Free Isomerism MCQs with Answers

1. Compounds with the same molecular formula but different structural formulae are called

• A. isotopes
• B. isomers
• C. homologues
• D. allotropes

Explanation: Isomers share the molecular formula but differ in how the atoms are connected or arranged in space, so they are genuinely different substances with different properties. Butane and 2-methylpropane both have the formula C4H10 yet boil at different temperatures. Homologues differ by CH2 and so have different molecular formulae.

2. Butan-1-ol and butan-2-ol are examples of

• A. chain isomers
• B. functional group isomers
• C. position isomers
• D. geometric isomers

Explanation: The carbon skeleton and the functional group are the same in both, and only the position of the hydroxyl group along the chain differs, which defines position isomerism. Chain isomers differ in the branching of the skeleton, and functional group isomers carry different groups altogether, as an alcohol and an ether do. Position isomerism is why butan-1-ol oxidises to an aldehyde while butan-2-ol gives a ketone.

3. Ethanol and dimethyl ether both have the molecular formula C2H6O. They are

• A. position isomers
• B. chain isomers
• C. optical isomers
• D. functional group isomers

Explanation: Ethanol contains a hydroxyl group while dimethyl ether contains an oxygen bridging two carbons, so the functional group itself differs and the two compounds behave completely differently, ethanol boiling at 78 degrees Celsius against minus 25 for the ether. That large difference is due to hydrogen bonding, which the ether cannot do. Aldehydes with ketones and acids with esters are the other standard pairs.

4. Geometric or cis trans isomerism arises in an alkene because

• A. the double bond prevents free rotation, fixing the groups on either side
• B. the molecule contains a chiral carbon
• C. the double bond rotates freely
• D. alkenes contain a ring

Explanation: The pi bond locks the two carbons in a fixed relative orientation, so substituents are held either on the same side, cis, or on opposite sides, trans, and the two forms cannot interconvert without breaking the pi bond. Each doubly bonded carbon must also carry two different groups, otherwise the two arrangements are identical. Single bonds rotate freely, so alkanes show no such isomerism.

5. A carbon atom that gives rise to optical isomerism must be attached to

• A. two identical groups and two different ones
• B. four different atoms or groups
• C. at least one double bond
• D. three hydrogen atoms

Explanation: A carbon with four different substituents has no plane of symmetry, so its mirror image cannot be superimposed on it and the two forms are optical isomers that rotate plane polarised light in opposite directions. Such a carbon is described as chiral or asymmetric. If any two of the groups are the same, the mirror image is identical to the original and the molecule is achiral.

6. The number of structural isomers of C4H10 is

• A. one
• B. two
• C. three
• D. four

Explanation: Butane has an unbranched chain of four carbons and 2-methylpropane has a three carbon chain with a methyl branch, and no other arrangement is possible, so there are two. Pentane, C5H12, has three isomers, which is why three is offered here as a distractor. The number of isomers rises very steeply as the chain lengthens.

7. Which pair of compounds are functional group isomers?

• A. Propan-1-ol and propan-2-ol
• B. Butane and 2-methylpropane
• C. Propanal and propanone
• D. Ethane and propane

Explanation: Propanal is an aldehyde and propanone a ketone, yet both have the molecular formula C3H6O, so they differ in functional group. The two propanols differ only in the position of the same hydroxyl group, making them position isomers, and butane with 2-methylpropane are chain isomers. Ethane and propane are homologues, not isomers, since their formulae differ.

8. Which compound would show optical isomerism?

• A. CH3CH2OH
• B. CH3CHClCH2CH3
• C. CH3CH2CH3
• D. CH2Cl2

Explanation: In CH3CHClCH2CH3 the second carbon carries a methyl group, a chlorine, a hydrogen and an ethyl group, four different substituents, so it is chiral and the molecule exists as two non superimposable mirror images. In every other option at least two of the four groups on each carbon are identical. Checking each carbon for four different groups is the whole test.