Amines (Chemical Reactions)

Chemical Reactions

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Amines exhibit a variety of chemical reactions, primarily due to the presence of the lone pair of electrons on the nitrogen atom and the presence of N-H bonds in primary and secondary amines.

Basic Character Of Amines

Lewis Basicity: Amines act as Lewis bases because the nitrogen atom has a lone pair of electrons that can be donated to an electron acceptor (Lewis acid).

Brönsted-Lowry Basicity: Amines act as Brönsted-Lowry bases by accepting a proton ($H^+$) from an acid, forming ammonium ions ($RNH_3^+, R_2NH_2^+, R_3NH^+$).

$RNH_2 + H^+ \rightarrow RNH_3^+$

Basicity Order:

• Aliphatic Amines: Secondary amines are generally more basic than primary amines, which are more basic than tertiary amines in aqueous solution. However, in the gaseous phase, basicity order is Tertiary > Secondary > Primary. The aqueous basicity order is due to a combination of inductive effect, solvation effects, and steric hindrance.
• Aromatic Amines: Aromatic amines (like aniline) are much weaker bases than aliphatic amines because the lone pair of electrons on the nitrogen atom is delocalized into the benzene ring through resonance, making it less available for protonation.

Alkylation

Description: Reaction of amines with alkyl halides ($R-X$) results in the substitution of hydrogen atoms on the nitrogen with alkyl groups. This process is called alkylation.

Mechanism: It's an $S_N2$ reaction where the amine acts as a nucleophile.

• Primary Amine: Reacts with alkyl halide to give secondary amine, then tertiary amine, and finally quaternary ammonium salt.

$RNH_2 + RX \rightarrow R_2NH \xrightarrow{RX} R_3N \xrightarrow{RX} [R_4N]^+X^-$

• Tertiary Amine: Reacts with alkyl halide to form quaternary ammonium salt.

$R_3N + R'X \rightarrow [R_3R'N]^+X^-$

Acylation

Description: Reaction of amines with acylating agents (like acid chlorides or acid anhydrides) results in the substitution of hydrogen atoms on the nitrogen with an acyl group ($-COR$). This process is called acylation.

Reaction: Primary and secondary amines react readily with acyl halides or acid anhydrides to form amides.

• With Acid Chlorides:

$RNH_2 + R'COCl \rightarrow RNHCOR' + HCl$

$R_2NH + R'COCl \rightarrow R_2CONR' + HCl$

• With Acid Anhydrides:

$RNH_2 + (R'CO)_2O \rightarrow RNHCOR' + R'COOH$

$R_2NH + (R'CO)_2O \rightarrow R_2CONR' + R'COOH$

Note: Tertiary amines do not react because they lack a hydrogen atom on the nitrogen that can be substituted.

Schotten-Baumann Reaction: Acylation of primary and secondary amines, especially aromatic amines, is often carried out in the presence of aqueous alkali or pyridine.

Carbylamine Reaction

Description: This is a characteristic reaction of primary amines (aliphatic and aromatic). When primary amines are heated with chloroform ($CHCl_3$) and an alcoholic solution of $KOH$, they form isocyanides (carbylamines), which have a foul smell.

$RNH_2 + CHCl_3 + 3KOH \xrightarrow{heat} RNC + 3KCl + 3H_2O$

Note: Secondary and tertiary amines do not give this reaction. It is also known as the isocyanide test.

Reaction With Nitrous Acid

The reaction of amines with nitrous acid ($HNO_2$, usually prepared in situ from $NaNO_2$ and $HCl$ at 0-5°C) depends on the class of the amine:

1. Primary Amines:

• Aliphatic Primary Amines: Form unstable diazonium salts which decompose to give alcohols, nitrogen gas, and $HCl$.

$RNH_2 + HNO_2 \xrightarrow{0-5^\circ C} [RNH_3]^+NO_2^- \rightarrow ROH + N_2(g)$

• Aromatic Primary Amines: Form relatively stable diazonium salts at low temperatures, which are important intermediates in organic synthesis.

$ArNH_2 + HNO_2 + HCl \xrightarrow{0-5^\circ C} [ArN_2]^+Cl^- + 2H_2O$

2. Secondary Amines: Form N-nitrosamines, which are oily yellow liquids.

$R_2NH + HNO_2 \rightarrow R_2N-NO + H_2O$

3. Tertiary Amines:

• Aliphatic Tertiary Amines: Form nitrosoammonium compounds.

$R_3N + HNO_2 \rightarrow [R_3NH]^+NO_2^- \rightarrow R_2NR(NO) + H_2O$

• Aromatic Tertiary Amines: Undergo electrophilic substitution on the aromatic ring, usually at the para position, to form $p$-nitrosodialkylanilines.

$C_6H_5N(CH_3)_2 + HNO_2 \rightarrow p-(CH_3)_2NC_6H_4NO + H_2O$

Reaction With Arylsulphonyl Chloride

Description: This reaction is used to distinguish between primary, secondary, and tertiary amines, known as the Hinsberg Test.

• Reaction with Benzene Sulphonyl Chloride ($C_6H_5SO_2Cl$):
• Primary Amine: Reacts to form an $N$-alkylbenzenesulfonamide, which is soluble in alkali due to the presence of the acidic N-H proton.

$RNH_2 + C_6H_5SO_2Cl \xrightarrow{KOH} C_6H_5SO_2NHR \xrightarrow{KOH} C_6H_5SO_2N^-R K^+ \text{(soluble)}$

• Secondary Amine: Reacts to form an $N,N$-dialkylbenzenesulfonamide, which is insoluble in alkali.

$R_2NH + C_6H_5SO_2Cl \xrightarrow{KOH \ or \ Pyridine} R_2N-SO_2C_6H_5$ (Insoluble)

• Tertiary Amine: Does not react with benzene sulfonyl chloride under these conditions (no hydrogen on nitrogen to replace).

Electrophilic Substitution

Description: Aromatic amines (like aniline) undergo electrophilic substitution reactions more readily than benzene.

Reason: The amino group ($-NH_2$) is a strongly activating group due to the lone pair of electrons on nitrogen participating in resonance with the aromatic ring, increasing electron density especially at the ortho and para positions.

Reactions:

• Halogenation: Aniline reacts readily with bromine water to give a precipitate of 2,4,6-tribromoaniline.

$C_6H_5NH_2 + 3Br_2(aq) \rightarrow C_6H_2(Br_3)NH_2(s) + 3HBr$

• Nitration: Direct nitration of aniline with a mixture of concentrated $HNO_3$ and $H_2SO_4$ is problematic because the amino group gets protonated under acidic conditions, forming anilinium ion ($C_6H_5NH_3^+$), which is a meta-director. To avoid this, the amino group is first acetylated.

$C_6H_5NH_2 \xrightarrow{CH_3COCl} C_6H_5NHCOCH_3 \xrightarrow{HNO_3/H_2SO_4} p-O_2NC_6H_4NHCOCH_3 \xrightarrow{H_3O^+} p-O_2NC_6H_4NH_2$

• Sulfonation: Reaction with concentrated sulfuric acid at 453 K (180°C) gives $p$-aminobenzenesulfonic acid (sulfanilic acid).

$C_6H_5NH_2 + H_2SO_4 \xrightarrow{453K} H_2N-C_6H_4-SO_3H(p) + H_2O$

• Friedel-Crafts Reactions: Aniline does not undergo Friedel-Crafts alkylation or acylation because the Lewis acid catalyst ($AlCl_3$) complexes with the basic nitrogen atom, deactivating the ring.