Question

How do you make aldehyde?

Answer 1

With difficulty; usually by oxidation of alcohol.

Explanation:

We start with a primary alcohol, `RCH_2OH`; (why can't we start with a secondary alcohol?).

`RCH_2OH + [O] rarr RC(=O)H rarr RC(=O)OH`

Now, there a variety of oxidizing agents, metal oxides, oxygen, chlorates for instance. The problem is that they tend to be indiscriminant. I have known very experienced organic chemists to use `K_2Cr_2O_7` to oxidize primary alcohols to aldehydes, but they would strictly monitor the reaction by `TLC`, and stop the oxidation at the aldehyde stage.