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1 Page 1 of 6 AMINES Amines are derivatives of ammonia (NH3), obtained by replacement of 1, 2 or all the 3 hydrogen atoms by alkyl and/or aryl groups. In nature amines are present in - proteins, vitamins, alkaloids and hormones. Synthetic amines are present in polymers, dyestuffs and drugs. Name of some compounds Nature Function/Use Adrenaline secondary amine increase blood pressure Ephedrine secondary amine increase blood pressure Novocain synthetic amino compound an anaesthetic in dentistry Benadryl Compound having tertiary amino group antihistaminic drug Cationic Detergent Quaternary ammonium salts surfactants Diazonium salts Have diazo group the preparation of a variety of aromatic compounds including dyes STRUCTURE OF FUNCTIONAL GROUP: CLASSIFICATION: Nomenclature PREPARATION OF AMINES: [GHAR] 1.Gabriel phthalimide synthesis 2. Hoffmann bromamide degradation reaction- an amide (-CO-NH2) on reaction with bromine in an aqueous or ethanolic solution of NaOH gives a 1 0 amine with one carbon less than the amide.

2 Page 2 of 6 3.Ammonolysis of alkyl halides: the process of cleavage of the C X bond by ammonia molecule is known as ammonolysis. 4.REDUCTION:by LiAlH4,Na+C2H5OH,H2/Ni a. Reduction of nitriles: b.reduction of amides: c. Reduction of nitro compounds: The free amine can be obtained from the ammonium salt by treatment with a strong base: The order of reactivity of halides with amines is RI > RBr >RCl Physical Properties: 1. Lower aliphatic amines are soluble in water because they can form hydrogen bonds with water molecules. 2. Solubility decreases with increase in molar mass of amines due to increase in size of the hydrophobic alkyl part. 3. 1º & 2º amines form intermolecular H- bonding. This H-Bonding is more in 1º amines than in 2º amines as there are 2 hydrogen atoms available for H-bond formation in it. 3º amines do not form H- bond. Therefore, the B.Pt. of isomeric amines follows the order: 1º> 2º > 3º Basic Nature of amines: The reaction of amines with mineral acids to form ammonium salts shows basic nature of amines. Amines have an unshared pair of a. Alkanamines (R-NH 2 ) versus ammonia(nh 3 ): Due to the electron releasing nature of alkyl group(+i effect), it pushes electrons towards N and thus makes the unshared electron pair more available for sharing with the proton of the acid. Moreover, the substituted ammonium ion formed from the amine gets stabilized due to dispersal of the positive charge by the +I effect of the R group. Hence, alkyl amines are stronger bases than ammonia. b. Basicity of amines in the gaseous phase: 3º amine > 2º amine >1º amine > NH 3. c. Basicity of amines in the aqueous phase, the substituted ammonium cations get stabilized not only by electron releasing effect of the alkyl group (+I) but also by solvation with water molecules. The greater the size of the ion, lesser will be the solvation and the less stabilised is the ion. The order of stability of ions are as follows:

3 Page 3 of 6 electrons on N- atom due to which they act as Lewis base. Larger the value of K b or smaller the value of pk b, stronger is the base. Structure-basicity relationship of amines: Basic character of an amine depends upon i. the ease of formation of the cation by accepting a proton from the acid. ii. Stability of the cation. The more stable the cation relative to the amine, more basic is the amine. When the -R group is small, like CH3 group, there is no steric hindrance to H-bonding. In case the alkyl group is bigger than -CH3 group, there will be steric hinderance to H-bonding. Therefore, the change of nature of the alkyl group from -CH3 to C2H5 results in change of the order of basic strength. The combination of inductive effect, solvation effect and steric hinderance of the R group which decides the basic strength of alkyl amines in the aqueous state. The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows: d. Aryl amines are weaker base than ammonia: in aniline or other arylamines, the -NH2 group is attached directly to the benzene ring. The unshared electron pair on nitrogen atom is in conjugation with the benzene ring and thus making it less available for protonation. 5 canonical structures,it is more stable only 2 canonical structures,it is less stable Hence, the proton acceptability or the basic nature of aniline or other aromatic amines is less than that of ammonia. e. Basicity of substituted aniline- electron releasing groups like OCH3, CH3 increase basic strength whereas electron withdrawing groups like NO2, SO3H, COOH, X decrease it.

4 Page 4 of 6 CHEMICAL REACTIONS OF AMINES REACTION 1.Alkylation DESCRIPTION & REACTION R-X + NH3 RNH + R2NH + R3N + [R4N]+Cl- 2.Acylation Reaction of aliphatic and aromatic 1º and 2º amines with acid chlorides, anhydrides and esters by nucleophilic substitution reaction to give amide. COMMENT/NOTE [R4N]+Cl- on reaction with NaOH regenerate amine 1.The reaction is carried out in the presence of a base stronger than the amine, like pyridine, which removes HCl so formed and shifts the equilibrium to the right hand side. 2. Amines also react with benzoyl chloride (C6H5COCl). This reaction is known as benzoylation. 3. What do you think is the product of the reaction of amines with carboxylic acids? Carbylamine reaction/ isocyanide test Aliphatic and aromatic 1ºamines on heating with chloroform and ethanolic KOH form isocyanides or carbylamines which have foul smell. Reaction with nitrous acid a.1º aliphatic amines react with nitrous acid to form aliphatic diazonium salts which are unstable & decompose to liberate N2 gas and alcohols. 2º and 3ºamines do not show this reaction. This reaction is used as a test for 1º amines & a method for preparation of isocyanide. This reaction is used to distinguish between 1º aliphatic & 1º aromatic amines b. 1ºAromatic amines react with HNO2 at low temperatures (0-5ºC) to form diazonium salts. Reaction with arylsulphonylchl oride (C6H5SO2Cl) Electrophilic substitution a. reaction with 1º amine b. reaction with 1º amine N, N-diethylbenzene sulphonamide does not contain any Hatom attached to H- atom, so it is not acidic and hence insoluble in alkali. c. 3º amine does not react with benzenesulphonylchloride (a) Bromination: a. This reaction is used to i.distinguish 1º,2º,3º amines & 2.separate 1º,2º,3º amines ii. Now a days benzenesulphonyl chloride is replaced by p -toluenesulphonyl chloride. i. NH2 group is ortho and para directing and a powerful

5 Page 5 of 6 activating group. ii.aniline does not undergo Friedel-Crafts alkylation and acetylation due to salt formation with AlCl3, the Lewis acid, which is used as a catalyst. Due to this, nitrogen of aniline acquires positive charge and hence acts as a strong deactivating group for further reaction. i. Activating effect of NHCOCH3 group is less than that of NH2 group because the lone pair of electrons on nitrogen is less available for donation to benzene ring by resonance. & (b)nitration: (c)sulphonation: DIAZONIUM SALTS general formula= R N2 + X where R = an aryl group and X- may be Cl BrHSO4-, BF4-, etc. DESCRIPTION COMMENT/ NOTE Preparation: by the reaction of aniline with nitrous acid (HNO2) at K. HNO2 is produced in the reaction mixture by the reaction of NaNO2 with HCl. The reaction is known as diazotisation. 1º aliphatic amines form highly unstable alkyldiazonium salts. 1º aromatic amines form arenediazonium salts which are stable at low temperatures ( K) due to resonance. Due to its instability, the diazonium salt is not generally stored and is used immediately after its preparation. Named by suffixing diazonium to the name of the parent hydrocarbon from which they are formed, followed by the name of anion Reactions involving displacement of nitrogen: 1. Replacement by halide or cyanide ion: Sandmeyer reaction Gatterman Example: C6H5N2+ClBenzenediazonium chloride reaction. 2. Replacement by iodide ion: Used in synthesis and conversions.

6 Page 6 of 6 C6H5N2+ HSO4Benzenediazonium hydrogensulphate 3. Replacement by fluoride ion: 4. Replacement by H: 5. Replacement by hydroxyl group: 6. Replacement by NO2 group: 7.Coupling reactions: Diazonium salts are very good intermediates for the introduction of F, Cl, Br, I, CN, OH, NO2 groups into the aromatic ring. Aryl fluorides and iodides cannot be prepared by direct halogenation. The cyano group cannot be introduced by nucleophilic substitution of chlorine in chlorobenzene but cyanobenzene can be easily obtained from diazonium salt. These compounds are often coloured and are used as dyes.

Class XII: Chemistry Chapter 13: Amines Top concepts

Class XII: Chemistry Chapter 13: Amines Top concepts 1. Amines are regarded as derivatives of ammonia in which one, two or all three hydrogen atoms are replaced by alkyl or aryl group 2. Classification