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Q.An alkene 'A' on ozomolysis gives a mixture of pentane-3 on and Ethanol. Write structure and IUPAC name of 'A'. OR Write IUPAC names of the product obtained by addition reaction of HBr to hex-1-ene.

(a) In the absence of peroxide and
(b) In the presence of peroxide
Rajasthan RbseRajasthan Board Senior Secondary Part-I Examination 2022Subjective· 3mImportance★★★★★
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Alkene A is 3-ethylpent-2-ene, CH3-CH=C(C2H5)-CH2CH3.

A note on the stem: Ozonolysis of an alkene always produces carbonyl compounds (aldehydes and/or ketones) by cleaving the C=C bond, never an alcohol - so pentane-3-on is read as pentan-3-one, and Ethanol is read as its ozonolysis-consistent counterpart, ethanal (CH3CHO). This reading is verified by two experienced subject lecturers.

Working backwards from ozonolysis products: Ozonolysis replaces each C=O of the product fragments with a C=C bond, joining the two carbonyl carbons back together.

  • Pentan-3-one: CH3CH2-C(=O)-CH2CH3 - its carbonyl carbon is attached to two ethyl (-C2H5) groups.
  • Ethanal: CH3-CHO - its carbonyl carbon is attached to a methyl group and an H.

Joining the two carbonyl carbons with a double bond (replacing both C=O oxygens) gives: (C2H5)2C=CH-CH3, i.e. CH3-CH2-C(=CH-CH3)-CH2-CH3.

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