Isomerism Paper

8/7/2019 Isomerism Paper

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Isomers are compounds that have the same molecular formula but different structures.

The different types of isomers can be summarized by the chart provided below. The three major

categories of isomers are conformational isomers, constitutional isomers and stereoisomers. Each

of the major categories can be further divided into subcategories.

Conformational isomers represent the first major category of isomers. Conformational

isomers are in reality a form of stereoisomers since they have the same molecular formula, same

connectivity and just a difference in 3D orientation. A Newman projection for butane is shown

below:

Isomers

Conformational Isomers

(R otation around single bonds)

Stereoisomers

(Same Connectivity)

Constitutional Isomers

(Different Connectivity)

Sk eletal

Positional

Functional

Without StereocentersWith Stereocenters

Enantiomers

(+/-) Diastereomers

MesoDiasteromers

Cis/trans isomers



All the structures represent butane, but in different conformations. The energy diagram below

illustrates the difference in energy for each of the conformation. It is clear that the anti

conformation of butane is the most energetically favorable, whereas the eclipsed steric strain of

the two methyl groups is the most energetically unfavorable.

Stereoisomers represent the second major category of isomers. Stereoisomers are

compounds with the same molecular formula, the same connectivity and just a difference in 3D



orientation. Stereoisomers, unlike conformational isomers, do not differ from each other as a

result of rotation around single (sigma) bonds. They represent different compounds.

The first sub-category of stereoisomers are known as enantiomers. Enantiomers are

molecules that are nonsuperimposable mirror images of one another. Optical isomers such as

enantiomers possess stereocenters and can rotate plane-polarized light. Isomers which rotate the

plane to the right (clockwise) are dextrorotatory and given the prefix (+). Isomers which rotate

the plane to the left (anticlockwise) are laevorotatory and given the prefix (-). If a solution

contains equal amounts of 2 optical isomers it is called a racemic mixture and will not be

optically active. Enantiomers have the same melting points, boiling points and solubilities. An

example of enantiomers is given below:

The second sub-category of stereoisomers are known as diastereomers. Diastereomers are

stereoisomers that not identical and not enantiomers. A pair of diastereomers has the same

configuration around at least one stereocenter and the opposite configuration around the other

stereocenters. If the the configuration differs at only one stereocenter then the diastereomer is

referred to as an epimer. Diastereomers are different compounds. Unlike enantiomers, they have

different melting points, boiling points and solubilities. Also unlike enantiomers, mixtures of

diastereomers can be separated much easier than enantiomers through physical means. An

example of diastereomers is given below:



Constitutional isomers are isomers with the same molecular formula, different

connections and so represent different compounds. The first sub-category of constitutional

isomers is functional and they represent compounds with the same molecular formula, but with

different functional groups. Alcohols and ethers tend to be closely related to this type of

isomerism. An example can be seen with C3H8O:

Compounds with the molecular formula C3H8O:

CH3CH2CH2OH and CH3OCH2CH3

1-propanol ethyl methyl ether

The second sub-category of constitutional isomers is positional isomers and they

represent compounds with the same molecular formula, same functional group, but different

position of that functional group. An example can be seen with the molecular formula C3H8O:

Positional Isomers of C3H8O:

CH3CH(OH)CH3 and CH3CH2CH2OH

2-propanol 1-propanol

Because both positional isomers have the same functional group both would have similar

chemical properties, but the difference in position would result in difference in physical

properties. One noted difference has to do with dehydration. Primary alcohols are known to be

easier to dehydrate than secondary alcohols.

The third sub-category of constitutional isomers is known as structural isomers.

Structural isomers represent compounds with the same molecular formula, but different



connections on the parent chain which result in different compounds. An example of the

constitutional isomers of hexane are given below:

All 5 constitutional isomers possess the molecular formula of C6H14. Since the all have the same

elements of carbon and hydrogen all would have similar chemical properties. Being alkanes they

are all near inert and undergo two main reactions: (1) radical halogenation and (2) combustion.

However, because of the difference in shape these isomers would have rather different physical

properties. For example, hexane has the least amount of branching and so it would have the

highest boiling point. On the other hand, 2,2-dimethylbutane and 2,3-dimethylbutane have much

more branching and so would have the lowest boiling points.

Isomerism Paper

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