23.5 Nucleophilic Substitution in Nitro-Substituted Aryl Halides

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1 23.5 Nucleophilic Substitution in Nitro-Substituted Aryl alides

2 nitro-substituted aryl halides do undergo nucleophilic aromatic substitution readily Cl OC 3 + NaOC 3 C 3 O 85 C + NaCl NO 2 NO 2 (92%)

3 Effect of nitro group is cumulative especially when nitro group is ortho and/or para to leaving group Cl Cl Cl Cl NO 2 O 2 N NO 2 NO 2 NO 2 NO x x too fast to measure

4 Kinetics follows second-order rate law: rate = k[aryl halide][nucleophile] inference: both the aryl halide and the nucleophile are involved in rate-determining step

5 Effect of leaving group unusual order: > Cl > Br > I X NO 2 X Cl Br I Relative Rate* *NaOC 3, C 3 O, 50 C

6 General Conclusions About Mechanism bimolecular rate-determining step in which nucleophile attacks aryl halide rate-determining step precedes carbon-halogen bond cleavage rate-determining transition state is stabilized by electron-withdrawing groups (such as NO 2 )

7 23.6 The Addition-Elimination Mechanism of Nucleophilic Aromatic Substitution

8 Addition-Elimination Mechanism Two step mechanism: Step 1) nucleophile attacks aryl halide and bonds to the carbon that bears the halogen (slow: aromaticity of ring lost in this step) Step 2) intermediate formed in first step loses halide (fast: aromaticity of ring restored in this step)

9 Reaction OC 3 + NaOC 3 C 3 O 85 C + Na NO 2 NO 2 (93%)

10 Mechanism Step 1 OC 3 bimolecular consistent with secondorder kinetics; first order in aryl halide, first order in nucleophile NO 2

11 Mechanism Step 1 OC 3 slow OC 3 NO 2 NO 2

12 Mechanism intermediate is negatively charged formed faster when ring bears electronwithdrawing groups such as NO 2 OC 3 NO 2

13 Stabilization of Rate-Determining Intermediate by Nitro Group OC 3 OC 3 O N + O O N + O

14 Mechanism Step 2 OC 3 fast OC 3 NO 2 NO 2

15 Leaving Group Effects > Cl > Br > I is unusual, but consistent with mechanism carbon-halogen bond breaking does not occur until after the rate-determining step electronegative stabilizes negatively charged intermediate

16 23.7 Related Nucleophilic Aromatic Substitution Reactions

17 Example: exafluorobenzene OC 3 NaOC 3 C 3 O 65 C (72%) Six fluorine substituents stabilize negatively charged intermediate formed in rate-determining step and increase rate of nucleophilic aromatic substitution.

18 Example: 2-Chloropyridine NaOC 3 N Cl C 3 O N OC 3 50 C 2-Chloropyridine reacts 230,000,000 times faster than chlorobenzene under these conditions.

19 Example: 2-Chloropyridine N Cl OC 3 N Cl OC 3 Nitrogen is more electronegative than carbon, stabilizes the anionic intermediate, and increases the rate at which it is formed.

20 23.8 The Elimination-Addition Mechanism of Nucleophilic Aromatic Substitution: Benzyne

21 Aryl alides Undergo Substitution When Treated With Very Strong Bases KN 2, N 3 Cl N 2 33 C (52%)

22 Regiochemistry new substituent becomes attached to either the carbon that bore the leaving group or the carbon adjacent to it C 3 Br NaN 2, N 3 33 C C 3 N2 + C 3 N 2

23 Regiochemistry new substituent becomes attached to either the carbon that bore the leaving group or the carbon adjacent to it C 3 C 3 C 3 NaN 2, N 3 33 C + N 2 Br N 2

24 Regiochemistry C 3 NaN 2, N 3 N 2 33 C C 3 C 3 C 3 N2 + + N 2 N 2

25 Same result using 14 C label * Cl KN 2, N 3 33 C N 2 * + * N 2 (52%) (48%)

26 Mechanism Step 1 Cl N 2 Cl N 2 compound formed in this step is called benzyne

27 Benzyne Benzyne has a strained triple bond. It cannot be isolated in this reaction, but is formed as a reactive intermediate.

28 Mechanism Step 2 N 2 N 2 Angle strain is relieved. The two sp-hybridized ring carbons in benzyne become sp 2 hybridized in the resulting anion.

29 Mechanism Step 3 N 2 N 2 N 2 N 2

30 ydrolysis of Chlorobenzene 14 C labeling indicates that the hightemperature reaction of chlorobenzene with NaO goes via benzyne. * Cl NaO, 2 O 395 C O * + * O (43%) (54%)

31 23.9 Diels-Alder Reactions of Benzyne

32 Other Routes to Benzyne Benzyne can be prepared as a reactive intermediate by methods other than treatment of chlorobenzene with strong bases. Another method involves loss of fluoride ion from the Grignard reagent of 1-bromo-2- fluorobenzene.

33 Other Routes to Benzyne Br Mg, T heat MgBr + MgBr

34 Benzyne as a Dienophile Benzyne is a fairly reactive dienophile, and gives Diels-Alder adducts when generated in the presence of conjugated dienes.

35 Benzyne as a Dienophile Br + Mg, T heat (46%)

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