15.10 Effect of Substituents on Reactivity and Orientation

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1 15.10 ffect of Substituents on Reactivity and Orientation

2 Z NO 3 2 SO 4 Z Z Z + + o- p- m- Z O Me CN o(%) p(%) <1 2 m(%) <

3 C 3 An activating group Ortho-para director An deactivating group Meta director More reactive than benzene toward electrophilic substitution Less reactive than benzene toward electrophilic substitution X An deactivating group Ortho-para directors Less reactive than benzene toward electrophilic substitution

4 Ortho-para directors NR 2 NR N 2 O OR NCOR OCOR R C 6 5 X Meta directors NR 3 C 3 CN SO 3 CO COR CO 2 R CON 2

5 15.11 Theory of Substituent ffects on lectrophilic Aromatic Substitution

6 Q Q Q δ δ + δ δ δ + Q releases electrons + δ Reaction is faster Q Q Q δ + δ δ + δ + δ Q withdraws electrons + δ Reaction is slower

7 R R R R R R R + + R R R

8 + +

9 + +

10 Orientation of Disubstituted Benzenes C 3 C 3 NO 3 2 SO 4 C 3 2 Fe 3 C 3 + C 3

11 C 3 CO 2 NCOC 3 N 2 C 3 CO 2 CO 2

12 C 3 CO 2 N 2 C 3 NCOC 3

13 15.12 Reactions of the Side Chain of Alkylbenzenes

14 C 3 C 2 C hv or heat + Br 2 hv or heat Br C 2 CC 3

15 15.13 Alkenylbenzenes

16 15.13A Stability of Conjugated Alkenylbenzene C C C more stable than C C C

17 15.13B Addition to the Double Bond of Alkenylbenzenes C CC 3 Br peroxide C 2 CC 3 Br C CC 3 Br C C 2 C 3 Br

18 15.13C Oxidation of the Side Chain C 3 C 2 C 3 KMnO4 CO 2 C(C 3 ) 2 C(C 3 ) 3 KMnO4

19 15.13D Oxidation of the Benzene Ring R C 6 5 (1) O 3, OAc (2) 2 O 2 R CO 2 KMnO 4 K 2 Cr 2 O 7 / + V 2 O 5 + O ~ 500 O O + CO O O Maleic anhydride

20 15.14 Synthetic Applications

21 When designing a synthesis of substituted benzenes, the order in which the substituents are introduced is crucial xample: Synthesize ortho-, meta-, and para-nitrobenzoic acid from toluene NO SO 4 Fe

22

23 15.14A Use of Protecting and Blocking Groups Strong activating groups such as amino and hydroxyl cause the aromatic ring to be so reactive that unwanted reactions can take place. These groups activate aromatic rings to oxidation by nitric acid when nitration is attempted; the ring is destroyed. O NO 3 2 SO 4 O O + + Oxidation products Low yield

24 N 2 NO 3 N 3 N 3 NO 3 2 SO 4 2 SO 4 An amino group or hydroxyl group can be protected (and turned into a moderately activating group) by acetylation. N 2 C 3 CO or (C 3 CO) 2 O NCOC 3 Base O Acylation OCOC 3 C 3 CO or (C 3 CO) 2 O Base Protecting groups

25 xample: The synthesis of p- and o- nitroaniline from aniline N 2 C 3 CO NCOC 3 NCOC 3 NCOC 3 Base NO 3 2 SO 4 + Aniline Acetanilide p-nitroacetanilide o-nitroacetanilide N 2 (90%) (trace) (1) 2 O, +, C 3 CO 2 + (2) O p-nitroaniline

26 A sulfonic acid group is used as a blocking group to force ortho substitution NCOC 3 concd. 2 SO 4 NCOC 3 NO 3 NCOC 3 2 SO 4 SO 3 SO 3 N 2 (1) 2 O, +, (2) O o-nitroaniline (56%)

27 15.14B Orientation in Disubstituted Benzenes When two substituents are present on the ring initially, the more powerful activating group generally determines the orientation of subsequent substitution Ortho-para directors determine orientation over meta directors Substitution does not occur between meta substituents due to steric hindrance

28 NCOC 3 2 C 3 CO 2 NCOC 3 + NCOC 3 C 3 C 3 (major product) C 3 (minor product) NO 3 2 SO 4 Br O 2 N + + Br Br Br (62%) (37%) (1%)

29 15.15 Allylic and Benzylic alides in Nucleophilic Substitution Reactions

30 Allylic and benzylic halides are classified in similar fashion to other halides C C C 2 X C R C C X R C C C X R' 1 Allylic 2 Allylic 3 Allylic R R Ar C 2 X Ar C X Ar C X R' 1 Benzylic 2 Benzylic 3 Benzylic

31 Both primary and secondary allylic and benzylic halides can undergo S N 1 or S N 2 reaction These primary halides are able to undergo S N 1 reaction because of the added stability of the allylic and benzylic carbocation Tertiary allylic and benzylic halides can only undergo S N 1 reaction

32

33 15.16 Reductions of Aromatic Compounds

34 Birch Reduction Ni, Pd, or Pt 180 ~ 250, P N 3 (liq.) + Na to K, Li OC 3 Na or Li N 3 (l), to OC 3 2 O + O CO 2 Na or Li N 3 (l), to CO 2

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