1 Radical Reactions Chapter 15 Smith

2 Introduction A radical is a chemical species with a single unpaired electron in an orbital. Two radicals arise when a bond is cleaved homolytically. Cl-Cl + heat  Cl + Cl

3 1 o, 2 o and 3 o Radicals Order of stability is the same as for carbocations: a tertiary radical is more stable than a secondary radical, and a secondary radical is more stable than a primary radical

4 Radical Reactions Radical reactions are initiated with heat (  ) or light (h ) and often with a peroxide (RO-OR) initiator. Once formed, a radical reacts to form a new radical. A radical formed by an initiation reaction may abstract an H radical from a C-H bond or bond to a  electron of a  bond. A new  bond is formed in both cases.

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6 Two Radicals May React with Each Other Compounds that prevent radical reactions are called radical inhibitors or radical scavengers. Vitamin E is a radical scavenger.

7 Monohalogenation of Alkanes (Replacement of one H with Br or Cl) Halogenation of an alkane is a substitution reaction. All Hydrogens are alike, replacement of any one gives the same product.

8 Show the monohalogenation products for the following reaction. Rule: We only make monohalogenation products in this course.

9 Example of a Radical Reaction Halogenation (bromination or chlorination) of alkanes. CH 4 + Br 2 + heat or UV light  CH 3 Br + HBr To do a halogenation, use a halogen (X 2 ).

10 How do halogenation reactions occur? Three essential steps : 1. Radical Initiation (heat or UV light) 2. Radical Propagation (two steps) 3. Radical Termination (three ways)

11 Radical Initiation Halogen + heat or UV light  radical Initiation: Two radicals are formed by homolysis of a  bond, starting the rxn.

12 Radical Propagation sp 3 sp 2 Propagation: A radical reacts with a reactant, forming a new  bond and a radical. (A radical makes a radical in propagation sub-steps.

13 Radical Termination Termination: Two radicals combine to form a stable bond.

14 Energy Profile of Propagation Two propagation steps; the first is rate determining.

15 The weaker a C-H bond, the easier it is to remove H Use this information to predict the product distribution when more than one kind of H is present in the substrate.

16 Predict which H is easiest to abstract in each compound. Tertiary H is easier to remove than secondary, and secondary is easier to remove than primary H.

17 Bromination vs Chlorination Bromination is slower and more selective than chlorination. Selectivity is in the order III o > II o > I o, the order of radical stability. The selectivity of bromination can be explained by Hammond’s postulate, because alkyl radical formation in bromination is endothermic and in chlorination is exothermic.

18 Halogenation in Synthesis Convert alkanes (usually symmetrical) into alkyl halides, from which alcohols, ethers and alkenes can be formed in one step.

19 Make trans-1,2-dibromocyclohexane from cyclohexene Classroom Activity

20 Stereochemistry of Halogenation Starting material Result a. Achiral (not chiral) Product is achiral or a racemic mixture b.1.Chiral w/rxn away from * Configuration at * (stereogenic center) is retained b.2. Chiral w/rxn at * center Configuration at * depends on the mechanism of the reaction.

21 Halogenation of an Achiral Compound Halogenation of a Chiral Compound Halogenation at I o carbon away from * retains the R configuration.

22 Problems Work problems 15.1 through in Smith.