Q.Classify the following reaction as addition, elimination or substitution, and name the nucleophile involved: CH3-CH2-Br + NaCN -> CH3-CH2-CN + NaBr.

[!TLDR] CN(-) replaces Br(-) on the same carbon skeleton, so this is a nucleophilic substitution reaction. [!ANSWER] This is a (nucleophilic) subs …

In $\text{CH}_3-\text{CH}_2-\text{Br} + \text{NaCN} \rightarrow \text{CH}_3-\text{CH}_2-\text{CN} + \text{NaBr}$, the bromine atom of ethyl bromide is replaced by a $-\text{CN}$ group, while the rest of the carbon skeleton ($\text{CH}_3-\text{CH}_2-$) stays exactly as it was and no multiple bond is created or destroyed. One group ($-\text{Br}$) is simply exchanged for another ($-\text{CN}$), which is the defining pattern of a substitution reaction. …

Check that one group ($-\text{Br}$) is replaced by another ($-\text{CN}$) with no change to the rest of the skeleton and no new multiple bond, confirming substitutio …

Failing to identify $\text{CN}^-$, rather than $\text{Na}^+$, as the reactive nucleophile; sodium is merely the spectator counter-ion in $\tex …