Which one of the following compounds is inactive towards S N 1 reaction?

Solution

Chemistry ma’am · 3 steps

For SN1, picture the carbocation first: it must go flat, and a bridgehead carbon cannot.

1. What SN1 needs. In SN1 the C-Cl bond breaks first and a carbocation forms. That carbocation is sp² and must become planar, with an empty p orbital. Anything that stabilises it, such as alkyl groups or resonance, speeds the reaction up.

   Option	Carbocation formed	Can it be planar and stabilised?	SN1
   (a) (CH₃)₃C-Cl	tertiary butyl	yes, three methyl groups donate electrons	active
   (b) CH₂=CH-CH₂Cl	allyl	yes, resonance with the C=C	active
   (c) 1-chloronorbornane	bridgehead	no, the rigid cage stops it from flattening	inactive
   (d) C₆H₅CH(Cl)CH₃	secondary benzylic	yes, resonance with the benzene ring	active

2. Bridgehead problem. In option (c) the chlorine sits on a bridgehead carbon of a bicyclic cage. Its three bonds are locked into a pyramid, so a planar carbocation would be badly strained. The cation does not form, and SN1 does not happen.
3. So. The bridgehead chloride, option (c), is inactive towards SN1.

The idea to keep. SN1 rate depends on the carbocation: tertiary, allylic and benzylic cations are stable. But if the carbon cannot become planar, as at a bridgehead, even a tertiary halide will not react by SN1.

Answer from the NTA final answer key. Solution by PrepWiser.