Alkanols can be manufactured from alkenes by the initial reaction of alkenes with

Analyze the question: We want to make an alkanol (alcohol) from an alkene. The question asks which reagent is used in the first (initial) step.

Concept: Industrially, alcohols such as ethanol are made from alkenes by hydration. In the laboratory and in some industrial processes, this happens in two stages:

1. Addition of concentrated H₂SO₄ (tetraoxosulphate(VI) acid) across the C=C double bond, forming an alkyl hydrogen sulphate: CH₂=CH₂ + H₂SO₄ → CH₃CH₂OSO₃H (ethyl hydrogen sulphate)

2. Hydrolysis by adding water and warming: CH₃CH₂OSO₃H + H₂O → CH₃CH₂OH + H₂SO₄

Check the other options:

• A. Br₂ in CCl₄: This is the test for unsaturation. It gives a dibromoalkane (e.g. 1,2-dibromoethane), not an alcohol.
• C. Aqueous KMnO₄: This oxidises the alkene to a diol (e.g. ethane-1,2-diol), not a simple alkanol. It is also the Baeyer test for unsaturation.
• D. NaOH solution: Alkenes do not react with it under normal conditions.

Answer: The initial reaction is with concentrated tetraoxosulphate(VI) acid, which is option B.

Tip: Remember "alkene → alcohol = acid first, then water." Br₂ means test for unsaturation, and KMnO₄ means diol or oxidation.