Find the correct stoichiometric coefficients in the following redox equation:
\(xH^+ + yC_6H_5CH_3 + wMnO_4^- \longrightarrow aH_2O + bMn^{2+} + tC_6H_5COOH\)
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → x = 18, y = 5, w = 6, a = 14, b = 6, t = 5
A redox equation is balanced by the half-reaction (ion-electron) method: balance each half for atoms and charge, make the electrons lost equal to the electrons gained, and add. Acidified permanganate oxidises the side chain of toluene to –COOH.
Oxidation half: the \( \ce{CH3} \) carbon goes from −3 to +3 in –COOH, a loss of 6 electrons:
\( \ce{C6H5CH3 + 2H2O -> C6H5COOH + 6H+ + 6e-} \)
Reduction half: Mn goes from +7 to +2, a gain of 5 electrons:
\( \ce{MnO4- + 8H+ + 5e- -> Mn^{2+} + 4H2O} \)
To make the electrons equal (30), multiply the oxidation half by 5 and the reduction half by 6:
\( \ce{5C6H5CH3 + 10H2O -> 5C6H5COOH + 30H+ + 30e-} \)
\( \ce{6MnO4- + 48H+ + 30e- -> 6Mn^{2+} + 24H2O} \)
Adding and cancelling 30 \( \ce{H^{+}} \) and 10 \( \ce{H2O} \) from both sides:
\( \ce{18H+ + 5C6H5CH3 + 6MnO4- -> 14H2O + 6Mn^{2+} + 5C6H5COOH} \)
So x = 18, y = 5, w = 6, a = 14, b = 6, t = 5.
Shortcut: check the charge. Left: 18 − 6 = +12; right: 6 × 2 = +12. Options (2) and (4) give 8 − 6 = +2 and 8 − 5 = +3 on the left against +12 and +10 on the right, so they fail. The oxygen balance (24 = a + 10) then gives a = 14, which rules out Option (3).
NCERT (2026-27 edition):
Methods of Preparation of Carboxylic Acids – …
