8-35 - Organic Chemistry (Mcmurry)

Question

10-Bromo-a-chamigrene, a compound isolated from marine algae, is thought to be biosynthesized from g-bisabolene by the following route:

Draw the structures of the intermediate bromonium and cyclic carbocation, and propose mechanisms for all three steps.

Step-by-Step Solution

Answer

Structures of the intermediate Bromonium Ion and Cyclic carbocation are:

Mechanism of the above reaction is:

Intermediates

Theoretically, when we give mechanism for any reaction we get a molecular entity that is formed between reactant and product that are known as Intermediates. But in chemical laboratory when we perform any reaction most of the times we are not able to isolate that molecular entity because these are highly unstable and short lived. These are generally free radicals or ions.

Carbocation Intermediate

Carbocation intermediate: We can obtain a carbocation intermediate by taking benzene ring as a reactant and any electrophile as a reagent. In the presence of any electrophile due to the presence of pi electrons in benzene ring, benzene ring will act as a nucleophile and an electrophile will be attracted towards benzene ring and we will get carbocation intermediate after the attachment of an electrophile on the benzene ring.

For example:  

 

Step-by-Step Mechanism of reaction

Step 1: Double bond will act as a nucleophile and will attack on (electrophile) and Bromonium Ion will form.

Step 2: In this step Intramolecular reaction will take place double bond of the molecule will attack on carbon attached with  and cyclic carbocation will form.

Step 3: In last step base will take the most acidic hydrogen of the molecule and we will get final product of the reaction.

                           Mechanism of the reaction