Question

reating cyclohexene with \(\mathrm{HBr}\) in the presence of acetic acid gives bromocyclohexane \((85 \%)\) and cyclohexyl acetate \((15 \%)\). BrC1CCCCC1 CC(=O)OC1CCCCC1 Cyclohexene \(\begin{array}{cc}\text { Bromocyclohexane } & \text { Cyclohexyl acetate } \\\ (85 \%) & (15 \%)\end{array}\) Propose a mechanism for the formation of the latter product.

Short answer

Question: Describe the mechanism for the formation of cyclohexyl acetate when cyclohexene reacts with HBr in the presence of acetic acid. Answer: The mechanism involves four main steps: 1) Electrophilic addition of HBr to cyclohexene to form a bromonium ion intermediate. 2) Formation of a carbocation by losing the bromide ion from the bromonium ion. 3) Reaction of the carbocation with acetic acid to form a new intermediate. 4) Formation of cyclohexyl acetate by losing a proton from the intermediate.

Step-by-step solution

Electrophilic addition of HBr to cyclohexene

First, the electrophile (HBr) reacts with the nucleophile (cyclohexene) in an electrophilic addition reaction. The pi bond of cyclohexene attacks the hydrogen atom in HBr, breaking the H-Br bond and forming a bromonium ion as an intermediate. Cyclohexene + HBr → Bromonium ion intermediate

Formation of a carbocation

In the next step, the bromonium ion loses the bromide ion, leaving behind a carbocation. This is a highly reactive intermediate and can react with other molecules present in the reaction mixture. Bromonium ion intermediate → Carbocation + Br⁻

Reaction with Acetic Acid

The carbocation formed in the previous step can now react with acetic acid. The negatively charged oxygen in the carboxylate group of the acetic acid attacks the positively charged carbon of the carbocation, yielding a new intermediate with the acetic acid attached to the molecule. Carbocation + Acetic Acid → Intermediate

Formation of Cyclohexyl Acetate

Lastly, the intermediate loses a proton and forms the final product, cyclohexyl acetate. Intermediate → Cyclohexyl Acetate Through this series of reaction steps, we have proposed a plausible mechanism for the formation of cyclohexyl acetate in this reaction.

Key concepts

Cyclohexene Reactions

Cyclohexene is a reactive alkene that participates in a variety of chemical reactions, with electrophilic addition being one of the most common. This type of reaction involves an electrophile, such as hydrogen bromide (HBr), attacking the electron-rich double bond in cyclohexene to form new products.

In the presence of an acid like acetic acid, cyclohexene undergoes electrophilic addition with HBr, which can lead to products like bromocyclohexane and cyclohexyl acetate. The reaction's pathway is influenced by the conditions set in the reaction environment, which helps explain the proportion of the observed products.

Bromocyclohexane Formation

The formation of bromocyclohexane from cyclohexene is the result of an electrophilic addition mechanism. This begins when the pi bond of cyclohexene attacks a molecule of HBr. As the bond between hydrogen and bromine breaks, a bromonium ion intermediate is formed.

The reaction's regiochemistry is determined by the formation of the most stable carbocation intermediate, which in this case is secondary. The subsequent attack by a bromide ion from the rear opens the three-membered bromonium ion ring, leading to the formation of bromocyclohexane. This reaction demonstrates the stereospecific nature of this kind of electrophilic addition.

Cyclohexyl Acetate Synthesis

The synthesis of cyclohexyl acetate is less straightforward and occurs via a reaction with acetic acid. Once the carbocation intermediate is formed from the initial electrophilic addition step, it is available to react with other nucleophiles.

The oxygen atom in the carboxyl group of acetic acid donates a pair of electrons to bond with the carbocation, expanding the molecule. This leads to the formation of an ester linkage and proton transfer occurs, which is the final step in the generation of cyclohexyl acetate. This pathway showcases the versatility of carbocation intermediates in electrophilic addition reactions.

Carbocation Intermediates

Carbocation intermediates play a pivotal role in electrophilic addition reactions. These positively charged, electron-deficient species are highly reactive and, as a result, tend to react quickly with nearby nucleophiles.

The stability of a carbocation is key to understanding the reaction pathway. It is affected by factors such as the number of alkyl groups attached to the positively charged carbon (tertiary carbocations are more stable than secondary, which are more stable than primary). Additionally, the presence of electron-donating groups in the vicinity of the carbocation can stabilize it through inductive effects, which influences the course of the reaction and the final products formed.