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From the following compounds, how many will produce effervescence (release CO2 gas) when treated with aqueous NaHCO3 solution? (i) 2-methylbenzoic acid (o-toluic acid) (ii) cyclohexylsulfonic acid (iii) 4-aminobenzaldehyde (iv) 4-hydroxycyclohexanone (v) cyclobutane-1,2,3,4-tetraol
- (A) 2
- (B) 3
- (C) 4
- (D) 5
Correct answer: (A) 2
Solution
NaHCO3 reacts with compounds whose pKa < ~6.4 to release CO2. (i) o-methylbenzoic acid is a carboxylic acid (pKa ~3.9) - REACTS. (ii) Cyclohexylsulfonic acid is a strong acid (pKa < 1) - REACTS. (iii) 4-aminobenzaldehyde has no acidic proton with pKa below 6.4 (the amino group is basic, the aldehyde is not acidic) - NO REACTION. (iv) 4-hydroxycyclohexanone: the alcohol OH has pKa ~15, the alpha-H of ketone has pKa ~20 - NO REACTION. (v) Cyclobutane-1,2,3,4-tetraol: alcohol groups with pKa ~15 - NO REACTION. From the 5 named compounds: 2 react. Note: compound (vi) was image-only and is excluded from evaluation.
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