The thermal cyclisation of trans, trans-2, 4-hexadiene gives:

Question

Accepted Answer

The thermal cyclisation of trans, trans-2, 4-hexadiene results in the formation of only trans-3, 4-dimethyl cyclobutene. Let's explore why Option 1 is the correct answer and why the other options are incorrect.

### Explanation for Option 1:

1. **Thermal Cyclisation Mechanism**: The thermal cyclisation of dienes like trans, trans-2, 4-hexadiene typically proceeds through a concerted mechanism, often involving a [2+2] cycloaddition. This type of reaction is symmetry-allowed under thermal conditions for trans, trans-configured dienes.

2. **Stereochemistry Consideration**: In the case of trans, trans-2, 4-hexadiene, the trans configuration of the double bonds leads to the formation of a cyclobutene ring where the substituents (methyl groups) are positioned in a trans configuration relative to each other. This is due to the spatial arrangement of the substituents during the cyclisation process.

3. **Product Formation**: The specific stereochemistry of the starting material dictates that the only feasible product is trans-3, 4-dimethyl cyclobutene. The trans configuration of the starting diene ensures that the resulting cyclobutene maintains this trans relationship between the methyl groups.

### Explanation for Why Other Options are Incorrect:

- **Option 2 (Only cis-3, 4-dimethyl cyclobutene)**: This option is incorrect because the starting material, trans, trans-2, 4-hexadiene, does not favor the formation of a cis product. The spatial arrangement of the substituents in the starting material leads to a trans product, not a cis one.

- **Option 3 (Both the above products in equal amount)**: This option is incorrect because the reaction conditions and the stereochemistry of the starting material do not support the formation of both cis and trans products. The reaction is stereospecific, leading exclusively to the trans product.

- **Option 4 (Cyclohexene)**: This option is incorrect because the thermal cyclisation of trans, trans-2, 4-hexadiene does not involve the formation of a six-membered ring. The reaction mechanism and the nature of the starting material favor the formation of a four-membered cyclobutene ring, not a six-membered cyclohexene.

In conclusion, the thermal cyclisation of trans, trans-2, 4-hexadiene specifically leads to the formation of only trans-3, 4-dimethyl cyclobutene, making Option 1 the correct answer.