Making nitriles from aldehydes and ketones

Making nitriles from aldehydes and ketones Aldehydes and ketones undergo an addition reaction with hydrogen cyanide. The hydrogen  cyanide adds across the carbon-oxygen double bond in the aldehyde or ketone to produce  a  hydroxynitrile. Hydroxynitriles used to be known as cyanohydrins.     For example, with ethanal (an aldehyde) you get 2-hydroxypropanenitrile: With propanone (a ketone) you get 2-hydroxy-2-methylpropanenitrile:     In every example of this kind, the -OH group will be on the number 2 carbon atom -  the one next to the -CN group. The reaction isn't normally done using hydrogen cyanide itself, because this is an extremely  poisonous gas. Instead, the aldehyde or ketone is mixed with a solution of sodium or   potassium cyanide in water to which a little sulphuric acid has been added. The pH of  the solution is adjusted to about 4 - 5, because this gives the fastest reaction. The  reaction happens at room temperature. The solution will contain hydrogen cyanide (from the reaction between the sodium or  potassium cyanide and the sulphuric acid), but still contains some free cyanide ions.   This is important for the mechanism.
These are useful reactions because they not only increase the number of carbon atoms in  a chain, but also introduce another reactive group as well as the -CN group. The -OH group  behaves just like the -OH group in any alcohol with a similar structure. For example, starting from a hydroxynitrile made from an aldehyde, you can quite easily  produce relatively complicated molecules like 2-amino acids - the amino acids which are  used to construct proteins.