Question: At what pH should imine formation be carried out if the amine’s protonated form has a Pk_a value of 9.0?

An aldehyde or a ketone can react with a compound containing an amine group to produce imines.

The compound formed from primary amines is called an imine, and other compounds containing amine groups like hydroxylamine and hydrazine react to produce imine derivatives.

The reaction is acid-catalyzed, and the -C=O bond of the carbonyl is converted to a –C=NR bond during imine formation. The reaction occurs between the nucleophilic nitrogen of the amine and electrophilic carbon of the carbonyl.

The pH-rate profile of a reaction is a plot of pH with the reaction rate. It shows the dependence of reaction rate on the pH of the solution. The reaction is carried out at different pH values and the rate is determined.

This plot is taken for a standard reaction and the optimum pH for similar reactions can be determined. This is done by calculating the difference between the pH giving the maximum reaction rate and the Pk_a value of the reactant. For imine formation reactions, the Pk_a value of the protonated amine is considered.

The optimum pH for an imine formation reaction is compared with the pH-rate profile of the reaction of hydroxylamine with acetone.

It can be seen that at low pH, the amine exists only in protonated form. The nucleophilic center of amine is required for the reaction, and therefore, the reaction rate decreases. The protonated intermediate is stabilized at high pH by deprotonation, making it less reactive. Therefore, the reaction rate decreases.

From the pH-rate profile, it can be seen that the rate of imine formation is maximum when pH is 1.5 less than the Pk_a of the protonated amine form. Therefore, if Pk_a of the protonated amine is 9.0, the pH at which the reaction can be carried out is calculated as:

pH =Pk_a -1.5

=9-1.5

=7.5

Therefore, imine formation can be carried out at a pH of 7.5.