Q.Explain the mechanism of esterification. Write the reactions involved in dehydration of 1°, 2° and 3° alcohols. OR What are vitamins? Name any two diseases caused by deficiency of vitamin A. Write the structures of: a. nucleoside b. nucleotide. How are 1-nitropropane, 2-nitropropane and 2-methyl-2-nitropropane are distinguished from each other using nitrous acid?

Answering either OR alternative: (1) esterification mechanism + alcohol dehydration order, or (2) vitamins/nucleic-acid structures + the HNO2 test for nitroalkanes.

Option 1 — Esterification mechanism: Fischer (acid-catalysed) esterification of a carboxylic acid with an alcohol, $RCOOH + R'OH \xrightleftharpoons{H^+} RCOOR' + H_2O$, proceeds by: (1) protonation of the carbonyl oxygen of the acid by $H^+$, making the carbonyl carbon more electrophilic; (2) nucleophilic attack of the alcohol's oxygen lone pair on this carbon, forming a tetrahedral intermediate; (3) proton transfer between oxygens; (4) elimination of a water molecule; (5) loss of the catalytic proton to give the neutral ester.

Dehydration of alcohols (acid-catalysed, e.g. conc. $H_2SO_4$, heat) proceeds mainly by an E1-type mechanism: the alcohol is protonated, water leaves to form a carbocation, and then a $\beta$-hydrogen is lost to give the alkene (Zaitsev product favoured). Tertiary alcohols dehydrate fastest and under the mildest conditions (most stable carbocation forms most easily); secondary alcohols dehydrate less readily; primary alcohols dehydrate least readily and generally need the highest temperature/strongest acid, sometimes proceeding through a more concerted E2-like pathway since the primary carbocation is unstable. E.g., ethanol (1°) needs conc. $H_2SO_4$ at 443 K to give ethene; tert-butanol (3°) dehydrates readily even with dilute acid.

Option 2 (OR) — Vitamins, nucleic acids, nitroalkane test: Vitamins are organic compounds required in small (trace) amounts in the diet, essential for normal growth, metabolism and health, that the body cannot synthesise adequately on its own. Vitamin A deficiency causes: (1) night blindness (nyctalopia) and (2) xerophthalmia (dryness/damage of the cornea and conjunctiva).

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