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Chemical Reactions of Amines

Last Updated : 16 Feb, 2022
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Amines are ammonia derivatives formed by replacing one, two, or all three hydrogen atoms with alkyl and/or aryl groups. Amines are organic compounds and functional groups that contain a nitrogen atom with a single electron pair. They are ammonia derivatives in which one or more hydrogen atoms have been replaced by substituent groups such as alkyl or aryl. These are differentiated by the presence of replaceable hydrogen atoms. The amine is primary when one of the three hydrogen atoms is replaced by an alkyl or aryl group. Secondary amines are formed when two out of three hydrogen atoms are replaced, and tertiary amines are formed when all three hydrogen atoms are replaced.

Chemical Reaction of Amines

Basic character of Amines

Amines show basic nature and they also react with acid to form a salt. Amine nitrogen has a tendency to donate its lone so that it is basic in nature. 

The salt which is formed during the reaction of amines and acids, on treatment with a base like NaOH again produce the same amine.

RNH3+X    +    OH     ⇢    R-NH2    +    H2O   +     X

Alkylation reaction of Amines

Ammonia or amines react with an alkyl halide, In which breaking the carbon-halogen bond and addition of alkyl group to the nitrogen of amines takes place. This process is also referred to as amino-de-halogenation or alkylation. When primary amine reacts with haloalkane, the secondary amine is formed. Secondary amine reacts with haloalkane to form tertiary amine. When tertiary amine reacts with haloalkane, it produces quaternary ammonium salt.

Acylation reaction

The nucleophilic substitution reaction occurs when aliphatic and aromatic primary and secondary amines react with acid chlorides, anhydrides, and esters. This is known as acylation. The products of the acylation reaction are known as amides. The reaction is carried out in the presence of a base stronger than the amine, such as pyridine.

Benzoylation reaction

Benzoylation is similar to an alkylation reaction. In the reaction, amines are treated with benzoyl chloride (C6H5COCl). This reaction is known as benzoylation.

Carbylamine reaction

Carbylamine reaction is also known as the Hoffman reaction. When aliphatic and aromatic primary amines are heated with chloroform and ethanolic potassium hydroxide, they form isocyanides or carbylamines, which have a foul odour. Secondary or tertiary amines do not give carbylamine reactions. This reaction, known as the carbylamine reaction or the isocyanide test, is used to detect primary amines.

Carbylamine reaction

Carbylamine reaction

Reaction with nitrous acid

Primary aliphatic amines react with nitrous acid to form aliphatic diazonium salts, which are unstable and release nitrogen gas and alcohols quantitatively.

Aromatic amines react with nitrous acid at low temperatures (273-278 K) to form diazonium salts, It is a very important compound and this method is also used in the preparation of diazonium salts.

Reaction with aryl sulphonyl chloride

Hinsberg’s reagent, benzene sulphonyl chloride (C6H5SO2Cl), reacts with primary and secondary amines to form sulphonamides. N-ethyl benzene sulphonyl amide is produced by reacting Hinsberg’s reagent with a primary amine.

N, N-diethyl benzene sulphonamide is formed during the reaction with secondary amine-

Tertiary amines are unaffected by benzene sulphonyl chloride. This property of amines reacting differently with benzene sulphonyl chloride is used to distinguish primary, secondary, and tertiary amines, as well as to separate a mixture of amines.

Electrophilic substitution Bromination

The Bromination reaction occurs when aniline is exposed to bromine. At room temperature, aniline reacts with bromine water to produce a white precipitate of 2,4,6-tribromo aniline.

To make a monosubstituted aniline derivative first, acetylate the -NH2 group with acetic anhydride, then perform the desired substitution followed by hydrolysis of the substituted amide to the substituted amine.

Electrophilic substitution Nitration

Direct nitration of aniline gives ortho, para and meta isomer of nitroaniline as a product. That is why the direct nitration of aniline is not useful for the desired product.

To obtain p-Nitroaniline as the major product, we first protect the –NH2 group by acetylation with acetic anhydride. The nitration reaction can then be controlled, and the p-nitro derivative can be obtained as the major product.

Electrophilic substitution Sulphonation

Aniline reacts with concentrated sulphuric acid to form anilinium hydrogen sulphate, which when heated with sulphuric acid at 453-473K produces the major product p-aminobenzene sulphonic acid, also known as sulphanilic acid.

 

Sample Questions

Question 1: Arrange the following in decreasing order of their basic strength:

  1. C6H5NH2
  2. C2H5NH2
  3. (C2H5)2NH, 
  4. NH3

Answer:

The basic strength of above compound is as follows:

(C2H5)2NH > C2H5NH2> NH3> C6H5NH2

Question 2: Complete the following acid-base reactions and name the products:

  1. CH3CH2CH2NH2   +   HCl  ⇢   product?
  2. (C2H5)3N  + HCl    ⇢    product?

Answer:

1. CH3CH2CH2NH2 + HCl    ⇢      CH3CH2CH2NH3+Cl

                                                       n-propyl ammonium chloride

2. (C2H5)3N  + HCl   ⇢      (C2H5)3NH+Cl

                                       tri-ethyl ammonium chloride

Question 3: What is Hinsberg’s reagent Draw structure?

Answer :

Benzenesulphonyl chloride (C6H5SO2Cl), which is known as Hinsberg’s reagent:

Question 4: How to get p-Nitroaniline as the major product in nitration of aniline?

Answer:

Direct nitration of aniline does not yield the desired product. To obtain p-Nitroaniline as the major product, we first protect the –NH2 group by acetylation with acetic anhydride. The nitration reaction can then be controlled, and the p-nitro derivative can be obtained as the major product.

Question 5: What is carbylamine reaction?

Answer:

When aliphatic and aromatic primary amines are heated with chloroform and ethanolic potassium hydroxide, they form isocyanides or carbylamines, which have a foul odour. This reaction does not occur in secondary or tertiary amines. This is known as the carbylamine reaction.

Carbylamine reaction

Carbylamine reaction



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