Why alpha carbon is must in aldol condensation?? And how we will know about alpha carbon in acetone
Answers
Answer:
the OP asked about the aldol condensation, not the aldol formation reaction. recall (or check wikipedia) that a condensation reaction is “an organic addition reaction that proceeds in a step-wise fashion to produce an addition product, usually in equilibrium, and a water molecule.” The term “condensation” actually historically results to the evolution of condensation a/k/a droplets of water on the reaction vessel.
the aldol condensation is a 2-part stepwise reaction: the first step is the aldol formation (that the other 3 posters seem to be discussing), then the elimination of water.
in an aldol condensation reaction (as well as the aldol formation reaction that is not capable of undergoing the final water-releasing step), an aldol is formed from two carbonyls (one of which must have a C-H alpha to the carbonyl, tis true).
however, the two differ as follows, among other things: in an aldol condensation, the carbonyl of the resultant aldol must ALSO have a proton on the carbon alpha to it. this is because at the very last step, a water molecule is released and a double bond formed. this is the condensation part. otherwise you’d still have an aldol (definition: aldehyde [traditionally, but other carbonyls work as well] group with alcohol group in the same molecule) instead of the condensed product (definition: carbonyl with alpha-beta double-bond in the same molecule).
at least one of the carbonyls MUST have a proton alpha to it in order for the condensation to occur. the condensation occurs when a proton sitting on the carbon alpha to the carbonyl and a hydroxyl sitting on the carbon beta to the carbonyl form a water molecule and leave behind a double bond. the electron-pushing is generally thought of as follows: a base absconds the proton alpha to the carbonyl (which is slightly acidic as others have said), the electron pair that formerly made up the C-H bond on the alpha carbon attacks the antibonding orbital of the hydroxyl group on the beta carbon, forming the alpha-beta olefin, and the electron pair that formerly made up the C-OH bond whose antibonding orbital just got entirely filled [recall that entirely filling an antibonding orbital breaks the corresponding bonding orbital’s bond] performs either a nucleophilic attack its own hydroxyl’s oxygen atom, forming OH-, or a nearby acidic proton, forming water.
in either case, the net result is the removal of water from the aldol and introduction of an alpha-beta olefin. SO TO SUMMARIZE: an aldol condensation DOES form an aldol as an intermediate but said aldol goes on to release water (and ceases being an aldol, instead becoming an alpha-beta unsaturated carbonyl).
if there was no proton present on the alpha carbon of the intermediate aldol, then there would be no way to abscond said [nonexistent] proton and the rest of the cascading chain of nucleophillic attacks could not occur.