Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics

Problem 46c

Chapter 4, Problem 46c

Predict the major monohalogenation product(s) of the following reactions. Indicate whether you think the reaction will be selective and justify your position.
(c)

Verified step by step guidance

Step 1: Recognize the reaction type. This is a free radical halogenation reaction involving chlorine (Cl₂) and heat. Free radical halogenation typically proceeds via a radical mechanism and is less selective compared to bromination.

Step 2: Analyze the structure of the starting compound. The molecule is 2,2,3-trimethylbutane, which has primary, secondary, and tertiary hydrogens available for substitution.

Step 3: Determine the relative reactivity of the hydrogens. In free radical halogenation, tertiary hydrogens are the most reactive, followed by secondary hydrogens, and then primary hydrogens. This is due to the stability of the resulting radical intermediates.

Step 4: Identify the possible sites for monohalogenation. The tertiary hydrogen on the central carbon (C3) is the most likely site for substitution due to the formation of a highly stable tertiary radical. Secondary hydrogens on adjacent carbons (C2) may also be substituted, but less likely.

Step 5: Evaluate the selectivity of the reaction. Chlorination is not highly selective, so multiple products may form. However, the major product will likely result from substitution at the tertiary position due to the stability of the tertiary radical intermediate.

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Key Concepts

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

Monohalogenation

Monohalogenation refers to the substitution reaction where one halogen atom replaces a hydrogen atom in an organic compound. This process typically occurs in alkanes and can be initiated by heat or light, leading to the formation of haloalkanes. Understanding the mechanism of this reaction, including radical formation and stability, is crucial for predicting the products.

Selectivity in Halogenation

Selectivity in halogenation describes the preference of a halogen to react with certain hydrogen atoms over others in a molecule. Factors influencing selectivity include the stability of the resulting radical intermediates and the nature of the halogen used. For example, bromination is generally more selective than chlorination due to the relative stabilities of the radicals formed.

Radical Mechanism

The radical mechanism involves the formation of free radicals during the halogenation process. This mechanism typically consists of three steps: initiation, propagation, and termination. Understanding how radicals are generated and how they react with substrates is essential for predicting the major products of monohalogenation reactions.

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Related Practice

Textbook Question

The radical fluorination of 2-methyl propane resulted in a 14:86 ratio of products.

(a) On the basis of this ratio, calculate the relative reactivity of 1° and 3° C―H bonds in the radical fluorination.

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Textbook Question

For each of the following acid–base reactions, (i) predict which side of the reaction you expect to be favored. If a pK_a is not one of the ten common ones we learned in Chapter 4, it will be given to you.

(a)

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Textbook Question

The radical fluorination of 2-methyl propane resulted in a 14:86 ratio of products.

(b) From the relative reactivity, calculate the difference in energy between the transition states of the first propagation steps leading to a 1° and 3° radical.

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Textbook Question

The following table of strain energies is associated with a variety of 1,2-gauche interactions. Use this table to answer the questions (i)–(iv).

For each of the bond rotations shown, (i) identify which you believe to be more stable, (ii) calculate ∆G°, (iii) calculate the equilibrium constant, and (iv) draw the transition state.

(c)

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Textbook Question

For each of the following acid–base reactions, (iv) draw the transition state, paying close attention to the degree of bond forming and breaking present in the transition state. If a pK_a is not one of the ten common ones we learned in Chapter 4, it will be given to you.

(a)

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Textbook Question

Predict the major monohalogenation product(s) of the following reactions. Indicate whether you think the reaction will be selective and justify your position.

(a)

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Predict the major monohalogenation product(s) of the following reactions. Indicate whether you think the reaction will be selective and justify your position.(c)

Verified video answer for a similar problem:

Key Concepts

Monohalogenation

Selectivity in Halogenation

Radical Mechanism

Predict the major monohalogenation product(s) of the following reactions. Indicate whether you think the reaction will be selective and justify your position.
(c)