In the following reaction
The % yield for reaction I is 60 % and that of reaction II is 50 %. The overall yield of the complete reaction is __________ %. [nearest integer]

Solution

Chemistry ma’am · 4 steps

Each step keeps only a fraction of what it gets, so multiply the two yields, never add them.

1. Follow one mole. Start with 1 mol of benzene. Each step converts one molecule into one molecule: benzene to benzenesulfonic acid, then benzenesulfonic acid to phenol.
2. Step I. Sulphonation with oleum has 60% yield, so 1 mol of benzene gives 0.60 mol of C₆H₅SO₃H.
3. Step II. Only that 0.60 mol goes into step II. Fusion with excess NaOH and then acidification has 50% yield, so it gives 0.60 × 0.50 = 0.30 mol of phenol.

   Overall yield = 0.60 × 0.50 × 100% = 30%

4. So. 1 mol of benzene finally gives 0.30 mol of phenol, so the overall yield is 30%.

The common slip: 55. Averaging the two yields gives 55, but step II only works on the 60% that step I produced, so the yields must be multiplied.

The idea to keep. In a multi-step synthesis the product of one step is the reactant of the next, so the overall yield is the product of the step yields as fractions. It is always lower than the worst single step.

Answer from the NTA final answer key. Solution by PrepWiser.