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Arrange the following carboxylic acids in the correct order of increasing acid strength: (i) CH3-CH2-COOH (propanoic acid) (ii) CH2=CH-COOH (acrylic acid / propenoic acid) (iii) HC≡C-COOH (propynoic acid / propiolic acid)

1. CH3-CH2-COOH < CH2=CH-COOH < HC≡C-COOH
2. HC≡C-COOH < CH2=CH-COOH < CH3-CH2-COOH
3. CH2=CH-COOH < CH3-CH2-COOH < HC≡C-COOH
4. CH3-CH2-COOH < HC≡C-COOH < CH2=CH-COOH

Correct answer: CH3-CH2-COOH < CH2=CH-COOH < HC≡C-COOH

Solution

Acid strength increases as the adjacent carbon's s-character increases: sp3 (propanoic) < sp2 (acrylic) < sp (propiolic). The highly electronegative sp carbon of the alkynyl group best stabilises the negative charge on the carboxylate, making propiolic acid the strongest. Approximate pKa values are 4.87, 4.25, and 1.84 respectively.

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