Amines are organic compounds derived from ammonia (NH₃) by replacing one or more hydrogen atoms with organic groups and undergo various chemical reactions due to the presence of a nitrogen atom with a lone pair of electrons.
Basic Characteristics of Amines
Amines show a basic nature, and they also react with acid to form a salt. Amine nitrogen has a tendency to donate its lone pair 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 produces 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 an alkyl group to the nitrogen of amines takes place. This process is also referred to as amino-de-halogenation or alkylation. When a primary amine reacts with a haloalkane, a 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 of Amines
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
The carbylamine reaction is also known as the Hofmann reaction. When aliphatic and aromatic primary amines are heated with chloroform and ethanolic potassium hydroxide, they form isocyanides or carbylamines, which have a foul odor. 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.
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.
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, benzenesulfonyl chloride (C₆H₅SO₂Cl), reacts with primary and secondary amines to form sulfonamides. N-ethyl benzenesulfonyl amide is produced by reacting Hinsberg's reagent with a primary amine.
N, N-diethyl benzenesulfonamide is formed during the reaction with secondary amine.
Tertiary amines are unaffected by benzenesulfonyl chloride. This property of amines reacting differently with benzenesulfonyl 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 isomers 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 sulfuric acid to form anilinium hydrogen sulfate, which, when heated with sulfuric acid at 453-473K, produces the major product p-aminobenzenesulfonic acid, also known as sulfanilic acid.
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Sample Questions
Question 1: Arrange the following in decreasing order of their basic strength:
- C₆H₅NH₂,
- C₂H₅NH₂,
- (C2H5)2NH,
- 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:
- CH₃CH₂CH₂NH₂ + HCl ⇢ product?
- (C₂H₅)₃N + HCl ⇢ product?
\begin{aligned}1.\quad &\text{CH}_3\text{CH}_2\text{CH}_2\text{NH}_2 + \text{HCl} \dashrightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{NH}_3^+\text{Cl}^- \\&\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad \textit{n-propyl ammonium chloride} \\\\2.\quad &(\text{C}_2\text{H}_5)_3\text{N} + \text{HCl} \dashrightarrow (\text{C}_2\text{H}_5)_3\text{NH}^+\text{Cl}^- \\&\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad \textit{tri-ethyl ammonium chloride}\end{aligned}
Question 3: What is Hinsberg’s reagent? Draw the structure?
Answer: Benzenesulphonyl chloride (C6H5SO2Cl), which is known as Hinsberg’s reagent:
Question 4: How to get p-nitroaniline as the major product in the 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.
Unsolved Problems
- Explain why amines form salts and how this reaction proves their basic nature.
- Why do tertiary amines not undergo acylation?
- Why is this reaction given only by primary amines and not by secondary or tertiary amines?
- Explain why aniline undergoes bromination easily even without a catalyst.
- Describe the Hinsberg test for distinguishing amines.