Each step in an acid catalysed synthesis of acetal is reversible. In fact this synthesis is not so easy , indeed it is very difficult because the K of this equilibrium
is certainly lower than 1. We can however move the rection toward the synthesis of acetal keeping attention to eliminate carefully water.
In fact this reaction, the hydrolysis of an acetal in presence of water, is much more efficient than the previous one:
1) A proton is gained by one oxygen atom of the acetal, turning RO in the much better leaving group RO+.
2)The loss of one molecule of alcohol is not given by a SN2 mechanism. The reaction is rather an SN1 and the loss of alcohol pass from the oxonium intermediate.
3) The oxonium ion is attached by a molecule of water. A π bond has been broken simultaneously to the formation of a new σ bond.
4)There's an intermolecular exchange of a proton that turn the RO remained in the better RO+ leaving group.
5) The oxonium ion loses the proton (regenerated) to give the aldehyde.
What is important to remember is that while acetal are formed only in presence of acid they can be hydrolysed only in presence of acid. Acetal are stable to base. In fact they can be used as protecting group in reaction carried out in basic environment.
