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Chapter 6: Problem 32

Treating cyclohexene with \(\mathrm{HBr}\) in the presence of acetic acid gives bromocyclohexane \((85 \%)\) and cyclohexyl acetate \((15 \%)\).

Short Answer

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Short answer: The reaction of cyclohexene with HBr and acetic acid mainly involves the electrophilic addition of HBr across the double bond of cyclohexene, forming bromocyclohexane (85%). However, due to the presence of acetic acid, a minor side reaction occurs where the carbocation intermediate formed during the electrophilic addition reacts with acetic acid through an esterification mechanism, forming cyclohexyl acetate (15%). The difference in product amounts is due to the faster and more favored electrophilic addition reaction with HBr compared to the side reaction with acetic acid.

Step by step solution

01

Identifying the main reaction

The reaction of cyclohexene with HBr in the presence of acetic acid (CH2COOH) involves the electrophilic addition of HBr across the double bond of cyclohexene, which results in the formation of bromocyclohexane as the major product. This is an example of an electrophilic addition reaction, where HBr acts as an electrophile and the double bond of cyclohexene acts as a nucleophile.
02

Understanding the electrophilic addition mechanism

The electrophilic addition mechanism consists of the following steps: 1. The π electrons of the double bond in cyclohexene attack the hydrogen of HBr, causing the H-Br bond to break and form a cyclohexyl carbocation. 2. The bromide ion (Br-) that was generated from the bond breaking in the previous step attacks the carbocation, forming bromocyclohexane.
03

Role of acetic acid in the reaction

Acetic acid plays a dual role in this reaction: it serves as a solvent and also provides an alternate electrophile for a side reaction. Although the reaction primarily involves the electrophilic addition of HBr to cyclohexene, there is also a minor side reaction due to the presence of acetic acid.
04

Understanding the side reaction mechanism with acetic acid

The side reaction involves the esterification of the carbocation intermediate with the acetic acid to form cyclohexyl acetate. The mechanism for this step is as follows: 1. The carbocation intermediate that was formed in Step 2 of the electrophilic addition reaction attacks the carbonyl carbon of acetic acid, causing the O-H bond to break and form a new C-O bond. 2. The resulting oxonium ion loses a proton to give the final product of cyclohexyl acetate.
05

Formation of products and their relative amounts

The primary product is bromocyclohexane, formed through the electrophilic addition of HBr to cyclohexene (85%). The minor product is cyclohexyl acetate, which is formed via the esterification of carbocation with acetic acid (15%). The difference in the amounts of these products is due to the fact that the electrophilic addition reaction with HBr is faster and more favored than the side reaction with acetic acid.

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Most popular questions from this chapter

When 2-pentene is treated with \(\mathrm{Cl}_{2}\) in methanol, three products are formed. Account for the formation of each product (you need not explain their relative percentages).

The 2-propenyl cation appears to be a primary carbocation, and yet it is considerably more stable than a \(1^{\circ}\) carbocation such as the 1 -propyl cation. $$ \begin{array}{lc} \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2}^{+} & \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}^{+} \\ \text {2-Propenyl cation } & \text { 1-Propyl cation } \end{array} $$ How would you account for the differences in the stability of the two carbocation?

Draw the structure of the chlorohydrin formed by treating l-methylcyclohexene with \(\mathrm{Cl}_{2} / \mathrm{H}_{2} \mathrm{O}\).

Treating 4-penten- \(1-\mathrm{ol}\) with bromine in water forms a cyclic bromoether.

Account for the fact that addition of HCl to 1-bromopropene gives exclusively 1-bromo1-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
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