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AMINES
Dr. Sheppard
CHEM 2412
Fall 2014
McMurry (8th ed.) sections: 24.2, 24.3, 24.4, 24.6, 24.7,
24.9, 24.10
Amines• Derivatives of ammonia• Neurotransmitters• Medicines• Amino acids, nucleic acids
Amines
I. Nomenclature Review
II. Structure
III. Spectroscopy
IV. Physical Properties
V. Basicity
VI. Preparation
VII. 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
NH2
2-butanamine
CH3 CH2 CH2 CH2 CH CH3
N
CH3 CH3
NH2
2-methyl-1-pentanamine
HO NH2
I. Nomenclature (Review)
• Common name system for amines• Name alkyl group(s)• List in alphabetical order• Add “amine”
• Examples:
Structure Name
CH3CH2NHCH3
(CH3CH2)2NH
I. Nomenclature (Review)• Aromatic amines
Structure Name
NH2
NH2
CH3
NH
CH3
I. Nomenclature (Review)• Pyridine
• If substituted, nitrogen is atom 1 of the ring. Number in direction of other substituents.
N
N
CH3
3-methylpyridine
II. Structure• Classification as 1°, 2°, 3°, 4°
II. Structure• Geometry
• Hybridization of N?• Electron arrangement = tetrahedral• Shape = pyramidal• Bond angle around N?
• Amines can be chiral• Not easily separated form enantiomer (unless 4° ammonium salt)• Nitrogen inversion (pyramidal inversion)
III. Spectroscopy of Amines: IR• Absorption(s) at 3300-3500 cm-1 for N-H
• 13C-NMR: • Atoms bonded to N are deshielded
• 1H-NMR: • N-H signal shifts around from d2-5• N-H signal is usually broad and shows no splitting • Hydrogens on carbons adjacent to N at d2.2-2.6
III. Spectroscopy of Amines: NMR
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
• Amines are strongly polar• Primary and secondary amines form H-bonds (neat)
• Tertiary amines only accept H-bonds from donor (e.g. H2O)
• Boiling points • Higher than hydrocarbons; lower than alcohols (why?)• Tertiary amines lower than primary or secondary• Quaternary salts = ionic = very high MP and BP
• Solubility• Low MW soluble in water; decreases as MW increases• Salts of amines are more soluble in water than neutral amines
IV. Physical Properties of Amines
• Odor• Low MW amines = fishy• Higher MW amines:
IV. Physical Properties of Amines
H2N NH2
cadaverine
H2N
NH2
putrescine
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 are better solvated, but those of tertiary amines are more stable through induction
• Primary, secondary and tertiary amines are all stronger bases than ammonia
R N H
H
H
R N H
R
R
Note pKa values for conjugate acidLower 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)• Loses its aromaticity when protonated
(Chapter 15 later this semester),
so less basic
N
H
N
H
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
CH3 C N
pKb = 24N
pKb = 8.75
N
H
pKb = 2.88
Note pKb values for amineLower pKb indicates stronger base
VI. Preparation of Amines
1. SN2 reactions with alkyl halides
• Often yields multiple substitution products • Products can continue to act as nucleophiles
VI. Preparation of Amines
2. Reduction of nitriles
3. Reduction of amides
VI. Preparation of Amines
4. Reduction of aromatic nitro groups
VI. Preparation of Amines
5. Reductive amination
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 in carboxylic acid derivative chapter