CBSE Class 12 Chemistry Amines Summary Set 02

CBSE Class 12 Chemistry Amines Summary (2). Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks. 

AMINES

Amines are an important topic of CBSE Class 12 Chemistry. These notes cover the structure, classification, nomenclature and isomerism of amines, their preparation, physical and basic properties, the reactions of amines, diazonium salts and coupling reactions, the uses of amines, and the tests that distinguish primary, secondary and tertiary amines.

AMINES

Amines are derivatives of ammonia (NH₃), obtained by replacing one, two or all three 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.

Table 1: Some amines, their nature and function

STRUCTURE OF THE FUNCTIONAL GROUP

The nitrogen orbitals in amines are sp³ hybridised and the geometry of amines is pyramidal.
Each of the three sp³ hybridised orbitals of nitrogen overlaps with orbitals of hydrogen or carbon, depending on the type of amine.
The fourth sp³ orbital of nitrogen contains a lone pair of electrons.
Because of this lone pair, the angle C-N-E (where E is C or H) is less than 109.5° (in trimethylamine it is 108°).
Because of the lone pair of electrons, amines are basic in nature.

CLASSIFICATION

If one hydrogen atom of ammonia is replaced by R or Ar, we get RNH₂ or ArNH₂, a primary (1°) amine.
If two hydrogen atoms of ammonia, or one hydrogen atom of RNH₂, are replaced by another alkyl/aryl group (R'), we get RNHR', a secondary (2°) amine.
If all three hydrogen atoms of ammonia, or two hydrogen atoms of RNH₂, are replaced by another alkyl/aryl group (R'), we get RNRR', a tertiary (3°) amine.
Amines are called 'simple' when all the alkyl or aryl groups are the same, and 'mixed' when they are different.
Amines in which the nitrogen atom is directly linked to 1, 2 or 3 (same or different) alkyl groups are called aliphatic amines. They may be 1°, 2° or 3°.
Amines in which the nitrogen atom is directly linked to 1, 2 or 3 (same or different) aromatic rings or aryl groups are called aryl amines, for example aniline, diphenylamine and triphenylamine. They may be 1°, 2° or 3°.
Amines in which the nitrogen atom is linked to side chains of 1, 2 or 3 (same or different) aromatic rings are called aralkyl amines, for example benzylamine and dibenzylamine. They may be 1°, 2° or 3°.

[Figure: Structures of aniline, diphenylamine and triphenylamine, and of benzylamine and dibenzylamine.]

NOMENCLATURE

Common name: alkyl amines are named by the name of the alkyl groups followed by 'amine', using the prefixes di- and tri- for identical groups.
CH₃CH₂NH₂ is ethylamine; (CH₃CH₂)₂NH is diethylamine; (CH₃CH₂)₃N is triethylamine.
The common name of mixed amines is the N-substituted derivative of the largest group of primary amine. For example, (C₂H₅)₂NCH₂CH₂CH₂CH₃ is N,N-diethylbutylamine.

IUPAC name: the final 'e' of the alkane name is replaced by 'amine', giving alkanamine. For example, CH₃NH₂ is methanamine.
If more than one amino group is present, their positions are given by numbers on the carbon atoms bearing the -NH₂ groups, with prefixes such as di- and tri-. The final 'e' of the hydrocarbon is retained. For example, H₂N-CH₂-CH₂-NH₂ is ethane-1,2-diamine.
When other functional groups such as -OH or a double bond are present, the priority order of nomenclature is followed. H₂NCH₂CH₂OH is 2-aminoethanol. H₂NCH₂CH=CH₂ is prop-2-en-1-amine.

Example: cyclohexyl ring bearing an -NH-C₂H₅ group is N-ethylcyclohexylamine (N-ethylcyclohexanamine).

ISOMERISM IN AMINES

PREPARATION OF AMINES

Memory aid: GHAR, where G = Gabriel, H = Hoffmann bromamide, A = Ammonolysis and R = Reduction.

1. Gabriel phthalimide synthesis: phthalimide on treatment with ethanolic KOH forms potassium phthalimide. On heating with an alkyl halide (R-X), followed by alkaline hydrolysis, it produces the corresponding primary amine.
This method is used only for preparing 1° amines. Aromatic primary amines cannot be prepared by this method, because aryl halides do not undergo nucleophilic substitution with the phthalimide anion.

2. Hoffmann bromamide degradation reaction: an amide on reaction with bromine in aqueous or ethanolic sodium hydroxide gives a primary amine with one carbon atom less than the amide.
CH₃CONH₂ + Br₂ + 4NaOH → CH₃NH₂ + Na₂CO₃ + 2NaBr + 2H₂O (ethanamide gives methylamine)
C₆H₅CONH₂ + Br₂ + 4NaOH → C₆H₅NH₂ + Na₂CO₃ + 2NaBr + 2H₂O (benzenamide gives aniline)
Significance: it is a method of preparing 1° amines and a step-down conversion.

3. Ammonolysis of alkyl halides: cleavage of the C-X bond of an alkyl halide by ammonia is called ammonolysis.
RX + NH₃ → RNH₂ + HX; HX + NH₃ → [RNH₃]⁺X^-
The free amine is obtained from the ammonium salt by treatment with a strong base.
The order of reactivity of halides is RI greater than RBr greater than RCl.
Aryl amines cannot be prepared by this method.
Limitation: the product is a mixture of 1°, 2° and 3° amines and quaternary ammonium salt.
Exhaustive alkylation is the process of converting an amine (1°, 2° or 3°) into its quaternary ammonium salt by treatment with excess alkyl halide.

4. By reduction:

(a) Reduction of nitriles (Mendius reaction): both aromatic and aliphatic nitriles on reduction with LiAlH₄, catalytic hydrogenation or Na/C₂H₅OH give 1° amines.
CH₃C≡N → (H₂/Ni) CH₃CH₂NH₂ (ethanenitrile gives ethylamine)
C₆H₅C≡N → (H₂/Ni) C₆H₅CH₂NH₂ (benzonitrile gives benzylamine)
Significance: this reaction is used for the ascent of the amine series, that is, to prepare an amine with one carbon atom more than the starting amine.

(b) Reduction of amides: amides on reduction with LiAlH₄ or Na/C₂H₅OH give 1° amines.

R-CO-NH₂ → (i) LiAlH₄, (ii) H₂O → R-CH₂-NH₂

(c) Reduction of nitro compounds: nitro compounds are reduced to 1° amines by
(i) passing H₂ gas in the presence of Ni, Pd or Pt;
(ii) metals in acidic medium (for example Sn + HCl);
(iii) LiAlH₄.

Examples: nitrobenzene on reduction with H₂/Pd in ethanol gives aniline; nitrobenzene with Sn-HCl or Fe-HCl also gives aniline.

(d) Reduction of oximes: oximes are reduced to 1° amines by reducing agents such as Na/C₂H₅OH or LiAlH₄.
RCH=NOH + 4[H] → (LiAlH₄) RCH₂NH₂ + H₂O

(e) Reductive amination of aldehydes and ketones: aldehydes and ketones react with NH₃, RNH₂ or R₂NH in the presence of a reducing agent such as H₂/Raney Ni or sodium cyanoborohydride (NaBH₃CN) to give 1°, 2° and 3° amines respectively.

5. Industrial preparation: aliphatic amines of low molecular mass are prepared on an industrial scale by passing a mixture of an alcohol and ammonia in the vapour phase over heated alumina (Al₂O₃) at 575 K.
ROH + NH₃ → (Al₂O₃, 575 K) RNH₂ → (ROH) R₂NH → (ROH) R₃N (R may be CH₃ or CH₃CH₂)

PHYSICAL PROPERTIES

1. Physical state: the lower aliphatic amines are gases with a fishy odour. 1° amines with three or more carbon atoms are liquids, and higher ones are solids.
2. Colour and odour: aniline and other aryl amines are usually colourless but become coloured on storage due to atmospheric oxidation.
3. Solubility: lower aliphatic amines are soluble in water because they form hydrogen bonds with water molecules. Solubility decreases as the molar mass of the amine increases, because the hydrophobic alkyl part becomes larger.
4. Boiling point: 1° and 2° amines form intermolecular hydrogen bonds. Hydrogen bonding is stronger in 1° amines than in 2° amines, because a 1° amine has two hydrogen atoms available for bonding. 3° amines do not form hydrogen bonds. Therefore, the boiling points of isomeric amines follow the order 1° greater than 2° greater than 3°.

BASIC NATURE OF AMINES

Amines have an unshared pair of electrons on the nitrogen atom, so they act as Lewis bases. They react with mineral acids to form ammonium salts. The ammonium salts can be converted back to amines by treatment with a strong base such as NaOH.
The equilibrium constant K_b of the reaction of an amine with water is a measure of its basic character. A larger value of K_b, or a smaller value of pK_b, means a stronger base.

\( \mathrm{R{-}NH_2 + H_2O \rightleftharpoons R{-}NH_3^+ + OH^-} \)

\( K = \dfrac{[\mathrm{RNH_3^+}][\mathrm{OH^-}]}{[\mathrm{RNH_2}][\mathrm{H_2O}]} \); or \( K[\mathrm{H_2O}] = \dfrac{[\mathrm{RNH_3^+}][\mathrm{OH^-}]}{[\mathrm{RNH_2}]} \); or \( K_b = \dfrac{[\mathrm{RNH_3^+}][\mathrm{OH^-}]}{[\mathrm{RNH_2}]} \), \( pK_b = -\log K_b \).

Reaction of an amine with a mineral acid: \( \mathrm{R{-}NH_2 + HX \rightleftharpoons R{-}NH_3^+X^-} \) (salt). For example, \( \mathrm{C_6H_5NH_2 + HCl \rightleftharpoons C_6H_5NH_3^+Cl^-} \) (aniline gives anilinium chloride).

Factors affecting the basic strength of amines:

(a) Inductive effect: alkyl groups release electrons, which increases the electron density on nitrogen. The lone pair is then more easily available for sharing with acids. The electron-releasing effect also stabilises the alkylammonium ion formed, which shifts the equilibrium in the forward direction. So alkylamines are stronger bases than ammonia.
In the gas phase, the expected order of basic strength is 3° amine greater than 2° amine greater than 1° amine greater than NH₃.

(b) Steric effect: crowding of alkyl groups around the nitrogen atom hinders the attack of a proton on the amine molecule and decreases its basic strength. As crowding increases from 1° to 3° amines, the basic strength is expected to decrease in the order 1° greater than 2° greater than 3°.

(c) Hydration effect: hydration refers to the stabilisation of the protonated amine by water molecules. Water molecules form hydrogen bonds with the protonated amine and release hydration energy. The greater the extent of hydrogen bonding in the protonated amine, the greater its stabilisation and the stronger the base.

Basicity of aliphatic amines in aqueous solution: the substituted ammonium cations are stabilised by the electron-releasing (+I) effect of alkyl groups and by solvation with water. The greater the size of the ion, the lesser the solvation and the less stable the ion. When the R group is small, like -CH₃, there is no steric hindrance to hydrogen bonding. When the alkyl group is bigger than -CH₃, such as -C₂H₅, steric hindrance to hydrogen bonding changes the order of basic strength. So the basic strength of alkylamines in aqueous solution is decided by the combination of inductive effect, solvation effect and steric hindrance of the R group.

The order of stability of the hydrated ammonium ions (by hydrogen bonding with water) is: 1° (R-NH₃⁺, three H-bonds) greater than 2° (R₂NH₂⁺, two H-bonds) greater than 3° (R₃NH⁺, one H-bond).

The order of basic strength of ethyl-substituted and methyl-substituted amines in aqueous solution is:
(C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃
(CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃

BASIC CHARACTER OF AROMATIC AMINES

Aromatic amines are weaker bases than ammonia and aliphatic amines.
(i) Delocalisation of the lone pair: in aniline, the lone pair of electrons on nitrogen is delocalised into the benzene ring by resonance. It is therefore less available for protonation than in ammonia or aliphatic amines. So aniline is a weaker base.
(ii) Lower stability of the anilinium ion: the anilinium ion formed by accepting a proton is not stabilised by resonance. It has only two canonical structures, so it is less stable than aniline. This makes aniline less likely to accept a proton.

Resonance structures of aniline: structure I has the lone pair on NH₂ attached to the benzene ring. Structures II, III and IV are formed by delocalising this lone pair into the ring. In each of them the nitrogen is =NH₂⁺ and the negative charge sits on the ortho (II), para (III) and other ortho (IV) carbon. These are in equilibrium with the original structure.

Anilinium ion: only two canonical structures, I and II (C₆H₅-NH₃⁺, the two Kekulé forms of the ring), are possible. So the anilinium ion is less stable than aniline.

Basicity of substituted anilines: an electron-releasing group (such as -OCH₃, -CH₃ or -NH₂), especially at the ortho or para position, stabilises the anilinium cation and increases the basic strength. Electron-withdrawing groups (such as -NO₂, -CN or halogens) destabilise the cation and decrease the basic strength.
Ortho effect: ortho-substituted anilines are generally weaker bases than aniline, whatever the electronic nature of the substituent. This is known as the ortho effect and is probably due to combined electronic and steric factors.

Examples: p-toluidine (p-CH₃C₆H₄NH₂) has K_b = 12 × 10^-10, aniline has K_b = 4.2 × 10^-10, and p-chloroaniline has K_b = 1 × 10^-10.

CHEMICAL REACTIONS OF AMINES

The reactions of amines are mainly due to the unshared pair of electrons on nitrogen, which makes them act as nucleophiles. In aromatic amines, the unshared pair of electrons on nitrogen also facilitates electrophilic substitution in the benzene ring. The number of hydrogen atoms on the nitrogen also affects the reactions.

CARBYLAMINE REACTION (ISOCYANIDE TEST)

Both 1° aliphatic and aromatic amines, on heating with chloroform and ethanolic KOH, form isocyanides (carbylamines), which have a foul smell.
Significance: it is a method of preparing isocyanides, and it is used as a test for 1° amines, because 2° and 3° amines do not show this reaction.

\( \mathrm{R{-}NH_2 + CHCl_3 + 3KOH \xrightarrow{heat} R{-}NC + 3KCl + 3H_2O} \)

REACTION WITH NITROUS ACID (HNO₂)

(a) 1° aliphatic amines react with nitrous acid to form aliphatic diazonium salts, which are unstable. They decompose to liberate N₂ gas and alcohols.
(b) 1° aromatic amines react with nitrous acid at low temperatures (0-5 °C) to form diazonium salts.
Significance: this reaction is used to distinguish between 1° aliphatic and 1° aromatic amines.

\( \mathrm{R{-}NH_2 + HNO_2 \xrightarrow{NaNO_2 + HCl} [R{-}N_2^+Cl^-] \xrightarrow{H_2O} ROH + N_2 + HCl} \)

\( \mathrm{C_6H_5{-}NH_2 + NaNO_2 + 2HCl \xrightarrow{273-278\,K} C_6H_5{-}N_2^+Cl^- + NaCl + 2H_2O} \) (aniline gives benzenediazonium chloride)

DIAZONIUM SALTS

General formula: ArN₂⁺X^-, where Ar is an aryl group and X^- may be Cl^-, Br^-, HSO₄^-, BF₄^-, etc. For example, C₆H₅N₂⁺Cl^- is benzenediazonium chloride, and C₆H₅N₂⁺HSO₄^- is benzenediazonium hydrogensulphate. They are named by adding 'diazonium' to the name of the parent hydrocarbon, followed by the name of the anion.

Preparation (diazotisation): aniline reacts with nitrous acid at 273-278 K. HNO₂ is produced in the reaction mixture by the reaction of NaNO₂ with HCl.
Note: 1° aliphatic amines form highly unstable alkyldiazonium salts. 1° aromatic amines form arenediazonium salts, which are stable at low temperatures (273-278 K) due to resonance. A diazonium salt cannot be stored, so it is used immediately after preparation.

\( \mathrm{C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{273-278\,K} C_6H_5N_2^+Cl^- + NaCl + 2H_2O} \)

Resonance in the benzenediazonium ion: C₆H₅-N⁺≡N: ↔ three structures in which the ring carbon bearing N₂ becomes C=N⁺=N^- (that is, the ring has a double bond to the exocyclic nitrogen, which carries the negative charge on the terminal N) and a positive charge sits on an ortho, para or the other ortho carbon of the ring. These structures account for the stability of arenediazonium salts.

REACTIONS INVOLVING DISPLACEMENT OF NITROGEN

1. Replacement by a halide or cyanide ion:
(i) Sandmeyer reaction: Cl^-, Br^- and CN^- are introduced into the benzene ring by treating benzenediazonium chloride with Cu₂Cl₂, Cu₂Br₂ or CuCN, in the presence of HCl, HBr or KCN respectively.
(ii) Gatterman reaction: Cl^- or Br^- is introduced by treating the diazonium salt solution with HCl or HBr in the presence of copper powder.
2. Replacement by iodide: the diazo group is replaced by iodide by warming with KI.
3. Replacement by fluoride (Balz-Schiemann reaction): when arenediazonium chloride is treated with fluoroboric acid, arenediazonium fluoroborate is precipitated. On heating, it decomposes to give aryl fluoride.
4. Replacement by hydrogen: mild reducing agents such as hypophosphorous acid (phosphinic acid) or ethanol reduce diazonium salts to arenes; they are themselves oxidised to phosphorous acid and ethanal respectively. The complete set of reactions starting from diazotisation of aniline, followed by reduction of the diazonium salt or replacement of the diazo group by hydrogen, is called deamination.
5. Replacement by -OH: on heating the diazonium salt solution up to 283 K, the salt is hydrolysed to phenol.
6. Replacement by -NO₂: when diazonium fluoroborate is heated with aqueous sodium nitrite in the presence of Cu, the diazonium group is replaced by -NO₂.
7. Replacement by phenyl group (Gomberg-Bachmann reaction): benzenediazonium chloride reacts with benzene in alkaline medium to give diphenyl.

Equations for the reactions above:

• Sandmeyer: \( \mathrm{ArN_2^+X^- \xrightarrow{Cu_2Cl_2/HCl} ArCl + N_2} \); \( \mathrm{ArN_2^+X^- \xrightarrow{Cu_2Br_2/HBr} ArBr + N_2} \); \( \mathrm{ArN_2^+X^- \xrightarrow{CuCN/KCN} ArCN + N_2} \)
• Gatterman: \( \mathrm{ArN_2^+X^- \xrightarrow{Cu/HCl} ArCl + N_2 + CuX} \); \( \mathrm{ArN_2^+X^- \xrightarrow{Cu/HBr} ArBr + N_2 + CuX} \)
• Iodide: \( \mathrm{ArN_2^+Cl^- + KI \rightarrow ArI + KCl + N_2} \)
• Fluoride: \( \mathrm{ArN_2^+Cl^- + HBF_4 \rightarrow Ar{-}N_2^+BF_4^- \xrightarrow{\Delta} Ar{-}F + BF_3 + N_2} \)
• Hydrogen: \( \mathrm{ArN_2^+Cl^- + H_3PO_2 + H_2O \rightarrow ArH + N_2 + H_3PO_3 + HCl} \); \( \mathrm{ArN_2^+Cl^- + CH_3CH_2OH \rightarrow ArH + N_2 + CH_3CHO + HCl} \)
• Hydroxyl: \( \mathrm{ArN_2^+Cl^- + H_2O \rightarrow ArOH + N_2 + HCl} \)
• Nitro: \( \mathrm{C_6H_5N_2^+Cl^- + HBF_4 \rightarrow C_6H_5N_2^+BF_4^- \xrightarrow{NaNO_2,\,Cu,\,\Delta} C_6H_5NO_2 + N_2 + NaBF_4} \)
• Gomberg-Bachmann: benzene + benzenediazonium halide (ArN₂⁺X^-) with OH^- gives diphenyl (biphenyl).
• Coupling: \( \mathrm{C_6H_5N{\equiv}N^+Cl^- + H{-}C_6H_4{-}OH \xrightarrow{OH^-} C_6H_5{-}N{=}N{-}C_6H_4{-}OH + Cl^- + H_2O} \) (p-hydroxyazobenzene, orange dye); \( \mathrm{C_6H_5N{\equiv}N^+Cl^- + H{-}C_6H_4{-}NH_2 \xrightarrow{H^+} C_6H_5{-}N{=}N{-}C_6H_4{-}NH_2 + Cl^- + H_2O} \) (p-aminoazobenzene, yellow dye)

COUPLING REACTIONS (DIAZO GROUP RETAINED)

Benzenediazonium chloride reacts with phenol. The phenol molecule is coupled with the diazonium salt at its para position to form p-hydroxyazobenzene. This is a coupling reaction, and it is an electrophilic substitution. Similarly, the diazonium salt reacts with aniline to give p-aminoazobenzene. p-Hydroxyazobenzene is an orange dye and p-aminoazobenzene is a yellow dye.
The azo products have an extended conjugated system, with both aromatic rings joined through the -N=N- bond. These compounds are often coloured and are used as dyes.

Importance of diazonium salts in synthesis: they are very good intermediates for introducing -F, -Cl, -Br, -I, -CN, -OH and -NO₂ groups into the aromatic ring.
Note: 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 easily be obtained from a diazonium salt.

LIEBERMANN NITROSO REACTION

Both aliphatic and aromatic secondary amines react with nitrous acid to give yellow oily nitrosoamines. For example, (C₂H₅)₂NH + HNO₂ → (C₂H₅)₂N-N=O (N-diethylnitrosoamine). Only the nitrosoamine of an aryl amine, on heating with phenol and conc. H₂SO₄, gives a green colour which soon changes to blue-green. On dilution it changes to red, and on reaction with alkali it changes to blue or violet.

USES OF AMINES

1. Methylamine (CH₃NH₂) is used as a refrigerant.
2. Ethylamine (CH₃CH₂NH₂) is used for solvent extraction, in the perfume industry, and as a stabiliser for rubber latex.
3. Aniline (C₆H₅NH₂) is used for preparing benzenediazonium salts, for synthesising azo dyes, and for preparing phenyl isocyanide.

Separation of primary, secondary and tertiary amines (flow chart)

• 1° + 2° + 3° amines + benzenesulphonyl chloride gives: products of 1° and 2° amines with benzenesulphonyl chloride + unreacted 3° amine. Treat this mixture with HCl and filter.
  • Filtrate: contains the 3° amine as its hydrochloride; add NaOH to get the TERTIARY AMINE (3° amine).
  • Solid residue: add aqueous KOH and filter.
    • Filtrate: acidify to get the sulphonamide of the 1° amine; hydrolyse to get the PRIMARY AMINE (1° amine, aliphatic or aromatic).
    • Insoluble residue: hydrolyse to get the SECONDARY AMINE (2° amine).

DISTINCTION BETWEEN 1°, 2° AND 3° ALIPHATIC AMINES

CONVERSIONS

Step-up conversion: the number of carbon atoms in the chain increases, while the functional group remains the same. For example, R-NH₂ into R-CH₂-NH₂.
Step-down conversion: the number of carbon atoms in the chain decreases, while the functional group remains the same. For example, R-CH₂-NH₂ to R-NH₂.

Step-up conversion scheme: R-NH₂ → (NaNO₂/HCl) ROH → (SOCl₂) RCl → (KCN) RCN → (H₂/Ni) RCH₂NH₂.
Example: CH₃NH₂ (methylamine) → (NaNO₂/HCl) CH₃OH (methanol) → (SOCl₂) CH₃Cl (chloromethane) → (KCN) CH₃CN (methyl cyanide) → (H₂/Ni) CH₃CH₂NH₂ (ethylamine).

Step-down conversion scheme: RCH₂NH₂ → (NaNO₂/HCl) RCH₂OH → ([O]) RCOOH → (NH₃, heat) RCONH₂ → (Br₂ + KOH) R-NH₂.
Example: CH₃CH₂NH₂ (ethylamine) → (NaNO₂/HCl) CH₃CH₂OH (ethanol) → ([O]) CH₃COOH (ethanoic acid) → (NH₃, heat) CH₃CONH₂ (ethanamide) → (Br₂ + KOH) CH₃NH₂ (methylamine).

Now practise the following conversions: methylamine into methanol, and similar ones.

DISTINCTION BETWEEN SOME IMPORTANT PAIRS OF COMPOUNDS

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