[chanman1]

I had this problem on a test and got it wrong.

CH3CH2CO2CH3 (reacts w/ what and in what order?) => CH3CH2(C6H5CH2)CHCO2CH3
Ethyl Methyl Ester                                                     1-benzyl propane methyl ester(?)

For my answer I wrote the reactants: 1) NaOEt and 2) C6H5CH2Br

I was marked wrong on the NaOEt and was told that LDA(lithium diisopropylamide) should be used instead in the first step. I know LDA is a much stronger base and therefore a lot stronger at pulling that alpha H off; I guess my question is when do we know when LDA should be used as opposed to another base. (I thought NaOEt was strong enough for this but apparently not)

[movies]

I think you have an extra carbon in your product.

Anyway, the problem with using ethoxide is that you would get significant amounts of trans-esterification (forming the ethyl ester instead of the methyl ester).  You would likely also produce a large amount of benzyl methyl ether.

LDA is a nice base to use for enolate alkylations because you completely form the reactive species (the enolate).  With alkoxide bases you only form a small amount of these enolates at a time.

[chanman1]

I was actually missing a carbon in my reactant... sorry I typed it wrong, should've been
CH3CH2CH2CO2CH3 (propyl methyl ester)

Anyway, I understand your response, thanks a lot! I guess the next question would be, is there a case where using LDA would not be good for this? for example, if there was a competing reaction on the molecule that you wanted to have the enolate form on? What would you use in that case?

[movies]

Generally, LDA is a good choice.  I have actually encountered a problem in my own research where I have a molecule with two potentially acidic sites and I end up getting a rather messy reaction and low yield.  I would say that this is more of an exception than the rule.  The characteristics and selectivity of LDA deprotonations are very well known because it is used so commonly.

With respect to other (non-deprotonation) types of reactions with LDA as base, there are very few.  The steric bulk of the base makes amidation and other types of nucleophilic attacks quite unfavorable.