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AMINES Dr. Sheppard CHEM 2412 Summer 2015 Klein (2nd ed.) sections: 23.1, 23.2, 23.3, 23.13, 23.4, 23.5, 23.6, 23.8 Amines • Derivatives of ammonia • Neurotransmitters • Medicines • Amino acids, nucleic acids Amines I. II. III. IV. V. VI. VII. Nomenclature Review Structure Spectroscopy Physical Properties Basicity Preparation Reactions I. Nomenclature (Review) 1. Parent chain is longest containing C bonded to –N 2. Change suffix “-e” to “-amine” 3. Number from end closest to –N. Show location of –N. 4. Name/number substituents 5. -NH2 as a substituent is “amino” • Order of precedence of functions I. Nomenclature (Review) • Common name system for amines • Name alkyl group(s) • List in alphabetical order • Add “amine” • Examples: Structure CH3CH2NHCH3 (CH3CH2)2NH Name I. Nomenclature (Review) • Aromatic amines Structure Name I. Nomenclature (Review) • Pyridine • If substituted, nitrogen is atom 1 of the ring. Number in direction of other substituents. II. Structure • Classification as 1°, 2°, 3°, 4° II. Structure • Geometry • Hybridization of N? • Electron arrangement = tetrahedral • Shape = pyramidal • Bond angle around N? II. Structure • Amines can be chiral • Not easily separated from enantiomer (unless 4° ammonium salt) • Nitrogen inversion (pyramidal inversion) III. Spectroscopy of Amines: IR • Absorption(s) at 3300-3500 cm-1 for N-H III. Spectroscopy of Amines: NMR • 13C-NMR: • Atoms bonded to N are deshielded • 1H-NMR: • N-H signal shifts around from d0.5-5 • N-H signal is usually broad and shows no splitting • Hydrogens on carbons adjacent to N at d2-3 III. Spectroscopy of Amines: NMR • 1H-NMR spectrum for 2-ethylaniline III. Spectroscopy of Amines: MS • Nitrogen rule: if a molecule has an odd number of nitrogen atoms, the mass will be an odd number • Odd masses have 1 or 3 nitrogen atoms • Even masses have 0 or 2 nitrogen atoms • Molecule fragments by carbon bonded to amine • a-cleavage III. Spectroscopy of Amines: MS • Ex: ethylpropylamine IV. Physical Properties of Amines • Amines are strongly polar • Primary and secondary amines form H-bonds (neat) • Tertiary amines only accept H-bonds from donor (e.g. H2O) • Solubility in water • Low MW amines are soluble in water • Solubility decreases as MW increases • Salts of amines are more soluble than neutral amines IV. Physical Properties of Amines • Boiling points • Higher than hydrocarbons; lower than alcohols (why?) • Tertiary amines lower than primary or secondary • Quaternary salts = ionic = very high MP and BP IV. Physical Properties of Amines • Odor • Low MW amines = fishy • Higher MW amines: V. Basicity of Amines • Amines have lone pair on N • More basic than alcohols (more stable conjugate acid) • More basic than amides (amides stabilized by resonance) V. Basicity of Amines • Trends in basicity • Tertiary, secondary, and primary amines are all approximately equal in basicity • Conjugated acids of primary amines are better solvated, but those of tertiary amines are more stable through induction • Primary, secondary and tertiary amines are all stronger bases than ammonia Note pKa values for conjugate acid Lower pKa indicates weaker base V. Basicity of Amines • Aromatic amines are weaker bases than aliphatic amines • Aniline • Stabilized by resonance • Less basic/nucleophilic • Pyrrole (a heterocycle) • Stabilized by resonance • Also loses its aromaticity when protonated (Chapter 18 later this semester), so less basic V. Basicity of Amines • sp-hybridized N’s are less basic than sp2-hybridized N’s, which are less basic than sp3-hybridized N’s • More s-character holds electrons more tightly • Electrons are less available/less basic Note pKb values for amine Lower pKb indicates stronger base VI. Preparation of Amines 1. Reduction of nitriles 2. Reduction of amides VI. Preparation of Amines 3. Reduction of aromatic nitro groups VI. Preparation of Amines 4. SN2 reactions with alkyl halides • Often yields multiple substitution products • Products can continue to act as nucleophiles VI. Preparation of Amines 5. Reductive amination (review) VII. Reactions of Amines 1. Alkylation (SN2 with alkyl halide) 2. Formation of imine (as in reductive amination) 3. Nucleophilic acyl substitution • Product = amide • Will see with carboxylic acid derivatives (Chapter 21)