hodges - 17-8-2009 at 13:52
Standard way to form esters involves adding concentrated H2SO4 to eliminate the water formed by the reaction. Recently I read that methyl sulfate is
extremely toxic, and it is formed by reacting methanol with H2SO4. When making an ester such as methyl salicylate, is there any danger that the
sulfate of the alcohol will be formed?
I've seen a lot of synthesis notes for esters, including at the high school lab level, and I've never seen this mentioned as a danger. So I assume
the alcohol does not form esters as easily with the sulfuric acid as with the organic acid. But why is this the case?
Thanks,
Hodges
Picric-A - 17-8-2009 at 15:05
I believe one of the uses of the Sulphuric acid is to form the sulphate itself...
I read somewhere (Organic Chemistry by P.J.DURRANT) it takes part in the reaction... for example the synthesis of ethyl ethanoate;
ethanol + sulphuric acid --> ethyl sulphate
ethyl Sulphate + ethanoic acid --> ethyl ethanoate + sulphuric acic + water
So accoridng to this the formation of the sulphate is a harmless side reaction so dont worry about it!
UnintentionalChaos - 17-8-2009 at 15:40
Acid catalysis is usually done with only a small amount of sulfuric acid.
Dimethyl sulfate is the dangerous alkylating agent. You are likely to have some methylsulfuric acid form, however, which is a rather weak alkylating
agent in comparison.
Generally, the stronger the acid being used, the more labile the ester bond (and more likely to alkylate DNA, for example). trimethyl orthophosphate,
dimethyl sulfate, and methyl tosylate are all esters of strong acids and notable alkylating agents. Most organic esters are of pitifully weak acids
and are for all intensive purposes harmless.
DJF90 - 17-8-2009 at 18:56
When working on a preparative scale, formation of methyl hydrogen sulfate and dimethyl sulfate are limited. The sulfuric acid only needs to be present
in catalytic amounts, and this minimises any dangers. I personally would not worry about it.