Compounds A and B both react with LDA (lithium diisopropylamide, a strong base) to form the same anion C with the formula $\ce{C4H5O^-Li^+}$.
I am unfamiliar with LDA. My first thought was that the base would attack the H of the carbonyl C, I presumed that this H would have the lowest $\mathrm{p}K_\mathrm{a}$ because it is an $\mathrm{sp^2}$ C and the C is slightly positive from the carbonyl.
However, the structures this gave were confusing and didn't give the same product for both A and B. I tried to google LDA which showed that it was a big bulky base so it can't attack hindered H, but this shouldn't be a problem as the carbonyl C H isn't hindered. However, I found one mechanism for a different aldehyde that had the H taken from the $\mathrm{sp^3}$ C next to the carbonyl.
Where would LDA preferentially attack an aldehyde and why?
Additional note: I have now played around with the structures to find that the only way LDA could react with A and B to form the same anion C if it in both cases it removes and $\ce{H+}$ from the $\mathrm{sp^3}$ carbon in the structure, these seems counter-intuitive! But perhaps LDA only attacks $\mathrm{sp^3}$ C-H for some reason?