[jumpinj_24]

Hello,

can anyone explain to me why in a Claisen reaction of a an ester with a ketone, produces a higher yield when mixing the carbonyl components first and then adding a base, rather than mixing the base with the ketone and then adding the ester?  Any help would be greatly appreciated!

[victor]

Ohh...can it?? as far as I know that Claisen condensation needs bases as the first initiator (to take the alpha hydrogen from alpha carbon)...please tell me if your procedure can be done...(mixing first, then base)..

[barcrphd]

i have not came across such an observation. anyway, one possible answer could be as follows.

both ketone as well as your ester contains alpha hydrogen (i suppose). which means both can undergo claisen condensation in presence of a base. so if you added base to ketone first, obviously, self condensation of ketone will start, decreasing the yield of your expected product.

on the otherhand, if you mix the ketone and ester first and then the base, chance for cross condensation increases. you should remember that even in this case, self condensation of ketone and ester cannot be avoided.

hope this reply will help you.

[movies]

I agree with barcrphd, the yield is probably lower because of self-aldol condensation of the ketone before you add the ester.  At least when the ester is present you have some chance to do the cross-Claisen condensation.