Nucleophiles and leaving groups reversible addition reactions

Nucleophiles and leaving groups reversible addition reactions 

In principle, all carbonyl addition reactions could be reversible but, in practice, many are essentially irreversible. Let us consider mechanisms for the reverse of the nucleophilic addition reactions given above. For the base-catalysed reaction, we would invoke the following mechanism  

For tire acid-catalysed reaction, we would write 

We can also rationalize why some addition reactions simply do not occur, e.g. halide ions do not add to carbonyl groups. Although we know that a halide such as bromide can act as an effective nucleophile in Sn 1 and Sn2 reactions (see Section 6.1.2), it is also a very good leaving group (pATa value for HBr —9). This means that the reverse reaction becomes very much more favourable than the forward reaction. In cases where both forward and reverse reactions are feasible, we can often usefully disturb the equilibrium by using an excess of one reagent (see below). 

The addition of 1 mol of an alcohol to an aldehyde gives a bemiacetal, and to a ketone a hemiketal. However, most chemists do not now differentiate between hemiacetals and hemiketals these are both termed hemiacetals. This reaction is usually catalysed 

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