Chapter 15, Problem 22P

16-27 Guanidine, p/C_a 13.6, is a very strong base, almost as basic as hydroxide ion.

NH

NH₂+

II

II

H₂N—C—NH₂ + H₂O H₂N—C—NH₂ + OH"

Guanidine

Guanidinium ion

pK_b = 13.6

1. Complete the Lewis structure for guanidine, showing all valence electrons.

The remarkable basicity of guanidine is attributed to the fact that the positive charge on the guanidinium ion is delocalized by resonance over the three nitrogen atoms. This delocalization increases the stability of the guanidinium ion relative to the ammonium ion or substituted ammonium ions.

Draw three equivalent contributing structures for the guanidinium ion and show by the use of curved arrows how these three contributors are related.

Propose an explanation for the fact that protonation occurs on the C=NH nitrogen rather than on one of the —NH., nitrogens. (Hint: Consider the resonance stabilization of the structure formed

by protonation on a —NH., nitrogen compared with the resonance stabilization of the structure formed by protonation on the =NH nitrogen.)

Predict the N—C—N bond angles in the hybrid.

Which is the stronger acid, the ammonium ion or the guanidinium ion?