Cyclohexene and cyclohexanol are both organic compounds that share a similar molecular structure, but they differ significantly in their boiling points. Cyclohexene, a hydrocarbon, has a lower boiling point than cyclohexanol, an alcohol. This difference can be explained by the distinct types of intermolecular forces present in each compound. In this article, we will explore the factors that contribute to the difference in boiling points, focusing on the nature of the intermolecular forces at play.

Understanding Cyclohexene and Cyclohexanol

Cyclohexene

Cyclohexene (C6H10) is a simple alkene with a six-membered carbon ring. It has a C=C double bond, which is characteristic of alkenes. This double bond makes cyclohexene a reactive molecule, but it also influences its physical properties, including its boiling point. Cyclohexene is a non-polar molecule due to its symmetrical structure and the absence of highly electronegative atoms or functional groups.

Cyclohexanol

Cyclohexanol (C6H12O) is an alcohol, similar in structure to cyclohexene but with a hydroxyl group (-OH) attached to one of the carbon atoms in the ring. The hydroxyl group is highly polar and allows cyclohexanol to form hydrogen bonds with other molecules, significantly influencing its physical properties, including its boiling point.

Boiling Point and Intermolecular Forces

The boiling point of a substance is determined by the amount of energy required to break the intermolecular forces between molecules in the liquid state so that the molecules can transition into the gas phase. The nature and strength of these intermolecular forces play a significant role in determining the boiling point. In the case of cyclohexene and cyclohexanol, the primary reason for the difference in their boiling points lies in the differences in their intermolecular forces.

Cyclohexene: Weak Intermolecular Forces

Cyclohexene, being an alkene, primarily experiences London dispersion forces (also known as Van der Waals forces). These forces arise from temporary fluctuations in electron density that create instantaneous dipoles in molecules. These temporary dipoles induce dipoles in neighboring molecules, leading to weak attractions between them. While these forces are present in all molecules, including cyclohexene, they are relatively weak in comparison to other types of intermolecular forces.

The lack of polarity in cyclohexene means that the intermolecular attractions are minimal, and as a result, only a small amount of energy is required to separate the molecules. This leads to a lower boiling point. Cyclohexene has a boiling point of around 83°C.

Cyclohexanol: Stronger Intermolecular Forces

Cyclohexanol, on the other hand, contains a hydroxyl group (-OH), which is highly polar. The oxygen atom in the hydroxyl group is more electronegative than the carbon and hydrogen atoms, creating a polar bond. This polarity allows cyclohexanol molecules to engage in hydrogen bonding with other cyclohexanol molecules or with other hydrogen bond donors (e.g., water).

Hydrogen bonding occurs when the hydrogen atom, attached to an electronegative atom (such as oxygen), forms a weak bond with another electronegative atom. In cyclohexanol, the hydrogen attached to the oxygen can form hydrogen bonds with the oxygen atom of another molecule. These bonds are significantly stronger than London dispersion forces, as hydrogen bonds involve the electrostatic attraction between the hydrogen and oxygen atoms, which is stronger and more stable than the transient dipoles created in cyclohexene.

As a result, cyclohexanol molecules are held together more tightly, and it requires more energy to break these hydrogen bonds. Therefore, cyclohexanol has a higher boiling point than cyclohexene. Cyclohexanol boils at around 161°C, significantly higher than cyclohexene’s boiling point.

Comparison of Boiling Points

To summarize, the boiling point of a substance is influenced by the strength of the intermolecular forces that hold its molecules together. In the case of cyclohexene and cyclohexanol:

• Cyclohexene has weaker intermolecular forces (London dispersion forces), which results in a lower boiling point of 83°C.
• Cyclohexanol has stronger intermolecular forces due to hydrogen bonding, which leads to a higher boiling point of 161°C.

This stark difference in boiling points is a direct result of the hydrogen bonding present in cyclohexanol and the absence of such interactions in cyclohexene. Cyclohexanol’s ability to form hydrogen bonds creates a more cohesive liquid structure, requiring more energy to overcome these forces during the transition from liquid to gas.

Conclusion

The boiling point of a substance is primarily influenced by the nature of the intermolecular forces between its molecules. Cyclohexene, a non-polar alkene, experiences only London dispersion forces, which are weak and result in a relatively low boiling point. In contrast, cyclohexanol, an alcohol, contains a hydroxyl group capable of hydrogen bonding, which is a much stronger intermolecular force. This hydrogen bonding explains why cyclohexanol has a significantly higher boiling point than cyclohexene. Understanding the relationship between molecular structure and boiling point provides valuable insight into the physical properties of organic compounds and the nature of intermolecular forces.