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1,3,5-trimethylbenzene (mesitylene) can be obtained by which of the following methods?

1. Heating acetone with concentrated H2SO4
2. Trimerisation of propyne (CH3-C≡C-H) over red-hot iron
3. Heating 1,3,5-tricarbonylbenzene derivative in an iron tube at 873 K followed by treatment with soda lime
4. Treating 1,3,5-triformylbenzene with Zn-Hg and HCl

Correct answer: Heating acetone with concentrated H2SO4

Solution

Acetone undergoes trimerisation in the presence of conc. H2SO4 on heating — three acetone molecules undergo aldol-type condensation and dehydration-aromatisation to give 1,3,5-trimethylbenzene (mesitylene). Trimerisation of propyne over Fe gives 1,3,5-trimethylbenzene too (option B is also correct). Zn-Hg/HCl reduces carbonyl to CH2 (Clemmensen), so 1,3,5-triformylbenzene (benzene-1,3,5-tricarbaldehyde) reduced by Clemmensen would give 1,3,5-tri(methyl)benzene only if the CHO is reduced to CH3 — but CHO -> CH3 requires two reductions; Clemmensen reduces CHO to CH2 (not CH3 if already CH2). Option D is not straightforward.

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