Question

Arrange the products obtained in the following cases in the increasing order of their pK_a values:

1. Oxidation of ethanol in the presence of acidified potassium dichromate.
2. Reaction of propanoic acid with Br₂ in the presence of red phosphorus.
3. Reaction of isopropyl magnesium bromide with carbon dioxide, followed by hydrolysis.
4. Reaction of propanoic acid with Cl₂ in the presence of red phosphorus.

Answer 1

1. \[\ce{CH3CH2OH ->[Acidified K2Cr2O7]CH3COOH}\]
2. \[\ce{CH3CH2COOH ->[Br2/Red P]CH3CHBrCOOH}\]
3. \[\ce{(CH3)2CHMgCl->[(i) CO2 (ii) H+, H2O](CH3)2CHCOOH}\]
4. \[\ce{CH3CH2COOH->[Cl2/Red P]CH3CHClCOOH}\]

Order of acidity: CH₃CHClCOOH > CH₃CHBrCOOH > CH₃COOH > (CH₃)₂CHCOOH

Order of pK_a values: CH₃CHClCOOH < CH₃CHBrCOOH < CH₃COOH < (CH₃)₂CHCOOH