Amines | CBSE - Wyatt's Notes

sources:

• text: Standard textbook reference

Amines are derivatives of ammonia where one or more H atoms are replaced by alkyl or aryl groups. They are classified as primary ($1°$), secondary ($2°$), or tertiary ($3°$).

• Primary amine: $\text{RNH}_2$, secondary: $\text{R}_2\text{NH}$, tertiary: $\text{R}_3\text{N}$
• Basicity: amines are Lewis bases (lone pair on nitrogen)
• Alkyl groups increase basicity (electron-donating), aryl groups decrease it
• Preparation: reduction of nitro compounds, ammonolysis of alkyl halides, Gabriel phthalimide synthesis
• Reactions: alkylation, acylation, carbylamine reaction, Hinsberg test
• Diazonium salts: $\text{ArN}_2^+\text{Cl}^-$ (formed from primary aromatic amines + NaNO$_2$/HCl at 0-5°C)
• Coupling reaction: diazonium salt + phenol/aniline $\rightarrow$ azo dye

Problem: Arrange methylamine, aniline, and dimethylamine in decreasing order of basicity in aqueous solution.

Solution:

Basicity depends on the availability of the lone pair on nitrogen.

• Dimethylamine ($(\text{CH}_3)_2\text{NH}$): Two electron-donating methyl groups increase electron density on N. Most basic. $pK_b = 3.27$

• Methylamine ($\text{CH}_3\text{NH}_2$): One methyl group provides moderate electron donation. $pK_b = 3.36$

• Aniline ($\text{C}_6\text{H}_5\text{NH}_2$): The lone pair on N delocalizes into the benzene ring, reducing availability. Least basic. $pK_b = 9.42$

Decreasing basicity: $(\text{CH}_3)_2\text{NH} > \text{CH}_3\text{NH}_2 > \text{C}_6\text{H}_5\text{NH}_2$

Common mistake: Assuming more alkyl groups always means higher basicity. Steric effects can reduce basicity in highly substituted amines in the gas phase, but in aqueous solution, solvation effects dominate.

Problem: How does the Hinsberg test distinguish between primary, secondary, and tertiary amines?

Solution:

The Hinsberg reagent is benzenesulfonyl chloride ($\text{C}_6\text{H}_5\text{SO}_2\text{Cl}$).

• Primary amine: Reacts to form a sulfonamide with an acidic N-H proton. Dissolves in NaOH: $\text{RNH}_2 + \text{C}_6\text{H}_5\text{SO}_2\text{Cl} \rightarrow \text{C}_6\text{H}_5\text{SO}_2\text{NHR} + \text{HCl}$ Product is soluble in NaOH.

• Secondary amine: Reacts to form a sulfonamide without acidic proton. Insoluble in NaOH: $\text{R}_2\text{NH} + \text{C}_6\text{H}_5\text{SO}_2\text{Cl} \rightarrow \text{C}_6\text{H}_5\text{SO}_2\text{NR}_2 + \text{HCl}$ Product is insoluble in NaOH.

• Tertiary amine: Does not react with benzenesulfonyl chloride.

Common mistake: Forgetting that the sulfonamide from a primary amine has an acidic proton that makes it soluble in base.

Problem: Write the coupling reaction of benzenediazonium chloride with phenol.

Solution:

$\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{C}_6\text{H}_5\text{OH} \xrightarrow{\text{NaOH}} \text{C}_6\text{H}_5-\text{N}=\text{N}-\text{C}_6\text{H}_4-\text{OH} + \text{NaCl} + \text{H}_2\text{O}$

Product: 4-hydroxyazobenzene (an orange-red azo dye)

The coupling occurs at the para position of phenol (activated ring).

Common mistake: Coupling at the meta position. The $-\text{OH}$ group is ortho/para-directing, so coupling occurs at para (or ortho if para is blocked).

1. Arrange pyridine, pyrrole, and piperidine in order of basicity.
2. Write the carbylamine reaction for ethylamine.
3. How would you convert aniline to phenol via diazonium salt?

Amines are found in amino acids, neurotransmitters, pharmaceuticals (ephedrine, amphetamine), and dyes. Understanding their chemistry is essential for drug design and biochemistry.

• Basicity: alkyl groups increase, aryl groups decrease
• Hinsberg test distinguishes primary, secondary, tertiary amines
• Diazonium salts are versatile intermediates for synthesis
• Azo dyes are formed by coupling reactions
• Carbylamine reaction is specific to primary amines (isocyanide test)

Amines are ammonia wearing different hats: Picture ammonia (NH₃) with its lone pair of electrons sitting on nitrogen like a raised hand ready to bond. When you replace one or more hydrogens with carbon groups, you get amines. The more alkyl groups you add, the more you push electron density onto nitrogen, making it more eager to accept protons — until steric crowding gets in the way.

Why it matters: Amines are everywhere in biology — amino acids, neurotransmitters, caffeine, and most drugs contain nitrogen. Understanding basicity trends helps predict drug absorption, and diazonium chemistry enables the synthesis of azo dyes used in textiles and food coloring.

The key insight: Basicity in aqueous solution is a three-way competition between electron donation (increases basicity), steric hindrance (decreases it), and solvation of the conjugate acid (stabilizes it).

Assuming more alkyl groups always means higher basicity. While alkyl groups are electron-donating and increase basicity in the gas phase, steric effects and solvation effects in aqueous solution can reduce basicity for highly substituted amines. For example, trimethylamine is less basic than dimethylamine in water despite having more methyl groups.

Forgetting that the Hinsberg test distinguishes primary and secondary amines by solubility. The sulfonamide from a primary amine has an acidic N-H proton that dissolves in NaOH, while the sulfonamide from a secondary amine lacks this proton and remains insoluble. Tertiary amines do not react with benzenesulfonyl chloride at all.

Coupling diazonium salts at the wrong position. The -OH and -NH2 groups on phenol and aniline are ortho/para-directing. Diazonium coupling occurs at the para position (or ortho if para is blocked). Students sometimes write coupling at the meta position, which is incorrect because the activating group directs to ortho/para positions.

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