1. CHAPTER TEN
Amines
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2. Structure & Classification
1°, 2°, or , 3° amines: Amines in which 1, 2, or 3 hydrogens of NH3 are replaced by alkyl or aryl groups.
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3. Structure & Classification
Amines are further divided into aliphatic, aromatic, and heterocyclic amines.
Aliphatic amine: An amine in which nitrogen is bonded only to alkyl groups.
Aromatic amine: An amine in which nitrogen is bonded to one or more aryl groups.
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4. Structure & Classification
Heterocyclic amine: An amine in which nitrogen is one of the atoms of a ring.
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5. Structure & Classification
Example: Classify each amino group by type.
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Nomenclature
Aliphatic Amines: Replace the suffix of the parent alkane by –amine.
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Nomenclature
The IUPAC system retains the common name aniline.
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Nomenclature
Among the various functional groups discussed in the text, -NH2 group has one of the lowest priorities.
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Nomenclature
Common names for most aliphatic amines are derived by listing the alkyl groups bonded to nitrogen in one word ending with the suffix -amine.
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Nomenclature
When four groups are bonded to the nitrogen atom, we name the compound as a salt of the corresponding amine.
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Physical Properties
Amines are polar compounds, and both 1° and 2° amines form intermolecular hydrogen bonds.
N-H- - - N hydrogen bonds are weaker than O-H O hydrogen bonds because the difference in electronegativity between N and H ( = 0.9) is less than that between O and H ( = 1.4).
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Basicity
All amines are weak bases, and aqueous solutions of amines are basic.
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Basicity
It is also common to discuss the basicity of amines by reference to the ionization constant Ka of its conjugate acid.
For any acid-conjugate base pair.
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Basicity
Using values of pKa, we can predict the position of equilibrium in acid-base reactions.
Acetic acid is the stronger acid and, therefore, the position of this equilibrium lies to the right.
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Basicity
Aliphatic amines have about the same base strength, pKb 3.0 – 4.0 and are slightly stronger bases than NH3.
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Basicity
Aromatic amines are considerably weaker bases than aliphatic amines.
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Basicity
Aromatic amines are weaker bases than aliphatic amines because the resonance stabilization of the free base is lost on protonation.
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Basicity
Electron-withdrawing groups, such as halogen, nitro, and carbonyl, decrease the basicity of aromatic amines by decreasing the availability of the electron pair on nitrogen.
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Basicity
Example: Select the stronger base in each pair of amines.
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Guanidine
Guanidine (pKb 0.4) is the strongest base among neutral organic compounds.
Its basicity is due to the resonance delocalization of the positive charge over the three nitrogen atoms.
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Reaction with Acids
All amines, whether soluble or insoluble in water, react quantitatively with strong acids to form water-soluble salts.
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Synthesis of 1°
Most 1° aromatic amines are synthesized by nitration of an aromatic ring followed by catalytic reduction of the nitro group to an amino group.
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23. Amines as Nucleophiles
Reaction of an amine with an alkyl halide can be used to form a new covalent bond.
Step 1: Reaction of a nucleophile with an electrophile to form a new covalent bond.
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24. Amines as Nucleophiles
Step 2: Take a proton away. Converts the amine salt to a free amine.
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25. Amines as Nucleophiles
In Step 2, a new 2° amine was produced, which can then act as a nucleophile with the result that the final reaction mixture will consist of varying ratios of RNH2, R2NH, R3N, and R4N+Cl–. These ratios are difficult to control and, therefore, we avoid using amines as nucleophiles in nucleophilic substitutions reactions.
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Amines
End Chapter 10
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