Aldol condensation

G1SG2

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If you have a ketone and an aldehyde undergoing aldol condensation, how do you determine which hydrogen gets abstracted by the base in the first step? Is it a hydrogen from the ketone or aldehyde?

Take a look at what's on the alpha carbon, and the molecule in general. If you have electron withdrawing groups on either the aldehyde or ketone, then that's good because they will stabilize the negative charge on the enolate ion once the alpha hydrogen comes off. Aldehydes are generally more acidic than ketones because the ketone has more electron donating alkyl groups destabilizing the conjugate base, but just look at what's on the molecule, and whether or not there are electron withdrawing groups.
 
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