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Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet SpectroscopyProblem 1b
Chapter 15, Problem 1b

Rank each group of compounds in order of increasing heat of hydrogenation.
(b)

Verified step by step guidance
1
Identify the compounds in the group and note the number of double bonds in each structure. The heat of hydrogenation is directly related to the number of double bonds present, as each double bond contributes to the overall energy released during hydrogenation.
Consider the degree of conjugation in the compounds. Conjugated double bonds (alternating single and double bonds) are more stable than isolated double bonds, and thus, compounds with conjugated systems will have a lower heat of hydrogenation compared to those with isolated double bonds.
Analyze the ring strain in the compounds. Smaller rings (e.g., cyclopentene) have higher ring strain compared to larger rings (e.g., cyclohexene). Higher ring strain increases the heat of hydrogenation.
Rank the compounds based on the number of double bonds, degree of conjugation, and ring strain. Compounds with fewer double bonds, more conjugation, and less ring strain will have a lower heat of hydrogenation.
For the given group of compounds, arrange them in order of increasing heat of hydrogenation by comparing their structural features as described in the previous steps.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Heat of Hydrogenation

The heat of hydrogenation is the amount of energy released when an alkene or alkyne is hydrogenated to form an alkane. It is a measure of the stability of the unsaturated compound; more stable compounds release less heat upon hydrogenation. Thus, compounds with higher degrees of substitution or resonance stabilization will have lower heats of hydrogenation compared to less stable, more strained structures.
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The definition of hydrogenation.

Stability of Alkenes

The stability of alkenes is influenced by the degree of substitution on the double bond. Tetrasubstituted alkenes are more stable than trisubstituted, which are more stable than disubstituted, and so on. This stability arises from hyperconjugation and the inductive effect of alkyl groups, which help to stabilize the double bond, leading to lower heats of hydrogenation.
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Understanding trends of alkene stability.

Resonance Effects

Resonance effects occur when a molecule can be represented by two or more valid Lewis structures, leading to delocalization of electrons. In the context of the compounds shown, resonance can stabilize certain structures, making them less reactive and thus lowering their heat of hydrogenation. Compounds with significant resonance stabilization will typically have lower heats of hydrogenation compared to those without such stabilization.
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Related Practice
Textbook Question

The central carbon atom of an allene is a member of two double bonds, and it has an interesting orbital arrangement that holds the two ends of the molecule at right angles to each other.

a. Draw an orbital diagram of allene, showing why the two ends are perpendicular.

b. Draw the two enantiomers of penta-2,3-diene. A model may be helpful.

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Textbook Question

Draw another resonance form for each of the substituted allylic cations shown in the preceding figure, showing how the positive charge is shared by another carbon atom. In each case, state whether your second resonance form is a more important or less important resonance contributor than the first structure. (Which structure places the positive charge on the more-substituted carbon atom?)

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Textbook Question

In a strongly acidic solution, cyclohexa-1,4-diene tautomerizes to cyclohexa-1,3-diene. Propose a mechanism for this rearrangement, and explain why it is energetically favorable.

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Textbook Question

Rank each group of compounds in order of increasing heat of hydrogenation.

(a) hexa-1,2-diene; hexa-1,3,5-triene; hexa-1,3-diene; hexa-1,4-diene; hexa-1,5-diene; hexa-2,4-diene

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