6 Physical Properties and Structure of Amines 2A. Physical Properties
7 2B. Structure of AminesN: sp3 hybridizedTrigonal pyramidalBond angles close to 109.5o
8 3o Amines with three different groups The two enantiomeric forms interconvert rapidly:Impossible to resolve enantiomers.Pyramidal or nitrogen inversion:Barrier ~ 25 kJ/mol.Enough to occur rapidly at room temperature.
9 Ammonium salts with four different groups. enantiomerscan be resolved
20 3C. Amines versus AmidesAmides are far less basic than amines (even less basic than arylamines). The pKa of the conjugate acid of a typical amide is about zeroLarger resonance stabilizationSmaller resonance stabilization
72 Analysis saltsof Amines 11A. Chemical AnalysisThey dissolve in dilute aqueous acidMoist pH paper is basicHinsberg test for 1o, 2o and 3o.1o aromatic amines form diazo salts. azo dye formation with 2-naphthol
73 11B. Spectroscopic Analysis IR1o amines3300 – 3555 cm-1 (N–H) two bands.2o amines one band only.3o aminesNo bands at 3300 – 3555 cm-1 region.
75 1H NMR spectra 1o and 2o amines N–H d (0.5 – 5 ppm), usually broad, exact position depends on the solvent, concentration, purity and temperature.N–H protons are not usually coupled to protons on adjacent carbons.Protons on the a carbon of an aliphatic amine are deshielded by the electron-withdrawing effect of the nitrogen and absorb typically in the d 2.2–2.9 region.
76 Protons on the b carbon are not deshielded as much and absorb in the range d 1.0–1.7
77 13C NMR chemical shifts (d) 13C NMR spectra23.034.014.329.742.513C NMR chemical shifts (d)
78 Mass spectraThe molecular ion in the mass spectrum of an amine has an odd number mass (unless there is an even number of nitrogen atoms in the molecule).The peak for the molecular ion is usually strong for aromatic and cyclic aliphatic amines but weak for acyclic aliphatic amines.Cleavage between the a and b carbons of aliphatic amines is a common mode of fragmentation.
79 Eliminations Involving Ammonium Compounds 12A. The Hofmann Elimination
81 Although most eliminations involving neutral substrates tend to follow the Zaitsev rule, eliminations with charged substrates tend to follow what is called the Hofmann rule and yield mainly the least substituted alkene