Account for the following observations:
i) pK_b for aniline is more than that for methylamine.
ii) A methylamine solution in water reacts with ferric chloride solution to give a precipitate of ferric hydroxide.
iii) Aniline does not undergo Friedel–Crafts reactions.
i) pK_b for aniline is more than that for methylamine.
ii) A methylamine solution in water reacts with ferric chloride solution to give a precipitate of ferric hydroxide.
iii) Aniline does not undergo Friedel–Crafts reactions.
Solution
(i) Aniline's nitrogen lone pair conjugates with the ring, so it protonates less readily than methylamine's — weaker base ⇒ \textbf{higher pK}.
(ii)
(iii) The basic –NH donates its lone pair to AlCl forming an N–Al adduct; the ring, now carrying a positive nitrogen, is deactivated — Friedel–Crafts fails.
(ii)
(iii) The basic –NH donates its lone pair to AlCl forming an N–Al adduct; the ring, now carrying a positive nitrogen, is deactivated — Friedel–Crafts fails.
CBSE Class 12 Board · 2008 · Delhi · Subjective