Why do we add water to ethyl acetate in curcumin synthesis?
Answers
Ethanoic acid can be esterificated by reacting it with an alcohol (e.g. ethanol) or an acyl chloride (e.g. ethanoyl chloride).
The reaction with an alcohol is reversible therefore has a lower yield (as the reaction does not go to completion). This reaction also requires a sulfuric acid catalyst to protonate the C=O group of ethanoic acid to produce a positively charged electrophile that is attacked by the lone pair on the oxygen atom of the OH group of the alcohol in nucleophilic substitution, forming a COOC ester linkage and eliminating a water molecule (as OH is lost from the acid and H is lost from the alcohol).
The reaction with an acyl chloride is irreversible therefore has a higher yield (as the waste product is a gas and can therefore be separated from the reaction mixture, unlike the H2O produced when reacting with an alcohol), produces a more concentrated solution that is more pure, and goes to completion. It also does not require a catalyst as the carbon atom of the C-Cl bond is very delta positive and Cl- is a better leaving group (species that departs with a pair of electrons in heterolytic fission) than OH- (as the HCl eliminated is more acidic than H2O) and the acyl chloride is attacked by the OH group of the acid in nucleophilic substitution forming a COOC link and eliminating a molecule of HCl. This process is therefore less expensive and less time consuming as a catalyst does not not need to be separated from the reaction mixture, however it does produce toxic HCl gas.
In both cases the end product is ethyl ethanoate.
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