Textbook Question
How many stereogenic double bonds are in octa-1,3,5-triene? How many stereocenters are there? Draw and name the four stereoisomers of octa-1,3,5-triene.
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Ch. 7 - Structure and Synthesis of Alkenes; Elimination
Wade9th EditionOrganic ChemistryISBN: 9780135213728
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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
Chapter 7, Problem 9b
Draw the six stereoisomers of octa-2,4,6-triene. Explain why there are only six stereoisomers, rather than the eight we might expect for a compound with three stereogenic double bonds.
Verified step by step guidance1
Step 1: Understand the structure of octa-2,4,6-triene. It is a linear hydrocarbon with three conjugated double bonds located at positions 2, 4, and 6. Each double bond can exhibit cis (Z) or trans (E) stereochemistry, which contributes to the stereoisomer count.
Step 2: Recall that for a molecule with three stereogenic double bonds, we might initially expect 2^3 = 8 stereoisomers (since each double bond can independently be cis or trans). However, conjugation and molecular symmetry reduce the number of stereoisomers.
Step 3: Consider the conjugation of the double bonds. The π-electron system in conjugated double bonds imposes restrictions on the spatial arrangement of the molecule, limiting the possible combinations of cis and trans configurations.
Step 4: Analyze the symmetry of the molecule. Octa-2,4,6-triene has a symmetrical structure, which means some stereoisomers are identical due to reflection symmetry. This further reduces the number of unique stereoisomers.
Step 5: Draw the six stereoisomers systematically. Assign cis (Z) or trans (E) configurations to each double bond while ensuring that you account for conjugation and symmetry. Label each stereoisomer clearly and verify that no duplicates are included in your set of six.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Stereoisomerism
Stereoisomerism refers to the phenomenon where compounds have the same molecular formula and connectivity of atoms but differ in the spatial arrangement of their atoms. In the case of octa-2,4,6-triene, the presence of double bonds introduces geometric isomerism, leading to different configurations (cis/trans) around each double bond. Understanding stereoisomerism is crucial for determining the number of unique isomers a compound can have.
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Determining when molecules are stereoisomers.
Stereogenic Centers
Stereogenic centers are atoms in a molecule that can lead to stereoisomerism when they are bonded to different substituents. In octa-2,4,6-triene, the double bonds can act as stereogenic centers, but not all double bonds contribute to the total count of stereoisomers due to symmetry and the presence of equivalent configurations. Recognizing which parts of a molecule are stereogenic is essential for calculating the total number of stereoisomers.
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Symmetry and Equivalent Configurations
Symmetry in a molecule can reduce the number of unique stereoisomers by creating equivalent configurations that do not result in distinct isomers. In octa-2,4,6-triene, the arrangement of double bonds and their substituents leads to certain configurations being indistinguishable from one another. This concept explains why, despite having three stereogenic double bonds, only six unique stereoisomers exist instead of the expected eight.
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Related Practice
Textbook Question
For each pair of compounds, predict the one with a higher boiling point. Which compounds have zero dipole moments?
b. cis- or trans-2,3-dichlorobut-2-ene
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Textbook Question
Some of the following examples can show geometric isomerism, and some cannot. For the ones that can, draw all the geometric isomers, and assign complete names using the E-Z system.
(i)
Textbook Question
Teflon-coated frying pans routinely endure temperatures that would cause polyethylene or polypropylene to oxidize and decompose. Decomposition of polyethylene is initiated by free-radical abstraction of a hydrogen atom by O2. Bond-dissociation energies of C—H bonds are about 400 kJ/mol, and C—F bonds are about 460 kJ/mol. The BDE of the H—OO bond is about 192 kJ/mol, and the F—OO bond is about 63 kJ/mol. Show why Teflon (Figure 7-5) is much more resistant to oxidation than polyethylene is.
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Textbook Question
For each pair of compounds, predict the one with a higher boiling point. Which compounds have zero dipole moments?
a. cis-1,2-dichloroethene or cis-1,2-dibromoethene
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Textbook Question
Some of the following examples can show geometric isomerism, and some cannot. For the ones that can, draw all the geometric isomers, and assign complete names using the E-Z system.
(g)
(h)