Rules
According to the IUPAC system, the nomenclature of organic
compounds consists of the following parts:
1) Steps Involved
● Longest Chain Rule: In this step, all we have to do is identify
the parent hydrocarbon and give the name to it. The parent
chain of the compound is usually the longest chain of carbon
atoms. This chain could be straight or of a different shape.
● Lowest number of Locants: We start the numbering of the
carbon atoms in the longest chain from the end that gives the
lowest number to the carbon atoms carrying the substituents.
● Multiple Presence of the same substituent: Prefixes such as di,
tri, etc. are added to the substituents that are present twice,
thrice respectively in the parent chain.
● Naming the various substituents: If more than one substituent is
present, then we need to arrange the substituents in an
alphabetical order of their names.
● Naming different substituents at equivalent positions: If we
find the presence of two different substituents on the same
position from the two ends, what do we do? In such cases, the
substituent first in the alphabetical order gets the lowest
number.
● The Naming of Complex Substituents: We name the complex
substituent when the substituent on the parent chain has a
branched structure (i.e complex structure). We name these
substituents as a substituted alkyl group. It is also important to
note that the carbon atom of this substituent gets the number 1.
We write the name of these type of substituents in brackets.
The final name will be in format : Locant + Prefix + Root + Locant +
Suffix. Now, we will look at some more details of nomenclature of
organic compounds.
2) Word root
It indicates the number of carbon atoms in the longest selected carbon
chain. For example, C1 is ‘Meth’ and C5 is ‘Pent’.
3) Suffix
A suffix is generally a functional group in the molecule which follows
the word root. We can divide it into:
● Primary suffix: We write it immediately after the word root.
For example, in alkanes the suffix is ane.
● Secondary suffix: We write it after the primary suffix. For
instance, if a compound has alkane and alcohol group attached
to it, the naming will be alkanol, -ol being the suffix for
alcohol.
4) Prefix
We add the prefix to the word root while naming the compound. It
indicates the presence of substituent groups or side chains in the
organic molecule. It reveals the cyclic and acyclic nature of the
compound.
● Primary prefix: Indicates whether the molecule is cyclic or not.
For example, for cyclic compounds the prefix used is cyclo.
● Secondary prefix: Indicates the presence of substituent groups
or any side chain. For example –CH3is known as Methyl and
–Br is Bromo.
Types of Chemical Nomenclature
1) Compositional Nomenclature of Organic Compounds
This term denotes the named constructions based on the composition
of species or substances being named, against the systems that involve
structural composition or information. One among them is the
generalised stoichiometric name. Substances or the elements are
named with multiple prefixes in order to give the overall
stoichiometry of an element or a compound.
When there are more components, then we divide them into 2 classes
namely, electropositive and electronegative components. These names
will sound like salt names and this does not imply the chemical nature
and behaviour of those species. Examples: Sodium Chloride – NaCl,
Trioxygen – O3, Phosphorous trichloride – PCl3
2) Substitutive Nomenclature of Organic Compounds
It is based on the approach where parent hydride is changed by
replacement of hydrogen atoms with other atoms or a group of atoms.
It is a system where we name the organic compounds using functional
groups as the suffix or prefix to the name of the parent compound. We
use this system in naming compounds derived from hydrides of
specific group elements in the periodic table.
Similar to that of carbon, these elements may form rings and chains
that will have many derivatives. Rules come in handy in naming the
parent or main compounds and their substituents. Hydrides belonging
to group 13-17 of the periodic table get the suffix – ane. For example
– Borane, Phosphane, and oxidane etc. Examples: 1, 1-difluoro
trisilane (SiH3.SiH2.Si.HF2), Trichlorophosphine(PCl3)
Solved Example for You
Q: Write a note on the additive nomenclature of organic compounds.
Ans: We use this method for the coordination compounds even though
it has wide applications. An example for its application is
pentaamminechlorocobalt (III) chloride – [CoCl(NH3)5]Cl2.
Chloride will have the prefix ‘chloro’ while ligand will have
‘chlorido’.
For example: PCl3 – trichloridophosphorus, [CoCl3 (NH3)3] –
tri-ammine-trichloridocobalt.
Purification of Organic Compounds
Almost everything that we see these days is impure, isn’t it? The water
we drink and the food we eat also need to go through levels of
purification processes. Similar is the case with organic compounds.
There are several methods of purification of organic compounds. Why
are these important and how do we do it? Let us learn all that in this
chapter!
Types of Purification
A large number of methods are available for the purification of
substances. The choice of method, however, depends upon the nature
of substance (whether solid or liquid). It also depends on the type of
impurities present in it. We commonly use these methods for
purification of substances:
● Simple crystallisation
● Fractional crystallisation
● Sublimation
● Simple distillation
● Fractional distillation
● Distillation under reduced pressure
● Steam distillation
● Azeotropic distillation
● Chromatography
Browse more Topics under Organic Chemistry
● General Introduction to Organic Compounds
● Classification of Organic Compounds
● Isomerism
● Nomenclature of Organic Compounds
● Qualitative Analysis of Organic Compounds
● Quantitative Analysis of Organic Compounds
● Structural Representations of Organic Compounds
● Types of Organic Reactions
● Fundamental Concepts of Organic Reaction Mechanism
Let us now study about these methods in brief for better
understanding.
Simple Crystallisation
This is the most common method that we use to purify organic solids.
For crystallisation, a suitable solvent is one
● which dissolves more of the substance at a higher temperature
than at room temperature
● in which impurities are either insoluble or dissolve to an extent
that they remain in solution (in the mother liquor) upon
crystallisation
● which is not highly inflammable and
● which does not react chemically with the compound to be
crystallized. The most commonly-used solvents for
crystallisation are water, alcohol, ether, chloroform, carbon-
tetrachloride, acetone, benzene, petroleum ether etc.
Fractional Crystallisation
It is the process of separation of different components of a mixture by
repeated crystallisations. In the first step, we dissolve the mixture in a
solvent in which the two components have different solubilities. When
we cool a hot saturated solution of this mixture, the less soluble
component crystallises out first while the more soluble substance
remains in solution.
The mother liquor left after crystallisation of the less soluble
component is again concentrated and then we allow it to cool. Hence,
we obtain the crystals of the more soluble component.
Sublimation
Certain organic solids on heating directly change from solid to vapour
state without passing through a liquid state. These substances are
sublimable. This process is sublimation.
We use this process for the separation of sublimable volatile
compounds from non-sublimable impurities. We use this for the
purposes of purification of camphor, naphthalene, anthracene, benzoic
acid, Iodine and salicylic acid etc containing non-volatile impurities.
Simple Distillation
Distillation is the joint process of vapourisation and condensation. We
use this method for the purification of liquids which boil without
decomposition and contain non-volatile impurities. We can also use
this method for separating liquids having sufficient difference in their
boiling points.
Fractional Distillation
We can use this process to separate a mixture of two or more miscible
liquids which have boiling points close to each other. We carry out
this process by using fractionating columns. The fractionating column
is a special type of long glass tube that has obstructions to the passage
of the vapour upwards and that of liquid downwards. This method can
separate a mixture of acetone (b. p. 330 K) and methyl alcohol (b. p.
338 K) or a mixture of benzene and toluene.
Distillation under Reduced Pressure
We use this method for the purification of high boiling liquids and
liquids which decompose at or below their boiling points. Practical
examples include the crude oil industry, sugarcane industry etc.
Steam Distillation
This method is applicable for the separation and purification of those
organic compounds (solids or liquids) which:
● are insoluble in water
● are volatile in steam
● possess a high vapour pressure (10-15 mm Hg) at 373 K and
● contain non-volatile impurities.
Azeotropic Distillation
An azeotropic mixture is a mixture having a constant boiling point.
The most familiar example is a mixture of ethanol and water in the
ratio of 95.87: 4.13 (a ratio present in rectified spirit). It boils at
78.13oC. We can’t separate the constituents of an azeotropic mixture
by fractional distillation. Hence, we have to use a special type of
distillation (azeotropic distillation) for separating the constituents of
an azeotropic mixture.
In this method, we use the third compound in distillation. The process
uses the fact that dehydrating agents like diethyl ether etc. depress the
partial pressure of one of the original components. As a result, the
boiling point of that component raises sufficiently and thus, the other
component will distil over.
Chromatography
This is a modern method that we can use for the separation of
mixtures into its components, purification of compounds and also test
the purity of compounds. The name chromatography comes from the
Greek word ‘chroma’ meaning colour and ‘graphy’ for writing
because the method was first used for the separation of coloured
substances found in plants. This method was described by Tswett in
1906.
Principle of Chromatography
The technique of chromatography uses the difference in the rates at
which the components of a mixture move through a porous medium
(stationary phase) under the influence of some solvent or gas (moving
phase).
Thus, this technique consists of two phases- one is a stationary phase
of the large surface area while the second is a moving phase which is
allowed to move slowly over the stationary phase. The stationary
phase is either a solid or a liquid while the moving phase may be a
liquid or a gas. There are also some other methods of purification like
differential extraction and other chemical methods.
Solved Examples for You
Question: Give two practical applications of simple crystallisation.
Answer: Practical applications of simple crystallisation include:
● Sugar having an impurity of common salt can be crystallized
from hot ethanol since sugar dissolves in hot ethanol but
common salt does not.
● A mixture of benzoic acid and naphthalene can be separated
from hot water in which benzoic acid dissolves but naphthalene
does not.
Qualitative Analysis of Organic Compounds
How do you think scientists can recognise the various organic
compounds? Is it easy? Well, ‘easy’ is a relative term! However, the
qualitative analysis that helps them know what compound is what. Just
like your teachers conduct tests to know which subject you are bad at,
scientists also have a number of tests to know which is which
compound. In this chapter, we will study the Qualitative analysis of
organic compounds. However, before we start, let us go through all
the basic tests first.
Qualitative Analysis of Elements
The most commonly occurring elements in organic compounds are
carbon, hydrogen, oxygen, nitrogen, sulphur and halogen elements.
There is no direct method for the detection of oxygen. For detecting
nitrogen, sulphur and halogens, we can use the sodium fusion test
(Lassaigne’s test).
Sodium Fusion Test
This test is used for the qualitative analysis of elements nitrogen,
sulphur and halogen in Organic compounds. In order to detect them, it
is necessary to convert them into ionisable inorganic substances.
Browse more Topics under Organic Chemistry
● General Introduction to Organic Compounds
● Classification of Organic Compounds
● Isomerism
● Nomenclature of Organic Compounds
● Purification of Organic Compounds
● Quantitative Analysis of Organic Compounds
● Structural Representations of Organic Compounds
● Types of Organic Reactions
● Fundamental Concepts of Organic Reaction Mechanism
1) Test for Nitrogen
We can detect cyanide ion and hence, nitrogen ion in the sample by
the Prussian blue test. The filtered alkaline solution resulting from the
action of water upon the sodium fusion is treated iron (II) sulphate and
thus, forms sodium hexacyanoferrate (II).
Upon boiling the alkaline iron (II) salt solution, some iron (III) ions
are insensibly produced by the action of air. Now, we add dilute
sulphuric acid to dissolve the iron (II) and (III) hydroxides. The
hexacyanoferrate (II) reacts with the iron (III) salt, producing iron (III)
hexacyanoferrate (II), Prussian blue. A Prussian blue precipitate or
colouration indicates that nitrogen is present.
FeSO4 + 6NaCN → Na4[Fe(CN)6] + Na2SO4
3Na4[Fe(CN)6] + 2Fe2(SO4)3 → Fe4[Fe(CN)6]3 +
6Na2SO4
2) Test for Halogens (Nitrogen and Sulphur Absent)
We acidify a portion of the fusion solution with dilute nitric acid. We,
then add an excess of silver nitrate solution. A precipitate indicates the
presence of a halogen. We decant the mother liquor and treat the
precipitate with dilute aqueous ammonia solution. If the precipitate is
white and readily soluble in ammonia solution, chlorine is present. In
case, it is pale yellow and difficulty soluble, bromine is present. If it is
yellow and insoluble, then iodine and bromine may be confirmed by
some more tests.
3) Test for Halogens (Nitrogen and/or Sulphur Present)
Cyanide and sulphide ions both interfere with this test for halide by
forming silver cyanide and silver sulphide precipitates. If nitrogen or
sulphur is present, we must remove the interfering ions. To remove
cyanide and sulphide ions, we have to acidify the fusion solution with
dilute nitric acid. Then, we have to evaporate it to half of the original
volume to expel hydrogen cyanide and/or hydrogen sulphide which
may be present.
Qualitative Analysis of Functional Groups
1) Alcoholic –OH group
We can detect the alcoholic group by the following tests:
● Sodium Metal Test: We conduct this test on the basis of the
appearance of effervescence due to the liberation of hydrogen
gas in reactions of sodium with alcohol.
2R – OH + 2Na → 2RONa + H2
● Acetyl Chloride Test: Acetyl chloride reacts vigorously with
primary and secondary alcohols with the evolution of hydrogen
chloride. The hydrogen chloride gives white fumes of
ammonium chloride with ammonium hydroxide.
● Ceric Ammonium Test: To the sample, we add a few drops of
ceric ammonium nitrate and shake well. The appearance of
pink or red colour indicates the presence of an alcoholic group.
2ROH + (NH4)2Ce(NO3)6 → (ROH)2Ce(NO3)4 +
2NH4NO3
2) Carbonyls (Aldehydes and Ketones)
● 2,4-dinitrophenyl hydrazine test: We add a small amount (2
drops or 0.05 – 0.1g) of the substance to 3 ml of
2,4-dinitrophenyl hydrazine reagent and shake well. A
crystalline precipitate indicates the presence of a carbonyl
compound. Occasionally the precipitate is oily at first but this
becomes crystalline upon standing.
3) Aldehydes
● Schiff’s Test: We dissolve the given compound in alcohol and
then add 1-2ml of Schiff’s reagent. The appearance of pink, red
or magenta colour confirms the presence of aldehyde group.
● Tollen’s Test (Silver Mirror Test): We add 3-4 drops of the
liquid to the Tollen’s reagent. We heat the container. A shining
mirror precipitate confirms the presence of the aldehyde.
2Ag(NH3)2+ + RCHO + 3OH– → RCOO– + 2Ag¯ +
4NH3 + 2H2O
4) Carboxyl Group
We can identify Carboxylic acid by the following tests:
● Sodium Bicarbonate test: We add Sodium bicarbonate
(NaHCO3) to the 1 ml of the sample. A pinch of effervescence
indicates the presence of a carboxylic group.
RCO2H + NaHCO3 → RCOONa + CO2 + H2O
● Ester Test: We warm a small amount of the acid with two parts
of absolute ethanol and one pare of concentrated sulphuric acid.
We cool the solution and pour it continuously into aqueous
Na2CO3 solution. A sweet, fruity smell of an ester confirms the
presence of ester.
5) Amino Group
The most important basic nitrogen compounds are the primary,
secondary and tertiary amines and they dissolve in mineral acids and
change red litmus to blue.
● Chemical classification of the amine function: The
classification of primary, secondary or tertiary amines is done
by means of the reaction with nitrous acid.
● Nitrous Acid Test: We add 2g of the substance to 5 ml of 2 M
HCl acid. Then, we cool it and add 2 ml of ice-cold 10%
aqueous NaNO2 solution slowly by means of a dropper. If we
obtain a clear solution, with a continuous evolution of nitrogen
gas, the substance is a primary amine.
RNH2 + HONO → ROH + H2O + N2
Solved Example for You
Q: Fehling’s test is used for the detection of what?
Ans: Fehling’s reagent is a blue coloured basic solution of
bistartratocuprate(II) complex. Fehling’s test is used to detect the
presence of an aldehyde group. It differentiates between aldehyde and
ketone group present in the same organic compound.
Quantitative Analysis
We might have learnt about the qualitative analysis of various organic
compounds. However, without knowing the exact quantity of each of
the compounds, we won’t be able to use them in the best possible way.
So, how do we do that? Quantitative analysis will help us! This form
of analysis also involves certain tests! Yeah, you can never escape
tests in chemistry! Let us start with the basics of what quantitative
analysis is.
What is Quantitative Analysis?
Quantitative analysis is an analysis method that we can use to
determine the quantity of the elements or molecules produced during
the reaction. Organic compounds consist of carbon and hydrogen. It
also comprises of elements like oxygen, nitrogen, phosphorus, sulphur
and halogens. We will now explain the various methods for the
measurement of percentage composition of elements in an organic
compound.
Quantitative Analysis of Halogens
Carius Method
Here, we heat a mixture of organic compound and fuming nitric acid
in the presence of silver nitrate contained in a tube (hard glass). This
tube is known as the Carius tube in a furnace. Carbon and hydrogen
present in the organic compound oxidise to carbon dioxide and water
respectively.
The halogens present in the organic compound reacts with the silver
nitrate to form the corresponding silver halide (AgX). After this, we
filter, wash, dry and weigh them.
Calculations
Let us decide the mass of the given organic compound as mg. Suppose
the total mass of the compound, AgX formed = m1 g. We know that 1
mol of AgX has 1 mol of X. So, in m1 g of AgX,
Mass of halogen = (atomic mass of X × m1 g)(molecular mass of
AgX)
Percentage of halogen = (atomic mass of X × m1 × 100)(molecular
mass of AgX × m)
Quantitative Analysis of Sulphur
In this segment, we continue to a process of heating a fixed mass of an
organic compound containing sulphur in a Carius tube. This tube has
sodium peroxide or fuming nitric acid. Sulphur present in the organic
compound oxidises into sulphuric acid. It, then, precipitates as barium
sulphate by the addition of barium chloride solution in water.
We then wash, filter, dry and weigh the precipitate. The mass of
barium sulphate that we use in calculating the percentage of sulphur in
the given organic compound.
Calculations
Let us consider the mass of organic compound = mg. Let the mass of
the compound, barium sulphate formed = m1 g. We know that 32 g
sulphur is present in 1 mol of BaSO4. Therefore, 233 g BaSO4
contains 32 g sulphur
⇒ M1 g of BaSO4 contains 32 × m1/233 g of sulphur
∴ Percentage of sulphur = 32 × m1 × 100/233 × m
Solved Example for You
Q: What is Aluise’s method?
Ans: It is a method that we use to calculate the weight of oxygen in an
organic compound. In this method, we heat the organic compound
containing oxygen with graphite. CO gets formed. We convert this CO
quantitatively into CO2 on reaction with I2O5. Thus, we get the weight
of the oxygen in the compound.
Structural Representations of Organic Compounds
All elements around us display certain properties. This is largely
dependent on their structure and chemical properties. In order to study
and understand the various concepts of such elements in a holistic
manner, it is pertinent to take a view of the structural representation of
such elements and compounds. This will help us in determining the
arrangement of atoms within an element. It will also enhance our
understanding of elements and compounds. Let us understand the
structural representation of different organic compounds.
Understanding the Structural Representation of Organic Compounds
We can represent organic compounds in various ways when it comes
to their structure. There are different types of structures of organic
compounds. Depending on the convenience of making the structure of
an organic compound, one can choose the structure that he wants to
use to depict the compound.
The Lewis structure is considered to be one of the most popular ways
to depict the structure of an organic compound. However, as the size
of the compound increases, it becomes difficult to portray it with the
use of Lewis structure. This is why many other structures have been
introduced, which can be used to portray the structure of an organic
compound. Let us take a look at some of these structures.
Browse more Topics under Organic Chemistry
● General Introduction to Organic Compounds
● Classification of Organic Compounds
● Isomerism
● Nomenclature of Organic Compounds
● Purification of Organic Compounds
● Qualitative Analysis of Organic Compounds
● Quantitative Analysis of Organic Compounds
● Types of Organic Reactions
● Fundamental Concepts of Organic Reaction Mechanism
The Complete Structural Formulae
The Lewis structures can be simplified easily by representing the two
– electron covalent bond by a single dash. Such a type of structural
formula focuses on the electrons involved in the bond formation.
A single dash represents a single bond, a double dash represents a
double bond and a triple dash represents a triple bond respectively.
Lone pairs of electrons on heteroatoms such as oxygen, halogens and
more, may or may not be shown.
Condensed Structural Formulae
We can further abbreviate complete structural formulae by omitting
some or all the dashes that represent covalent bonds. Identical
repetitive units are put in parenthesis and subscripts are used to
indicate their repetition. For example, we can further condense
CH3CH2CH2CH2CH2CH3 to CH3(CH2)4CH3.
The role of parentheses is very important in condensed structural
formulae. We can further condense these structural formulae by
enclosing the repetitive structural unit within a bracket and placing an
integer as a subscript that indicates the number times the structural
unit gets repeated. Further examples of this phenomenon are:
● CH3CH2CH2CH2CH2CH2CH3 changes to CH3 (CH2)5CH3
● CH3CH2CH2CH2CH2CH2CH2COOH changes to
CH3(CH2)6COOH
Bond Line Structural Representation
In a Bond line structural representation, carbon and hydrogen atoms
are not shown and the lines representing carbon-carbon bonds are
drawn in a zig-zag fashion. Only heteroatoms are written in bond line
representation. The terminals in the bond – line structures denote
methyl (CH3) groups unless indicated otherwise by a functional group,
while the line junctions denote carbon atoms bonded to an appropriate
number of hydrogen molecules required to satisfy the valency of the
carbon atoms.
Cyclic compounds are usually represented by bond line formulae as
well. It is a simple, short and convenient method of representing
organic molecule. Each carbon on the line end or intersection is
attached to the required number of hydrogen atoms. Thus, the terminal
denotes CH3 group and an unsubstituted intersection denotes a CH2
group.
Polygon Formulae
There are many organic compounds, in which carbon atoms are not
joined in a chain but they are actually joined in a ring. These cyclic
compounds are usually represented by polygon without showing
carbon and hydrogen atoms.
The corners of a polygon represent a carbon atom and the sides of a
polygon denote a carbon-carbon bond. Similarly, if an atom or a group
of atoms other than hydrogen is attached to carbon, then that atom or a
group of atoms manifest in this structure.
A Solved Question for You
Q: Discuss the concept of wedge and dash representation and
sawhorse projection.
Ans: Wedge and dash projection is a means of representing a molecule
(drawing). In this three types of lines come in use to represent the
three-dimensional structure:
● solid lines to represent bonds which are in the plane of the
paper;
● dashed lines to represent bonds that extend away from the
viewer;
● wedge-shaped lines to represent bonds oriented facing the
viewer.
Sawhorse projection a way of representing an organic compound from
a rather oblique angle to study its conformations. Therefore, it is a
very efficient way of studying conformations of a compound along
with its optical character. This is because it is easily convertible into
Fischer projection and Newman projections. In this representation, we
observe two carbon atoms bonded to each other along with the groups
attached to them from an edge view unlike that in Newman projection
in which we observe it from the front view.
Types of Organic Reactions
Reactions happen all around us. Do you think there are types of
organic reactions? Yes. When someone tries to study organic
chemistry, the study of the types of organic reactions becomes an
inherent part of such study module. Let us take a look at the types of
organic reactions that scientists have been able to decipher until now.
Types of Organic Reactions
There are five main types of organic reactions that can take place.
They are as follows:
● Substitution reactions
● Elimination reactions
● Addition reactions
● Radical reactions
● Oxidation-Reduction Reactions
Let us study each of these reactions in detail, to understand more
about them.
(Source: wikihow)
1) Substitution Reactions
In a substitution reaction, generally, one atom or a group of atoms
take place of another atom or a group of atoms which leads to the
formation of an altogether new substance. We can take an example of
C – Cl bond, in which the carbon atom usually has a partial positive
charge due to the presence of highly electronegative chlorine atoms.
In a nucleophilic substitution reaction, it is important that the
nucleophile must have a pair of electrons and it also should have a
high affinity for the electropositive species in comparison to the
substituent which was originally present in the element. In order for
the substitution reaction to occur, there are certain conditions that have
to be present such as maintaining low temperatures same as room
temperature.
Also, a strong base such as NaOH has to be in dilute form because
suppose if the base is of higher concentration, there are chances of
dehydrohalogenation taking place in the reaction. And, the solution
needs to be in an aqueous state such as water for the reaction to take
place. The types of Substitution reactions include nucleophilic
substitution reactions and electrophilic substitution reactions.
Browse more Topics under Organic Chemistry
● General Introduction to Organic Compounds
● Classification of Organic Compounds
● Isomerism
● Nomenclature of Organic Compounds
● Purification of Organic Compounds
● Qualitative Analysis of Organic Compounds
● Quantitative Analysis of Organic Compounds
● Structural Representations of Organic Compounds
● Fundamental Concepts of Organic Reaction Mechanism
2) Elimination Reactions
There are certain reactions which involve the elimination and removal
of the adjacent atoms. After these multiple bonds are simultaneously
formed and there is a release of small molecules as products as a
result. One of the examples of a typical elimination reaction is the
conversion of ethyl chloride to ethylene.
CH3CH2Cl → CH2= CH2 + HCl
In the above reaction, the eliminated molecule is HCl, which can form
out of the combination of H+ from the carbon atom which is on the
left side and Cl– from the carbon atom which is on the right side.
3) Addition Reactions
An addition reaction is simply just the opposite of an elimination
reaction. In an addition reaction, the components or molecules of A
and B are added to the carbon-carbon multiple bonds and this is called
an addition reaction. In the reaction given below when HCl is added to
ethylene, it will give us ethylene chloride.
HCl + CH2 = CH2 → CH3CH2Cl
4) Radical Reactions
Most of the organic reactions involve radicals and their movement.
Addition of a halogen to a typically saturated hydrocarbon involves
free radical mechanism. There are usually three stages involved in a
radical reaction which are, initiation, propagation, and termination.
Initially when the weak bond is broken initiation of the reaction takes
place with the formation of free radicals. After that when the halogen
is added to the hydrocarbon a radical is produced and finally, it gives
alkyl halide.
A Solved Question for You
Q: In the context of an organic reaction, explain why such reactions
take place.
Ans: In case of all organic reactions we can study that during a
reaction, the solution will reach an equilibrium that favours the more
stable side. In most cases, these will be reaction products as written
from left to right, however, sometimes we must predict which side is
favoured as in the case of acid-base reactions.
This can be demonstrated through DGº = – RT. In Keq = DH – TDS. If
Keq > 1, in this case, the energy is released to the surroundings which
are an exergonic reaction, the negative value of DGº, reaction
favoured. If Keq, < 1, energy is absorbed from the surroundings which
demonstrate an endergonic reaction, positive value of DGº, reaction
not favoured.
Energy changes in a reaction can be illustrated by Energy Diagrams in
which the highest energy point in a reaction step is called the
transition state and the energy required to go from reactant to
transition state is the activation energy. If a reaction occurs in more
than one step, it must involve species that are neither the reactant nor
the final product. Each step has its own free energy of activation.
Fundamental Concepts of Organic Reaction Mechanism
As we know the branch of chemistry that deals with the study of
hydrocarbons and their derivatives is known as Organic Chemistry.
But are you guys aware regarding the concepts of organic reaction
mechanism? In this chapter, let us now understand the fundamental
concepts of organic reaction mechanism in detail.
Introduction to Organic Chemistry
The Shapes of Carbon Compounds
Both ‘s’ and ‘p’ orbitals are involved in hybridization, inorganic or
carbon compounds which further leads to three types of hybridization
which are sp3 (in alkanes) – Tetrahedral in shape sp2 (in alkenes) –
Linear molecule – Planar structure sp (in alkynes).
Functional Group
The functional groups are atom or group of atoms which are further
joined in a specific manner and determines the chemical properties of
the organic compound. Few of the examples are the hydroxyl group
(—OH), carboxylic acid group (—COOH) and aldehyde group
(—CHO) etc.
Homologous Series
A homologous series refers to as a family of organic compounds that
have similar chemical properties, the same functional group, and the
successive members differ from each other in the molecular formula
by —CH2 units. By the same general molecular formula, the members
of a homologous series can be represented.
Nomenclature of Organic Compounds
Common name: Organic compounds were named after the sources of
origin, before the IUPAC system of nomenclature. For example, urea
was so named because it was obtained from the urine of mammals.
Formic acid was so named since it was extracted from red ants called
Formica.
The Fundamental Concepts in Organic Reaction Mechanism
Fission of a Covalent Bond
A covalent bond can undergo Fission in two ways:
● Homolytic Fission: Also referred to as Homolysis, Homolytic
fission refers to the process wherein each of the atoms acquires
one of the bonding electrons.
● Heterolytic Fission: Also referred to as Heterolysis, Heterolytic
Fission refers to the process wherein when the bond is broken,
one of the atoms acquires both of the bonding electrons.
In case B is more electronegative than A, further to acquires both the
bonding electrons and becomes negatively charged. The products of
heterolytic fission are ions.
Organic Reaction Mechanism: Inductive Effect
The frequently observed electron displacement effects in the substrate
molecules are as following: it is a permanent effect which comes into
existence when an electron is withdrawing or an electron donating
group is attached to a chain of singly bonded carbon atoms.
The displacement of sigma-electrons along a saturated carbon chain
due to the presence of an electron withdrawing group or electron
repelling group at one end of the chain resulting in the development of
partial positive or partial negative charges in the decreasing order of
magnitude is called an inductive effect or I effect.
Organic Reaction Mechanism: Electromeric Effect
Electromeric effect or E effect refers to the complete transfer of the
shared pair of pie electrons of multiple bonds to one of the shared
atoms in the presence of an attacking reagent.
Resonance Effect (Mesomeric Effect)
Resonance refers to the phenomenon in which a molecule is
represented by several electronic structures which do not differ much
in their energy contents and are obtained by the oscillation of pie
electrons. Such structures are called canonical forms and the molecule
is said to be a resonance hybrid of these canonical forms.
The permanent effect involving the transfer of electron relayed
through pie electrons of multiple bonds in a chain of carbon atoms in a
molecule is called the mesomeric effect. The mesomeric effect is a
permanent effect and comes into existence in the following two cases:
● when electron withdrawing or electron pumping group is in
conjugation with a pie bond.
● when an atom or group having at least one lone pair of electron
is in conjugation with a pie bond.
Solved Examples for You
Question 1. Which of the following represents the correct IUPAC
name for the compounds concerned?
a. 2, 2-Dimethylpentane or 2-Dimethylpentane
b. 2, 4, 7-Trimethyloctane or 2, 5, 7- Trimethyloctane
c. 2-Chloro-4-methylpentane or 4-Chloro-2-methylpentane
d. But-3-yn- l-ol or But-4-ol-yne.
Answer:
a. 2, 2-Demethylpentane
b. 2, 4, 7-Trimethyloctane.
c. Alphabetical order of substituents: 2- Chloro-4-methylpentane
d. But-3-yn-l-ol. Lower locant for the principal functional group,
i.e., alcohol.
Question 2. Draw formulas for the first five members of each
homologous series beginning with the following compounds,
a. H—COOH
b. CH3COCH3
c. H—CH=CH2
Answer:
(a) CH3—COOH
CH3CH2—COOH CH3CH2CH2—COOH
CH3CH2CH2CH2—COOH
(b) CH3COCH3
CH3COCH2CH3
CH3COCH2CH2CH3
CH3COCH2CH2CH2CH3
CH3CO(CH3)4CH3
(c) H—CH=CH2
CH3CH=CH2
CH3CH2CH=CH2
CH3CH2CH2CH=CH2
CH3CH2CH2CH2CH=CH2
Question 3. Which of the two: O2NCH2CH2O– or CH3CH2O– is
more stable and why?
Answer: O2N——<——- CH2——<——- CH2 —<——- O– is more
stable than CH3——<——-CH2——<——-O- because NO2 group
has -I-effect and hence it tends to disperse the -ve charge on the
O-atom. In contrast, CH3CH2 has +I-effect. It, therefore, tends to
intensify the -ve charge and hence destabilizes it.
Question 4. Explain why (CH3)3C+ is more stable than CH3C+H2.
Answer: (CH3)3C+ has nine alpha hydrogens and has nine
hyperconjugation structures while CH3C+H2 has three alpha
hydrogens and has three hyperconjugation structures,
therefore(CH3)3C+ is more stable than CH3C+H2.