Chapter 10 Radical Reactions. 1.Introduction: How Radicals Form and How They React  Heterolysis  Homolysis © 2014 by John Wiley & Sons, Inc. All rights.

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Presentation transcript:

Chapter 10 Radical Reactions

1.Introduction: How Radicals Form and How They React  Heterolysis  Homolysis © 2014 by John Wiley & Sons, Inc. All rights reserved.

1A.Production of Radicals  Homolysis of covalent bonds ●Need heat or light (h ) (alkoxyl radical) (chlorine radical) © 2014 by John Wiley & Sons, Inc. All rights reserved.

1B.Reactions of Radicals  Almost all small radicals are short-lived, highly reactive species © 2014 by John Wiley & Sons, Inc. All rights reserved.

2.Homolytic Bond Dissociation Energies (  H°) Bond formation is an exothermic process. Reactions in which only bond breaking occurs are always endothermic. © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Relative Stability ●Carbon radicals are considered to be electron deficient (similar to carbocations), thus electron donating groups stabilize radicals  3 o > 2 o > 1 o © 2014 by John Wiley & Sons, Inc. All rights reserved.

3.Reactions of Alkanes with Halogens  Alkanes have no functional groups, are inert to many reagents, and do not undergo many reactions  Halogenation of alkanes is one of the most typical free radical reactions © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Alkanes react with molecular halogens to produce alkyl halides by a substitution reaction called radical halogenation © 2014 by John Wiley & Sons, Inc. All rights reserved.

3A.Multiple Halogen Substitution © 2014 by John Wiley & Sons, Inc. All rights reserved.

4.Chlorination of Methane: Mechanism of Reaction  Most radical reactions include 3 stages, or steps (1) chain initiation (2) chain propagation (3) chain termination © 2014 by John Wiley & Sons, Inc. All rights reserved.

 Mechanism of Free Radical Chlorination of CH 4 (1) Chain initiation ●Radicals are created in this step © 2014 by John Wiley & Sons, Inc. All rights reserved.

(2) Chain propagation ●Repeating (i) and (ii) in a chain reaction provides the product CH 3 Cl ●In chain propagation, one radical generates another and the process goes on © 2014 by John Wiley & Sons, Inc. All rights reserved.

(2) Chain propagation ●Other than CH 3 Cl, other chlorination products can be formed in the chain propagation step © 2014 by John Wiley & Sons, Inc. All rights reserved.

(2) Chain propagation © 2014 by John Wiley & Sons, Inc. All rights reserved.

(3) Chain termination © 2014 by John Wiley & Sons, Inc. All rights reserved.

(3) Chain termination ●Free radical reactions cannot be completed without chain termination ●All radicals are quenched in this step ●Radical reactions usually provide mixtures of many different products ●Relatively controlled synthesis of CH 3 Cl or CCl 4 is possible using different amounts of reactants (CH 4 and Cl 2 ) © 2014 by John Wiley & Sons, Inc. All rights reserved.

e.g.: CH 4 (large excess) + Cl 2 CH 4 + Cl 2 (large excess) CH 3 Cl (mainly) h CCl 4 (mainly) h © 2014 by John Wiley & Sons, Inc. All rights reserved.

5.Halogenation of Higher Alkanes  Mechanism for radical halogenation of ethane © 2014 by John Wiley & Sons, Inc. All rights reserved.

6.The Geometry of Alkyl Radicals  Planar, similar to carbocation p-orbital sp 2 hybridized © 2014 by John Wiley & Sons, Inc. All rights reserved.

12C.Combustion of Alkanes © 2014 by John Wiley & Sons, Inc. All rights reserved.

 One product of the second chain- propagating step is R―OOH, called an alkyl hydroperoxide. The oxygen– oxygen bond of an alkyl hydroperoxide is quite weak, and it can break and produce radicals that can initiate other chains © 2014 by John Wiley & Sons, Inc. All rights reserved.

END OF CHAPTER 10 © 2014 by John Wiley & Sons, Inc. All rights reserved.