The ΔH^∘ of hydrogenation is the heat liberated when a...

The $\Delta H^{\circ}$ of hydrogenation is the heat liberated when a compound undergoes catalytic hydrogenation. Consider the $\Delta H^{\circ}$ values for hydrogenation of the following three alkenes: 3 -methyl-l-butene, $-126.8 \mathrm{~kJ} \mathrm{~mol}^{-1}$ $\left(-30.3\right.$ kcal $\left.\mathrm{mol}^{-1}\right) ; 2$ -methyl-1-butene, $-119.2 \mathrm{~kJ} \mathrm{~mol}^{-1}$ $\left(-28.5 \mathrm{kcal} \mathrm{mol}^{-1}\right) ;$ and 2 -methyl-2-butene, $-112.6 \mathrm{~kJ} \mathrm{~mol}^{-1}$ $\left(-26.9\right.$ kcal $\left.\mathrm{mol}^{-1}\right)$ (a) Draw an energy diagram in which the three alkanes are placed on the same energy seale along with 2-methylbutane. (b) Use these data to rank the three alkenes in order of stability, most stable first. Explain how you reached your conclusion. (c) By how much do the heats of formation of the three alkenes differ? Explain. (d) Explain why this stability order is expected.

The $\Delta H^{\circ}$ of hydrogenation is the heat liberated when a compound undergoes catalytic hydrogenation. Consider the $\Delta H^{\circ}$ values for hydrogenation of the following three alkenes: 3 -methyl-l-butene, $-126.8 \mathrm{~kJ} \mathrm{~mol}^{-1}$ $\left(-30.3\right.$ kcal $\left.\mathrm{mol}^{-1}\right) ; 2$ -methyl-1-butene, $-119.2 \mathrm{~kJ} \mathrm{~mol}^{-1}$
$\left(-28.5 \mathrm{kcal} \mathrm{mol}^{-1}\right) ;$ and 2 -methyl-2-butene, $-112.6 \mathrm{~kJ} \mathrm{~mol}^{-1}$
$\left(-26.9\right.$ kcal $\left.\mathrm{mol}^{-1}\right)$
(a) Draw an energy diagram in which the three alkanes are placed on the same energy seale along with 2-methylbutane.
(b) Use these data to rank the three alkenes in order of stability, most stable first. Explain how you reached your conclusion.
(c) By how much do the heats of formation of the three alkenes differ? Explain.
(d) Explain why this stability order is expected.

Key Concepts

Hydrogenation Reaction

A hydrogenation reaction involves the addition of hydrogen (H?) across a multiple bond, such as a carbon-carbon double bond, using a catalyst. It is an exothermic process where the loss of energy (heat) during the reaction is indicative of the stability of the original compound, with less energy released suggesting a more stable starting material.

Heat of Hydrogenation

The heat of hydrogenation is the amount of energy released when a compound, typically an alkene, is hydrogenated. This measure is used as an indirect indicator of alkene stability; lower (less negative) values imply that the alkene is more stable because it resides at a lower energy level compared to its less substituted, higher energy counterparts.

Alkene Stability

Alkene stability refers to the relative energy level of alkenes, which is often influenced by the number and nature of substituents attached to the double bond. More substituted alkenes are generally more stable due to hyperconjugation and inductive effects, and this increased stability is reflected in smaller heats of hydrogenation.

Energy Diagram Representation

An energy diagram visually represents the relative energies of reactants and products in a chemical reaction. In the context of hydrogenation, it illustrates the energy levels of different alkenes along a common scale, allowing for direct comparison of their stabilities based on the relative energy gaps to the saturated alkane products.

Heats of Formation

Heats of formation indicate the enthalpy change associated with forming a compound from its constituent elements. Differences in heats of hydrogenation among alkenes can be correlated to differences in their heats of formation, where a more stable alkene has a lower heat of formation, thus releasing less energy upon hydrogenation.

Substituent Effects in Alkenes

Substituent effects, including hyperconjugation and inductive stabilization, play a crucial role in determining alkene stability. The presence and position of alkyl groups on the double bond can stabilize the molecule by delocalizing charge and electrons, leading to lower energies and smaller heats of hydrogenation.

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