Q.Draw the cis and trans structures of hex-2-ene. Which isomer will have higher b.p. and why?

Concept: Geometrical Isomerism and Dipole Moment

Hex-2-ene has a double bond at C-2, creating two geometrical isomers based on the arrangement of substituents around the rigid C=C bond.

Structures:

cis-Hex-2-ene: Both alkyl chains (methyl and propyl) on the same side of the double bond.

trans-Hex-2-ene: Alkyl chains on opposite sides.

Boiling Point Comparison: …

Understanding Geometric Isomerism in Hex-2-ene

Hex-2-ene has a carbon-carbon double bond at the second position, which creates a rigid planar geometry. Unlike single bonds that rotate freely, the $\pi$-bond in a double bond locks the molecule into a fixed arrangement. When two different groups are attached to each carbon of the double bond, we get geometric isomers—molecules with the same connectivity but different spatial arrangements.

The two isomers differ in whether the larger substituents (the alkyl chains) lie on the same side or opposite sides of the double bond.

Drawing the Structures

1. cis-Hex-2-ene

In the cis-isomer, the two alkyl groups (the methyl $\ce{-CH3}$ and the propyl $\ce{-CH2CH2CH3}$) are on the same side of the double bond:

The molecule adopts a bent or "kinked" shape because the bulky alkyl chains are forced to the same side.

2. trans-Hex-2-ene

In the trans-isomer, the alkyl groups are on opposite sides:

This arrangement is more linear and extended.

Comparing Boiling Points

3. Molecular polarity and shape

Although both isomers have the same molecular formula ($\ce{C6H12}$) and similar London dispersion forces, their shapes create different dipole moments.

In cis-hex-2-ene, the two C–C bonds on the same side create a net dipole moment. The electron density is unevenly distributed, making one side of the molecule slightly more electron-rich than the other. This permanent dipole allows for dipole-dipole interactions between molecules.

In trans-hex-2-ene, the molecule is more symmetrical. The dipole moments of the C–C bonds on opposite sides largely cancel each other out, resulting in a near-zero net dipole moment. The molecule is essentially non-polar.

4. Intermolecular forces

The cis-isomer experiences both London dispersion forces and dipole-dipole interactions. The trans-isomer relies almost entirely on London dispersion forces.

Dipole-dipole interactions are stronger than dispersion forces alone, so molecules of cis-hex-2-ene attract each other more strongly.

5. Effect on boiling point …