Question

Account for the fact that addition of \(\mathrm{HCl}\) to 1 -bromopropene gives exclusively 1 -bromo-1-chloropropane. $$ \mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHBr}+\mathrm{HCl} \longrightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHBrCl} $$ 1-Bromopropene 1-Bromo-1-chloropropane

Short answer

Answer: The addition of HCl to 1-bromopropene results in the exclusive formation of 1-bromo-1-chloropropane because the secondary carbocation intermediate, formed during the reaction, is more stable than the primary one. This stability is due to the inductive effect of the adjacent methyl group. The chloride anion then attacks the more stable carbocation, leading to the observed product, 1-bromo-1-chloropropane.

Step-by-step solution

Identify the type of reaction

In this case, we are dealing with an electrophilic addition reaction, where HCl acts as the electrophile and the double bond in 1-bromopropene acts as the nucleophile.

Formation of carbocation intermediate

When the electrophile (HCl) approaches the double bond in 1-bromopropene, the bond will break, resulting in the formation of a carbocation intermediate. Since there are two carbons in the double bond, there are two possible carbocations that can form: a primary carbocation on the carbon with the Br attached and a secondary carbocation on the adjacent carbon. Primary carbocation: $$ \mathrm{CH}_{3}\mathrm{CH}^{+}\mathrm{CHBr}\ \mathrm{[-HCl]} $$ Secondary carbocation: $$ \mathrm{CH}_{3}\mathrm{C}^{+}\mathrm{CH}_{2}\mathrm{CHBr} \ \mathrm{[-HCl]} $$

Compare the stability of carbocations

The stability of carbocations follows the order: tertiary > secondary > primary. In this case, the secondary carbocation is more stable than the primary carbocation due to the inductive effect of the adjacent methyl group, which can donate electron density to the positively charged carbon. Therefore, the formation of the secondary carbocation is favored over the primary carbocation.

Addition of Cl to the carbocation

The chloride anion (generated from the dissociation of HCl) will now attack the positively charged carbon in the secondary carbocation, forming a single bond and resulting in the formation of 1-bromo-1-chloropropane: $$ \mathrm{CH}_{3}\mathrm{C}^{+}\mathrm{CH}_{2}\mathrm{CHBr} + \mathrm{Cl}^{-} \longrightarrow \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CHBrCl} $$

Conclusion

The addition of HCl to 1-bromopropene leads to the exclusive formation of 1-bromo-1-chloropropane because the secondary carbocation intermediate is more stable than the primary one. The chloride anion then attacks the more stable carbocation, resulting in the observed product.