Question

Propose a mechanism for each reaction, showing explicitly how the observed mixtures of products are formed.

a) $3-\mathrm{methyl}-\mathrm{but}-2-\mathrm{en}-1-\mathrm{ol}+\mathrm{HBr}\to 1-\mathrm{bromo}-2-\mathrm{ene}+3-\mathrm{bromo}-3-\mathrm{methyl}-1-\mathrm{ene}$

b)$2-\mathrm{methyl}-\mathrm{but}-3-\mathrm{en}-2\mathrm{a}-\mathrm{ol}+\mathrm{HBr}\to 1-\mathrm{bromo}-2-\mathrm{ene}+3-\mathrm{bromo}-3-\mathrm{methyl}-1-\mathrm{ene}$c)$\mathrm{cyclopenta}-1,3-\mathrm{diene}+{\mathrm{Br}}_2\to 3,4-\mathrm{dibromocyclopent}-1-\mathrm{ene}+3,5-\mathrm{dibromocyclopent}-1-\mathrm{ene}$

d)$1-\mathrm{cholorobut}-2-\mathrm{ene}+{\mathrm{AgNO}}_3+{\mathrm{H}}_2\mathrm{O}\to \mathrm{but}-2-\mathrm{en}-1-\mathrm{ol}+\mathrm{but}-3-\mathrm{en}-2-\mathrm{ol}$

e)$3-\mathrm{cholorobut}-1-\mathrm{ene}+{\mathrm{AgNO}}_3+{\mathrm{H}}_2\mathrm{O}\to \mathrm{but}-2-\mathrm{en}-1-\mathrm{ol}+\mathrm{but}-3-\mathrm{en}-2-\mathrm{ol}$

Short answer

a)

b)

c)

d)

e)

Step-by-step solution

Allylic carbocations

Allylic carbocations are resonance-stabilized, and there are two forms of resonance with a +1 charge on an allylic carbon.The nucleophile can attack either one.

Mechanism of the reaction

1. When is$3-\mathrm{methyl}-\mathrm{but}-2-\mathrm{en}-1-\mathrm{ol}$ reacted with HBr, two allylic cations are formed, and the nucleophile can attack either one. The mechanism of the reaction is shown as follows:

Mechanism of the reaction

Mechanism of the reaction

1. When $2-\mathrm{methyl}-\mathrm{but}-3-\mathrm{en}-2\mathrm{a}-\mathrm{ol}$is reacted with HBr, two allylic cations are formed, and the nucleophile can attack either one. The mechanism of the reaction is shown as follows

Mechanism of the reaction

Mechanism of the reaction

When$\mathrm{cyclopenta}-1,3-\mathrm{diene}$ isreacted with and${\mathrm{Br}}_2$, two allylic carbocations are formed, and the nucleophile can attack either one. The mechanism of the reaction is shown as follows:

Mechanism of the reaction

Mechanism of the reaction

When is $1-\mathrm{cholorobut}-2-\mathrm{ene}$reacted with${\mathrm{AgNO}}_3$and${\mathrm{H}}_2\mathrm{O}$, the same allylic carbocations as in are formed. The nucleophile can attack either one. The mechanism of the reaction is shown as follows:

Mechanism of the reaction

Mechanism of the reaction

When $3-\mathrm{cholorobut}-2-\mathrm{ene}$is reacted with${\mathrm{AgNO}}_3$and${\mathrm{H}}_2\mathrm{O}$, the same allylic carbocations as in are formed. The nucleophile can attack either one. The mechanism of the reaction is shown as follows:

Mechanism of the reaction