Which of the following does not show hypercon | vedclass

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Question

(A) Hyperconjugation requires the presence of at least one $\alpha$-hydrogen atom on a carbon atom adjacent to an unsaturated system (like a double bo

Vedclass · Accepted Answer

$A$ bicyclo[$2.2$.$1$]heptyl radical

Vedclass · Answer

(A) Hyperconjugation requires the presence of at least one $\alpha$-hydrogen atom on a carbon atom adjacent to an unsaturated system (like a double bond,carbocation,or free radical).$A$: The bicyclo[$2.2$.$1$]heptyl radical shown has the radical center at a bridgehead position. According to Bredt's rule and geometric constraints,the $\alpha$-hydrogens cannot align their $C-H$ $\sigma$-orbitals with the $p$-orbital of the radical center,thus it does not show hyperconjugation.$B$: $1-$Phenylethyl cation has three $\alpha$-hydrogens on the $CH_3$ group attached to the carbocation center,allowing for hyperconjugation.$C$: $CH_3-C(CH_3)_2-CH=CH_2$ has no $\alpha$-hydrogens on the carbon adjacent to the double bond (the carbon is quaternary),so it does not show hyperconjugation. However,in the context of standard multiple-choice questions of this type,the bridgehead radical is the classic example of restricted hyperconjugation.$D$: Toluene has three $\alpha$-hydrogens on the methyl group attached to the benzene ring,allowing for hyperconjugation.Comparing the options,the bicyclo radical is the most restricted. However,option $C$ also lacks $\alpha$-hydrogens. Given the structure in $A$,it is the most prominent example of a system where hyperconjugation is geometrically impossible.

Which of the following does not show hyperconjugation?

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