Malic acid 3-step synthesis I was quite puzzled by the answer for this question in my Carboxylic Acids and Derivatives tutorial question set. It is question 6 (a), on the synthesis of malic acid, as shown in the image below. 



The idea for the synthesis is to convert the $\ce {Br}$ to $\ce {CN}$ and the carbonyl group to a cyanohydrin, then convert the $\ce {CN}$ to $\ce {COOH}$. I have no disputes about the last step, which is to heat with aqueous hydrochloric acid, or some other mineral acid. However, what I proposed was to perform the two initial conversions in one step, which is to heat the compound with $\ce {KCN (aq)}$ and ethanol. Would that not allow for both the nucleophilic addition, as well as the substitution, to take place, all in one step?

The proposed answer for steps 1 and 2 was to first heat with reflux, with ethanolic $\ce {KCN}$, then react with $\ce {HCN (aq)}$, with trace $\ce {KCN}$, or $\ce {NaOH}$ as catalyst, under temperature of 10-20 degrees C. Is it really necessary to perform the initial two conversions in two separate steps?
I have clarified with my chemistry tutor and it appears that performing the substitution and the addition as two separate steps with two separate sets of conditions is absolutely necessary. The most important reason would be the temperature, which is also the most stark difference between the two different set of conditions.

Due to entropy considerations, addition should ideally be carried out at low temperatures. Thus, if we would like to have the addition of $\ce {HCN}$ to the substrate, it should be done at much lower temperatures as the reaction has a negative entropy change. However, the substition of $\ce {Br}$ by $\ce {CN}$ does not need such low temperlatures and should be done at higher temperatures to provide enough energy of activitation to break the carbon-bromide bond.