Provide the structures of the 1,2- and 1,4- addition products obtained from the following reaction.

Question

Provide the structures of the 1,2- and 1,4- addition products obtained from the following reaction. Make sure to label which is 1,2- and which is 1,4-, which is kinetic and which is the thermodynamic product.

Answer 1

When 2,3-dimethyl-1,3-butadiene undergoes an electrophilic addition reaction with HCl, two different products can be formed: 1,2-addition and 1,4-addition products. This is because the double bonds in the diene are electrophilic, and react with HCl to form a proton and a chloride ion. The reaction proceeds as follows:

1. The alkene's electrons in the pi orbital attack the H+ to form a carbocation intermediate. The carbocation intermediate is initially formed at the tertiary position, which is more stable than the secondary carbocation that would be formed in the case of the 1,4-addition product.
2. The stability of the carbocation intermediate determines the product distribution. The more stable the carbocation intermediate, the faster the reaction. In the case of the reaction of 2,3-dimethyl-1,3-butadiene with HCl, the more stable carbocation intermediate is formed at the tertiary position, making the 1,2-addition product the kinetic product.
3. The allylic resonance form stabilizes the carbocation intermediate and is responsible for the 1,4-addition product. The less stable carbocation intermediate formed during the 1,4-addition reaction requires a higher activation energy, making it the thermodynamic product.
4. The kinetic product is the 1,2-addition product and forms via a more stable carbocation intermediate. It results in a less substituted alkene and is less stable than the thermodynamic product.
5. The thermodynamic product is the 1,4-addition product and forms via a less stable carbocation intermediate (which is at the allylically stabilized primary/end carbon). It results in a more substituted alkene and is, therefore the more stable product. At higher temperatures, where the reaction is thermodynamically favored, the thermodynamic product is formed as it has the lower overall free energy and is the more stable alkene.

It is important to note that the reaction temperature can influence the product distribution, with lower temperatures favoring the kinetic product and higher temperatures favoring the thermodynamic product.