Question 1

What is Markovnikov's rule for the addition of water to an alkene?

Accepted Answer

In the acid-catalysed addition of water, hydrogen attaches to the alkene carbon already bearing more hydrogen atoms, while the hydroxyl group attaches to the carbon bearing fewer hydrogens. This regiochemistry is called Markovnikov addition.

Question 2

Why does propene form propan-2-ol during acid-catalysed hydration?

Accepted Answer

Protonation at the terminal carbon forms a secondary carbocation at the middle carbon, which is more stable than the alternative primary carbocation. Water attacks the middle carbon, so deprotonation produces propan-2-ol.

Question 3

What product is formed when ethene reacts with acidified water?

Accepted Answer

Ethene is symmetrical because both double-bond carbons contain two hydrogens. Addition of water therefore gives ethanol, C2H6O, regardless of which alkene carbon receives hydrogen.

Question 4

What is the product of acid-catalysed hydration of 2-methylbut-2-ene?

Accepted Answer

Hydrogen adds to the alkene carbon already bearing one hydrogen, while OH adds to the carbon bearing no hydrogen. The product is 2-methylbutan-2-ol with molecular formula C5H12O.

Question 5

Does acid-catalysed hydration of an alkene involve a carbocation?

Accepted Answer

Yes. Protonation of the double bond produces a carbocation intermediate, after which water attacks the positively charged carbon. Loss of a proton from the resulting oxonium ion gives the alcohol and regenerates the acid catalyst.

Acid-catalysed Hydration of Alkenes: Putting Water on the Right Carbon

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