30: Isomerism

Question 1

What is isomerism?

Answer 1

Isomerism is the property of two or more compounds (isomers) that have the same molecular formula but different arrangement of atoms.

Question 2

What are two types of isomerism?

Answer 2

Structural isomerism and stereoisomerism.

Question 3

What are structural isomers?

Answer 3

Structural isomers are two or more compounds that have the same molecular formula but the atoms are bonded together in different orders with different structures.

Question 4

What are three types of structural isomerism?

Answer 4

1. Chain isomerism, where there is a different arrangement of carbon atoms in the longest carbon chain.
2. Position isomerism, where the same functional group is attached to the carbon chain at different point.
3. Functional group isomerism, where the functional groups are different.

Question 5

What are the differences in properties of chain isomers of alkanes?

Answer 5

They have similar chemical properties. However, the more branched the isomers, the weaker the van der Waals’ forces between their molecules, and the lower the boiling point.

Question 6

What are stereoisomers?

Answer 6

Stereoisomers are compounds which have the same structural formula, but their atoms are arranged differently in space.

Question 7

What are two types of stereoisomerism?

Answer 7

Cis-trans isomerism and enantiomerism.

Question 8

What are the criteria for cis-trans isomerism?

Answer 8

1. The compound contains a carbon-carbon double bond.
2. There are two different atoms or groups on each carbon atom of the carbon-carbon double bond.

Question 9

Explain the difference in melting points of cis and trans isomers.

Answer 9

The trans isomer has a more regular and symmetrical structure than the cis isomer. Molecules of the trans isomer can pack more compactly in the solid. More heat is required to overcome the van der Waals’ forces between the molecules, hence the trans isomer has a higher melting point than the cis isomer.

Question 10

What are enantiomers?

Answer 10

Enantiomers are two molecules that are non-superimposable mirror images of each other.

Question 11

What is the criteria for a molecule to be chiral?

Answer 11

A molecule having a carbon atom with four different atoms / groups attached to it is chiral, with the carbon atom being the chiral centre.

Question 12

What is a chiral molecule?

Answer 12

A chiral molecule is a molecule that cannot be superposed on its mirror image.

Question 13

What is an achiral molecule?

Answer 13

An achiral molecule is a molecule that can be superposed on its mirror image.

Question 14

What is the shortest chiral alkane?

Answer 14

3-methylhexane (with a methyl group, an ethyl group, and a propyl group attached to the chiral carbon)

Question 15

How can we test for chirality?

Answer 15

Chirality occurs when the molecule does not have a plane of symmetry.

Question 16

What are the differences of enantiomers?

Answer 16

Enantiomers have identical physical and chemical properties, except that they interact differently with plane-polarised light and other chiral reagents.

Question 17

What is plane-polarised light?

Answer 17

Plane-polarised light is light that passes through a polariser, leaving only vibrations in one plane.

Question 18

What is an optically active substance?

Answer 18

An optically active substance can rotate the plant of polarisation of plane-polarised light.

Question 19

How do enantiomers interact differently with plane-polarised light?

Answer 19

One isomer rotates the plane of polarisation clockwise while the other rotates the plane of polarisation anticlockwise by the same extent.