4-Isopropylcyclohexene

At equilibrium, the product mixture contains about 30% isopropylidenecyclohexane and about 70% 1-isopropylcyclohexene. 

In the simplest picture of optimum resonance-derived stabilization, the two carbons of the double bond, the amine nitrogen and all five atoms affixed to these three enamine atoms lie in a common plane. Much the same demand for planarity exists for the exocyclic isopropylide-necycloalkanes. For these hydrocarbons, one finds the rearrangement of 1-isopropylcyclohexene to isopropylidenecyclohexane is exothermic by 3.1 0.6 kJ mol-1 but that of 1-isopropylcyclo-pentene to isopropylidenecyclopentane is exothermic by 4.3 0.3 kJ mol -1 [D. B. Bigley and R. W. May, J. Chem. Soc. (B), 1761 (1970)]. If one ignores error bars, the 3.1 - (-4.3) s 7 kJ mol -1 net destabilization for the exocyclic double bond in the cyclopentene vs cyclohexene hydrocarbon case is essentially identical to the 51 — 45 6 kJ mol -1 for the cyclopentenyl and cyclohexenyl enamines. 

Addition of HCl to 1-isopropylcyclohexene yields a rearranged product. Propose a mechanism, showing tlie structures of the intermediates and using curved arrows to indicate electron flow in each step. 

The required anti and coplanar transition state geometry can also be used to predict the regiochemistry of E2 elimination in halocyclohexanes such as chlorocyclohex-anes. In these molecules, anti and coplanar correspond to trans and diaxial. Consider the E2 reaction of the cis isomer of l-chloro-2-isopropylcyclohexane.The major product is 1-isopropylcyclohexene, the more substituted cycloalkene. 

When isopropylidenecyclohexane is treated with strong acid at room temperature, isomerization occurs by the mechanism shown below to yield 1-isopropylcyclohexene ... 

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