Isomerism

Isomerism of organic compound

What are isomers?

Isomers are molecules with the same

molecular formula, but different

arrangements of atoms. There are different

types of isomers, shown by the diagram.

Isomerism

Structural Isomerism

Structural isomerism occurs when two or more organic compounds have the same molecular formulae, but different structures. These differences tend to give the molecules different chemical and physical properties. There are three types of structural isomerism that you need to be aware of at course: chain isomerism, positional isomerism and functional isomerism. There is a fourth type, known as tautomerism (where there are two isomers are known as the keto and enol isomers).

Chain Isomerism

Chain isomerism occurs when the way carbon atoms are linked together is different from compound to compound. It is an example of structural isomerism, and is also called nuclear isomerism.

see the isomers of pentane pentane 2-methylbutane 2,2-dimethylpropane

C5H12

Positional Isomerism

Positional isomerism, an example of

structural isomerism, occurs when

functional groups are in different positions

on the same carbon chain. You usually

meet positional isomers of alcohols and

alkenes.

butan-1-ol butan-2-ol

but-1-ene but-2-ene

Note: this is cis-

but-2-ene, which

has a geometric

isomer called

trans-but-2-ene

(select here to find

out more)

2-methylphenol 3-methylphenol 4-methylphenol

Functional Isomerism

Functional isomerism, an example of

structural isomerism, occurs substances

have the same molecular formula but

different functional groups. This means that

functional isomers belong to different

homologous series. There are two functional

group isomers of which you need to be

aware:

alcohols and ethers

aldehydes and ketones

Ethanol methoxymethane C2H6O Alcohols have the

hydroxyl group,

–OH.

Ethers have the

functional group

R–O–R'.

Propanal propanone (acetone) C3H6O

Aldehydes and ketones both have the carbonyl group C=O. In ketones this is attached to two carbon atoms; in aldehydes it is attached to 1 or 2 hydrogen atoms.

Stereo-isomerism

Stereo-isomerism occurs when the atoms in a molecule can have different arrangements in space. There are two types of stereo-isomerism: geometrical isomerism and optical isomerism. Geometrical isomers can have very different physical properties, such as different melting points, but they tend to have the same chemical properties. Optical isomers have the same chemical and physical properties, except that one structure rotates the plane of polarised light to the right and the other rotates it to the left.

Geometrical Isomerism

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.

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!

C C

CH3

H

CH3

H

C C

H

H

CH3

CH3

C C

H

H

H

CH2 CH3

Optical Isomerism

Optical isomerism is an example of stereoisomerism. It occurs when substances have the same molecular and structural formulae, but one cannot be superimposed on the other. Put simply, they are mirror images of each other (see the diagram).

No matter how hard you try, the molecule on the left will not turn into the molecule on the right – unless you break and make some bonds! Molecules like this are said to be chiral (pronounced ky-ral), and the different forms are called enantiomers

Optical isomers can occur when there is an asymmetric carbon atom.

An asymmetric carbon atom is one which is bonded to four different

groups. The groups can be something hideously complex, or

something comfortably simple like a hydrogen or chlorine atom.

Remember:

* there must be four groups, and

* they must be different.

Tro, Chemistry: A Molecular

Approach

15

Nonsuperimposable Mirror

Images

mirror image cannot be rotated so all its atoms align

with the same atoms of the original molecule

Tro, Chemistry: A Molecular

Approach

16

Chirality

any molecule with a nonsuperimposable mirror image is said to be chiral

any carbon with 4 different substituents will be a chiral center

a pair of nonsuperimposable mirror images are called a pair of enantiomers

Tro, Chemistry: A Molecular

Approach

17

Optical Isomers of 3-methylhexane

Tro, Chemistry: A Molecular

Approach

18

Plane Polarized Light

light that has been filtered so that only

those waves traveling in a single plane are

allowed through

Tro, Chemistry: A Molecular

Approach

19

Optical Activity

a pair of enantiomers have all the same physical properties except one – the direction they rotate the plane of plane polarized light

each will rotate the plane the same amount, but in opposite directions

dextrorotatory = rotate to the right

levorotatory = rotate to the left

an equimolar mixture of the pair is called a racemic mixture

rotations cancel, so no net rotation

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Chemical Behavior of

Enantiomers

a pair of enantiomers will have the same chemical

reactivity in a non-chiral environment

but in a chiral environment they may exhibit different

behaviors

enzyme selection of one enantiomer of a pair

Optical isomers can rotate the plane of polarisation of plane-polarised light:

one enantiomer rotates the polarised light clockwise (to the right) and is the (+) enantiomer;

the other rotates the polarised light anticlockwise (to the left) and is called the (–) enantiomer.

A mixture containing equal concentrations of the (+) and (–) enantiomers is not optically active (it will not rotate the plane of polarisation). It is called a racemic mixture or racemate.

(+)-lactic acid (–)-lactic acid

Lactic acid (2-hydroxypropanoic acid) is a fairly

common and simple example of optical isomerism. The

(+) enantiomer of lactic acid is found in muscle. Sour

milk contains a racemic mixture of the two enantiomers.

D-glyceraldehyde L-glyceraldehyde

2,3-dihydroxypropanal (glyceraldehyde) is a fairly common and simple example of optical isomerism, too. This is because glyceraldehyde is used as a standard by which other chiral molecules are compared. There are two enantiomers of glyceraldehyde, depending on the position of the –OH (hydroxyl) group on the molecule. These are known as D-glyceraldehyde and L-glyceraldehyde.

The little capital letters D and L are deliberate. The positions of the hydroxyl groups on other chiral molecules can be compared with glyceraldehyde to see if they are the D-enantiomer or the L-enantiomer. This is very common in Biology (natural sugars are D-enantiomers and amino acids are L-enantiomers). However, knowing whether a molecule is the D or L-enantiomer does not tell us whether it is the (+) or (–) enantiomer – so be careful!

MOLECULES WITH TWO OR MORE

STEREOCENTERS

To generalize, for a molecule with n stereocenters, the

maximum number of stereoisomers possible is 2n2.

For a molecule with one srereocenter, 21 = 2

stereoisomers are possible, for a molecule with two

srereocenters, 22 = 4 stereoisomers are possible.

Enantiomers and Diastereoisomers

C

C

CHO

OHH

H OH

CH2OH

C

C

CHO

HHO

HO H

CH2OH

C

C

COH

HHO

H OH

CH2OH

C

C

CHO

OHH

HO H

CH2OH

(a) (b)

One pair of enantiomers(Erythrose)

A second pair of enantiomers(Erythrose)

(d)(c)

Stereoisomers (a) and (b) are nonsuperposable mirror images and are,

therefore, a pair of enantiomers. Stereoisomers (c) and (d) are also

nonsuperposable mirror images and are a second pair of enantiomers.

Diastereoisomers are stereoisomers that are not mirror images.

Compound (a) and (c), (a) and (d), (b) and (c), (b) and (d) are

diastereisomers.

26

Figure 5.8 Summary: The four

stereoisomers of 2,3-

dibromopentane

Meso Compounds

C

C

COOH

OHH

H OH

COOH

C

C

COOH

HHO

HO H

COOH

C

C

COOH

HHO

H OH

COOH

C

C

COOH

OHH

HO H

COOH

(a) (b)

A pair of enantiomers A meso compound

(d)(c)

A plane of symmetry

In tartaric acid, carbon 2 and 3 are stereocenters, and, using the 2n rule, the maximum number of stereoisomers possible is

22 = 4. Structures (a) and (b) are nonsuperposable mirror images and are, therefore, a pair of enantiomers. Structures (c)

and (d) are also mirror images, but they are superposable. Therefore, (c) and (d) are not different molecules; they are the

same molecule, just oriented differently. Because (c) and its mirror image are superposable, (c) is achiral.

Another way to determine that (c) is achiral is to see that it has a plane of symmetry that bisects the molecule in such a way

that the top half is the reflection of the bottom half. Thus, even though (c) has two stereoisomers, it is achiral . The

stereoisomer of tartaric acid represented by (c) or (d) is called a meso compound. A meso compound is an achiral

compound that contains two or more stereocenters.

28

Figure 5.9 Summary: The three

stereoisomers 2,3-

dibromobutane

• Larger organic molecules can have two, three or

even hundreds of stereogenic centers.

30

How many stereogenic centers does each molecule have?

a)

Br

Br

b)

31

c)

H2N

HN

NH

OH

CO2H

O

SH

O

O

Only carbons attached to four different groups.

• Stereogenic centers may also occur at carbon

atoms that are part of a ring.

• To find stereogenic centers on ring carbons, always

draw the rings as flat polygons, and look for

tetrahedral carbons that are bonded to four different

groups.

33

Locate the stereogenic center in the following:a)

and b)

No stereogenic

centers.

b)

O

a)