Organic chemistry question

[traitorman]

When you react a ketone or aldehyde with a strong base, how do you know whether the base acts as a nucleophile and attacks the carbonyl carbon or when it removes a proton in order for the reaction to undergo aldol condensation?

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[BerkReviewTeach]

When you react a ketone or aldehyde with a strong base, how do you know whether the base acts as a nucleophile and attacks the carbonyl carbon or when it removes a proton in order for the reaction to undergo aldol condensation?

If the base is large, then it will be limited to deprotonation, being that its bulk will sterically hinder attack of the carbonyl. This is why LDA and t-butOK are often chosen when the desired reaction involves reacting with the alpha-carbon. When the base is small, such as NaOEt, NaOMe, or NaOH, then it is capable of both attacking the carbonyl (to form a hemiacetal or hemiketal) or deptotonating the alpha carbon. If you look at the yields for these reaction, they often show that there are mutliple, competing reaction pathways going on. If you can isolate the aldol product, then you can make that the ultimate pathway. Look for the commonality in reaction conditions for various Aldol reactions, and you will have your answer.

 

[traitorman]

hey thanks for the quick response and explanation.

i figured the bulkiness would play a role but i wasn't sure what would stop a smaller base from deprotonating instead of attacking the carbonyl carbon. but i guess ultimately a base would want to attack the carbonyl carbon first and only deprotonate when this pathway is unattainable