Title SP-3_Ch-19_Amines.pdf ✅

Chapter Contents Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Chapter 19 Amines  Introduction  Structure of Amines  Classification  Nomenclature  Preparation of Amines  Physical Properties  Chemical Reactions  Diazonium Salts  Nitro Compounds  Cyanides and Isocyanides Introduction Functional groups containing nitrogen are present in a variety of naturally occurring and man-made organic compounds. These groups are responsible for their unique chemical reactivity patterns and play crucial roles in the preparation of drugs, agrochemicals, dyes and molecules of life. Some categories of compounds based on these functional groups include nitro compounds, amines, cyanides, isocyanides and diazo compounds. Amines are organic derivatives of ammonia in which one or more than one hydrogen atoms are replaced by other atoms or group of atoms. They are classified as primary amines (RNH2 or 1°), secondary amines (R2NH or 2°) or tertiary amines (R3N or 3°) depending upon one, two or all the three hydrogen atoms of ammonia displaced by alkyl and/ or aryl groups. STRUCTURE OF AMINES As ‘N’ atom in NH3 molecule is sp3 hybridised and tetrahedral in geometry, similarly ‘N’ atom in all amines is also sp3 hybridised which has tetrahedral geometry but their shape is pyramidal. Out of four hybrid orbitals of nitrogen, three orbitals overalp with hydrogen or carbon atom of alkyl/aryl group depending upon the composition of amines, whereas fourth hybrid orbital contains lone pair of electrons. Repulsion between lone pair and bond pair electrons supresses the bond angle which becomes lesser than 109.5°. Example, NNN RRR HRR HHR 1° amine RNH2 2° amine R NH 2 3° amine R N3
312 Amines NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 CLASSIFICATION (a) Primary Amines : R NH2 CH CH CH NH 3222 CH CH CH NH 3 22 CH3 (b) Secondary Amines : R NH R CH CH NHC 32 3 H (CH CH ) NH 3 22 (c) Tertiary Amines : R N R R CH CH CH N(CH )CH CH 322 3 23 (CH ) N 3 3 (d) Arylamines : Arylamines have NH2 group directly attached to the benzene ring. They are named as derivatives of aniline or benzenamine. NH2 Benzenamine (Aniline) NH2 2-methyl aniline o-toluidine CH3 N-Isopropyl benzenamine NHCHCH3 CH3 N,N-Dimethylbenzenamine N(CH )3 2 Diphenylamine NH (e) Heterocyclic Amines : N Pyrrole  N Pyridine N Piperidine H (f) Simple and mixed amines : If all the alkyl groups attached to ‘N’ atom are similar, this is a simple amine and if these groups are different, it is a mixed amine. Simple amines – (CH3) 2NH, (CH3CH2) 3N, (C6H5) 3N etc. Mixed amines – CH3NHC2H5, C6H5NHC2H5, CH N C H 3 37 C H2 5 etc. NOMENCLATURE In common system an aliphatic amine is named by prefixing alkyl group to amine i.e., alkylamine as one word. CH CH CH NH 3 2 2 2 Propylamine CH CH CH 3 3 NH2 Isopropylamine
NEET Amines 313 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 CH N CH 3 3 H Dimethylamine CH N C 3 3 H CH3 Trimethylamine C H N CH CH CH CH 25 2 2 2 3 CH3 N-Ethyl-N-methylbutylamine In IUPAC system, amines are named as alkanamines, derived by replacement of e– of alkane by the word amine. Example : CH3NH2 Methane + amine Methanamine  CH CH CH CH 3 23 NH2 1 23 4 Butane + amine = Butan-2-amine CH N CH CH CH 3 223 C H2 5 123 N-Ethyl, N-methylpropane + amine = N-Ethyl-N-methylpropan-1-amine NH CH CH CH CH 222 2 1 2 34 but + ene + amine = But-3-en-1-amine 3 H N CH CH CH NH 2 2222 2 1 Propane-1, 3-diamine PREPARATION OF AMINES Amines are prepared by following methods : 1. Reduction of nitro compounds : Nitro compounds are reduced to amines by passing hydrogen gas in the presence of finally divided Ni, Pd or Pt and also by reduction with metals in acidic medium. R NO + 3H 2 2 RNH + 2H 2 2O Ni Sn/HCl Base RNO2 32 RNH RNH    NO2 NH2 Sn/HCl or Fe/HCl NO2 NH2 H /Pd 2 Ethanol 2. Ammonolysis of alkyl halides : Alkyl halides undergo nucleophilic substitution reaction by SN2 mechanism with NH3 forming primary amines. RNH3 + – NH + R X 3 X RNH + NH X 2 4  Nucleophile Ammonium salt 1° amine NH3 The reaction does not stop at this stage. Primary amines being more basic than ammonia further reacts with alkyl halide forming secondary amine (2°), tertiary amine (3°) and eventually quaternary ammonium
314 Amines NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 salt, if the alkyl halide is present in excess. Hence, we obtain a mixture of 1°, 2°, 3° amines and quaternary ammonium salt. Quaternary ammonium salt R NH 2 R N3 R N4 + – RNH2 X  R X R X R X –HX –HX We can make primary amine as the major product by carrying out the reaction with excess ammonia. The order of reactivity of halides with amines is RI > RBr > RCl. 3. Reduction of nitriles : Nitriles on reduction with LiAlH4 or catalytic hydrogenation produce primary amines. R C N RCH NH 2 2 H /Ni 2 or LiAlH or Na(Hg)/C H OH 4 2 5 H /Pt 2 C N CH NH 2 2 When Na and C2H5OH is used as a reducing agent, it is called Mendius reduction. 4. Reduction of amides : The amides on reduction with LiAlH4 yield amines. RCONH2 RCH NH 2 2 (ii) H O2 (i) LiAlH4 LiAlH /Ether 4 CONH2 CH NH 2 2 H O2 5. Gabriel phthalimide synthesis : Gabriel synthesis is used for the preparation of primary amines. Phthalimide on treatment with ethanolic potassium hydroxide forms potassium salt of phthalimide which on heating with alkyl halide followed by alkaline hydrolysis produces the corresponding primary amine. Aromatic primary amines can not be prepared by this method because aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide. C C C C NH N K– + O O O O KOH R X 2KOH (aq) N-Alkylphthalimide C C NR O O 1° amine COO K COO K RNH2 + 6. Hoffmann bromamide degradation reaction : Hoffmann developed a method for preparation of primary amines by treating an amide with bromine in an aqueous or ethanolic solution of sodium hydroxide. RCONH + Br + 4NaO 2 2 H RNH + Na CO + 2NaBr + 2H O 2 23 2 CONH2 NH2 + 4NaOH + Br2 + 2NaBr + Na CO + 2H O 23 2 It is a decent of series in which product has one ‘C’ atom lesser than reactant.