Question 8.5: Give the structures of the likely products from reaction of ...
Predicting the Product of Electrophilic Addition to a Conjugated Diene
Give the structures of the likely products from reaction of 1 equivalent of HCl with 2-methylcyclohexa-1,3-diene. Show both 1,2- and 1,4-adducts.
S t r a t e g y
Electrophilic addition of HCl to a conjugated diene involves the formation of an allylic carbocation intermediate. Thus, the first step is to protonate the two ends of the diene and draw the resonance forms of the two allylic carbocations that result. Then allow each resonance form to react with Cl^-generating a maximum of four possible products.
In the present instance, protonation of the C1–C2 double bond gives a carbocation that can react further to give the 1,2-adduct 3-chloro-3-methyl-cyclohexene and the 1,4-adduct 3-chloro-1-methylcyclohexene. Protonation of the C3–C4 double bond gives a symmetrical carbocation, whose two reso- nance forms are equivalent. Thus, the 1,2-adduct and the 1,4-adduct have the same structure: 6-chloro-1-methylcyclohexene. Of the two possible modes of protonation, the first is more likely because it yields a tertiary allylic cation rather than a secondary allylic cation.
