CHAPTER 1

Introduction to Organic Chemistry

Chapter 1 2

Introduction

Organic Chemistry

The chemistry of the compounds of carbonThe human body is largely composed of

organic compoundsOrganic chemistry plays a central role in

medicine, bioengineering etc.

Chapter 1 3

Structural TheoryCentral Premises

Valency: atoms in organic compounds form a fixed number of bonds

Carbon can form one or more bonds to other carbons

Chapter 1 4

Isomers

Isomers are different molecules with the same molecular formula Many types of isomers exist Example: C2H6O

Chapter 1 5

Constitutional Isomers

They are different compounds that have the same molecular formula but different connectivity of atoms

They often differ in physical properties (e.g. boiling point, melting point, density) and chemical properties

Chapter 1 6

Three Dimensional Shape of Molecules

Virtually all molecules possess a 3-D shape which is often not accurately represented by drawings

It was proposed in 1874 by van’t Hoff and le Bel 4 bonds around carbon where not all in a plane, but rather in a

tetrahedral arrangement

Chapter 1 7

Chemical Bonds

Octet Rule

Atoms form bonds to produce the electron configuration of a noble gas (because the electronic configuration of noble gases is particularly stable)

For most atoms of interest this means achieving a valence shell configuration of 8 electrons corresponding to that of the nearest noble gas

Atoms close to helium achieve a valence shell configuration of 2 electrons

Atoms can form either ionic or covalent bonds to satisfy the octet rule

Chapter 1 8

Electronegativity

Electronegativity is the ability of an atom to attract electrons It increases from left to right and from bottom to top in the periodic

table (noble gases excluded)

Chapter 1 9

Ionic Bonds

When ionic bonds are formed atoms gain or lose electrons to achieve the electronic configuration of the nearest noble gas

In the process the atoms become ionic The resulting oppositely charged ions attract and form ionic bonds This generally happens between atoms of widely different

electronegativities

Chapter 1 10

Covalent Bonds

Covalent bonds occur between atoms of similar electronegativity (close to each other in the periodic table)

Atoms achieve octets by sharing of valence electrons Molecules result from this covalent bonding Valence electrons can be indicated by dots (electron-dot formula

or Lewis structures) The usual way to indicate the two electrons in a bond is to use a

line (one line = two electrons)

Chapter 1 11

Representations of Structural Formulas

Dot formulas are more cumbersome to draw than dash formulas and condensed formulas

Lone-pair electrons are often (but not always) drawn in, especially when they are crucial to the chemistry being discussed

Chapter 1 12

Dash formulas

Each dash represents a pair of electrons This type of representation is meant to emphasize connectivity

and does not represent the 3-dimensional nature of the molecule

Constitutional isomers Constitutional isomers have the same molecular formula but different

connectivity Propyl alcohol (above) is a constitutional isomer of isopropyl alcohol

(below)

Chapter 1 13

Condensed Structural Formulas

In partially condensed structures all hydrogens attached to an atom are simply written after it but some or all of the other bonds are explicitly shown

In fully condensed structure all bonds are omitted and atoms attached to carbon are written immediately after it

For emphasis, branching groups are often written using vertical lines to connect them to the main chain

Chapter 1 14

Bond-Line Formulas

A further simplification of drawing organic molecules is to completely omit all carbons and hydrogens and only show heteroatoms (e.g. O, Cl, N) explicitly

Chapter 1 15

Cyclic compounds are condensed using a drawing of the corresponding polygon

Multiple bonds are indicated by using the appropriate number of lines connecting the atoms

Chapter 1 16

Three-Dimensional Formulas

Since virtually all organic molecules have a 3-dimensional shape it is often important to be able to convey their shape

The conventions for this are: Bonds that lie in the plane of the paper are indicated by a simple line Bonds that come forward out of the plane of the paper are indicated

by a solid wedge Bonds that go back out of the plane of the paper are indicated by a

dashed wedge

Chapter 1 17

Trigonal planar arrangements of atoms can be drawn in 3-D in the plane of the paper

Bond angles should be approximately 120o

These can also be drawn side-on with the central bond in the plane of the paper, one bond forward and one bond back

Linear arrangements of atoms are always best drawn in the plane of the paper

Chapter 1 18

Carbon-carbon Covalent Bonds

Carbon forms strong covalent bonds to other carbons and to other elements such as hydrogen, oxygen, nitrogen and sulfur

Organic compounds are grouped into functional group families

A functional group = a specific grouping of atoms (e.g. carbon- carbon double bonds are in the family of alkenes)

Chapter 1 19

Hydrocarbons: Representative Alkanes, Alkenes Alkynes, and

Aromatic Compounds

Hydrocarbons contain only carbon and hydrogen atoms Subgroups of Hydrocarbons:

Alkanes contain only carbon-carbon single bonds Alkenes contain one or more carbon-carbon double bonds Alkynes contain one or more carbon-carbon triple bonds Aromatic hydrocarbons contain benzene-like stable structures

Saturated hydrocarbons: contain only carbon-carbon single bonds e.g. alkanes

Unsaturated hydrocarbons: contain double or triple carbon-carbon bonds e.g. alkene, alkynes, aromatics

Chapter 1 20

Representative Hydrocarbons

Alkanes Principle sources of alkanes are natural gas and petroleum

Smaller alkanes (C1 to C4) are gases at room temperature

Methane: Major component of natural gas Produced by primitive organisms called methanogens found in mud,

sewage and cows’ stomachs

Chapter 1 21

Alkenes Ethene (ethylene) - a major industrial feedstock

Used in the production of ethanol, ethylene oxide and the polymer polyethylene

Propene (propylene) - also very important in industry Molecular formula C3H6

Used to make the polymer polypropylene and is the starting material for acetone

Many alkenes occur naturally

Chapter 1 22

Alkynes Ethyne (acetylene) is used in welding torches because it burns at

high temperature

Many alkynes are of biological interest Capillin is an antifungal agent found naturally Dactylyne is a marine natural product Ethinyl estradiol is a synthetic estrogen used in oral contraceptives

Chapter 1 23

Benzene Benzene is the prototypical aromatic compound

The Kekulé structure (named after August Kekulé who formulated it) - a six-membered ring with alternating double and single bonds

Resonance theory explains this by suggesting there are two resonance hybrids that contribute equally to the real structure

The real structure is often depicted as a hexagon with a circle in the middle

Chapter 1 24

Functional Groups

Functional group families are characterized by the presence of a certain arrangement of atoms called a functional group

A functional group is the site of most chemical reactivity of a molecule

The functional group is responsible for many of the physical properties of a molecule

Alkanes do not have a functional groups Carbon-carbon single bonds and carbon-hydrogen bonds are

generally very unreactive

Chapter 1 25

Alkyl Groups and the Symbol R Alkyl groups are obtained by removing a hydrogen from an alkane Often more than one alkyl group can be obtained from an alkane by removal of different

kinds of hydrogens

R is the symbol to represent a generic alkyl groups

Chapter 1 26

A benzene ring with a hydrogen removed is called a phenyl and can be represented in various ways

Toluene (methylbenzene) with its methyl hydrogen removed is called a benzyl group

Chapter 1 27

Alkyl Halides

In alkyl halides, halogen (F, Cl, Br, I) replaces the hydrogen of an alkane

They are classified based on the carbon the halogen is attached to

Chapter 1 28

Alcohols

In alcohols the hydrogen of the alkane is replaced by the hydroxyl (-OH) group

Alcohols are also classified according to the carbon the hydroxyl is directly attached to

Chapter 1 29

Ethers

Ethers have the general formula R-O-R or R-O-R’ where R’ is different from R

These can be considered organic derivatives of water in which both hydrogens are replaced by organic groups

The bond angle at oxygen is close to the tetrahedral angle

Chapter 1 30

• Amines

Amines are organic derivatives of ammonia They are classified according to how many alkyl groups replace the

hydrogens of ammonia This is a different classification scheme than that used in alcohols

Chapter 1 31

Aldehydes and Ketones

Both contain the carbonyl group

Aldehydes have at least one carbon attached to the carbonyl group

Ketones have two organic groups attached to the carbonyl group

The carbonyl carbon is sp2 hybridized

Chapter 1 32

Carboxylic Acids, Esters and Amides

All these groups contain a carbonyl group bonded to O or N Carboxylic Acids

Esters

Chapter 1 33

Amide

Nitriles An alkyl group is attached to a carbon triply bonded to a nitrogen

This functional group is called a cyano group

Chapter 1 34

Summary of Important Families of Organic Compounds

Chapter 1 35

Chapter 1 36

Physical Properties and Molecular Structure

The strength of intermolecular forces (forces between molecules) determines the physical properties (i.e. melting point, boiling point and solubility) of a compound

Stronger intermolecular forces result in high melting points and boiling points

The type of intermolecular forces important for a molecule are determined by its structure

Chapter 1 37

Chapter 1 38

Dipole-Dipole Forces

Dipole-dipole forces are between molecules with permanent dipoles

There is an interaction between + and - areas in each molecule; these are much weaker than ion-ion forces

Molecules align to maximize attraction of + and - parts of molecules

Example: acetone

Chapter 1 39

Hydrogen Bonds

Hydrogen bonds result from very strong dipole-dipole forces There is an interaction between hydrogens bonded to strongly

electronegative atoms (O, N or F) and nonbonding electron pairs on other strongly electronegative atoms (O, N or F)

Chapter 1 40

Example Ethanol (CH3CH2OH) has a boiling point of +78.5oC; its isomer

methyl ether (CH3OCH3) has a boiling point of -24.9oC Ethanol molecules are held together by hydrogen bonds whereas

methyl ether molecules are held together only by weaker dipole-dipole interactions

Chapter 1 41

van der Waals Forces (London or Dispersion Forces)

Van der Waals forces result when a temporary dipole in a molecule caused by a momentary shifting of electrons induces an opposite and also temporary dipole in an adjacent molecule

These temporary opposite dipoles cause a weak attraction between the two molecules

Molecules which rely only on van der Waals forces generally have low melting points and boiling points

Chapter 1 42

Solubilities

Water dissolves ionic solids by forming strong dipole-ion interactions

These dipole-ion interactions are powerful enough to overcome lattice energy and interionic interactions in the solid

Chapter 1 43

Generally like dissolves like Polar solvents tend to dissolve polar solids or polar liquids Methanol (a water-like molecule) dissolves in water in all proportions

and interacts using hydrogen-bonding to the water

A large alkyl group can overwhelm the ability of the polar group to solubilize a molecule in water

Decyl alcohol is only slightly soluble in water The large alkyl portion is hydrophobic (“water hating”) and

overwhelms the capacity of the hydrophilic (“water loving”) hydroxyl

Chapter 1 44

Summary of Attractive Electric Forces