1 mole of a symmetrical alkene on ozonolysis gives 2 moles of an aldehyde with molecular mass 44 u. The alkene is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → But-2-ene
Ozonolysis followed by Zn/H2O cuts the C=C bond and turns each doubly bonded carbon into a C=O group, so a symmetrical alkene gives two identical carbonyl molecules. The aldehyde of molar mass 44 u is ethanal:
\( \displaystyle M(\ce{CH3CHO}) = 2(12) + 4(1) + 16 = 44\ \mathrm{u} \)
Joining two \( \ce{CH3CHO} \) units at their carbonyl carbons by a C=C bond gives but-2-ene:
\( \displaystyle \ce{CH3CH=CHCH3} \xrightarrow[\text{(ii) }\ce{Zn/H2O}]{\text{(i) }\ce{O3}} \ce{2CH3CHO} \)
Why the other options are incorrect
Option (1) Ethene — Incorrect. \( \ce{CH2=CH2} \) gives 2 moles of methanal, HCHO (molar mass 30 u).
Option (2) Propene — Incorrect. \( \ce{CH3CH=CH2} \) is not symmetrical; it gives one mole each of ethanal (44 u) and methanal (30 u).
Option (3) But-1-ene — Incorrect. \( \ce{CH3CH2CH=CH2} \) is not symmetrical; it gives propanal (58 u) and methanal (30 u).
NCERT (2026-27 edition):
By Ozonolysis of Alkenes – “As we know, ozonolysis of alkenes followed by reaction with zinc dust and water gives aldehydes, ketones or a mixture of both depending on the substitution pattern of the alkene (Unit 9, Class XI).”