Question

conversion of alcohol to aldehyde?

Answer 1

One can use a mild or weak oxidising agent to convert an alcohol to an aldehyde.
Mind you the alcohol should be a primary alcohol and not a secondary or tertiary one.
The reaction goes as follows:-
Eg
Methanol to Methanal

CH3OH ---------Mild oxidising Agent----》HCHO

Mechanism:

The reaction is in aqueous solution, so one of the water molecules bonds with one of the hydrogen atoms connected to the carbon having the alcohol group connected to it. This happens as the O in H2O has lone pairs of electrons. This new group causes the O in the water molecule to have a partial positive charge since it now has 3 bonds instead of 2. So the H3O+ radical gets separated from the main group leaving a positive charge on the carbon atom. Now the oxygen atom in the alcohol group attracts the bond pair of electrons between it and hydrogen atom. So the H+ gets separated. It later bonds with OH- in the aqeous solution to form water. Now the carbon atom has a positive charge and the oxygen atom has a negative charge so another bond is formed between the two of them.

And Voila you have converted an Alcohol to an Aldehyde.

Additionally the same reaction can be used on an aldehyde to acquire a carboxylic acid