i had a doubt regarding the mechanism of aldol say for example,of acetaldehyde.After OH- takes away an acidic hydrogen,the formed -CH2CHO attacks the C of C=O of another acetaldehyde mol but
1)isnt the reso structure of -CH2CHO more stable i.e CH2=CH-O-.y isnt that formed?
(-ve charge on more electro-ve element)
2)why does the second attack take place on C=O bond and not acidic hydrogen of second mol again??
39Carbanion formation is an equilibrium process. If the carbanion abstracts a proton from another mol of acetaldehyde, the equilibrium goes backward and it forms acetaldehyde again, so that is redundant. Instead if the carbanion attacks by electromeric effect, the equilibrium moves vastly forward.
1see C∂+ is a soft centre and C- is also soft ...hence soft soft attack is preferred so carbon centre attacks n not the oxygen centre....
wen we have OH- even then carbon could have been attacked but it is not attacked since oxygen is a hrd centre n H is a hard centre too.......by hard soft acid base thery hard hard n soft soft combination is preffered due to energy matching......
1as u r saying oxygen reso form is more stable n should attack then....oxygen being a hard centre would again abstract a hydrogen as ur doubt in 2nd part f ur question in which case we would again get back the original reactants....n hence no fate of the reaction....
39But a carbocation is a HARD ACID....why would it combine with a carbanion? Remember, electromeric effect leads to the formation of not delta but full fledged charges for a short duration as per attacking conditions.
C+- O- is formed, and the charge is full, not partial. Even I thought it would be like you said.
1yeh...but C is a soft centre owing to its size..!!we take FON as the hard bases!!
39http://en.wikipedia.org/wiki/HSAB_theory (Hard Soft Acid Base theory)
