Isomers

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Isomers are basically two or more compounds that have the same molecular structure but are different in their structural formulas and have different chemical properties. An example of an isomer would be butane and isobutane.

There are two main types of isomerism: structural isomerism and stereoisomerism. Structural isomerism includes chain, position and functional group isomerism, whereas stereoisomerism refers to geometric isomerism and optical isomerism[1].

The number of structural isomers possible in organic chemistry increases dramatically in relation to increasing the length of the carbon chain, for example, decane (containing 10 carbons) has 75 structural isomers, whereas a chain consisting of 40 carbon atoms will have 62,000,000,000 structural isomers. Position isomers and functional group isomers refer to the position of the functional group on the carbon chain and also the type of functional group present in the molecule. Chain isomerism, as one may expect, denotes changes to the structure of the carbon chain.

Geometric isomerism is also referred to as cis-trans or E-Z isomerism. It relies on the restricted rotation of the carbon-carbon double bond, as different isomers arise from the positions of similar or dissimilar groups on the carbons attached by the double bond. Optical isomerism requires a chiral carbon to be bonded to 4 different groups in a molecule. Optical isomers are non-superimposable on one another and are often referred to as right hand and left-hand enantiomers.

There are two main types of isomerism: structural isomerism and stereoisomerism. Structural isomerism includes chain, position and functional group isomerism, whereas stereoisomerism refers to geometric isomerism and optical isomerism[2].

Structural Isomerism

The number of structural isomers possible in organic chemistry increases dramatically in relation to increasing the length of the carbon chain, for example, decane (containing 10 carbons) has 75 structural isomers, whereas a chain consisting of 40 carbon atoms will have 62,000,000,000 structural isomers.

Position isomerism is shown in molecules that have the same length carbon chain and the same functional groups but the different location of the functional groups. An Example of this is shown between primary and secondary alcohols, eg. propan-1-ol and propan-2-ol.

Functional group isomerism refers to the type of functional group present in the molecule. One example of this is found in aldehydes and ketones, eg. butanone and butanal. This type of isomerism can also be found between alkenes and cycloalkanes, eg. hex-1-ene and cyclohexane.

Chain isomerism denotes changes to the structure of the carbon chain but maintains the same functional group. One isomer may be branched while another may be a straight chain. For example, butane and methylpropane[3].

Stereoisomerism

Stereoisomers fall into two categories:

Geometric isomerism is also referred to as cis-trans or E-Z isomerism. It relies on the restricted rotation of the carbon-carbon double bond, as different isomers arise from the positions of similar or dissimilar groups on the carbons attached by the double bond.

Optical isomerism requires a chiral carbon to be bonded to 4 different groups in a molecule. Optical isomers are non-superimposable on one another and are often referred to as right hand and left-hand enantiomers. Meaning they are the mirror image of each other but can have completely different effects in the body. Thalidomide is an example of a drug which has two optical isomers, R and S.

References

  1. A Brief Guide to Types of Isomerism in Organic Chemistry - Compound interest, May 22nd, 2014. Available at http://www.compoundchem.com/2014/05/22/typesofisomerism/
  2. A Brief Guide to Types of Isomerism in Organic Chemistry - Compound interest, May 22nd, 2014. Available at http://www.compoundchem.com/2014/05/22/typesofisomerism/
  3. A-level Year 1 and AS Chemistry the complete course for AQA, 1st Edition, UK, CGP, 2015.
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