Introduction
Amines are an important class of organic compounds, characterized by the presence of a nitrogen atom bonded to one or more alkyl or aryl groups. They are derivatives of ammonia ($NH_3$) where one or more hydrogen atoms have been replaced by alkyl or aryl groups. Amines are broadly classified into primary (1°), secondary (2°), and tertiary (3°) amines based on the number of carbon-containing groups attached to the nitrogen atom.
Classification of Amines
Primary Amines (1°)
Primary amines have one alkyl or aryl group attached to the nitrogen atom. The general formula is $RNH_2$.
ExampleExample: Methylamine ($CH_3NH_2$)
Secondary Amines (2°)
Secondary amines have two alkyl or aryl groups attached to the nitrogen atom. The general formula is $R_2NH$.
ExampleExample: Dimethylamine ($(CH_3)_2NH$)
Tertiary Amines (3°)
Tertiary amines have three alkyl or aryl groups attached to the nitrogen atom. The general formula is $R_3N$.
ExampleExample: Trimethylamine ($(CH_3)_3N$)
Quaternary Ammonium Salts
Although not true amines, quaternary ammonium salts are worth mentioning. They have four alkyl or aryl groups attached to the nitrogen atom, giving the nitrogen a positive charge. The general formula is $R_4N^+X^-$.
ExampleExample: Tetramethylammonium chloride ($(CH_3)_4N^+Cl^-$)
Structure and Bonding in Amines
Bond Angles and Geometry
Amines have a trigonal pyramidal shape due to the lone pair of electrons on the nitrogen atom. The bond angles are approximately $107^\circ$, slightly less than the tetrahedral angle of $109.5^\circ$ due to the lone pair-bond pair repulsion.
NoteThe lone pair of electrons on the nitrogen atom in amines significantly influences their chemical properties, including basicity and nucleophilicity.
Hybridization
The nitrogen atom in amines is $sp^3$ hybridized, with one of the hybrid orbitals occupied by a lone pair of electrons.
Physical Properties of Amines
Boiling Points
Amines have higher boiling points than hydrocarbons of similar molar mass due to hydrogen bonding but lower than alcohols because the $N-H$ bond is less polar than the $O-H$ bond.
Solubility
Lower aliphatic amines are soluble in water due to hydrogen bonding with water molecules. Solubility decreases with increasing molecular mass.
TipThe solubility of amines in water can be remembered by the phrase "like dissolves like," meaning polar amines dissolve well in polar solvents like water.
Basicity of Amines
Amines are basic due to the lone pair of electrons on the nitrogen atom, which can accept a proton ($H^+$). The basicity of amines is influenced by the alkyl groups attached to the nitrogen atom.
Comparative Basicity
- Aliphatic Amines: More basic than ammonia due to the electron-donating effect of alkyl groups.
- Aromatic Amines: Less basic than aliphatic amines due to the delocalization of the lone pair of electrons into the aromatic ring.
Example: Aniline ($C_6H_5NH_2$) is less basic than ethylamine ($C_2H_5NH_2$) because the lone pair on nitrogen in aniline is delocalized into the benzene ring.
Basicity in Aqueous Solution
The basicity of amines in aqueous solution can be compared using pKb values. Lower pKb indicates a stronger base.
$$ NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^- $$
$$ RNH_2 + H_2O \rightleftharpoons RNH_3^+ + OH^- $$
Preparation of Amines
Reduction of Nitro Compounds
Nitro compounds can be reduced to primary amines using reducing agents like $H_2$ in the presence of a catalyst or by using metals in acidic medium.
$$ RNO_2 + 6[H] \rightarrow RNH_2 + 2H_2O $$
Ammonolysis of Alkyl Halides
Alkyl halides react with ammonia to form primary amines. Excess ammonia is used to prevent further substitution.
$$ RX + NH_3 \rightarrow RNH_2 + HX $$
Reduction of Nitriles
Nitriles can be reduced to primary amines using hydrogen in the presence of a catalyst or by using lithium aluminum hydride ($LiAlH_4$).
$$ RCN + 4[H] \rightarrow RCH_2NH_2 $$
Gabriel Phthalimide Synthesis
This method is used to prepare primary amines by treating phthalimide with potassium hydroxide to form potassium phthalimide, which then reacts with an alkyl halide, followed by hydrolysis.
$$ C_6H_4(CO)_2NH + KOH \rightarrow C_6H_4(CO)_2NK + H_2O $$
$$ C_6H_4(CO)_2NK + RX \rightarrow C_6H_4(CO)_2NR + KX $$
$$ C_6H_4(CO)_2NR + 2H_2O + HCl \rightarrow RNH_2 + C_6H_4(CO)_2H + KCl $$
Chemical Reactions of Amines
Alkylation
Amines react with alkyl halides to form higher amines and quaternary ammonium salts.
$$ RNH_2 + RX \rightarrow R_2NH + HX $$
Acylation
Amines react with acyl chlorides to form amides.
$$ RNH_2 + RCOCl \rightarrow RCONHR + HCl $$
Reaction with Nitrous Acid
Primary aliphatic amines react with nitrous acid to form alcohols, while aromatic amines form diazonium salts.
$$ RNH_2 + HNO_2 \rightarrow ROH + N_2 + H_2O $$
$$ C_6H_5NH_2 + HNO_2 + HCl \rightarrow C_6H_5N_2^+Cl^- + 2H_2O $$
NoteDiazonium salts are important intermediates in the synthesis of azo dyes and other aromatic compounds.
Summary
Amines are versatile compounds with significant importance in organic chemistry and various industrial applications. Understanding their structure, properties, and reactions is crucial for mastering the topic in NEET Chemistry.
TipPractice drawing the structures of different amines and writing their IUPAC names to reinforce your understanding.
Common MistakeA common mistake is confusing the basicity of aromatic and aliphatic amines. Remember that aromatic amines are less basic due to the delocalization of the lone pair of electrons.
