Question

Show the main products of the reactions of \((E, E)-2,4\)-hexadiene under the following conditions. There may be more than one product in some cases. (a) \(\mathrm{HCl}\), cold, short time (b) one equivalent \(\mathrm{H}_{2} / \mathrm{Pd}\) (c) one equivalent \(\mathrm{Cl}_{2} / \mathrm{CCl}_{4}\) (d) \(\Delta, \mathrm{H}_{3} \mathrm{COOC}-\mathrm{C} \equiv \mathrm{C}-\mathrm{COOCH}_{3}\)

Short answer

(a) 3-chloro-2-hexene (b) (E)-2-hexene (c) 3,4-dichlorohex-1-ene and 1,4-dichlorohex-2-ene (d) Dimethyl (4E, 8aE)-4,7,8,8a-tetrahydronaphthalene-1,2-dicarboxylate

Step-by-step solution

Reaction with HCl, cold, short time

The reaction of \(E, E\)-2,4-hexadiene with HCl under cold and short time conditions results in the Markovnikov addition of HCl. The regioselectivity of this reaction leads to the formation of 3-chloro-2-hexene as the major product.

Hydrogenation with one equivalent of \(H_{2} / Pd\)

The hydrogenation of \(E, E\)-2,4-hexadiene with one equivalent of hydrogen in the presence of a palladium catalyst leads to the selective hydrogenation of one of the double bonds. This reaction forms (E)-2-hexene as the major product.

Halogenation with one equivalent of \(Cl_{2} / CCl_{4}\)

The reaction of \(E, E\)-2,4-hexadiene with one equivalent of \(Cl_{2}\) in \(CCl_{4}\) forms a mixture of 3,4-dichlorohex-1-ene and 1,4-dichlorohex-2-ene due to addition of chlorine across the double bonds.

Diels-Alder reaction with \Delta, H_{3}COOC-C \equiv C-COOCH_{3}\

The Diels-Alder reaction involves the addition of a diene (here \(E, E\)-2,4-hexadiene) and a dienophile (here dimethyl acetylenedicarboxylate) under heat \Delta). This cycloaddition results in the formation of a six-membered ring product, specifically dimethyl (4E, 8aE)-4,7,8,8a-tetrahydronaphthalene-1,2-dicarboxylate.

Key concepts

Markovnikov addition

Markovnikov addition is a fundamental concept in organic chemistry describing the addition of protic acids (like HCl) to alkenes. This rule states that during the reaction, the hydrogen atom (from the acid) is added to the carbon atom that has the greater number of hydrogen atoms already attached to it.
In our example involving (E, E)-2,4-hexadiene with HCl under cold and short time conditions, we see this principle at work. Here’s how it operates:
- The double bonds in the hexadiene offer sites for addition reactions.
- The HCl breaks apart into H⁺ and Cl⁻.
- According to Markovnikov's rule, the H⁺ attaches to the carbon with more hydrogen atoms, leading to the most stable carbocation intermediate.
- Finally, the Cl⁻ bonds to the carbocation, resulting in the major product being 3-chloro-2-hexene.
This regioselective behavior ensures that the addition occurs in such a way that the more substituted, and thus more stable, carbocation intermediate is formed.

Hydrogenation

Hydrogenation is another crucial reaction in organic chemistry where hydrogen (H₂) is added to unsaturated compounds, typically alkenes or alkynes, in the presence of a catalyst like palladium (Pd). This process generally converts double bonds to single bonds (alkenes to alkanes).
In the case of (E, E)-2,4-hexadiene with one equivalent of hydrogen and a Pd catalyst:
- The diene contains two double bonds.
- The hydrogenation selectively reduces one of these double bonds since only one equivalent of H₂ is used.
- The Pd catalyst facilitates this process by adsorbing the hydrogen gas and the diene onto its surface, allowing the H atoms to add across one of the double bonds.
The product of this selective hydrogenation is (E)-2-hexene. This specific outcome highlights the efficiency of catalytic hydrogenation in controlling the number of double bonds reduced.

Halogenation

Halogenation involves the addition of halogen atoms (like chlorine, Cl₂) to alkenes or alkynes. It is an essential reaction for the synthesis of halogenated organic compounds.
In the example of (E, E)-2,4-hexadiene with one equivalent of Cl₂ in CCl₄:
- The double bonds in the diene react with chlorine.
- Each chlorine molecule (Cl₂) breaks into two Cl atoms, which then add across the double bonds.
- Since only one equivalent of Cl₂ is used, it leads to the formation of multiple products:

• 3,4-dichlorohex-1-ene
• 1,4-dichlorohex-2-ene

This reaction demonstrates the complexity and product diversity in halogenation reactions of dienes, where different addition products can form due to multiple possible reaction sites.

Diels-Alder reaction

The Diels-Alder reaction is a powerful synthetic tool in organic chemistry, facilitating the creation of six-membered rings through cycloaddition. It involves a conjugated diene reacting with a dienophile (an electron-poor alkene or alkyne).
For (E, E)-2,4-hexadiene reacting with dimethyl acetylenedicarboxylate under heat (∆):
- The electron-rich diene (2,4-hexadiene) reacts with the electron-poor dienophile (dimethyl acetylenedicarboxylate).
- The reaction leads to the formation of a new six-membered ring structure.
- The major product is dimethyl (4E, 8aE)-4,7,8,8a-tetrahydronaphthalene-1,2-dicarboxylate.
This cycloaddition product shows the efficacy of the Diels-Alder reaction in synthesizing complex cyclic structures from simpler organic molecules.