Question

In the preparation of aldehydes from primary alcohols, aldehydes formed must be distilled out as soon as they are formed. Why?

Answer 1

Oxidising alcohols to make aldehydes and ketonesGeneralThe oxidising agent used in these reactions is normally a solution of sodium or potassium dichromate(VI) acidified with dilute sulphuric acid. If oxidation occurs, the orange solution containing the dichromate(VI) ions is reduced to a green solution containing chromium(III) ions.The net effect is that an oxygen atom from the oxidising agent removes a hydrogen from the -OH group of the alcohol and one from the carbon to which it is attached.[O] is often used to represent oxygen coming from an oxidising agent.R and R' are alkyl groups or hydrogen. They could also be groups containing a benzene ring, but I'm ignoring these to keep things simple.If at least one of these groups is a hydrogen atom, then you will get an aldehyde. If they are both alkyl groups then you get a ketone.If you now think about where they are coming from, you will get an aldehyde if your starting molecule looks like this:In other words, if you start from a primary alcohol, you will get an aldehyde.You will get a ketone if your starting molecule looks like this:. . . where R and R' are both alkyl groups.Secondary alcohols oxidise to give ketones.Making aldehydesAldehydes are made by oxidising primary alcohols. There is, however, a problem.The aldehyde produced can be oxidised further to a carboxylic acid by the acidified potassium dichromate(VI) solution used as the oxidising agent. In order to stop at the aldehyde, you have to prevent this from happening.