Editing Acetal (section)

==Acetalisation and ketalization==
{{anchor|Acetalisation}}
Acetalisation and ketalization are the [[organic reaction]]s that involve the formation of an acetal (or ketals) from aldehydes and ketones, respectively. These conversions are [[acid]] [[catalysis|catalysed]].  They eliminate water. Since each step is often a rapid equilibrium, the reaction must be driven by removal of water.  Methods for removing water include [[azeotropic distillation]] and trapping water with desiccants like [[aluminium oxide]] and [[molecular sieve]]s.  Steps assumed to be involved: protonation of the carbonyl oxygen, addition of the alcohol to the protonated carbonyl, protonolysis of the resulting [[hemiacetal]] or hemiketal, and addition of the second alcohol.  These steps are illustrated with an aldehyde RCH=O and the alcohol R'OH:
:{{chem2| RCH\dO  +  H+  <->  RCH\dOH+ }}
:{{chem2|RCH\dOH+  +  R'OH  <->  RCH(OH)(OR')  +  H+}}
:{{chem2| RCH(OH)(OR')  +  H+  <->  RC+H(OR')  +  H2O}}
:{{chem2|RC+H(OR')  +  R'OH  <->  RCH(OR')2  +  H+}}

Another way to avoid the entropic cost is to perform the synthesis by acetal exchange, using a pre-existing acetal-type reagent as the OR'-group donor rather than simple addition of alcohols themselves. One type of reagent used for this method is an orthoester. In this case, water produced along with the acetal product is destroyed when it hydrolyses residual orthoester molecules, and this [[side reaction]] also produces more alcohol to be used in the main reaction.