In step 1 the alpha bromination of ketones is acid catalyzed. What acid is forme

Question

In step 1 the alpha bromination of ketones is acid catalyzed. What acid is formed as Step 1 progresses? Why are alpha bromination reactions acid catalyzed? Step 1 of the reaction, alpha bromination, created a new stereocenter (shown by asterisk). Do you expect the brominated product to be a single stereoisomer or a racemic mixture? Explain. Substitution of bromine by t-bulyl amine is the second step of the reaction today. Secondary alkyl halides can udergo substitution by Sn1 or Sn2 mechanisms. Explain (in terms of electronic interactions) why an Sn2 mechanism may be more favored in this case when the bromide is alpha to a carbonyl.

Explanation / Answer

1b) Ketones undergo keto-enol tautomerization in which keto form is more predominant. The addition of acid as catalyst promotes the enolization thus alpha bromination takes place fastly.

2) In alpha bromination, enol form acts nucleophile and it is planar. Thus it may attack the bromine molecule from either side of the plane equally results a racemix mixture.

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