We saw that putting an aldehyde in an alcoholic solution we can observe a new compound known as hemiacetal.
This was the acid catalysed formation of hemiacetal:
The acid catalysis work on the rate of the reaction and not on the equilibrium. But even if we can't isolate hemiacetals, in a mixture of aldehyde (or ketone), alcohol and catalytic H+a new compound has formed instead. It's called acetal.
This reaction goes on because, in presence of acid, OH can be turned in a better leaving group by protonation (OH2+). When the hemiacetal lose a molecule of water it became a great electrophile and suffers the attack of another molecule of alcohol (R''OH).
As every equilibrium, we can make the reaction go towards the formation of tha acetal distilling out water (le chatelier).
Here is the mechanism of acetal formation from hemiacetal under acid conditions:
The mechanism is very simple. The stages are:
1) The hydroxyl group is protonated, becoming a good leaving group.
Since it has a positive charge on oxygen, this makes the carbonyl very reactive, much more electrophile (δ+)
3) The oxonium ion is attached by another molecule of alchohol. The π bond is broken, and the more electronegative oxygen get back its electrons.
4)The proton is loss (regenerated). We synthesized an acetal.
Here is the synthesis of an acetal from aldehyde and the hydrolisis of the same acetal, changing opportunely the reaction conditions.