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Rhodium:

4-bromo-2,5-dimethoxybenzaldehyde [ Org Prep Proced Int 23(4), 419-424 (1991) ]

The bromination of 2,5-dimethoxybenzaldehyde with elemental bromine in glacial 
acetic acid gives rise to two different products. The predominant one (4-bromo-
2,5-dimethoxybenzaldehyde) is formed in 87% yield, while the other (6-bromo-
2,5-dimethoxybenzaldehyde) is formed to an extent of only 5%. The selectivity 
should be much better when using NBS instead of elemental bromine, and repeated 
recrystallization from ethanol (or perhaps 95% MeOH or IPA will do just as well) 
should work satisfactorily to remove any 6-bromo-2,5-dimethoxybenzaldehyde 
present (which is easily detected with TLC or melting point determination).  

Experimental:

A cold solution of 20.0g (0.12 mol) of 2,5-dimethoxybenzaldehyde in glacial 
acetic acid (115ml) was treated with 20.0g of bromine in 60ml glacial acetic 
acid. The solution was stirred at room temperature for two to three days and 
then dilute with ice water. The yellow precipitate was collected by filtration 
and dried (28g, 95%), mp 118-126°C (lit. 125-126°C). Recrystallization from 
95% ethanol gave 15.20g of 4-bromo-2,5-dimethoxybenzaldehyde, mp 132-133°C 
(lit. 132-133°C). Ethanol was removed from the mother liquor in vacuo and the 
residue was subjected to column chromatography (silicagel, benzene). 
The solid (10.4g) collected in the initial fractions consisted of 
4-bromo-2,5-dimethoxy-benzaldehyde, total yield 25.6g (87%), mp 132-133°C. 
Subsequent fractions gave 1.5g (5%) of 6-bromo-2,5-dimethoxybenzaldehyde, 
mp 102-103°C after recrystallization from ethanol. 

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