Would a thioester hydrolyse faster or slower than a normal ester and why?
I think the thioester is slower, but I'm not sure why.
To determine whether a thioester hydrolyses faster or slower than a normal ester, let's first understand the difference between the two.
An ester is a compound formed by the reaction between an alcohol and an acid, resulting in the formation of an ester linkage. The general structure of an ester is R-COOR', where R is an alkyl or aryl group and R' is another alkyl or aryl group.
On the other hand, a thioester is similar to an ester, but it contains a thiol group (-SH) instead of an alcohol group (-OH). The general structure of a thioester is R-COSR', where R and R' represent alkyl or aryl groups.
Now, let's consider why a thioester would hydrolyze slower than a normal ester.
In an ester hydrolysis reaction, water (H2O) reacts with the ester to produce an alcohol and a carboxylic acid. This reaction is catalyzed by either an acid or a base. Both acid-catalyzed and base-catalyzed ester hydrolysis reactions are reasonably fast under appropriate conditions.
However, in the case of thioesters, the presence of a sulfur (S) atom changes the reactivity of the compound. The C-S bond in a thioester is less polar than the C-O bond in an ester. This reduced polarity makes the thioester less susceptible to nucleophilic attack by water compared to an ester.
Furthermore, the sulfur atom in a thioester can participate in resonance, which delocalizes the negative charge between the carbonyl oxygen and sulfur atoms. This resonance stabilization also slows down the hydrolysis reaction.
Therefore, due to the reduced reactivity and the resonance stabilization of the thioester, it hydrolyzes slower than a normal ester.
In summary, a thioester hydrolyzes slower than a normal ester due to the reduced polarity of the C-S bond and the resonance stabilization of the negative charge on the carbonyl oxygen and sulfur atoms.