Question

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

Short answer

Cis-hex-2-ene likely has a higher boiling point due to stronger Van der Waals forces.

Step-by-step solution

Understand the Chemical Structure

Hex-2-ene is an alkene with a six-carbon chain where the double bond is located between the second and third carbon atoms. Isomers occur due to the different spatial arrangements of atoms around the double bond.

Identify Cis and Trans Configurations

In the cis configuration, the highest priority groups (e.g., larger alkyl groups or atoms with higher atomic numbers) on each carbon involved in the double bond are on the same side. In the trans configuration, these groups are on opposite sides of the double bond.

Draw the Cis Structure of Hex-2-ene

For cis-hex-2-ene, the `CH3` and `CH2-CH2-CH3` groups attached to the carbon atoms of the double bond are on the same side. The structure is: \( \text{CH}_3-\text{CH} = \text{CH}-\text{CH}_2-\text{CH}_2-\text{CH}_3 \).

Draw the Trans Structure of Hex-2-ene

For trans-hex-2-ene, the `CH3` and `CH2-CH2-CH3` groups attached to the carbon atoms of the double bond are on opposite sides. The structure is: \( \text{CH}_3-\text{CH} = \text{CH}-\text{CH}_2-\text{CH}_2-\text{CH}_3 \), but with the groups on opposite positions across the double bond.

Analyze Boiling Points

The boiling point of a compound is influenced by the strength of intermolecular forces. Cis isomers typically have stronger Van der Waals forces due to their shape which allows the molecules to pack closer together, potentially resulting in higher boiling points compared to trans isomers.

Key concepts

Cis-Trans Isomerism

Cis-trans isomerism, also known as geometric isomerism, is a fascinating aspect of chemistry. It arises due to the different spatial arrangements of atoms around a double bond in alkenes. The rigid nature of the carbon-carbon double bond prevents rotation, leading to two unique isomers:

• Cis Isomer: Here, identical or priority groups are on the same side of the double bond.
• Trans Isomer: Here, the same groups are on opposite sides of the double bond.

In the case of hex-2-ene, this distinct difference in spatial arrangement between cis and trans isomers influences their physical properties, such as boiling points.

Alkenes

Alkenes are a class of hydrocarbons that contain at least one carbon-carbon double bond. This bond is key to their reactivity and physical characteristics. The double bond is rigid and is responsible for the formation of different isomers, such as cis and trans.

• General Formula: Alkenes generally have the formula \( C_nH_{2n} \), indicating unsaturation.
• Properties: Alkenes are known for their reactivity, allowing them to participate in various chemical reactions, such as polymerization and addition reactions.

In hex-2-ene, the double bond between the second and third carbon atoms allows for the existence of either cis or trans conformations, impacting the compound's boiling point and other properties.

Boiling Points Comparison

The boiling point is a critical property of substances, influenced by molecular structure and intermolecular forces. When discussing cis and trans isomers like those of hex-2-ene, these differences become more pronounced. Cis isomers often have higher boiling points than trans isomers due to the following:

• Closer Packing: Cis molecules pack more tightly, enhancing Van der Waals forces.
• Polarity: The cis arrangement may result in a more polar molecule, amplifying dipole-dipole interactions.

In contrast, trans isomers have more linear structures, reducing these forces and generally leading to lower boiling points. Therefore, in most cases, cis-hex-2-ene will have a higher boiling point compared to its trans counterpart.