alpha halogenation

Problem # 748

Alpha bromination is usually carried out under acidic conditions via the enol intermediate.

Alpha bromination is uncontrollable under basic conditions, which goes through the enolate intermediate. Let's explore why.

a) Rank each carbonyl below in order of decreasing alpha-proton acidity (1= most acidic). Explain.

b) based on a), why does the reaction below lead to polyhalogenation?

Solution

a) Bromine is an electron withdrawing group (EWG), which makes nearby protons more acidic. So the carbonyl with two bromines is the most acidic.

b) To perform alpha bromination, the enolate (or enol) must first be formed, which then attacks Br₂.

The problem with doing this under basic conditions is that each successive bromination leads to a carbonyl that is more acidic, and so forms an enolate even more easily. So once this reaction starts, it's difficult to control.

Enols are less nucleophilic than enolates, so this react can be controlled under acidic conditions. (Br₂/H3O⁺ instead of Br₂/NaOH).

Problem Details

MendelSet practice problem # 748 submitted by Matt on July 27, 2011.

Subject: Organic Chemistry

Subject Topic(s): Carbonyl Alpha-Substitution Reactions (Enols and Enolates, Malonic Ester Syn, Stork)

Question Type: Concept (explain why this is so..)

Keyword(s): acid strength, EDG and EWG, carbonyl alpha substitution, alpha halogenation

Problem # 741

Like enolates (Q740), enols can also act as nucleophiles and make new bonds at the alpha position.

Let's go through the mechanism for how this happens, using alpha bromination as our example.

In the reaction below, the carbonyl form is in equilibrium with its enol form (Q738), which can then attack molecular bromine.

Complete the structure of the protonated carbonyl, and show the curved arrows that transform it into the final carbonyl product.

Solution

Enols are just like enolates in that they attack electrophiles and add to the alpha position.

Problem Details

MendelSet practice problem # 741 submitted by Matt on July 27, 2011.

Subject: Organic Chemistry

Subject Topic(s): Carbonyl Alpha-Substitution Reactions (Enols and Enolates, Malonic Ester Syn, Stork)

Question Type: Mechanism (show a mechanism using curved arrows..)

Keyword(s): enols, carbonyl alpha substitution, alpha halogenation

alpha halogenation

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