Amines are organic bases. Methylamine (CH3NH2) is the simplest amine. — Organic Chemistry / Acid-Base Chemistry Question
Problem Context
Amines are organic bases. Methylamine (CH3NH2) is the simplest amine.
Write Lewis structures for methylamine and its conjugate acid, the methylammonium ion.
Describe the local geometry and hybridization of nitrogen in methylamine and the methylammonium ion.
Model Answer
In methylamine: N is trigonal pyramidal with sp3 hybridization
In methylammonium ion: N is tetrahedral with sp3 hybridization
Methylamine is a much stronger base than aniline (C6H5NH2). Account for this observation on the basis of structure and bonding.
Model Answer
In aniline, the lone pair is less accessible for donation, due to resonance between the lone pair on N and the benzene ring (see below).
This resonance interaction is lost on protonation, which destabilizes the conjugate acid form and thus makes aniline less basic.
Less important considerations include: (1) A phenyl group is less electron donating than methyl since the sp2-hybridized carbon is more electronegative than the sp3 carbon in methylamine. (2) The slightly larger size of phenyl vs. methyl might interfere with solvation of the charged conjugate acid, destabilizing the conjugate acid form of aniline relative to methylamine.
There are four isomeric amines with the formula C3H9N. Write condensed structural formulas for each and identify each structure as a primary, secondary, or tertiary amine.
Model Answer
CH3CH2CH2NH2 : primary amine (propylamine)
CH3CH(CH3)NH2 : primary amine (isopropylamine)
CH3NHCH2CH3 : secondary amine (ethyl methyl amine)
(CH3)3N : tertiary amine (trimethylamine)