a frequently discussed issue on the hive is how to make
a benzaldehyde from the corresponding benzoic acid. especially
the conversion of trimethyl gallic acid into 3,4,5TMBA or
piperic acid into piperonal (the "black pepper route") is
a matter of interest.
to the existing routes (rosenmund-reduction of the acid chloride,
via ester-->benzyl alcohol) i would like to add one, that has
yet been overlooked (afaik). in Berichte 59 , pp 727-737, (1926),
they make 3,4,5-TMBA from the hydrazide of 3,4,5 TMbenzoic acid.
procedure is as follows:
1 g hydrazide is dissolved in 70 ml of water along with 6times the
molar amount of ammonia. to the mixture is gradually added 3times
the molar
amount of potassium ferricyanide keeping the temperature as low as
possible (ice bath).
after filtering off 10% of acylhydrazone, 56 - 59% of the aldehyd
is extracted by means of ether.
the article is very interesting, as it gives some background notes
on this topic. the hydrazide is prepared by 8 hour reflux of TMBenzoic
acid methyl ester with hydrazine hydrate.
otto