Filemon
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Acetalisation and Ketalization
The books that I have consulted don't give details and little information to make acetals or ketals. Why is it used dimethyl acetal (or ketal) in time
diethyl acetal (or ketal)? Would it be enough 3N of glacial acetic acid or is a too much weak acid for the acetalisation or transacetalisation?
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Klute
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Acetals are amde by addition of an alcohol on a carbonyl, and "etherification" for the hemiacetal with another molecule of alcohol. The recation is
acied-catalyzed, but you might need soemthing a little stronger than AcOH, like H2SO4 or TsOH.
Reflux with exces MeOH, EtOH or ethylene glycol and a drop or two of H2SO4 should do the trick. Using EG, a Dean Stark is very usefull to drive the
reaction and to moniter it.
I think Vogels has a short section on acetals.
[Edited on 14-8-2008 by Klute]
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Filemon
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I have seen methods using I2 as catalyst instead of an acid. How does it work? For the reaction with the alcohol and conversion of I2 in HI?
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Nicodem
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I2 is a Lewis acid so it can catalyze the acetal formation. Essentially any suitable acid could be used: ZnCl2, CF3COOH, H2SO4, HCl, etc., acidic
clays, zeolites and polymeric sulfonic acids can also be used. Acetic acid can not be used since it can esterify (ethyl acetate is thermodinamicaly
bellow a diethyl acetal). But for reasons of nonvolatility and perfect performance tosylic acid is used most commonly by removing water with toluene
azeotrope (this is particularly suitable when doing acetals with ethylene glycol).
Acetals of volatile alcohols like MeOH are most commonly done via transacetalization using 2,2-dimethoxypropane or trimethylorthoformate. Some methyl
acetals form particularly easily and require only anhydrous CuSO4 as mildly acidic dehydrating salt in methanol.
However, the decision on which method to use depends on the substrate/alcohol.
PS: The term "ketal" is being discouraged from use by IUPAC.
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