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JEE Main levelNumerical answerDifficulty 2 of 3o2013

Consider the acyclic alkenes of molecular formula , with (E) and (Z) isomers counted separately (six alkenes: pent-1-ene, (E)-pent-2-ene, (Z)-pent-2-ene, 2-methylbut-1-ene, 2-methylbut-2-ene, 3-methylbut-1-ene). Assume that in hydroboration the boron goes entirely to the less substituted alkene carbon when the two carbons differ in substitution, and gives a mixture when they are equally substituted. How many of these six alkenes give only one constitutional alcohol on treatment with followed by ?

Numerical answer type, tolerance 0.5.

Answer: 4

Pent-1-ene gives pentan-1-ol; 2-methylbut-1-ene gives 2-methylbutan-1-ol; 2-methylbut-2-ene gives 3-methylbutan-2-ol (B on the CH carbon); 3-methylbut-1-ene gives 3-methylbutan-1-ol. Each gives a single alcohol. (E)- and (Z)-pent-2-ene have equally substituted carbons and give pentan-2-ol plus pentan-3-ol. The count is 4.
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