Question

Mono addition of HCl to unsymmetrical dienes generally produces four products - two 1,2-addition products and two 1,4-addition products. In some cases, such as the following, a 1,2-addition product and a 1,4-addition product may have the same structure, and thus only three distinct products are formed. Draw the reactant diene that forms the following three products, and identify the addition route that forms each product.

Answer 1

Concepts and reason

• Unsymmetrical dienes are alkenes (unsaturated) compounds that contain two carbon-carbon double bonds which undergo 1, 2- and 1, 4-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

The dienes are more reactive than alkanes and hence usually undergo addition reaction in which the pi bond is broken and two new groups are added across the carbon-carbon double bond to give again unsaturated compound.

An addition reaction goes via a two-step process:

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

2.The counter ion attacks the carbocation intermediate and forms a saturated product.

The addition of an electrophile to a double bond is regioselective which is provided by the two atoms/groups when added to a double bond are different.

A regioselective addition reaction can follow one of the two rules when the two atoms/groups that add to the double bond are different:

1.Markovnikov’s Rule:

The electrophile adds to more substituted hydrogen atoms while the nucleophile adds to less substituted hydrogen atoms.

2.Anti-Markovnikov’s Rule:

The electrophile adds to less substituted hydrogen atoms while the nucleophile adds to more substituted hydrogen atoms.

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

Ans:

The structure of the diene is