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Author: Subject: Hofmann rearrangement of 3,4,5-trimethoxybenzamide using TCCA
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[*] posted on 12-11-2019 at 04:37
Hofmann rearrangement of 3,4,5-trimethoxybenzamide using TCCA

A while ago I posted about the preparation of the n-chloro-3,4,5-trimethoxybenzamide using trichloroisocyanuric acid, and subsequent attempts to perform a Hofmann rearrangement on that to end up with the aniline.

I've had zero success with that reaction :(, but have since attempted the reaction directly on the benzamide with TCCA and had modest success. The procedure is based on other TCCA-based ones others have presented in this forum and others eg:

Dissolve 20g of NaOH in 200ml water.

Dissolve 7g of TCCA (0.03 mol) in the NaOH solution. The TCCA used is nominally 90%, so I estimated this was a total of 0.08 mol of chlorine.

The solution was chilled down in an ice bath to between 5 -10C, and 17.1g (0.08 mol) of the 3,4,5-trimethoxybenzamide was added in portions with magnetic stirring. The suspension eventually all dissolves leaving a slightly cloudy golden yellow solution.

This was stirred cold for 1 hour. Some insoluble material was removed by filration, and this also removed the cloudiness from the solution. The solution was then allowed to reach room temperature and stirred for a further hour. After this period, the temperature was increased rapidly to between 70 - 75C for 30 minutes.

As the temperature passed approx 30C, a precipitate began to form. One the final temperature was reached a thick suspension was present. The liquid meanwhile had gone from yellow to red to black.

After 30 minutes the result was allowed to cool to room temperature and then chilled in the fridge overnight.

The solids were filtered with vacuum filtering whilst washing with cold water, then re-crystallised from hot water as long fine needles that accumulated into a felted mass. They retained a purplish colour that diminished on a second re-crystallisation, but was not eliminated. See notes below.

Total yield was 8.2g @ 183.204 = 0.045mol = 56%

MP 109C - 110C (lit 110 - 113).


I saw no temperature rise when adding the benzamide into solution. On later attempts, I added it all at once with no drop in yield.

On a second attempt, I omitted the prolonged stirring at room temperature and went straight to the heating step after filtering. No precipitate was noted as the temperature went past 30C, but after some minutes at 75C the ppt began to form. Yield was this approach was 59%.

I tried some decolourizing carbon in the first recrystalissation, but it didn't really help eliminate the colour.

What will I do with this? Eventually try a Borsche-Beech Aldehyde Synthesis. I'm using the aldehyde to get oxazolones, then phenylpyruvic acids.
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