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SolutionsCarboxylic Acids

1. Which is more acidic?

Arrange in order of decreasing acidity: water, ethanol, phenol and acetic acid.

Acetic acid (pKa ≈ 4.7) > phenol (≈ 10) > water (15.7) > ethanol (≈ 16).

Acidity follows the stability of the conjugate base: acetate shares the charge between two equivalent oxygens; phenoxide delocalizes it into the ring, but onto carbons; ethoxide keeps it on a single oxygen.

2. The reactive centres

Identify in propanoic acid, CH3CH2COOH: a) the most acidic hydrogen; b) the electrophilic centre; c) the basic centres; d) the weakly acidic hydrogens.

  1. The OH hydrogen.
  2. The carbonyl carbon.
  3. The two oxygens, especially the C=O oxygen.
  4. The hydrogens of the CH2 next to the carboxyl group (α position).

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