Question

For the following reaction, show the product of one mole of reagent adding across the triple bond, and then show the product of a second mole of reagent.

Answer 1

Concepts and reason

Alkenes and alkynes contain unsaturated double bonds which undergo addition reactions. In addition reactions, the weaker pi bond converts into two new stronger sigma bonds. Addition reactions are regioselective, based on the reagent used.

Fundamentals

Alkenes and alkynes are more reactive than alkanes; hence, they usually undergo addition reactions in which the pi bond is broken and two new groups are added across the double bond to give a saturated compound.

An addition reaction goes via a two-step process:

1.The electrophile attacks the double bond, forming a carbocation intermediate.

2.In the second step, the counter ion attacks the carbocation intermediate and forms a saturated product.

The addition of an electrophile to a double bond is regioselective, provided the two atoms that add to the double bond are different.

The general mechanism is shown below:

An addition reaction can follow one of these two rules when the two atoms that add to the double bond are different:

1.Markovnikov’s Rule:

The electrophile adds to the less hindered carbon atom, and the nucleophile adds to the more hindered carbon atom of the double bond.

2.Anti-Markovnikov’s Rule:

The electrophile adds to the more hindered carbon atom, and the nucleophile adds to the less hindered carbon atom of the double bond.

Markovnikov’s addition gives a stable carbocation intermediate over anti-Markovnikov’s addition. Hence, Markovnikov’s addition gives a more stable and major product, and anti-Markovnikov’s addition gives a less stable and minor product.

The rearrangement of the carbocation intermediate may take place in the case where a stable carbocation formation is possible on rearrangement.

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