A type of isomerism that ClCH=CHCl can exhibit is

Analyze the question

ClCH=CHCl is 1,2-dichloroethene. We need to find which type of isomerism it can show.

Explain the concept

Each type of isomerism has its own requirement:

• Chain isomerism: different carbon skeletons (branching). Needs at least 4 carbons, so it is not possible with 2 carbons.
• Positional isomerism: the same carbon skeleton, but the substituent or double bond is in a different position.
• Functional group isomerism: the same molecular formula but different functional groups.
• Optical isomerism: needs a chiral carbon (four different groups on one carbon).
• Geometric (cis-trans) isomerism: needs a C=C bond with two different groups on each carbon. This is not among the options.

Solution

1. ClCH=CHCl has the formula C₂H₂Cl₂, with a C=C double bond. Each carbon carries one H and one Cl.
2. This is the setup for cis-trans (geometric) isomerism (cis-1,2-dichloroethene and trans-1,2-dichloroethene). However, geometric isomerism is not listed.
3. Chain isomerism: not possible with only 2 carbons.
4. Optical isomerism: there is no chiral carbon, because the sp² carbons each have only three different attachments. So no.
5. Functional group isomerism: C₂H₂Cl₂ could be written differently, but the question is about what this structure can exhibit with its isomers. No other functional group arrangement is realistic here.
6. Positional isomerism: the formula C₂H₂Cl₂ has an isomer 1,1-dichloroethene (Cl₂C=CH₂). Here the two Cl atoms are on the same carbon rather than on different carbons. This is a change in the position of the substituents on the same skeleton, which is positional isomerism.

Answer

The best choice among the options is B. positional, because ClCH=CHCl (1,2-dichloroethene) and Cl₂C=CH₂ (1,1-dichloroethene) differ only in the position of the chlorine atoms.

Tip

When the options don't include "geometric", check whether moving a substituent gives another isomer. For small molecules with 2 carbons, chain isomerism is never possible, and optical isomerism needs a carbon with four different groups.