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Consider the heterolytic dissociations producing carbocations (each releasing Cl-): (I) cyclohexyl chloride -> cyclohexyl cation, delta-H1 (II) cyclohexenyl (allylic) chloride -> cyclohexenyl cation, delta-H2 (III) benzyl chloride -> benzyl cation, delta-H3 (IV) chlorobenzene -> phenyl cation, delta-H4 Arrange the reaction enthalpies in correct decreasing order.

1. delta-H4 > delta-H1 > delta-H2 > delta-H3
2. delta-H1 > delta-H2 > delta-H3 > delta-H4
3. delta-H3 > delta-H2 > delta-H1 > delta-H4
4. delta-H2 > delta-H1 > delta-H4 > delta-H3

Correct answer: delta-H4 > delta-H1 > delta-H2 > delta-H3

Solution

Reaction enthalpy for ionization is smallest for the most stable carbocation and largest for the least stable. The phenyl cation is highly unstable (needs most energy), while the resonance-stabilized benzyl cation needs least, giving the decreasing-enthalpy order delta-H4 > delta-H1 > delta-H2 > delta-H3.

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