Question

Which cycloalkanc bas the lowest heat of combustion per \(\mathrm{CII}_{2}\) group? (1) cyclopropane (2) cyclobutane (3) cyclopentane (4) cyclohexane

Short answer

Cyclohexane has the lowest heat of combustion per \( \text{CH}_2 \) group.

Step-by-step solution

Understanding the Heat of Combustion

The heat of combustion refers to the amount of energy released when a compound is completely burned. For cycloalkanes, it is influenced by the ring strain. Lower ring strain results in a lower heat of combustion.

Analyzing Ring Strain in Cycloalkanes

Among cycloalkanes, cyclopropane and cyclobutane have significant ring strain due to their smaller ring sizes, making them less stable. Cyclopentane and cyclohexane have less ring strain, with cyclohexane having the least due to its near-perfect hexagonal shape.

Comparing Heat of Combustion Per \( \text{CH}_2 \) Group

Given cyclohexane has the least ring strain, it releases the least amount of energy per \( \text{CH}_2 \) group when burned, indicating a lower heat of combustion compared to the other cycloalkanes listed.

Conclusion

Based on the information about ring strain and stability, cyclohexane has the lowest heat of combustion per \( \text{CH}_2 \) group among the options given.

Key concepts

heat of combustion

The heat of combustion is the energy released when a compound is completely burned in the presence of oxygen. For cycloalkanes, this energy release is closely related to the structure and stability of the molecule. When we talk about the heat of combustion per \(\text{CH}_2\) group, we are referring to the amount of energy released for each methylene group in the cycloalkane structure.
Cyclohexane, with its lowest ring strain among the provided options, releases the least energy per methylene group. This makes it the cycloalkane with the lowest heat of combustion per \(\text{CH}_2\) group.

ring strain

Ring strain is the tension or stress in the molecular structure of cycloalkanes caused by angle strain, torsional strain, and steric strain. Smaller ring sizes like cyclopropane and cyclobutane experience significant ring strain. This is because their bond angles deviate largely from the ideal tetrahedral angle of 109.5 degrees.
Cyclopentane and cyclohexane have much lower ring strain. Cyclohexane, in particular, adopts a chair conformation that minimizes the strain, approximating the ideal bond angles and reducing both torsional and steric strain. This lower ring strain leads to greater stability and therefore a lower heat of combustion.

cycloalkane stability

Cycloalkane stability is closely linked to ring strain. Molecules strive for conformations that minimize internal stressors. Cyclopropane and cyclobutane, due to their high ring strain, are more reactive and less stable. In contrast, cyclopentane and cyclohexane are significantly more stable.
Cyclohexane, with its nearly perfect hexagonal ring and chair conformations, exhibits the highest stability among the provided cycloalkanes. This stability correlates to a lower heat of combustion since less energy is stored in the molecule's bonds.

organic chemistry

In organic chemistry, the study of hydrocarbons like cycloalkanes is fundamental. Understanding the concepts of combustion, ring strain, and stability are key to exploring the properties and reactivity of these compounds.
Cycloalkanes are saturated hydrocarbons with unique properties due to their ring structures. These properties influence their reactivity and the energy they release upon combustion. Through exercises like analyzing the heat of combustion of cycloalkanes, students can deepen their understanding of molecular stability and energy dynamics in organic molecules.

• Ring strain affects molecular stability.
• Lower ring strain results in a lower heat of combustion.
• More stable molecules release less energy when burned.