Organic Chemistry-Package(Custom)
Organic Chemistry-Package(Custom)
4th Edition
ISBN: 9781259141089
Author: SMITH
Publisher: MCG
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Chapter 15, Problem 15.55P

Consider the following bromination: ( CH 3 ) 3 CH + Br 2 Δ ( CH 3 ) 3 CBr + HBr .

a. Calculate Δ H ο for this reaction by using the bond dissociation energies in Table 6.2.

b. Draw out a stepwise mechanism for the reaction, including the initiation, propagation, and termination steps.

c. Calculate Δ H ο for each propagation step.

d. Draw an energy diagram for the propagation steps.

e. Draw the structure of the transition state of each propagation step.

Expert Solution
Check Mark
Interpretation Introduction

(a)

Interpretation: The value of ΔHο for the given reaction by the use of bond dissociation energies is to be calculated.

Concept introduction: The value of ΔHο is calculated by the formula,

ΔHο=BDE(B.B.)BDE(B.F.)

Answer to Problem 15.55P

The value of ΔHο for the given reaction by the use of bond dissociation energies is 16kcal/mol.

Explanation of Solution

The given bromination reaction is,

(CH3)3CH+Br2Δ(CH3)3CBr+HBr

In the given reaction, BrBr and CH bond is broken and CBr and HBr bond is formed.

The value of ΔHο is calculated by the formula,

ΔHο=BDE(B.B.)BDE(B.F.)

Where,

BDE(B.B.) is the sum of bond dissociation energy of bond broken.

BDE(B.F.) is the sum of bond dissociation energy of bond formed.

The bond dissociation energy of (CH3)3CH bond is 91kcal/mol.

The bond dissociation energy of (CH3)3CBr bond is 65kcal/mol.

The bond dissociation energy of BrBr bond is 46kcal/mol.

The bond dissociation energy of HBr bond is 88kcal/mol.

Substitute the bond dissociation energy of (CH3)3CH, (CH3)3CBr, BrBr and HBr in the above formula.

ΔHο=(91kcal/mol+46kcal/mol)(65kcal/mol+88kcal/mol)=16kcal/mol

Therefore, the value of ΔHο for the given reaction by the use of bond dissociation energies is 16kcal/mol.

Conclusion

The value of ΔHο for the given reaction by the use of bond dissociation energies is 16kcal/mol.

Expert Solution
Check Mark
Interpretation Introduction

(b)

Interpretation: The stepwise mechanism for the given reaction which includes initiation, propagation and termination steps is to be drawn.

Concept introduction: The general steps followed by free-radical reaction are stated below:

1. First step is initiation that involves formation of radical.

2. Second step is propagation.

3. Third step is the termination that involves the formation of stable bond.

Answer to Problem 15.55P

The stepwise mechanism for the given reaction which includes initiation, propagation and termination steps is,

Organic Chemistry-Package(Custom), Chapter 15, Problem 15.55P , additional homework tip  1

Figure 1

Explanation of Solution

The initiation step involves the generation of bromine radical by hemolytic cleavage of Br2. In the propagation step, bromine radical on reaction with (CH3)3CH generates tertiary radical which on reaction with Br2 converts into (CH3)3CBr. In the termination step, all free radicals combine to form stable molecules. The stepwise mechanism for the given reaction which includes initiation, propagation and termination steps is shown below.

Organic Chemistry-Package(Custom), Chapter 15, Problem 15.55P , additional homework tip  2

Figure 1

Conclusion

The stepwise mechanism for the given reaction which includes initiation, propagation and termination steps is shown in Figure 1.

Expert Solution
Check Mark
Interpretation Introduction

(c)

Interpretation: The value of ΔHο for each propagation step is to be calculated.

Concept introduction: The general steps followed by free-radical reaction are stated below:

1. First step is initiation that involves formation of radical.

2. Second step is propagation.

3. Third step is the termination that involves the formation of stable bond.

Answer to Problem 15.55P

The value of ΔHο for each propagation step is 3kcal/mol and 19kcal/mol.

Explanation of Solution

The propagation step of given reaction involves two steps. In the first step (CH3)3CH bond breaks and HBr bond is formed. In the second step, BrBr bond breaks and (CH3)3CBr bond is formed.

The value of ΔHο is calculated by the formula,

ΔHο=BDE(B.B.)BDE(B.F.)

Where,

BDE(B.B.) is the sum of bond dissociation energy of bond broken.

BDE(B.F.) is the sum of bond dissociation energy of bond formed.

The bond dissociation energy of (CH3)3CH bond is 91kcal/mol.

The bond dissociation energy of (CH3)3CBr bond is 65kcal/mol.

The bond dissociation energy of BrBr bond is 46kcal/mol.

The bond dissociation energy of HBr bond is 88kcal/mol.

The value of ΔHο for first propagation step is,

ΔHο=91kcal/mol88kcal/mol=3kcal/mol

The value of ΔHο for second propagation step is,

ΔHο=46kcal/mol65kcal/mol=19kcal/mol

Conclusion

The value of ΔHο for each propagation step is 3kcal/mol and 19kcal/mol.

Expert Solution
Check Mark
Interpretation Introduction

(d)

Interpretation: The energy level diagram for the propagation steps is to be drawn.

Concept introduction: The graphical representation of chemical reaction in which x-axis represents energy of the reaction and y-axis represents the reaction coordinate is called energy profile diagram.

Answer to Problem 15.55P

The energy level diagram for the propagation steps is,

Organic Chemistry-Package(Custom), Chapter 15, Problem 15.55P , additional homework tip  3

Figure 2

Explanation of Solution

The energy level diagram for the propagation steps is shown in Figure 2.

Organic Chemistry-Package(Custom), Chapter 15, Problem 15.55P , additional homework tip  4

Figure 2

Conclusion

The energy level diagram for the propagation steps is shown in Figure 2.

Expert Solution
Check Mark
Interpretation Introduction

(e)

Interpretation: The structure of transition state of each propagation step is to be drawn.

Concept introduction: The general steps followed by free-radical reaction are stated below:

1. First step is initiation that involves formation of radical.

2. Second step is propagation.

3. Third step is the termination that involves the formation of stable bond.

Answer to Problem 15.55P

The structure of transition state of first and second propagation step is,

Organic Chemistry-Package(Custom), Chapter 15, Problem 15.55P , additional homework tip  5

Figure 3

Explanation of Solution

The propagation step of given reaction involves two steps. In the first step (CH3)3CH bond breaks and HBr bond is formed. In the second step, BrBr bond breaks and (CH3)3CBr bond is formed.

The transition state of the first propagation step is,

Organic Chemistry-Package(Custom), Chapter 15, Problem 15.55P , additional homework tip  6

Figure 4

The transition state of the first propagation step is,

Organic Chemistry-Package(Custom), Chapter 15, Problem 15.55P , additional homework tip  7

Figure 5

Conclusion

The structure of transition state of first and second propagation step is shown in Figure 4 and Figure 5, respectively.

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Chapter 15 Solutions

Organic Chemistry-Package(Custom)

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