Ch. 11 - Properties and Synthesis of Alkyl Halides: Radical Reactions

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

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Mullins 1st Edition

Ch. 11 - Properties and Synthesis of Alkyl Halides: Radical Reactions

Problem 62a

Chapter 10, Problem 62a

A halogenation intended to make compound A formed B instead.

(a) Suggest a mechanism for the intended formation of A.

Verified step by step guidance

Step 1: Identify the starting compound and the halogenation reagent. Halogenation typically involves the addition of a halogen (such as Cl₂ or Br₂) to an organic compound, often in the presence of a catalyst like FeCl₃ or light.

Step 2: Consider the type of halogenation reaction. For example, if the compound is an alkane, the reaction might proceed via a radical mechanism. If it's an alkene or aromatic compound, it might proceed via electrophilic addition or substitution.

Step 3: For radical halogenation, initiate the reaction by homolytic cleavage of the halogen molecule to form two halogen radicals. This can be represented as:

{Cl}_{2} \to {Cl}_{•} + {Cl}_{•}

Step 4: Propagation steps involve the halogen radical reacting with the organic compound to form a new radical and a halogenated product. For example,

{Cl}_{•} + RH \to R •_{1} + HCl

, followed by

R •_{1} + {Cl}_{2} \to RCl + {Cl}_{•}

Step 5: Termination steps involve the combination of radicals to form stable products, ending the chain reaction. For example,

{Cl}_{•} + {Cl}_{•} \to {Cl}_{2}

R •_{1} + {Cl}_{•} \to RCl

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Key Concepts

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

Halogenation Reaction

Halogenation is a chemical reaction that involves the addition of one or more halogens to a compound. In organic chemistry, it typically refers to the substitution of hydrogen atoms in hydrocarbons with halogen atoms, often using reagents like chlorine or bromine. Understanding the conditions and reagents used is crucial for predicting the outcome of such reactions.

Reaction Mechanism

A reaction mechanism is a detailed step-by-step description of how a chemical reaction occurs at the molecular level. It includes the sequence of elementary steps, the intermediates formed, and the transition states. Mechanisms help in understanding the pathway from reactants to products, which is essential for predicting and controlling chemical reactions.

Regioselectivity

Regioselectivity refers to the preference of a chemical reaction to occur at one location over another within a molecule. In halogenation, regioselectivity determines which hydrogen atom is replaced by a halogen, influenced by factors like steric hindrance and electronic effects. Understanding regioselectivity is key to predicting the major product of a reaction.

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Related Practice

Textbook Question

Tributyltin hydride (Bu₃SnH) is often used as a 'radical carrier' in radical reactions. Which bond would you expect to be weaker, Sn–H or C–H? How might this relate to radical stability? Explain your answer.

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Textbook Question

Haloalkanes can be reduced to alkanes using radical reactions involving Bu₃SnH and a catalytic amount of AIBN. Suggest a mechanism for this reaction. [Pay close attention to the bonds formed and broken in developing your mechanism.]

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Textbook Question

Cyclizations can be carried out under radical conditions involving Bu₃SnH and a catalytic amount of AIBN. Suggest a mechanism for this reaction. [Pay close attention to the bonds formed and broken in developing your mechanism. It may be helpful to number the carbons.]

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Textbook Question

A halogenation intended to make compound A formed B instead.

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Textbook Question

A halogenation intended to make compound A formed B instead.

(d) Without looking it up, would you expect C–H_a or C–H_b to have the lower bond-dissociation energy?

Textbook Question

A halogenation intended to make compound A formed B instead.

(b) Suggest a mechanism that rationalizes the formation of B.

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