when we react Grignard with a ketone or an aldehyde, why does neucleophilic addition take place? why doesn't R- behave as a base and have an acid-base reaction with the enolisable hydrogen (provided that one exists) of the ketone or the aldehyde?

Question

alphadxg · Answer

I have to go, you can use this site for a good reference. http://www.chemguide.co.uk/mechanisms/nucadd/hcntt.html

anonymous · Answer

i couldn't really find an answer to my question on the website. also, the link is for HCN nucleophilic addition. i'm trying to figure out as to why doesn't R- behave as a base and react with the acidic hydrogen in the ketone. i'd be glad if you could help.

anonymous · Answer

just to make the question clear, this is what i mean. please don't mind the crappy resolution.

anonymous · Answer

here, R- inspite of being a strong base doesn't react with the acidic hydrogen in the compound. (as contrasted with aldol conversion where OH- inspite of being a weaker base reacts with the acidic hydrogen). WHY?

aaronq · Answer

if i remember correctly, it has to do with several factors. One is the coordination of the$Mg^{2+}$ to the carbonyl oxygen that has productive proximity effects for the attack and the second is that it's charge controlled, both the carbanion, $R^-$, and the carbonyl carbon are charged, $^{\delta+}C=O^{~\delta-}$. There may be other reasons, read the section in Carey's Mechanism and reactivity on carbanion nucleophiles.

aaronq · Answer

You can also look into explanations using MO perturbation theory, but that's advanced stuff that may you not understand yet.

anonymous · Answer

thank you so much. also, could you attempt to give an explanation in terms of MO perturbation theory in simple terms. i do know the basics of it, so i could probably understand. thanks.