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Magazine Chapter 2: 40P (page 139)
The following compound can become protonated on any of the three nitrogen atoms. One of these nitrogens is much more basic than the others, however.
- Draw the important resonance forms of the products of protonation on each of the three nitrogen atoms.
- Determine which nitrogen atom is the most basic.
Short Answer
(a)
(b)
(b)
Step by step solution
Resonance structures
A single structural formula sometimes cannot explain all the properties of a compound that is given. In such cases, the compound may be represented by two or more structural formulae which differ from each other only in the arrangement of electrons. None of these structural formulae alone can explain all the observed properties of the compound. The compound is then said to show resonance. The various structures are known as resonating structures. The true structure of the molecule is not represented by any of the resonating structures but is considered to be a resonance hybrid of the various resonating structures.
Resonance effect on basicity
The basicity of common organic compounds is affected by resonance stabilization. Resonance delocalization of charges on a base helps in making the base more stabilized, and hence making it a weaker base and conjugate acid is strengthened.
Identifying the nitrogen that is more basic by protonation
Basicity mostly depends on electronegativity and delocalization of the negative charge of the base (anion).
(a)
(b) Most basic is the second nitrogen atom (centered one) because the conjugate base which is formed by protonation of this nitrogen atom is stabilized by delocalization of positive charge.
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