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🥼organic chemistry review

Heats of Hydrogenation

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

Heats of hydrogenation refer to the amount of energy released or absorbed when a compound undergoes hydrogenation, the chemical reaction where hydrogen gas is added to a compound. This term is particularly relevant in the context of understanding the stability of alkenes and conjugated dienes.

5 Must Know Facts For Your Next Test

  1. The heat of hydrogenation is a measure of the enthalpy change (ΔH) that occurs when a compound is hydrogenated.
  2. Alkenes with more substituted double bonds have lower heats of hydrogenation, indicating greater stability.
  3. Conjugated dienes have lower heats of hydrogenation compared to isolated dienes, due to increased stabilization from delocalization of π-electrons.
  4. The heat of hydrogenation is directly related to the stability of the compound, with more stable compounds having lower heats of hydrogenation.
  5. The heat of hydrogenation can be used to predict the relative stability of different alkenes and conjugated dienes.

Review Questions

  • Explain how the heat of hydrogenation is related to the stability of alkenes.
    • The heat of hydrogenation is a measure of the enthalpy change that occurs when a compound is hydrogenated. For alkenes, more substituted double bonds have lower heats of hydrogenation, indicating greater stability. This is because the more substituted alkenes are more stable due to increased electron delocalization and steric effects. The lower the heat of hydrogenation, the more stable the alkene, as less energy is required to add hydrogen and saturate the double bond.
  • Describe how the heat of hydrogenation can be used to understand the stability of conjugated dienes based on molecular orbital theory.
    • According to molecular orbital theory, conjugated dienes are more stable than isolated dienes due to the delocalization of π-electrons across the entire system. This delocalization results in conjugated dienes having lower heats of hydrogenation compared to isolated dienes. The decreased heat of hydrogenation for conjugated dienes indicates that less energy is required to saturate the double bonds, reflecting their greater inherent stability. This stability arises from the increased electron delocalization and the ability to distribute the energy of the π-bonds more evenly across the molecule.
  • Evaluate how the heat of hydrogenation can be used to predict and compare the relative stability of different alkenes and conjugated dienes.
    • The heat of hydrogenation provides a quantitative measure of the stability of alkenes and conjugated dienes. By comparing the heats of hydrogenation for different compounds, one can predict and evaluate their relative stability. For alkenes, those with more substituted double bonds will have lower heats of hydrogenation and are therefore more stable. For conjugated dienes, the delocalization of π-electrons results in lower heats of hydrogenation compared to isolated dienes, indicating greater stability. This relationship between heat of hydrogenation and stability can be used to make informed predictions about the reactivity and behavior of these compounds in various chemical processes and reactions.