Comments on: Reactions on the “Benzylic” Carbon: Bromination And Oxidation

By: Jozef Prachár

Jozef Prachár — Sat, 09 Jan 2021 22:25:26 +0000

Hi James,
I have one coment to the last question in your quiz. You mentioned that the best synthetic route to get m-nitrobenzoic acid is the second one. It is possible to provide FC alkylation even on the relative electron-poor nitrobenzen (compared to e.g. benzen)? Because, some textbooks (such as e.g., McMurry) mentions that (in general) this type of reaction is not carried out. Thank you for your coment.

The last route is wrong because you can´t get n-propyl/isopropyl benzen using methyl chloride.

By: James Ashenhurst

James Ashenhurst — Mon, 23 Mar 2020 01:44:57 +0000

In reply to Robert. Hi Robert - The molecule would have two carboxylic acids, one at the benzylic position and one at the end of the chain (on the carbon 2 carbons over from the dimethylated benzylic carbon).

By: Robert

Robert — Sat, 21 Mar 2020 21:48:36 +0000

What happens with the carbons? I was thinking, what if we took a dimethyl substituted version of tetralin where one of the benzylic positions had both its H’s replaced by methyl groups, then oxidized it? That position would be unaffected in the reaction, but the other position would turn into a carboxylic acid, and I imagine the chain would unravel. But what would be at the end of that chain?

By: Dom

Dom — Tue, 17 Jul 2018 07:29:51 +0000

In reply to Victor. I've read that the radical bromination of primary carbon isn't as effective as the corresponding chlorination. With the fairly low concentration of Br2 created by NBS, which result in an even lower concentration of Br radical, is it possible for NBS to brominate the primary carbons? For example in such compounds as t-butyl benzene, ethane, & neo-pentane?

By: Matthew Pierce

Matthew Pierce — Sat, 30 Jun 2018 19:40:26 +0000

In reply to James.

I wonder if it is possible to obtain a good yield of mono alkylation product by using a large excess of benzene? A professor claimed that in at least one class. In that case the reaction might work fine for simple synthesis like making toluene.

But vast majority of professors would accept monalkylation to make toluene. Otherwise you would need to write “obtain by distillation from hydrocarbons obtained through fracking”. Or “buy from Aldrich.”

Or something else awkward like formylation/reduction or carboxylation/reduction. Or organometallic transformation that few students have learned.

By: James

James — Fri, 29 Jun 2018 21:33:21 +0000

In reply to Victor. Thanks Victor. 1) I was not previously aware of the neophyl radical rearrangement. Thank you. 2) Guilty as charged re: the FC alkylation. Good point.

By: Victor

Victor — Wed, 20 Jun 2018 14:25:22 +0000

Definitely a worthy topic to discuss. Thank you, James. A couple of notes though. For the fairness sake, I have to note, that when the benzylic hydrogen is not available, the radical brominatiln with NBS is still quite possible, won’t give you the substitution at benzylic position, but still the reaction does occur, so it might be misleading saying that it fails completely and there’s no reaction. When it’s done with t-Bu-Ph, it also gives a radical rearrangement that in fact does yield the benzylic radical.

Also, I believe you’ve mentioned in one of your previous posts that FC alkylation often yields multiple alkylation products and is generally not a good reaction for the actual synthetic purposes, yet you use it here as the first step in your synthesis. Tut-tut ?

By: James

James — Thu, 14 Jun 2018 20:17:35 +0000

In reply to mamid. Thank you very much.

By: mamid

mamid — Wed, 13 Jun 2018 13:53:04 +0000

As usual, excellent post. Some minor corrections are needed, though:
– “here’s are the products” (under “Benzylic Bromination”);
– in the scheme under “When Does It NOT Work?” a 3 is missing in the structure (CH instead of CH3);
– “So why might allylic bromination be useful?” – it should read “benzylic bromination”;
– “Benzylic oxidation With KMnO4 (or H2CrO4)” – it should be “Benzylic Oxidation” (all right, that’s a really small one);
– under this title: “oxidized up to the alcohol/ketone” – it should read “aldehide/ketone”.