Textbook Question
Rank each group of compounds in order of increasing heat of hydrogenation.
(b)
1542
views
Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy
Wade9th EditionOrganic ChemistryISBN: 9780135213728
Not the one you use?Change textbookSelect a different textbook
Table of contents
- Ch.1 - Structure and Bonding107
- Ch. 2 - Acids and Bases; Functional Groups115
- Ch.3 - Structure and Stereochemistry of Alkanes100
- Ch.4 - The Study of Chemical Reactions99
- Ch.5 - Stereochemistry93
- Ch.6 - Alkyl Halides; Nucleophilic Substitution118
- Ch. 7 - Structure and Synthesis of Alkenes; Elimination158
- Ch.8 - Reactions of Alkenes171
- Ch.9 - Alkynes91
- Ch.10 - Structure and Synthesis of Alcohols143
- Ch.11 - Reactions of Alcohols104
- Ch. 12 - Infrared Spectroscopy and Mass Spectrometry22
- Ch. 13 - Nuclear Magnetic Resonance Spectroscopy45
- Ch. 14 - Ethers, Epoxides, and Thioethers72
- Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy89
- Ch. 16 - Aromatic Compounds74
- Ch. 17 - Reactions of Aromatic Compounds126
- Ch. 18 - Ketones and Aldehydes193
- Ch. 19 - Amines129
- Ch. 20 - Carboxylic Acids77
- Ch. 21 - Carboxylic Acid Derivatives141
- Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds175
- Ch. 23 - Carbohydrates and Nucleic Acids93
- Ch. 24 - Amino Acids, Peptides, and Proteins54
All textbooks
Wade 9th EditionCh. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet SpectroscopyProblem 1a
Wade 9th EditionCh. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet SpectroscopyProblem 1aChapter 15, Problem 1a
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
Verified step by step guidance1
Step 1: Understand the concept of heat of hydrogenation. Heat of hydrogenation is the energy released when a molecule undergoes hydrogenation (addition of H₂ across double bonds). Molecules with more stable double bonds (due to conjugation or resonance) will have lower heats of hydrogenation.
Step 2: Analyze the structures provided. The compounds are hexa-1,2-diene, hexa-1,3,5-triene, hexa-1,3-diene, hexa-1,4-diene, hexa-1,5-diene, and hexa-2,4-diene. Note the number and arrangement of double bonds in each structure.
Step 3: Identify conjugation in the molecules. Conjugated double bonds (alternating single and double bonds) are more stable due to delocalization of π-electrons. For example, hexa-1,3,5-triene has fully conjugated double bonds, making it the most stable and thus having the lowest heat of hydrogenation.
Step 4: Rank the compounds based on stability. Molecules with isolated double bonds (e.g., hexa-1,2-diene) are less stable and will have higher heats of hydrogenation. Partially conjugated systems (e.g., hexa-1,3-diene, hexa-1,4-diene, hexa-1,5-diene, hexa-2,4-diene) will have intermediate heats of hydrogenation.
Step 5: Arrange the compounds in order of increasing heat of hydrogenation: hexa-1,3,5-triene (lowest heat of hydrogenation due to full conjugation), followed by partially conjugated systems (hexa-1,3-diene, hexa-1,4-diene, hexa-1,5-diene, hexa-2,4-diene), and finally hexa-1,2-diene (highest heat of hydrogenation due to isolated double bonds).
Verified video answer for a similar problem:
This video solution was recommended by our tutors as helpful for the problem above.
Video duration:
9mWas this helpful?
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 is converted to an alkane through the addition of hydrogen. This value is indicative of the stability of the alkene; more stable alkenes release less heat upon hydrogenation. Thus, the heat of hydrogenation can be used to compare the relative stabilities of different alkenes, with less heat indicating greater stability.
Recommended video:
Guided course
00:48
The definition of hydrogenation.
Alkene Stability and Substitution
Alkene stability is influenced by the degree of substitution at the double bond. More substituted alkenes (those with more alkyl groups attached to the carbon atoms of the double bond) are generally more stable due to hyperconjugation and the inductive effect. This stability affects the heat of hydrogenation, where more substituted alkenes will have a lower heat of hydrogenation compared to less substituted ones.
Recommended video:
Guided course
04:28Understanding trends of alkene stability.
Conjugation and Resonance
Conjugation occurs when double bonds are separated by a single bond, allowing for delocalization of π electrons across the molecule. This delocalization stabilizes the alkene, leading to lower heat of hydrogenation. Resonance structures can also contribute to the stability of certain alkenes, making them less reactive and thus requiring less energy to hydrogenate.
Recommended video:
Guided course
05:29Definition of Conjugation
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.
2373
views
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.
1601
views