GAMSAT Section III – Isomerism in GAMSAT Organic Chemistry

It is often noticed that isomerism or the state, quality, or relation of two or more isomeric compounds, have been a favourite topic for the makers of GAMSAT questions. Students often tend to study this chapter superficially but as evident from the GAMSAT chemistry sample test papers issued by ACER and various other tutoring services, it can be clearly noted that most of the organic chemistry questions focus on stereoisomerism.

Now you might wonder (since the topic sounds really vast), where to begin from? Or, which are the topics of utmost importance under this chapter. Well, do not start panicking. Under isomerism, for GAMSAT chemistry, you need to have a clear idea about the three basic types of isomerism: geometrical isomerism, stereoisomerism and conformational isomerism. You don’t need to study all of these by-heart just keep in mind a few of the vital topics, that is, cis-trans or E-Z isomerism under geometrical isomerism and dl or R and S configuration under stereoisomerism.

Isomerism in Organic Reactions: Tautomerism & Stereoisomerism

Isomerism can be defined as the phenomenon by virtue of which two or more compounds having same molecular formula can be assigned different molecular structures.

Isomerism can be broadly classified into two categories:

  • Structural isomerism/constitutional isomerism
  • Stereoisomerism

These are further divided into various sub-categories which are illustrated as follows:

Structural Isomerism

The isomers have different molecular structures due to the arrangement of atoms in their molecules. The structural isomers have same molecular formulae.


Tautomerism is a type of structural isomerism where isomers exist simultaneously in dynamic equilibrium with each other. Such isomers which are present in equilibrium with each other are known as tautomers . For example, acetylacetone exists in two forms, as shown below:



Isomers have the same structural formula but differ in relative arrangements of the atoms or groups in space within the molecule.
The spatial arrangement of atoms or groups is also referred to as configuration of the molecule and thus we can say that the stereoisomers have the same structural formula but different configuration. Geometrical and optical isomerism are two types of stereoisomerism.

Geometrical Isomerism

In this type of isomerism, the isomers possess the same structural formula but differ in the spatial arrangement of the groups around the double bond.

This isomerism is exhibited by alkenes or their derivatives. When similar groups lie on the same side of the double bond they are called as cis-isomers, while when the similar groups lie on opposite sides with respect to the double bond, they are called as trans-isomer. Trans isomers are more stable than cis isomers. For example,



Optical Isomerism

This type of isomerism arises from different arrangement of atoms or groups in three dimensional space resulting in two isomers which are mirror images of each other. Optical isomers contain an asymmetric/chiral carbon atom in their molecules. For example, lactic acid having four different groups on the central carbon atom is optically active, while succinic acid having two similar atoms on the central carbon atom is optically inactive.


The red dot in lactic acid molecule indicates the chiral carbon. The only essential condition for a chiral molecule is that it does not superimpose on its mirror image. Optical isomers differ only in their behaviour towards plane polarised light.

Isomer which rotates the plane polarised light towards left is known as laevo rotatory (l or ‘-’ sign) while that which rotates the plane polarised light to the right is known as dextro rotatory (d or ‘+’ sign).


A clear cut knowledge of the topics mentioned above will surely boost your score in GAMSAT exams. Try to avoid losing too much time on one question. If one of the questions doesn’t seem straightforward then move forth and leave it for later on. Come back to it when you think you have some extra time in hand to afford.

GAMSAT preparation for chemistry can get a lot easier or stress-free keeping these guidelines in mind. Bear in mind that in order to crack GAMSAT, apart from subject-based knowledge, you need to have a remarkable aptitude. So do not waste any more time and start your preparation from now on. Remember, better late than never.

All the best!!!

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