Question

Why is the conjugate acid of morpholine more acidic than the conjugate acid of piperidine?

Short answer

Inductive electron withdrawal of oxygen in morpholine helps to increase the acidity of protonated morpholine.

Step-by-step solution

Step-by-Step SolutionStep 1:Structure of morpholine and piperidine

The structure of morpholine is given below:

Structure of morpholine

The structure of piperidine is given below:

Structure of piperidine

Step 2: Conjugate acid formation of morpholine and piperidine

The conjugate acid of morpholine is formed by accepting a proton, i.e. a proton gets added to the nitrogen atom. Its $\mathrm{pKa}$ value is shown below:

Conjugate acid of morpholine

The conjugate acid of piperidine is also formed similarly as that of morpholine. Its formation and $\mathrm{pKa}$value is shown below:

Conjugate acid of piperidine

Comparing the acidity between conjugate acids of morpholine and piperidine

Thevalue of conjugated acid of morpholine is$\mathbf{9}\mathbf{.}\mathbf{28}$and that of piperidine isrole="math" localid="1652370624470" $\mathbf{11.12}$. From these values, it is clear that protonatedmorpholine is stronger than protonatedpiperidine sinceacidity is inversely proportional to the$\mathbf{pKa}$value.

Moreover, in protonated morpholine, there is an oxygen atom which withdraws electrons inductively while this is not possible in protonated piperidine because there is no oxygen atom.

So, protonated morpholine is a stronger acid than protonated piperidine.