Question

Which of the following is/are examples of pseudo unimolecular reactions? (This question has multiple correct options)
\[\begin{align}
  & a)~~~~C{{H}_{3}}C{{O}_{2}}{{C}_{2}}{{H}_{5}}+{{H}_{2}}O\xrightarrow{{{H}^{+}}}C{{H}_{3}}C{{O}_{2}}H+{{C}_{2}}{{H}_{5}}OH \\
 & b)\text{ }{{C}_{12}}{{H}_{22}}{{O}_{11}}+{{H}_{2}}O\xrightarrow{{{H}^{+}}}{{C}_{6}}{{H}_{12}}{{O}_{6}}\left( Glucose \right)\text{ }+\text{ }{{C}_{6}}{{H}_{12}}{{O}_{6}}\left( Fructose \right) \\
 & c)\text{ }C{{H}_{3}}COCl+{{H}_{2}}O\to C{{H}_{3}}C{{O}_{2}}H+HCl \\
 & d)\text{ }C{{H}_{3}}C{{O}_{2}}{{C}_{2}}{{H}_{5}}+{{H}_{2}}O\xrightarrow{O{{H}^{-}}}C{{H}_{3}}C{{O}_{2}}H+{{C}_{2}}{{H}_{5}}OH \\
\end{align}\]

Answer 1

Hint: Reactions which look like second order reactions but behave like first order reactions and follow the first order rate law are referred to as pseudo unimolecular reactions. With this definition, try and narrow down all the possible answers.

Complete step-by-step answer:
Let us first try and understand what a pseudo unimolecular reaction really is before then applying this knowledge and these concepts to help find the answers to the given question.
In a chemical reaction if two or more reactants are involved but the rate of reaction depends only upon the concentration of one of the reactants and is independent of the others, then it is said to be a pseudo unimolecular reaction.
The rate constant of a pseudo unimolecular reaction has the units of a second order reaction as a pseudo unimolecular reaction is a reaction of second order in which one of the reactants is present in large excess.
Let us now analyse the given options and with the help of the aforementioned information, try and solve this question.
We observe that the reactions in options A, B and C represent pseudo unimolecular reactions as water is present in large excess. Hence, the concentration of water remains practically unchanged and does not affect the rate of the reaction making them pseudo unimolecular reactions by definition.

Hydrolysis of ester in acidic medium is one of the most widely known examples of pseudo unimolecular reactions, however the same process in alkaline medium is bimolecular. Inversion of cane sugar and the hydrolysis of acetyl chloride are also pseudo unimolecular.
Thus, we can safely conclude that the answers to this question are a), b) and c).

Note: Pseudo-unimolecular reactions are generally bimolecular reactions but with one reactant in excess such that to whatever time the reaction proceeds concentration of excess reactant doesn't change, leaving the rate determined by the concentration of the other reactant. For example, in hydrolysis of an ester water is in excess so the rate of reaction is only dependent on the concentration of ester.