Solutions-Class-12-Chemistry-Chapter-13-Amines-Maharashtra Board

(c) sp³

(c) 1° amine

Explanation:

The general formula for isobutylamine is (CH₃)₂-CH-CH₂-NH₂. It is an ammonia derivative where the ammonia's solitary hydrogen atom is replaced with an isobutyl group. Therefore, in addition to the two N - H bonds that are present and belong to a primary amine, isobutylamine also contains a secondary carbon atom.

(a) n-Butylamine

(d) Dimethylamine

(a) Primary amine

(a) Primary amine

(c) (CH₃-CH₂)₃N

(a) Ethylamine

(d) CH₃-CH₂-OH

(b) p-Hydroxyazobenzene

2 moles of methyl bromide are required to convert ethanamine to N,N-dimethyl ethanamine.

Propanamide (C₂H₅CONH₂) produces ethanamine by Hofmann bromamide degradation reaction.

When propanamide is treated with bromine and aqueous or alcoholic sodium hydroxide, ethanamine is obtained which has one carbon atom less.

The order of basicity of aliphatic alkyl amines in the gaseous follows the order :

tertiary amine > secondary amine > primary amine > NH₃.

The alkyl group in primary amines has + I effect i.e. (electron releasing).

The alkyl group tends to increase the electron density on the nitrogen atom. As a result, amines can donate the lone pair of electrons on nitrogen more easily than ammonia.

The amine being a base, can donate a pair of electrons to an acid. The alkyl group with + I effect will disperse the positive charge on the cation more than ammonia.

Due to + I effect of alkyl group cation formed by primary amine is more stable compared to cation formed from ammonia. Also it is seen that observed increasing basic strength from ammonia to primary amine is explained on the basis of increased stabilization of conjugate acids by + I effect for the presence of alkyl (R) groups. Hence, primary or aliphatic amine is a stronger base than ammonia.

The product is aniline/

The IUPAC name is Phenylmethanamine.

Amines in an increasing order of boiling points : trimethyl amine, ethyl methyl amine, n-propyl amine.

2(C₂H₃)₂NH + H₂SO₄ →  [(C₂H₅)₂N⁺H₂]₂SO₂^-

Diethyl amine                         diethylammonium sulphate

Cyclohexyl amine (_pK_b 3.34), aniline (_pK_b 9.13) 4-nitroaniline (_pK_b 12.99).

(A) C₆H₅CH₂CN phenyl acetonitrile

(B) C₆H₅CH₂-CH₂-NH₂ ß-phenyl ethyl amine

The basic nature of amines is due to presence of a lone pair of electrons on the nitrogen atom.

Lewis theory : Amines are bases because they can share a lone pair of electrons on ‘N’ atom with an electron deficient species.

Example : Trimethylamine shares its lone pair of electrons with the electron deficient boron trifluoride.

Basic strength of aliphatic amines :

Basic strength of amines is expressed quantitatively as K_b or _pK_b value. In terms of Lowry-Bronsted theory, the basic nature of amines is explained by writing the following equilibrium.

• In this equilibrium amine accepts H⁺, hence an amine is a Lowry-Bronsted base.
• The nitrogen atom in amines has a lone pair of electrons, which can be donated to suitable acceptor like proton H⁺.
• The aqueous solutions of amines are basic in nature due to release of free OH^— ions in solutions. Hence amines are Lewis bases. There exists an equilibrium in their aqueous solutions as follows :

• Since OH^— is a stronger base, equilibrium shifts towards left hand side giving less concentration of OH^—.
• Here, K_b value is smaller and _pK_b value is larger. Hence amines are weak bases.

Diazotisation : The conversion of primary aromatic amine into diazonium salts is called diazotisation.

Aniline reacts with nitrous acid in cold to form diazonium salt which has reasonable solubility at 273 K.

• Diazonium salts react with certain aromatic compounds having an electron-rich group (e.g.—OH, —NH₂, etc.) to form azo compounds. This reaction is an electrophilic substitution and is called coupling reaction.
• Azo compounds are brightly coloured and are used as dyes and indicators. Coupling reaction is an electrophilic substitution reaction.

Examples :

(a) Benzene diazonium chloride reacts with alkaline solution of phenol to give p-hydroxy azo benzene (orange dye).

(b) Benzene diazonium chloride reacts with aniline in mild alkaline medium to give p-aminobenzene (yellow dye).

This method is used for the synthesis of primary amine. It involves the following three stages.

• Formation of potassium salt of phthalimide from phthalimide on reaction with alcoholic potassium hydroxide.
• Formation of N-alkyl phthalimide from the potassium salt by reaction with alkyl halide.
• Alkaline hydrolysis of N-alkyl phthalimide to form the corresponding primary amine.

• Aromatic amines cannot be prepared by this method because aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide.

Aliphatic or aromatic primary amines on heating with chloroform and alcoholic potassium hydroxide solution form carbyl amines or alkyl/aryl isocyanides with extremely unpleasant (foul) smell. This reaction is a test for primary amines. Secondary and tertiary amines do not give this test.

Example :

(i) methanamine into ethanamine :

(ii) Aniline into p-bromoaniline :

(a) C₆H₅N₂⁺Cl^- + C₂H₅OH → C₆H₅-N=N-CH₂-CH₂-OH + HCl

(b)

• When alkyl halide is heated with an alcoholic solution of excess ammonia, it undergoes a nucleophilic substitution reaction in which the halogen atom is replaced by an amino (-NH₂) group to form primary amine.
• This process of breaking of the C- X bond by ammonia is known as ammonolysis. The reaction is also known as the alkylation of ammonia. The reaction is carried out in a sealed tube at 373 K.
• The primary amine obtained in the1st step is stronger nucleophile than ammonia. Hence, it further reacts with an alkyl halide to form secondary and tertiary amines and finally quaternary ammonium salt if NH₃ is not used in large excess.

• The order of reactivity of alkyl halides with ammonia is R-I > R-Br > R-Cl.

(a) N-Ethyl, N-methyl propan-1-amine

(b) 3, 3-Dimethylbutan-1-amine

(c) N-Ethyl propan-2-amine

Amines : The alkyl or aryl derivatives of ammonia in which one, two or all the three hydrogen atoms attached to nitrogen are replaced by same or different alkyl or aryl groups are called amines. OR Amines are nitrogen containing organic compounds having basic character.

• Amines are present in structure of many natural compounds like proteins, vitamins, hormones and many plant products like nicotine.

Example : methyl amine : CH₃-NH₂

Classification of amines :

• Amines are classified on the basis of the number of hydrogen atoms of ammonia that are replaced by alkyl group.
• Amines are classified as primary (1°), secondary (2°) and tertiary (3°).

Primary amines (1° amines) : The amines in which only one hydrogen atom of ammonia is replaced by an alkyl group or aryl group are called primary (1°) amines.

Examples :

Secondary amines (2° amines) : The amines in which two hydrogen atoms of ammonia are replaced by two, same or different alkyl or aryl groups are called secondary (2°) amines.

Examples :

Tertiary amines (3° amines) : The amines in which all the three hydrogen atoms of ammonia are replaced by three same or different alkyl or aryl groups are called tertiary (3°) amines.

Examples :

Secondary and tertiary amines are further classified as (i) simple / symmetrical amines and (ii) mixed / unsymmetrical amines.

(i) Simple or symmetrical amines : When all the alkyl or aryl groups on nitrogen are same, it is a simple amine.

Examples :

(ii) Mixed or unsymmetrical amines : When all the alkyl or aryl groups on nitrogen are different, then the amine is a mixed amine.

Examples :

Amines are also divided into two major classes, namely, aliphatic and aromatic amines on the basis of nature of the groups attached to the nitrogen atom.

(a) Hexan-1, 6-diamine

(b) 2, 4-Dimethylaniline

(c) Benzene-1, 4-diamine

(a) Ethanamine from Acetonitrile :

(b) Ethanamine from Nitroethane :

(c) Ethanamine from Propionamide :

(a) Ethyl amine on reaction with acetic anhydride forms monoacetyl derivative, N-acetylethylamine.

(b) Diethyl amine (a secondary amine) on reaction with acetic anhydride forms a monoacetyl derivative, N-acetyldiethyl amine (or N, N-diethyl acetamide).

(c) Triethyl amine does not react with acetic anhydride as it does not have any H atom attached nitrogen atom of amine

(a) Primary amine (like ethyl amine) is treated with Hinsberg’s reagent (benzene sulphonyl chloride) forms N-alkyl benzene sulphonamide which dissolve in aqueous KOH solution to form a clear solution of potassium salt and upon acidification gives insoluble N-alkyl benzene sulphonamide.

(b) Secondary amine like diethyl amine is treated with benzene sulphonyl chloride forms N,N-diethyl benzene which sulphonyl amide remains insoluble in aqueous KOH and does not dissolve in acid.

(b) Tertiary amine like triethyl amine does not react with benzene sulphonyl chloride and remains insoluble in KOH, however it dissolves in dil. HCl to give a clear solution due to formation of ammonium salt.