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Radical Reactions

Radical Reactions. Chapter 15 Smith. 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•. 1 o , 2 o and 3 o Radicals.

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Radical Reactions

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  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. 1o, 2o and 3o 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 (D) or light (hn) 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 p electron of a p bond. A new s bond is formed in both cases.

  5. 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.

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

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

  8. Example of a Radical Reaction • Halogenation (bromination or chlorination) of alkanes. • CH4 + Br2+ heat or UV light  CH3Br + HBr • To do a halogenation, use a halogen (X2).

  9. 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)

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

  11. Radical Propagation sp2 sp3 Propagation: A radical reacts with a reactant, forming a new s bond and a radical. (A radical makes a radical in propagation sub-steps.

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

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

  14. 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.

  15. 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.

  16. Bromination vs Chlorination • Bromination is slower and more selective than chlorination. • Selectivity is in the order IIIo > IIo > Io, 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.

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

  18. Make trans-1,2-dibromocyclohexane from cyclohexene Classroom Activity

  19. Stereochemistry of Halogenation

  20. Halogenation of an Achiral Compound Halogenation of a Chiral Compound Halogenation at Io carbon away from * retains the R configuration.

  21. Problems • Work problems 15.1 through 15.18 in Smith.

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