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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: raaaainm on September 01, 2013, 05:08:38 PM

Title: Racemic mixture upon treatment with base?
Post by: raaaainm on September 01, 2013, 05:08:38 PM

I'm studying for the GRE and encountered this problem:

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FLEwEzUx.png&hash=c5d9b849d6debe5a6753ae69477bf4b285ebe73c)

The answer is stated to be A. What does the base do? I cannot imagine what reactions could occur, aside from an attack on the ketone to produce a diol.

Title: Re: Racemic mixture upon treatment with base?
Post by: discodermolide on September 01, 2013, 06:18:38 PM

What other reactions of ketones and base do you know?

Title: Re: Racemic mixture upon treatment with base?
Post by: raaaainm on September 01, 2013, 07:18:53 PM

Treatment of A with NaOH could produce an enolate with a tertiary negative charge. Can this react with a second molecule of A to undergo an aldol condensation? If so, is that racemic (inclined to say yes!)

If an enolate is formed, then does that mean that the destabilizing inductive effect is lesser than the stabilizing delocalization of the negative charge across the ketone?

Title: Re: Racemic mixture upon treatment with base?
Post by: discodermolide on September 01, 2013, 07:23:01 PM

If an enolate is formed and then quenched the product will be racemic.
This because the enolate is planar and the proton can attack from either the top or bottom face leading to a racemic product.

Title: Re: Racemic mixture upon treatment with base?
Post by: raaaainm on September 01, 2013, 07:37:53 PM

Thank you. If that is the case, then why is B not also a correct answer?

Edit: is it because the enolate would instead be selectively formed on the opposite side of the molecule? (i.e. not the chiral site)

Edit2: its not even chiral, question self-answered!

Thank you again :)