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Esters can be formed in two ways according to the syllabus I'm following:
(1) Mixing an alcohol with a carboxylic acid in the presence of sulfuric acid; and
(2) Mixing an alcohol with an acid chloride.
My question is:
Which method produces the highest yield of esters?
I think method (2) does because the $\ce{Cl-}$ will attach with the $\ce{-OH}$ in alcohol. However, I'm not exactly sure of the reason. Can someone explain it clearly with full details please?
Option 2 generally gives higher yields.
This is because option 1 (Fischer esterification) usually results in an equilibrium mixture which contains significant amounts of products and reactants, limiting the yield. This is mitigated by using a stoichiometric amount of a concentrated acid catalyst, such as sulfuric acid, which is also a strong dehydrating agent, and therefore removes the water from the equilibrium as it forms, favouring the products. Additionally, the reaction mixture can be heated to evaporate the water, or you can use a large excess of one of the reactants, typically the alcohol because it also acts as the solvent.
Conversely, option 2, more easily gives high yields because the reaction is more or less complete, with very little reactants left at the end. This is because the reaction produces hydrogen chloride gas instead of water, which is lost from the reaction mixture, driving the reaction forward. The downside of this option is that acid chlorides are generally more dangerous than carboxylic acids and are harder to prepare, or more expensive to buy. Aditionally, hydrogen chloride gas is produced which must be disposed of safely.
Another, often used method, is esterification using acid anhydrides. These are intermediate in reactivity between carboxylic acids and acid chlorides and are easier to use than acid chlorides, but still provide excellent yields. A classic example of this is the acetylation of salycylic acid with acetic anhydride to produce Aspirin.
Also, for methyl esters one of the cleanest methods is to use diazomethane.
Of course, diazomethane is pretty nasty, but there are nice kits that can safely generate it in small quantities.
