a. Propose a mechanism for the following reaction:b. Use the bond-dissociation enthalpies given in Table 4-2 (page 167) to calculate the value of ΔH° for each step shown in your mechanism. (The BDE for CH2=CHCH2―Br is about 280 kJ/mol, or 67 kcal/mol.) Calculate the overall value of ΔH° for the reaction. Are these values consistent with a rapid free-radical chain reaction?

Ch.6 - Alkyl Halides; Nucleophilic Substitution

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

All textbooks

Wade 9th Edition

Ch.6 - Alkyl Halides; Nucleophilic Substitution

Problem 6a,b

Chapter 6, Problem 6a,b

a. Propose a mechanism for the following reaction:

b. Use the bond-dissociation enthalpies given in Table 4-2 (page 167) to calculate the value of ΔH° for each step shown in your mechanism. (The BDE for CH₂=CHCH₂―Br is about 280 kJ/mol, or 67 kcal/mol.) Calculate the overall value of ΔH° for the reaction. Are these values consistent with a rapid free-radical chain reaction?

Verified step by step guidance

Step 1: Analyze the reaction mechanism. This reaction involves a free-radical chain mechanism initiated by light (hv). The bromine molecule (Br2) undergoes homolytic cleavage under light to form two bromine radicals. This is the initiation step.

Step 2: In the propagation step, one bromine radical reacts with the allylic hydrogen of H2C=CH—CH3, forming an allylic radical (H2C=CH—CH2•) and HBr. The bond-dissociation enthalpy (BDE) for breaking the allylic C—H bond is 372 kJ/mol (89 kcal/mol).

Step 3: The allylic radical (H2C=CH—CH2•) reacts with another Br2 molecule, forming the product H2C=CH—CH2Br and regenerating a bromine radical. The BDE for forming the C—Br bond in H2C=CH—CH2Br is approximately 280 kJ/mol (67 kcal/mol).

Step 4: Calculate ΔH° for each step using the bond-dissociation enthalpies provided. For the initiation step, breaking the Br—Br bond requires 190 kJ/mol (45 kcal/mol). For the propagation steps, subtract the energy released in bond formation from the energy required for bond breaking.

Step 5: Sum the ΔH° values for all steps to determine the overall ΔH° for the reaction. Compare the calculated ΔH° to the typical energy profile of a rapid free-radical chain reaction. If the overall ΔH° is negative and the steps are energetically favorable, the reaction is consistent with a rapid free-radical chain mechanism.

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above.

Video duration:

10m

Was this helpful?

Key Concepts

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

Free Radical Mechanism

The free radical mechanism involves the formation of reactive species called free radicals, which are atoms or molecules with unpaired electrons. In this reaction, bromine (Br2) is homolytically cleaved under light (hv) to generate two bromine radicals. These radicals then react with the alkene, leading to the addition of bromine across the double bond and the formation of a bromoalkane.

Bond Dissociation Enthalpy (BDE)

Bond dissociation enthalpy (BDE) is the energy required to break a specific bond in a molecule, resulting in the formation of free radicals. In this context, BDE values are used to calculate the energy changes associated with each step of the reaction mechanism. By summing the BDEs of bonds broken and formed, one can determine the overall enthalpy change (ΔH°) for the reaction, which provides insight into its feasibility and kinetics.

Enthalpy Change (ΔH°)

Enthalpy change (ΔH°) is a measure of the heat absorbed or released during a chemical reaction at constant pressure. It is calculated by considering the energy of bonds broken and formed during the reaction. A negative ΔH° indicates an exothermic reaction, while a positive value suggests an endothermic process. Understanding ΔH° is crucial for evaluating whether the reaction is energetically favorable and consistent with the characteristics of a rapid free-radical chain reaction.

Recommended video:

Guided course

04:38

Calculating Enthalpies

Related Practice

Textbook Question

Show how you might use S_N2 reactions to convert 1-chlorobutane into the following compounds.

a. butan-1-ol

views

Textbook Question

The reaction of an amine with an alkyl halide gives an ammonium salt.R3N amine + R'—X alkyl halide —> R3(N+)—R' X- ammonium salt The rate of this SN2 reaction is sensitive to the polarity of the solvent. 1. Draw an energy diagram for this reaction in a nonpolar solvent and another in a polar solvent. 2. Consider the nature of the transition state, and explain why this reaction should be sensitive to the polarity of the solvent. 3. Predict whether it will be faster or slower in a more polar solvent.

For each pair of compounds, predict which compound has the higher boiling point. Check [TABLE 6-2] to see if your prediction was right; then explain why that compound has the higher boiling point.

b. isopropyl chloride and tert-butyl bromide

views

Textbook Question

For each pair of compounds, predict which compound has the higher boiling point. Check [TABLE 6-2] to see if your prediction was right; then explain why that compound has the higher boiling point.

a. isopropyl bromide and n-butyl bromide

views

Textbook Question

Under appropriate conditions, (S)-1-bromo-1-fluoroethane reacts with sodium methoxide to give pure (S)-1-fluoro-1-methoxyethane.

a. Why is bromide rather than fluoride replaced?

b. Draw perspective structures (as shown on the previous page for 2-bromobutane) for the starting material, the transition state, and the product.

c. Does the product show retention or inversion of configuration? d. Is this result consistent with reaction by the SN2 mechanism?

views