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Tributyltin hydride ( Bu3SnH ) is used synthetically to

Organic Chemistry | 8th Edition | ISBN: 9780321768414 | Authors: L.G. Wade Jr ISBN: 9780321768414 33

Solution for problem 54SP Chapter 4

Organic Chemistry | 8th Edition

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Organic Chemistry | 8th Edition | ISBN: 9780321768414 | Authors: L.G. Wade Jr

Organic Chemistry | 8th Edition

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Problem 54SP

Tributyltin hydride ( Bu3SnH ) is used synthetically to reduce alkyl halides, replacing a halogen atom with hydrogen. Free radical initiators promote this reaction, and free-radical inhibitors are known to slow it or stop it. Your job is to develop a mechanism, using the following reaction as the example.

The following bond-dissociation enthalpies may be helpful:

Br- Br 192 kJ/mol

H- Br 368 kJ/mol

Bu3Sn-H 310 kJ/mol

Bu3Sn-Br 552 kJ/mol

(a) Propose initiation and propagation steps to account for this reaction.

(b) Calculate values of  for your proposed steps to show that they are energetically feasible. (Hint: A trace of Br2 and light suggests it’s there only as an initiator, to create Br. radicals. Then decide which atom can be abstracted most favorably from the starting materials by the Br. radical. That should complete the initiation. Now decide what energetically favored propagation steps will accomplish the reaction.)

Step-by-Step Solution:
Step 1 of 3

Given:

The bond-dissociation enthalpies:

Br- Br 192 kJ/mol

H- Br 368 kJ/mol

Bu3Sn-H 310 kJ/mol

Bu3Sn-Br 552 kJ/mol

Step-1

Initiation:

In this step Br2 dissociate to form Br  free radicals and this Br. will react with tributyltin hydride ( Bu3SnH ) and forms  tributyltin free radical.

Propagation:

The  propagation reaction involves two steps. In the 1st step the bromobenzene under goes free radical substitution reaction...

Step 2 of 3

Chapter 4, Problem 54SP is Solved
Step 3 of 3

Textbook: Organic Chemistry
Edition: 8
Author: L.G. Wade Jr
ISBN: 9780321768414

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