Question

Indicate which nitrogen in each of these bases is the one most likely to be protonated? Are they more basic than regular amines?
Choose the correct options:

• A. Guanidine is the most basic of the three
• B. DABCO is the most basic of the three
• C. DBN is the most basic of the three
• D. These are all less basic than amines.

Answer 1

The correct option is A Guanidine is the most basic of the three

As you can see – each of these given compounds contain the amidine group:

Let us start with the guanidine. Suppose we protonate the one of the singly bonded nitrogen atoms:

The above structure doesn’t have any stabilizing features with respect to the positive charge on the protonated nitrogen atom. Now, compare this with the structure having the double-bonded nitrogen atom protonated:

The positive charge is spread across 4 different atoms such that there are several contribution resonance structures. This makes the protonation on the doubly bonded nitrogen atom more viable in terms of stability. So in each case, the protonation is more likely on the doubly bonded nitrogen atom.
Likewise, for DBN:

Likewise, for DABCO:
Compared to amines, these compounds (the ones having the amidine group) are even more basic. This is because of the conjugation between the Nitrogen – Carbon – Nitrogen skeleton where the charge density on the doubly bonded nitrogen is greater owing to conjugation.