Identify the wrong statement about alkenes.

Cis isomers have lower boiling points than trans isomers

The correct answer is option 3. Cis isomers have lower boiling points than trans isomers.

Let us delve into the details of why cis isomers of alkenes generally have higher boiling points than trans isomers. Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C). The boiling points of alkenes depend largely on their molecular structure, which includes the arrangement of atoms around the double bond.

Cis and Trans Isomers

Cis Isomers:

In cis isomers, the substituent groups attached to the carbon atoms of the double bond are on the same side of the molecule. This results in a more asymmetric or less symmetrical shape of the molecule. Examples include cis-but-2-ene where both methyl groups are on the same side of the double bond.

Trans Isomers:

In trans isomers, the substituent groups are on opposite sides of the molecule, across the double bond. This configuration leads to a more symmetrical shape of the molecule. Example includes trans-but-2-ene where the methyl groups are on opposite sides of the double bond.

Boiling Points Comparison

The boiling points of cis and trans isomers of alkenes differ due to the following reasons:

Intermolecular Forces:

Cis isomers tend to have higher boiling points compared to trans isomers. This is because cis isomers are less symmetrical, leading to stronger intermolecular forces such as dipole-dipole interactions or van der Waals forces. The asymmetry of cis isomers allows for closer packing in the liquid phase, which requires more energy to overcome during boiling, hence higher boiling points.

Symmetry and Packing Efficiency:

Trans isomers are more symmetrical, resulting in less efficient packing in the liquid phase. This reduces the strength of intermolecular forces and lowers the boiling points compared to their cis counterparts. Example: But-2-ene Isomers

Cis-But-2-ene: Has two methyl groups on the same side of the double bond. The cis isomer is less symmetrical and has stronger dipole-dipole interactions or van der Waals forces. Consequently, it has a higher boiling point than trans-but-2-ene.

Trans-But-2-ene: Has two methyl groups on opposite sides of the double bond. The trans isomer is more symmetrical and has weaker intermolecular forces. It generally has a lower boiling point compared to cis-but-2-ene.

Conclusion:

In summary, cis isomers of alkenes typically have higher boiling points than their trans isomers. This is primarily due to their less symmetrical structure, which results in stronger intermolecular forces and more efficient packing in the liquid phase. Therefore, statement (3) that cis isomers have lower boiling points than trans isomers is incorrect.