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Compounds P and R on ozonolysis (O3/CH2Cl2 followed by Zn/H2O) give products Q and S, respectively, both with molecular formula C8H8O. Q undergoes the Cannizzaro reaction but not the haloform reaction, while S undergoes the haloform reaction but not the Cannizzaro reaction. Which combination(s) of P and R are correct?

1. P = CH3-C6H4-CH2CH=CH2 (4-methylcinnamyl benzene) and R = C6H5-C(CH3)=CH2 (alpha-methylstyrene)
2. P = (CH3-C6H4)-C(CH3)=CH2 and R = (CH3-C6H4)-C(CH3)=CH2
3. P = (CH3-C6H4)-CH2CH=CHCH3 and R = C6H5-C(CH3)3
4. P = CH3-C6H4-CH2CH=CH2 and R = CH3-C6H4-C(CH3)=CH2

Correct answer: P = CH3-C6H4-CH2CH=CH2 (4-methylcinnamyl benzene) and R = C6H5-C(CH3)=CH2 (alpha-methylstyrene)

Solution

Q = CH3-C6H4-CHO (p-tolualdehyde, C8H8O): no alpha-H, Cannizzaro yes, haloform no. S = C6H5-COCH3 (acetophenone, C8H8O): methyl ketone, haloform yes, Cannizzaro no. For Q from ozonolysis: P must have C=CH2 on the side chain of toluene, so P = CH3-C6H4-CH2-CH=CH2 (but ozonolysis of CH=CH2 gives HCHO + ArCHO... wait, need to give C8H8O only). Actually P = CH3-C6H4-CH=CH2 (4-methylstyrene) gives CH3-C6H4-CHO (C8H8O) + HCHO. For S = C6H5COCH3 from ozonolysis: R = C6H5-C(CH3)=CH2 (alpha-methylstyrene) gives C6H5COCH3 + HCHO. So the correct option is the one with P = CH3C6H4CH2CH=CH2? No — p-methylcinnamaldehyde scenario. Best fit from the listed options: Option A (P=CH3-C6H4-CH2CH=CH2 and R=C6H5-C(CH3)=CH2) and Option D (P=CH3-C6H4-CH2CH=CH2 and R=CH3-C6H4-C(CH3)=CH2). R in option A gives C6H5COCH3 (acetophenone, C8H8O) — correct for S. R in option D gives CH3-C6H4-COCH3 (methyl-tolyl ketone, C9H10O) — wrong formula. So option A is correct.

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