11. Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations

1. 11. Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations Based on McMurry’s Organic Chemistry, 6th edition

2. Nucleophiles and Leaving Groups:

3. Alkyl Halides React with Nucleophiles • Alkyl halides are polarized at the carbon-halide bond, making the carbon electrophilic • Nucleophiles will replace the halide in C-X bonds of many alkyl halides(reaction as Lewis base) • Nucleophiles that are strong Brønsted bases can produce elimination

4. Reaction Kinetics • The study of rates of reactions is called kinetics • The order of a reaction is sum of the exponents of the concentrations in the rate law – the first example is first order, the second one second order.

5. 11.4 The SN1 and SN2 Reactions • Follow first or second order reaction kinetics • Ingold nomenclature to describe characteristic step: • S=substitution • N (subscript) = nucleophilic • 1 = substrate in characteristic step (unimolecular) • 2 = both nucleophile and substrate in characteristic step (bimolecular)

6. Stereochemical Modes of Substitution • Substitution with inversion: • Substitution with retention: • Substitution with racemization: 50% - 50%

7. SN2 Process • The reaction involves a transition state in which both reactants are together

8. “Walden” Inversion

9. SN2 Transition State • The transition state of an SN2 reaction has a planar arrangement of the carbon atom and the remaining three groups

10. Steric Effects on SN2 Reactions The carbon atom in (a) bromomethane is readily accessible resulting in a fast SN2 reaction. The carbon atoms in (b) bromoethane (primary), (c) 2-bromopropane (secondary), and (d) 2-bromo-2-methylpropane (tertiary) are successively more hindered, resulting in successively slower SN2 reactions.

11. Steric Hindrance Raises Transition State Energy • Steric effects destabilize transition states • Severe steric effects can also destabilize ground state Very hindered

12. Order of Reactivity in SN2 • The more alkyl groups connected to the reacting carbon, the slower the reaction

13. Sensitive to steric effects Methyl halides are most reactive Primary are next most reactive Secondary might react Tertiary are unreactive by this path No reaction at C=C (vinyl halides) 11.5 Characteristics of the SN2 Reaction

14. The SN1 Reaction • Tertiary alkyl halides react rapidly in protic solvents by a mechanism that involves departure of the leaving group prior to addition of the nucleophile • Called an SN1 reaction – occurs in two distinct steps while SN2 occurs with both events in same step

15. Stereochemistry of SN1 Reaction • The planar intermediate leads to loss of chirality • A free carbocation is achiral • Product is racemic or has some inversion

16. SN1 in Reality • Carbocation is biased to react on side opposite leaving group • Suggests reaction occurs with carbocation loosely associated with leaving group during nucleophilic addition

17. Effects of Ion Pair Formation • If leaving group remains associated, then product has more inversion than retention • Product is only partially racemic with more inversion than retention • Associated carbocation and leaving group is an ion pair

18. SN1 Energy Diagram • Rate-determining step is formation of carbocation Step through highest energy point is rate-limiting (k1 in forward direction) k1 k-1 k2 V = k[RX]

19. 11.9 Characteristics of the SN1 Reaction • Tertiary alkyl halide is most reactive by this mechanism • Controlled by stability of carbocation

20. Delocalized Carbocations • Delocalization of cationic charge enhances stability • Primary allyl is more stable than primary alkyl • Primary benzyl is more stable than allyl

21. Comparison: Substitution Mechanisms • SN1 • Two steps with carbocation intermediate • Occurs in 3°, allyl, benzyl • SN2 • Two steps combine - without intermediate • Occurs in primary, secondary

22. The Nucleophile • Neutral or negatively charged Lewis base • Reaction increases coordination at nucleophile • Neutral nucleophile acquires positive charge • Anionic nucleophile becomes neutral • See Table 11-1 for an illustrative list

23. Relative Reactivity of Nucleophiles • Depends on reaction and conditions • More basic nucleophiles react faster (for similar structures. See Table 11-2) • Better nucleophiles are lower in a column of the periodic table • Anions are usually more reactive than neutrals

25. The Leaving Group • A good leaving group reduces the barrier to a reaction • Stable anions that are weak bases are usually excellent leaving groups and can delocalize charge

26. “Super” Leaving Groups

27. Poor Leaving Groups • If a group is very basic or very small, it is prevents reaction

28. Effect of Leaving Group on SN1 • Critically dependent on leaving group • Reactivity: the larger halides ions are better leaving groups • In acid, OH of an alcohol is protonated and leaving group is H2O, which is still less reactive than halide • p-Toluensulfonate (TosO-) is excellent leaving group

29. Allylic and Benzylic Halides • Allylic and benzylic intermediates stabilized by delocalization of charge (See Figure 11-13) • Primary allylic and benzylic are also more reactive in the SN2 mechanism

31. The Solvent • Solvents that can donate hydrogen bonds (-OH or –NH) slow SN2 reactions by associating with reactants • Energy is required to break interactions between reactant and solvent • Polar aprotic solvents (no NH, OH, SH) form weaker interactions with substrate and permit faster reaction

33. Polar Solvents Promote Ionization • Polar, protic and unreactive Lewis base solvents facilitate formation of R+ • Solvent polarity is measured as dielectric polarization (P)

34. Solvent Is Critical in SN1 • Stabilizing carbocation also stabilizes associated transition state and controls rate Solvation of a carbocation by water

35. Effects of Solvent on Energies • Polar solvent stabilizes transition state and intermediate more than reactant and product

36. Form dipoles that have well localized negative sides, poorly defined positive sides. Examples: DMSO, HMPA (shown here) Polar aprotic solvents - + + +

37. Common polar aprotic solvents

38. - - - - - - - Cl + + + + + + + + + + + + + + + + + + Polar aprotic solvents solvate cations well, anions poorly + + + + - + + - Na + good fit! bad fit!

39. SN1: Carbocation not very encumbered, but needs to be solvated in rate determining step (slow) Polar protic solvents are good because they solvate both the leaving group and the carbocation in the rate determining step k1! The rate k2 is somewhat reduced if the nucleophile is highly solvated, but this doesn’t matter since k2 is inherently fast and not rate determining.

40. SN2: Things get tight if highly solvated nucleophile tries to form pentacoordiante transition state Polar aprotic solvents favored! There is no carbocation to be solvated.

41. Nucleophiles in SN1 • Since nucleophilic addition occurs after formation of carbocation, reaction rate is not affected normally affected by nature or concentration of nucleophile

42. 11.10 Alkyl Halides: Elimination • Elimination is an alternative pathway to substitution • Opposite of addition • Generates an alkene • Can compete with substitution and decrease yield, especially for SN1 processes

43. Zaitsev’s Rule for Elimination Reactions (1875) • In the elimination of HX from an alkyl halide, the more highly substituted alkene product predominates

44. Mechanisms of Elimination Reactions • Ingold nomenclature: E – “elimination” • E1: X- leaves first to generate a carbocation • a base abstracts a proton from the carbocation • E2: Concerted transfer of a proton to a base and departure of leaving group

45. 11.11 The E2 Reaction Mechanism • A proton is transferred to base as leaving group begins to depart • Transition state combines leaving of X and transfer of H • Product alkene forms stereospecifically

46. Geometry of Elimination – E2 • Antiperiplanar allows orbital overlap and minimizes steric interactions

47. E2 Stereochemistry • Overlap of the developing  orbital in the transition state requires periplanar geometry, anti arrangement Allows orbital overlap

49. Predicting Product • E2 is stereospecific • Meso-1,2-dibromo-1,2-diphenylethane with base gives cis 1,2-diphenyl • RR or SS 1,2-dibromo-1,2-diphenylethane gives trans 1,2-diphenyl (E)-1bromo-1,2-diphenylethene