Which one of the following is not correct with respect to properties of amines?
- $pK_b$ of aniline is more than that of methylamine.
- Ethylamine is soluble in water whereas aniline is not
Aniline does not undergo Friedel-Crafts reaction.
- Ethylamine reacts with nitrous acid to give ethanol.
Ethanamide on reaction with $Br_2$ and NaOH gives ethylamine.
The Correct Option is
Solution and Explanation
Let’s analyze each statement about amines:
\( pK_b \) of aniline is more than that of methylamine: True. Aniline is a weaker base than methylamine due to resonance delocalization of the lone pair on nitrogen into the benzene ring. Hence, it has a higher \( pK_b \).
Ethylamine is soluble in water whereas aniline is not: True. Ethylamine, being a small and polar molecule, forms hydrogen bonds with water and is soluble. Aniline is less polar and has limited solubility because of the bulky aromatic ring.
Ethanamide on reaction with \( Br_2 \) and NaOH gives ethylamine: False. This is incorrect. The reaction of ethanamide with bromine and sodium hydroxide (Hofmann bromamide reaction) gives methylamine, not ethylamine.
Ethylamine reacts with nitrous acid to give ethanol: True. Primary aliphatic amines like ethylamine react with nitrous acid to form ethanol through diazonium salt decomposition.
Aniline does not undergo Friedel–Crafts reaction: True. The lone pair on nitrogen in aniline interacts with the Lewis acid catalyst (such as AlCl₃), which deactivates the aromatic ring and prevents the Friedel–Crafts reaction.
Correct Answer: (C)
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Amines are usually formed from amides, imides, halides, nitro compounds, etc. They exhibit hydrogen bonding which influences their physical properties. In alkyl amines, a combination of electron releasing, steric and H-bonding factors influence the stability of the substituted ammonium cations in protic polar solvents and thus affect the basic nature of amines. Alkyl amines are found to be stronger bases than ammonia. Amines being basic in nature, react with acids to form salts. Aryldiazonium salts, undergo replacement of the diazonium group with a variety of nucleophiles to produce aryl halides, cyanides, phenols and arenes.
How can you convert the following:
(i) Ethanoic acid to methanamine
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