Do aldehydes need to be protected from NBS (i.e Bromine)? Im planning to conduct a benzylic bromination on methyl benzaldehyde. I dont see why it
would need to be protected, but Im not sure. I cant seem to find an answer or procedure that would calrify this for me.Metacelsus - 27-6-2014 at 15:44
The only time aldehydes need to be protected is if they have an alpha hydrogen AND you are using bromine (not NBS).
Your reaction should work fine. You could even use elemental bromine if you wanted.
Edit: The aldehyde will probably be oxidized (see Nicodem's post).
[Edited on 28-6-2014 by Cheddite Cheese]AvBaeyer - 27-6-2014 at 18:57
Aldehydes such as your substrate can be oxidized to the acyl halide by agents like NBS, NCS, TCCA and the like. I can attest to this from personal
experience. See the discussion section on "solid state vs solution reactivity" regarding compound 25U.
By the way, I found this reference on Google within a matter of minutes searching for reaction of aldehydes with N-bromosuccinimide. Additional
information can be found in Fieser - Reagents for Organic Synthesis if you put in the time.
The only time aldehydes need to be protected is if they have an alpha hydrogen AND you are using bromine (not NBS).
Your reaction should work fine. You could even use elemental bromine if you wanted.
He is not talking about an electrophilic bromination. Like AvBaeyer said, aldehydes get easily oxidized during radical brominations. This reaction is
relatively fast and probably occurs faster than the desired benzylic bromination. The literature about radical halogenation of aldehydes has been
partially reviewed in another thread.
