These notes were written for the old IB syllabus (2009). The new IB syllabus for first examinations 2016 can be accessed by clicking the link below.

IB syllabus for first examinations 2016

Isomerism


There are four types of isomerism, broadly grouped into two different concepts:

1. Structural isomerism - Due to a change in the arrangement of the atoms in a structure.

¨ Positional
¨ Functional group

2. Stereoisomerism - Due to the spacial properties of the molecule itself.

¨ Geometrical
¨ Optical


1. Structural isomerism


Positional isomerism

The molecular formula is the same but the arrangement of atoms in the molecule is different although there is no change in functional group. There may be a difference in the branching of a hydrocarbon, or the position of an adduct to the chain may be different. Structural isomers have different physical and chemical properties.

Methyl propane Butane
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In this instance the physical properties show a difference in boiling point with the more branched isomer (the methyl propane) having the lower b.p. The explanation for this is that this molecule is more spherical and has less surface area for the Van der Waal's forces to act.

1- chloro propane 2- chloro propane
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In this case the two isomers will have different reaction rates with sodium hydroxide (nucleophilic substitution by the OH- group). The 2- chloro isomer reacts faster and more easily than the 1- chloro isomer.


Functional group isomerism

The molecular formula is the same but the atoms are arranged in such a way that there is a change in the functional group. This of course produces radical change in both the physical and chemical properties of the isomers.

Example

ethenol and ethanal



Example

ethoxy ethane and butanol



2. Stereoisomerism

Geometrical isomerism

In this type of isomerism there is a lack of rotation between two asymmetric sides which produces different chemical (and physical) environments. The simplest case is that of an alkene where there is no rotation possible about the double bond (because it would result in rupture of the pi orbital). If there are two different attachments on each side of the double bond then geometrical isomerism results. If the main groups seem to "cross" the double bond then the prefix trans- is used. If they seem to be on the same (lateral) side of the double bond then the prefix cis- is used. In geometrical isomers there are differences in both physical and chemical properties.


Example e) but - 2 - ene


Other situations where rotation is restricted also produce geometric isomerism. This may be due to steric hindrance or rings of carbon atoms. Find one example of each of these situations.


Optical isomerism

When four separate groups are attached to a carbon atom it is possible to have a non-superimposible mirror image of the molecule. This is called optical isomerism.

Each of the mirror images is called an enantiomer. A solution of an enantiomer (or a pure liquid enantiomer) will rotate the plane of polarised light either to the left (laevorotatory) or to the right (dextrorotatory). The degree of rotation is determined by the nature of the molecule and the concentration of the solution.

Specific rotation applies to a 1 molar solution at RTP. An equimolar solution of two enantiomers has no optical properties as the laevo rotatory form counteracts the effects of the dextro form. Such a solution is said to be a racemate or racemic solution.

Optical isomers have identical physical properties (except for polarised light) and identical chemical properties (unless reacting with other optical isomers when the orientation of the two molecules could have a bearing on the final result, for example in biological systems. Note that this fact is used to separate enantiomers to obtain pure optical forms).


Resources

 


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