Transcript Drawing Organic Structures Functional Groups Constitutional Isomers

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)