Amines Lecture Notes PDF

Aromatic Amines  Amines Amines are classified into: Primary amines 1ry, Secondary amines 2ry Tertiary amines 3ry R R NH2 R NH R N R...

Aromatic Amines  Amines Amines are classified into: Primary amines 1ry, Secondary amines 2ry Tertiary amines 3ry R R NH2 R NH R N R R On the basis of the number of of organic groups attached to the nitrogen. Also, amines are classified based on the type of the carbon atoms attached to the nitrogen into alkyl-, aryl- or aralkyl- amines  Nomenclature of amines: Primary amines: Common nomenclature: most 1ry amines are named as alkylamines Systematic nomenclature: By adding the suffix amine to the chain or ring system to which the NH2 group is attached with elision of the final (e) Amines Alkyl- Aryl- Aralkyl- Heterocyclic R R N Ar N Ar N N  NH2 CH3CH2NH2 Ethylamine Cyclohexylamine Ethanamine Cyclohexanamine Secondary and tertiary amines: Most of them are named in the same general way. In common nomenclature we either designate the organic groups individually if the are different, or use the prefixes di- or tri- if they are the same. CH3CH2NHCH2CH3 CH3CH2NHCH3 Diethylamine Ethylmethylamine N-Ethylethanamine N-Methylethanamine In systematic IUPAC we use the locant N to designate substituents attached to a nitrogen atom. In the IUPAC, the substituent NH2 is called amino group. OHCH2CH2NH2 HOOCCH2CH2CH2NH2 2-Aminothanol 4-Aminobutanoic acid Aryl amines: H3C NH2 NH NH2 CH3 Aniline N-Methylaniline o-, m- or p-toluidine Benzamine N-Methylbenzamine 4-Methylbenzamine Order of Basicity: In gas phase: (CH3)3N > (CH3)2NH > CH3NH2 > NH3 In aq. Phase: (CH3)2NH > CH3NH2 > (CH3)3N > NH3 The differences in the basicity order in the gas phase and aqueous solutions are the result of solvation effects. Amines in water solution exist as ammonium ions. In water, the ammonium salts of primary and secondary amines undergo solvation effects (due to hydrogen bonding) to a much greater degree than ammonium salts of tertiary amines. These solvation effects increase the electron density on the amine nitrogen to a greater degree than the inductive effect of alkyl groups. Basicity of amines: Arylamines are weaker bases than cyclohexylamines because of resonance. Aniline, a typical arylamine, exhibits the resonance structures shown in the following figure. Preparation of Aromatic Amines: 1. Reduction of nitro compounds (tin (Sn), iron (Fe) or zinc (Zn), with hydrochloric or acetic acid) Metal/ acid Ar NO2 Ar NH2 + H2O 2. From carboxylic acids and its derivatives Hofmann Amide Degradation (Hofmann Rearrangement) Mechanism Reaction of Aromatic amines Due to the unshared electron pair, amines can act as both bases and nucleophiles. As base: Amines are basic compounds and react with acids to form salts. The basicity of amines related to the availability of lone pair (the more available the more basic), so aliphatic amines are more basic than aromatic amines due to contribution of the lone pair of N in ring resonance in case of aromatic amine while in aliphatic they are localized. On the other hand alkyl gp have +I effect so ↑ electron density on N, where Ph have –I effect so  electron density on N and so  basicity. - Electron donating gp (+I , +M) → ↑ basicity. - Electronwithdrawing gp (-I , -M) →  basicity.  Reaction as nucleophile: A- Alkylation: (aromatic amines undergo multiple alkylation with alkyl halide RX to give finally quaternary ammonium salt). Acylation: 1ry and 2ry aromatic amines reacted with acid anhydride or acid halide to form amides Electrophilic substitution reaction Aromatic amines are more reactive than benzene toward SE reaction due to activation of the ring toward SE by NH2 effect (+M). - Amino gp o,p directing. Halogenation: Halogenation of aromatic amines doesn’t need lewis acid and afford polyhalogenated product, due to strong activating power of NH2. Nitration: We can’t make direct nitration for aromatic amines because cHNO3 will oxidize aniline to afford benzoquinone ???? So nitration of aromatic amine can done by Nitration in the presence of excess acid (afford m-nitroaniline) Protection of the amino gp by acetylation ( afford o-,p-nitroaniline) Indirect preparation of m-nitroaniline nitration Diazotization Reaction of 1ry aromatic amines with Nitrous acid HNO2 amine reacted with nitrous acid (prepared by reaction of NaNO2 and HCl) to afford benzene diazonium salt Sandmeyer Reaction