Chapter 7 Alkyl Halides

1. Chapter 7 Alkyl Halides

2. 吸入性麻醉劑 消毒,殺蟲劑

3. 7.1 Naming Alkyl Halides

7. 7.2 Preparing Alkyl Halides

8. The most general method for preparing alkyl halides is to make them from alcohols,

9. The reaction works best with tertiary alcohols. Primary and secondary alcohols react much more slowly.

10. Primary and secondary alcohols are best converted into alkyl halides by treatment with either thionyl chloride (SOCl2) or phosphorus tribromide (PBr3).

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13. 7.3 Reactions of Alkyl Halides: Grignard Reagents Organometallic compounds

14. Because of its electron-rich carbon, a Grignard reagent is both a base and a nucleophile; it therefore reacts with acids and electrophiles. It is a useful method for converting an organic halide into a hydrocarbon.

18. 7.4 Nucleophilic Substitution Reactions In 1896, the German chemist Paul Walden’s cycle of reactions interconverting (+)- and (-)-malic acids 蘋果酸

19. Nucleophilic substitutions occur by two major pathways, named the SN1 reaction and SN2 reaction. Nu:- : nucleophile X:- : leaving group

22. 7.5 The SN2 Reaction An SN2 reaction takes place in a single stepwithout intermediates when the entering nucleophile attacks the substrate from a direction 180o away from the leaving group.

24. Rate of SN2 reactions : the rate of the reaction depends on the concentrations of two substances  bimolecular. Stereochemistry of SN2 reactions : inverting the configuration at carbon.

26. Steric effects in SN2 reactions

27. SN2 reactivity

28. Vinylic (R2C=CRX) and aryl (Ar-X) halides are NOTshown on this reactivity list because they are completely unreactive toward SN2 displacement.

29. The leaving group in SN2 reactions The best leaving groups are those that give the most stable anions (anions of strong acids)

30. 7.6 The SN1 Reaction SN1 reactions take place only on tertiary substrates and only under neutral or acidic conditions in a hydroxylic solvent such as water or alcohol. For example, that alkyl halides can be prepared from alcohols by treatment with HCl or HBr. An SN1 reaction occurs by spontaneous loss of the leaving group before the incoming nucleophile approaches. SN1 reactions can occur only when stable carbocation intermediates are formed.

31. intermediate

32. Rates of SN1 depends only on the concentration of the substrate and is independent of the nucleophile concentration. Stereochemistry of SN1 reactions Carbocations are planar and sp2-hybridized, they are achiral. The positive charged carbon can therefore react with a nucleophile equally well from either top or bottom face, leading to a racemic mixture of enantiomers.

34. The leaving group in SN1 reactions The best leaving group in SN reactions are those that give the most stable anions.

35. 7.7 Eliminations: The E2 Reaction

36. Elimination reactions almost always give mixturesof alkene products, and the best we can usually do is to predict which will be the major one. But-2-ene But-1-ene Zaitsev’s rule: In the elimination of HX from an alkyl halide, the more highly substituted alkene product predominates. Zaitsev, Russian chemist in 1875

37. Mechanism of E2 Alkyl halide treated with a strong base

41. 7.8 Eliminations: The E1 Reaction intermediate

42. The best E1 substrates are also the best SN1 substrates, and mixtures of substitution and elimination products are usually obtained. 2-Methylpropan-2-ol SN1 E1

43. 7. 9 A Summary of Reactivity: SN1, SN2, E1, E2

46. 7.10 Substitution Reactions in Living Organisms 正腎上腺素 腺甘酸 腎上腺素