10.26Bond formation takes place at the end of the diene system to give a bridged bicyclic ring system.10.27The two Diels–Alder adducts formed in the reaction of 1,3-pentadiene with acrolein arise by the twoalignments shown:10.28Compound B arises by way of a Diels–Alder reaction between compound A and dimethylacetylenedicarboxylate. Compound A must therefore have a conjugated diene system.10.29The reaction is a nucleophilic substitution in which the nucleophile (C6H5S2) becomes attached tothe carbon that bore the chloride leaving group. Allylic rearrangement is not observed; therefore,it is reasonable to conclude that an allylic carbocation is notinvolved. The mechanism is SN2.10.30(a)Solvolysis of in ethanol proceeds by an SN1 mechanism and involves acarbocation intermediate.This carbocation has some of the character of a tertiary carbocation. It is more stable and istherefore formed faster than allyl cation,
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