Class 11 Chemistry – Chapter 20: Nitrogen-Containing Compounds (FBISE)
This section provides complete, exam-oriented notes for Class 11 Chemistry Chapter 20 – Nitrogen-Containing Compounds strictly according to the Federal Board (FBISE) syllabus. The chapter focuses on structure, preparation, properties, and reactions of nitrogen-based organic compounds.
Major topics include amines, diazonium salts, cyanides, isocyanides, nitriles, basicity of amines, and important reactions and uses. Reaction mechanisms, comparison tables, and exam-focused synthesis points are included for effective learning.
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20.1 Amines: Definition and Classification
Amines are organic compounds containing nitrogen linked with alkyl or aryl groups. They are considered derivatives of ammonia ($NH_{3}$) where one or more hydrogen atoms are replaced by organic groups.
Classification
- Primary ($1^{\circ}$) Amines: One alkyl group attached to nitrogen. General formula: $R-NH_{2}$.
- Secondary ($2^{\circ}$) Amines: Two alkyl groups attached to nitrogen. General formula: $R-NH-R'$.
- Tertiary ($3^{\circ}$) Amines: Three alkyl groups attached to nitrogen. General formula: $R_{3}N$.
Structure and Bonding
The nitrogen atom in amines is $sp^{3}$-hybridized, resulting in a nearly tetrahedral structure. One of the $sp^{3}$ orbitals contains a non-bonding lone pair of electrons, making amines both bases and nucleophiles.
20.2 Physical Properties
- Polarity: The $N-H$ bond is polar due to electronegativity differences.
- Hydrogen Bonding: Amines can form intermolecular hydrogen bonds, leading to higher boiling/melting points than alkanes of similar mass.
- Solubility: Lower amines are highly soluble in water due to hydrogen bonding with water molecules.
20.3 Preparation of Amines
- Reaction of Alkyl Halides with Ammonia: Heating an alcoholic solution of $NH_{3}$ with alkyl halides produces a mixture of primary, secondary, and tertiary amines, as well as quaternary ammonium salts ($[R_{4}N]^{+}X^{-}$).
- Reduction of Nitriles: $R-C \equiv N$ can be reduced to $R-CH_{2}-NH_{2}$ using $LiAlH_{4}$ or sodium in ethanol.
- Reduction of Amides: Treatment with $LiAlH_{4}$ reduces the $-CO-$ group to $-CH_{2}-$.$$CH_{3}CONH_{2} + 4[H] \xrightarrow{LiAlH_{4}} CH_{3}CH_{2}NH_{2} + H_{2}O$$
- Hoffmann Degradation: Amides react with $Br_{2}$ and $KOH$ to form a primary amine with one fewer carbon atom than the parent amide.
20.4 Basicity
Basicity is measured by $pK_{b}$. A lower $pK_{b}$ indicates a stronger base.
- Methylamine ($pK_{b} = 3.38$) is a stronger base than Ammonia ($pK_{b} = 4.76$) because the methyl group is electron-donating ($+I$ effect), increasing electron density on Nitrogen.
- Order of Basicity: Secondary amine > Primary amine > Ammonia.
20.5 Reactions of Amines
- Acylation: Reaction with acid halides ($RCOCl$) to form secondary amides.
- Alkylation: Reaction with alkyl halides to step-wise form $2^{\circ}$, $3^{\circ}$ amines and quaternary salts.
- Diazotization: Primary aromatic amines (like Aniline) react with $HNO_{2}$ and $HCl$ below $10^{\circ}C$ to form stable Benzene diazonium chloride ($C_{6}H_{5}N_{2}^{+}Cl^{-}$), used to make azo dyes.
Review Questions & Answers
Q1: Why are tertiary amines with three different groups optically active?A1: Because the nitrogen atom is $sp^{3}$ hybridized and the four "groups" (including the lone pair) are different, creating a chiral center. However, in many cases, rapid "nitrogen inversion" can make separation difficult.
Q2: How does Hoffmann Degradation differ from $LiAlH_{4}$ reduction of amides?A2: $LiAlH_{4}$ reduction keeps the carbon chain length intact (reduces $C=O$ to $CH_{2}$), whereas Hoffmann Degradation removes the carbonyl carbon entirely, resulting in an amine with one less carbon atom.
Q3: Why is Benzene diazonium chloride more stable than aliphatic diazonium salts?A3: The diazonium group on a benzene ring is stabilized by the delocalization of electrons (resonance) into the ring system.