Textbook Question
Rank the following radicals in decreasing order of stability. Classify each as primary, secondary, or tertiary.
a. The isopentyl radical,
b. The 3-methyl-2-butyl radical,
c. The 2-methyl-2-butyl radical,
d.
1388
views
Ch.4 - The Study of Chemical Reactions
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
Chapter 4, Problem 25b,c
In the presence of a small amount of bromine, cyclohexene undergoes the following light-promoted reaction:
b. Draw the structure of the rate-limiting transition state.
c. Use Hammond's postulate to predict which intermediate most closely resembles this transition state.
Verified step by step guidance1
Step 1: Understand the reaction mechanism. The reaction involves the addition of bromine to cyclohexene under light (hv), which promotes a radical chain reaction. The first step is the homolytic cleavage of Br2 to form two bromine radicals (Br•). These radicals initiate the reaction by abstracting a hydrogen atom from cyclohexene, forming a cyclohexenyl radical.
Step 2: Identify the rate-limiting step. In radical chain reactions, the rate-limiting step is typically the step that forms the most stable intermediate. Here, the rate-limiting step is the formation of the cyclohexenyl radical, which occurs when a bromine radical abstracts a hydrogen atom from cyclohexene.
Step 3: Draw the structure of the rate-limiting transition state. The transition state for the hydrogen abstraction step will involve partial bond breaking of the C-H bond in cyclohexene and partial bond formation between the hydrogen atom and the bromine radical. Represent this structure with dashed lines to indicate the bonds in the process of breaking and forming.
Step 4: Apply Hammond's postulate. Hammond's postulate states that the structure of the transition state resembles the structure of the species (reactants or products) to which it is closer in energy. Since the formation of the cyclohexenyl radical is endothermic, the transition state will more closely resemble the cyclohexenyl radical intermediate than the starting materials.
Step 5: Conclude the analysis. Based on Hammond's postulate, the transition state most closely resembles the cyclohexenyl radical intermediate. This is because the energy of the transition state is closer to the energy of the intermediate than to the energy of the reactants.
Verified video answer for a similar problem:
This video solution was recommended by our tutors as helpful for the problem above.
Video duration:
7mWas this helpful?
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Rate-Limiting Transition State
The rate-limiting transition state is the highest energy state along the reaction pathway, representing a critical point where bonds are partially broken and formed. It determines the overall rate of the reaction, as the energy required to reach this state is the largest barrier. Understanding this concept is essential for predicting reaction mechanisms and the stability of intermediates.
Recommended video:
Guided course
10:14
Limitations
Hammond's Postulate
Hammond's postulate states that the structure of the transition state resembles the structure of the closest stable species (intermediate) in energy. If the transition state is closer in energy to the reactants, it will resemble them more, and if it is closer to the products, it will resemble the products. This concept helps in predicting the characteristics of the transition state based on the stability of intermediates.
Recommended video:
Guided course
01:30Defining the Hammond Postulate.
Bromination of Alkenes
Bromination of alkenes involves the addition of bromine across the double bond, typically resulting in the formation of vicinal dibromides. In the case of cyclohexene, the reaction proceeds via a radical mechanism when light is present, leading to the formation of brominated products. Understanding the mechanism of bromination is crucial for predicting the products and the nature of the transition state.
Recommended video:
Guided course
00:54Mechanism of Allylic Bromination.
Related Practice
Textbook Question
Write an equation for the reaction of vitamin E with an oxidizing radical (RO•) to give ROH and a less reactive free radical.
1448
views
Textbook Question
Alkoxy radicals (R—O•) are generally more stable than alkyl (R•) radicals. Write an equation showing an alkyl free radical (from burning gasoline) abstracting a hydrogen atom from tert-butyl alcohol, (CH3)3COH. Explain why tert-butyl alcohol works as an antiknock additive for gasoline
1038
views
Textbook Question
Use the information in Table 4-2 to explain why toluene (PhCH3) has a very high octane rating of 111. Write an equation to show how toluene reacts with an alkyl free radical to give a relatively stable radical.
1374
views
Textbook Question
The triphenylmethyl cation is so stable that some of its salts can be stored for months. Explain why this cation is so stable.
1587
views
Textbook Question
In the presence of a small amount of bromine, cyclohexene undergoes the following light-promoted reaction:
a. Propose a mechanism for this reaction.
4927
views