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Consider the following reaction sequence in which P, Q, R, S, and T are products: HC≡CH --[red hot Fe]--> P --[HNO3/H2SO4]--> Q --[Fe/HCl]--> R --[NaNO2(aq)/HCl, 0-5 deg C]--> S --[(i) phenol, NaOH; (ii) H3O+]--> T T is a red-coloured compound. What is the degree of unsaturation (index of hydrogen deficiency) of T?

1. 7
2. 9
3. 11
4. 13

Correct answer: 9

Solution

The sequence converts HC≡CH -> benzene (P) -> nitrobenzene (Q) -> aniline (R) -> benzenediazonium chloride (S) -> p-hydroxyazobenzene (T, C12H10N2O). DoU = (2*12 + 2 + 2 - 10) / 2 = 18/2 = 9.

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