The heat of combustion of decahydronaphthalene (C10 H18) is -6286 kJ / mol. The heat of combust

The heat of combustion of decahydronaphthalene (C10 H18) is...

The heat of combustion of decahydronaphthalene $\left(\mathrm{C}_{10} \mathrm{H}_{18}\right)$ is $-6286 \mathrm{~kJ} / \mathrm{mol}$. The heat of combustion of naphthalene $\left(\mathrm{C}_{10} \mathrm{H}_{8}\right)$ is $-5157 \mathrm{~kJ} / \mathrm{mol}$. (In both cases $\mathrm{CO}_{2}(g)$ and $\mathrm{H}_{2} \mathrm{O}(l)$ are the products.) Using these data and data in Appendix $C$, calculate the heat of hydrogenation of naphthalene. Does this value provide any evidence for aromatic character in naphthalene?

Key Concepts

Thermochemical Cycles and Hess's Law

Thermochemical cycles, built on Hess's Law, allow chemists to calculate unknown thermodynamic quantities by combining known heats of reaction. In the analysis of hydrogenation reactions, these cycles help to deduce the stabilization energy of aromatic systems by comparing the energy changes of a compound and its saturated analogue. This methodology is key in determining subtle differences in energy that reflect aromatic stabilization effects.

Aromatic Stabilization

Aromatic stabilization is the extra stability observed in cyclic, planar, conjugated systems that follow Hückel's rule. This stabilization results from the delocalization of ?-electrons over the ring, which lowers the overall energy of the molecule compared to its non?aromatic counterparts. Evidence for aromatic character is often inferred by comparing experimental data, such as heats of hydrogenation, to theoretical values expected for non-aromatic analogues.

Heat of Combustion

This concept refers to the energy released when a substance is completely burned in oxygen to form products such as CO? and H?O. In a broader context, heats of combustion are used to compare the relative stabilities of different compounds by analyzing the energy content of their molecular structures. It plays a crucial role in thermochemistry and is a central tool for evaluating energy differences in chemical reactions.

Heat of Hydrogenation

Heat of hydrogenation is the energy released when unsaturated compounds, such as alkenes or aromatic systems, react with hydrogen to become saturated. It provides a measure of the intrinsic stability of unsaturated bonds—the lower the heat released relative to a typical unsaturated system, the more stable the original system is. This concept is pivotal in studying delocalization and conjugation effects, especially in aromatic compounds where the heat of hydrogenation is less exothermic due to aromatic stabilization.