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How many of the following reactions produce either decarboxylation or dehydration (or both) upon heating? (i) 1,2-cyclohexanedicarboxylic acid (cis, forming anhydride/decarboxylation) (ii) 4-hydroxybutanoic acid (HO-CH2-CH2-CH2-COOH) (iii) phenol (iv) 2-aminobutanoic acid (beta-amino acid: CH3-CH(NH2)-CH2-COOH) (v) benzene-1,2-diacetic acid (two -CH2COOH groups on adjacent carbons of benzene) (vi) but-3-enoic acid (CH2=CH-CH2-COOH)

1. 1
2. 2
3. 3
4. 4

Correct answer: 4

Solution

(i) cis-1,2-cyclohexanedicarboxylic acid: the two -COOH groups are close in space, undergo dehydration to form the cyclic anhydride on heating. Dehydration occurs. (ii) HO(CH2)3COOH = 4-hydroxybutanoic acid (gamma-hydroxy): forms gamma-butyrolactone (5-membered ring) by dehydration. (iii) Phenol: heating alone does not typically give decarboxylation or dehydration (it is already a stable compound; its -OH is on ring). No. (iv) CH3CH(NH2)CH2COOH = beta-amino acid: beta-amino acids can lose CO2 (decarboxylation) or form beta-lactam. Yes. (v) Benzene-1,2-diacetic acid (homophthalic acid): two CH2COOH on adjacent benzene carbons; on heating forms homophthalic anhydride (dehydration). Yes. (vi) CH2=CHCH2COOH = but-3-enoic acid (beta,gamma-unsaturated): heating can cause decarboxylation or isomerization, but simple carboxylic acids without special activation don't readily decarboxylate unless alpha-beta unsaturation. Not straightforward. Count: (i), (ii), (iv), (v) = 4 reactions.

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