LOVELY PROFESSIONAL UNIVERSITY

Phagwara(Punjab.)

TERM PAPER

SUB: -CHEMISTRY-101

TOPIC: - ISOMERISM(OPTICAL AND GEOMETRICAL).

SUBMITTED TO, SUBMITTED BY,

Mr. Rahul Mehta Omkar kumar jha

RH4901-A12

10902923

Mechanical Engg.. IV Sem(LEET)

INDEX

1. Acknowledgement

2. Introduction

3. Types (Isomerism)

4. STEREOISOMERISM

5. TYPES (STEREOISOMERISM)

6. OPTICAL ISOMERISM

7. GEOMETRICAL ISOMERISM

8. IN BRIEF

9. refrences

ACKNOWLEDGEMENT

I hereby submit my term paper given to me by my teacher

of the subject ‘CHE-101’ namely ‘Mr. Rahul Mehta’.I have

prepared this term paper under the guidance of my subject

teacher. I would thank my class teacher, my subject teacher

and my friends who have helped me to complete the term

paper. I am also highly thankful to all the staff and

executives of the esteemed university namely ‘LOVELY

PROFESSIONAL UNIVERSITY, PHAGWARA, JALANDHAR’.

Omkar kumar jha

RH-4901-A12

10902923

B-tech ME(IV Sem)

INTRODUCTION

The word is derived from the Greek ισομερης, isomerès; isos

= "equal", méros = "part".

And the the isomer’s are

Compounds having the same molecular formula, but

differing in physical and chemical properties are known as

isomers. This phenomenon is known as isomerism. The

isomers can be identified and distinguished from one

another because of difference in their physical and chemical

properties.

Or,

Some web pages includes as:-

Britannica Concise Encyclopedia: -

One of two or more substances with identical molecular formulas but

different configurations, differing only in the arrangement of their

component atoms. It usually refers to stereoisomer (rather than

constitutional isomers or tautomers; see isomerism, tautomerism), of

which there are two types. Optical isomers, or enantiomers, occur in

mirror-image pairs. Geometric isomers are often the result of rigidity

in the molecular structure; in organic compounds, this is usually due

to a double bond or a ring structure. In the case of a double bond

between two carbon atoms, if each has two other groups bonded to it

and all are rigidly in the same plane, the corresponding groups can be

on the same side of the C=C bond or across the C=C bond from each

other. An analogous distinction can be made for ring structures that

are all in a plane, between isomers whose substituent groups are on

the same side and isomers whose substituent groups are on both

sides of the plane. Diastereomers that are not enantiomers also fall

into this category. Most cis-trans isomers are organic compounds.

Food and Nutrition chemistry :-

Molecules containing the same atoms but differently arranged, so that

the chemical and biochemical properties differ. (1)In positional

isomers the functional groups are on different carbon atoms; e.g.

leucine and isoleucine.(2)D- and L-isomerism refers to the spatial

arrangement of four different chemical groups on the same carbon

atom (stereo-isomerism or optical isomerism). R- and S-isomerism is

the same, but determined by a set of systematic chemical rules. See D-

.(3)Cis- and trans-isomerism refers to the arrangement of groups

adjacent to a carbon-carbon double bond; in the cis-isomer the groups

are on the same side of the double bond, while in the trans-isomer

they are on opposite sides.

Columbia Encyclopedia: -

isomer (ī 'səmər), in chemistry, one of two or more compounds having

the same molecular formula but different structures (arrangements of

atoms in the molecule). Isomerism is the occurrence of such

compounds. Isomerism was first recognized by J. J. Berzelius in 1827.

Early work with stereoisomers was carried out by Louis Pasteur, who

separated racemic acid into its two optically active tartaric acid

components by crystallization (1848). Pasteur's results were given

theoretical basis by J. H. Van't Hoff and independently by J. A. le Bel

(1864).

+++++++++++++++++++++++++++++++++++++++++++

===========================================+++++++++++++++++++++++++++++++++++++++++++

A simple example of isomerism is given by propanol: it has the

formula C3

H8

O (or C3

H7

OH) and occurs as two isomers: propan-1-ol (n-

propyl alcohol; I) and propan-2-ol (isopropyl alcohol; II)

Note that the position of the oxygen atom differs between the two: it

is attached to an end carbon in the first isomer, and to the center

carbon in the second.

There is, however, another isomer of C3

H8

O which has significantly

different properties: methoxyethane (methyl-ethyl-ether; III). Unlike

the isomers of propanol, methoxyethane has an oxygen connected to

two carbons rather than to one carbon and one hydrogen. This makes

it an ether, not an alcohol, as it lacks a hydroxyl group, and has

chemical properties more similar to other ethers than to either of the

above alcohol isomers.

Examples of isomers having different medical properties can be easily

found. For example, in the placement of methyl groups. In substituted

xanthines, Theobromine, found in chocolate, is a vasodilator with

some effects in common with caffeine, but if one of the two methyl

groups is moved to a different position on the two-ring core, the

isomer is theophylline, which has a variety of effects, including

bronchodilation and anti-inflammatory action. Another example of

this occurs in the phenethylamine-based stimulant drugs.

Phentermine is a non-chiral compound with a weaker effect than

amphetamine. It is used as an appetite reducing medication and has

mild or no stimulant properties. However, a different rearrangement

of the same atoms gives dextromethamphetamine, which is more

potent than amphetamine; it is a very strong stimulant.

Allene and propyne are examples of isomers containing different bond

types. Allene contains two double bonds, while propyne contains one

triple bond.

TYPES OF ISOMERISM

Isomerism may be classified into two types :-

1. Structural isomerism

2. Stereo isomerism

OR,

HERE WE WILL STUDY IN DEPTH ABOUT STEREOISOMERISM: -

STEREOISOMERISM

In stereoisomer the bond structure is the same, but the geometrical

positioning of atoms and functional groups in space differs. This class

includes enantiomers where different isomers are non-

superimposable mirror-images of each other, and diastereomers when

they are not.Diastereomerism is again subdivided into "cis-trans

isomers"", which have restricted rotation within the molecule

(typically isomers containing a double bond) and "conformational

isomers" (conformers), which can rotate about one or more single

bonds within the molecule. An obsolete term for "cis-trans isomerism"

is "geometric isomers".

For compounds with more than two substituent E-Z notation is used in

stead of cis and trans. If possible, E and Z (written in italic type) is

also preferred in compounds with two substituent. In octahedral

coordination compounds fac- (with facial legends) and mer- (with

meridional legends) isomers occur. Note that although conformers can

be referred to as diastereomers, they are not stable diastereomers,

since bonds in conformers can be rotated to make them mirror

images. While structural isomers typically have different chemical

properties stereoisomer behave identically in most chemical

reactions, except in their reaction with other stereoisomer. Enzymes

however can distinguish between different enantiomers of a

compound, and organisms often prefer one isomer over the other.

Some stereoisomer also differ in the way they rotate polarized light.

TYPES OF STEREOISOMERISM

1. OPTICAL ISOMERISM

2. GEOMETRICAL ISOMERISM

OPTICAL ISOMERISM

Why optical isomers?

Optical isomers are named like this because of their effect

on plane polarised light. Simple substances which show

optical isomerism exist as two isomers known as

enantiomers.

A solution of one enantiomer rotates the plane of

polarisation in a clockwise direction. This enantiomer

is known as the (+) form.

For example, one of the optical isomers (enantiomers)

of the amino acid alanine is known as (+)alanine.

A solution of the other enantiomer rotates the plane of

polarisation in an anti-clockwise direction. This

enantiomer is known as the (-) form. So the other

enantiomer of alanine is known as or (-)alanine.

If the solutions are equally concentrated the amount of

rotation caused by the two isomers is exactly the same

- but in opposite directions.

When optically active substances are made in the lab,

they often occur as a 50/50 mixture of the two

enantiomers. This is known as a racemic mixture or

racemate. It has no effect on plane polarised light.

How optical isomers arise

The examples of organic optical isomers required at A' level all

contain a carbon atom joined to four different groups. These two

models each have the same groups joined to the central carbon atom,

but still manage to be different:

Obviously as they are drawn, the orange and blue groups aren't

aligned the same way. Could you get them to align by rotating one of

the molecules? The next diagram shows what happens if you rotate

molecule B.

They still aren't the same - and there is no way that you can rotate

them so that they look exactly the same. These are isomers of each

other.

They are described as being non-superimposable in the sense that (if

you imagine molecule B being turned into a ghostly version of itself)

you couldn't slide one molecule exactly over the other one. Something

would always be pointing in the wrong direction.

What happens if two of the groups attached to the central carbon

atom are the same? The next diagram shows this possibility.

The two models are aligned exactly as before, but the orange group

has been replaced by another pink one.

Rotating molecule B this time shows that it is exactly the same as

molecule A. You only get optical isomers if all four groups attached to

the central carbon are different.

Some real examples of optical isomers

1. Butan-2-ol

The asymmetric carbon atom in a compound (the one with four

different groups attached) is often shown by a star.

It's extremely important to draw the isomers correctly. Draw one of

them using standard bond notation to show the 3-dimensional

arrangement around the asymmetric carbon atom. Then draw the

mirror to show the examiner that you know what you are doing, and

then the mirror image.

Notice that you don't literally draw the mirror images of all the letters

and numbers! It is, however, quite useful to reverse large groups -

look, for example, at the ethyl group at the top of the diagram.

It doesn't matter in the least in what order you draw the four groups

around the central carbon. As long as your mirror image is drawn

accurately, you will automatically have drawn the two isomers.

So which of these two isomers is (+)butan-2-ol and which is (-)butan-2-

ol? There is no simple way of telling that. For A'level purposes, you

can just ignore that problem - all you need to be able to do is to draw

the two isomers correctly.

2. aminopropanoic acid (alanine)

This is typical of naturally-occurring amino acids. Structurally, it is

just like the last example, except that the -OH group is replaced by -

NH2

The two enantiomers are:

Only one of these isomers occurs naturally: the (+) form. You can't tell

just by looking at the structures which this is.

It has, however, been possible to work out which of these structures is

which. Naturally occurring alanine is the right-hand structure, and the

way the groups are arranged around the central carbon atom is known

as an L- configuration. Notice the use of the capital L. The other

configuration is known as D-.

So you may well find alanine described as L-(+)alanine.

That means that it has this particular structure and rotates the plane

of polarisation clockwise.

Even if you know that a different compound has an arrangement of

groups similar to alanine, you still can't say which way it will rotate

the plane of polarisation.

The other amino acids, for example, have the same arrangement of

groups as alanine does (all that changes is the CH3

group), but some

are (+) forms and others are (-) forms.

It's quite common for natural systems to only work with one of the

enantiomers of an optically active substance. It isn't too difficult to

see why that might be. Because the molecules have different spatial

arrangements of their various groups, only one of them is likely to fit

properly into the active sites on the enzymes they work with.

In the lab, it is quite common to produce equal amounts of both forms

of a compound when it is synthesised. This happens just by chance,

and you tend to get racemic mixtures.

GEOMETRICAL ISOMERISM

What is it?

Geometrical isomerism is an example of stereo-isomerism. This occurs

when substances have the same molecular formula, but a different

arrangment of their atoms in space. There are three ways that this can

happen:

where there is a C=C bond in the molecule;

where a molecule has rings; or

where there is a >C=N bond.

In AS and A2 Chemistry, we only need to know about geometrical

isomerism caused by a C=C bond in the molecule.

If you are studying Biology, you will meet geometrical isomerism

caused by rings when you look at sugars such as glucose, fructose,

mannose and galactose.

What is here?

I have put models of the geometrical isomers of but-1-ene and but-2-

ene here. But-1-ene does not form geometrical isomers, even though it

has a C=C bond, because one of the double-bonded carbon atoms has

two identical groups on it (hydrogen atoms in this case). But-2-ene

does form geometrical isomers because each double-bonded carbon

atom has two different groups on it. You should be prepared to spot

geometrical isomers for simple organic compounds like these for your

examinations, and you also need to be able to name the them.

cis-but-2-ene trans-but-2-ene Where like groups are on the

same side of the double

bond, we call it a cis isomer;

where they are on opposite

sides we call it a trans

isomer.

but-1-ene Although but-1-ene contains a C=C bond,

it does not form geometrical isomers.

Take care - look for different groups

on the double-bonded carbon atoms!

GEOMETRICAL ISOMERISM

A web site includes that “Isomers are two molecules with the same

molecular formula, but differ in the way the atoms are arranged.

Geometrical isomers are one form of stereoisomers which have

identical molecular formulae, the atoms are bonded together in the

same order but the arrangement of atoms in space are different.

Optical isomers are another form of stereoisomers. Geometrical

isomerism is also called cis-trans isomerism.

Cis-trans isomers occur in organic compounds which contain a

carbon-carbon double or triple bond. An example is in but-2-ene.

The cis form is where the substituent groups are on the same side of

the double bond. The trans form is where they on opposite sides.

Geometric isomers are possible for both square planar and

octahedral complexes, but not tetrahedral.

The number of geometric isomers expected for common

stereochemistries are as follows:

Square Planar:

CompoundNo. type of isomers

Ma2

b2

2(cis-andtrans-)

Mabcd 3(usecis-and rans- relations)

here a, b, c, and d refer to monodentate ligands.

A number of examples of these types have been isolated and

characterised and they show very different chemical and biological

properties. Thus for example, cis-PtCl2

(NH3

)2

is an anti-cancer agent

(cisplatin) whereas the trans- isomer is inactive against cancer (it is

toxic), and so not useful in Chemotherapy. cis- and trans- refer to the

position of 2 groups relative to each other. In the cis- isomer they are

"next to each other" i.e. at 90 degrees in relation to the central metal

ion, whereas in the trans- isomer they are "opposite each other", i.e. at

180 degrees relative to the central metal ion.

a

|

|

M ----b a----M----b

cis- trans-

The first report of the three geometric isomers being isolated and

characterised for complexes of the type [Mabcd] was by Il'ya

Chernyaev in 1928. The example above was reported by Anna Gel'man

in 1948.

IN BRIEF

We can now return to compounds that differ only in their 3-

dimensional structures. Geometric isomers have the same structural

formulas but differ in the arrangement of groups at a single atom, at

double bonds, or in rings. Cis- and trans-platin (see Figure 37) are

examples of geometric isomers based on the different arrangement of

groups at a single atom. Cis- and trans-2-butene differ in the

arrangement of the methyl groups about the double bonds.

Although geometric isomers have completely different physical and

chemical properties (for example, cis- and trans-2-butene have

different boiling points and densities), optical isomers (also called

enantiomers) differ in only one characteristic--their interaction with

plane polarized light. When a beam of light is passed through a

certain type of filter, all of the waves except those in one plane are

removed. Figure 39 shows this plane-polarized light impinging upon

and being rotated by two optical isomers. One of the optical isomers

rotates the light in one direction, the other rotates the light in the

opposite direction but by the same amount. In every other way, such

as boiling point, density, refractive index, viscosity, etc., the two

optical isomers are identical.

Figure 39. The interaction of optical isomers with plane-polarized

light.

What are these optical isomers? Optical isomers are mirror images

that are not superimposable. Your hands, assuming that we do not

examine them too closely for scratches and other imperfections, are

nonsuperimposable mirror images. Imagine that you approach a

mirror and raise your right hand in greeting so that you see the palm

of your right hand in the mirror. What you see in the mirror is the

mirror image of your right hand and it looks exactly like the palm of

your left hand. So your left hand is a mirror image of your right hand.

Now, try to superimpose your two hands by laying one atop the other

with the palms facing in the same direction. Notice that no matter how

you twist them, they cannot be oriented so that the right thumb is on

top of the left thumb, the right little finger on top of the left little

finger, etc.

If you now perform the same exercise with a pencil, you will notice

that the mirror image of a pencil is superimposable on the pencil.

Clearly, there must be something about the symmetry of your hands

that cause them to be nonsuperimposable.

On the molecular scale, most optical isomers are organic compounds,

and they have non superimposable mirror images when there are four

different groups on a carbon (the carbon is said to be chiral, from the

Greek meaning "handed"). Amino acids are good examples of

molecules that have nonsuperimposable mirror images. Alanine has

the structural formula

and the two mirror images shown in Figure 40. The peptides and

proteins that make up much of our bodies are constructed from

combinations of amino acids like alanine. One of the fascinating

aspects of chemical evolution is the fact that only one optical isomer

is used by natural organisms in producing biological materials. It is

also frequently the case that optical isomers differ in their biological

activity. We have already seen this in part I during our discussion of

levorphane and dextrophane.

Reference: 1. www.chemguide.co.uk/basicorg/isomerism/structural.html 2. www.britannica.com/EBchecked/topic/296365/isomerism 3. www.chem.uwimona.edu.jm/courses/IC10Kiso.html 4. www.goiit.com/.../0/content-isomerism-804382.htm? 5. www.chem.purdue.edu/gchelp/cchem/ 6. BOOK WRITTEN BY FORIGN WRITER OVER

WWW.BOOKS.GOOGLE.COM 7. IMAGES ARE SEARCHED OVER THE PARENT SITES AND OVER

THE GOOGLE IMAGES. 8. WWW.en.wikipedia.org/wiki/Isomer 9. www.chemguide.co.uk/basicorg/isomerism/optical.html