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Electrophilic addition reaction of a conjugated diene, also known as 鈥�1,2-addition & 1,4-addition鈥�, in which conjugate diene reacts with hydrogen halide(HBr/HCl) or halogen (${\text{Cl}}_{\text{2}}{\text{,Br}}_{\text{2}}$) results in the formation of 1,2 and 1,4-addition products.

E.g., when conjugate diene 1,3 butadiene reacts with HCl result in the formation of two product as follows:

• 1,2 addition product formed is 3-chlorobut-1-ene
• 1,4 addition product formed is 1-chlorobut-2-ene

Electrophilic addition of HCl to a conjugated diene 1-vinylcyclopent-1-ene involves the formation of four allylic carbocation intermediates. 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 $C{l}_2$, generating a maximum of four possible products.

The two adducts formed from 1,2 addition are 1-(1-chloroethyl)cyclopent-1-ene and 1-chloro-1-vinylcyclopentane.

Two adducts are formed from 1,4 addition are (z)-1-chloro-2ethylidenecyclopentane and (2-chloro

The four possible adducts formed by reacting the diene with 1 equivalent HCl are shown as follows:

Formation of all possible adduct