Learning Objectives: Chapter 15: Reactions of Substituted Benzenes 1. Be able to recognize and utilize the oxidative and reductive reactions involving the substituents on benzene. 2. Recognize whether a substituent on a benzene ring is activating or deactivating toward electrophilic aromatic substitution reaction, and why it is so. 3. Predict the effect a substituent will have on the regioselectivity of an electrophilic substitution reaction. 4. Predict the effect a substituent will have on pka. 5. Be able to design the synthesis of a multisubstituted benzene. 6. Be able to recognize and utilize the reactions involving arenediazonium salts. 7. Recognize and be able to write the mechanism of nucleophilic aromatic substitution. 8. Recognize the structure of benzyne, be able to explain how it is formed, and how it reacts. Sections: 15.1 Nomenclature of Disubstituted and Polysubstituted Benzenes 15.2 Some Substituents Increase the Reactivity of a Benzene Ring and Some Decrease Its Reactivity* 15.3 The Effect of Substituents on Orientation (Regioselectivity)* 15.4 The Effect Substituent on pka 15.5 The Ortho-Para ratio 15.6 Additional Considerations Regarding Substituent Effects 15.7 Designing a Synthesis III: Synthesis of monosubstituted and Disubstituted Benzenes* 15.8 Synthesis of Trisubstituted Benzenes 15.9 Synthesis of Substituted Benzenes Using Arenediazonium Salts* 15.10 The Arenediazonium Ion as an Electrophile* 15.11 Mechanism for the Reaction of Amines with Nitrous Acid* 15.12 Nucleophilic Aromatic Substitution Reactions: An Addition-Elimination Mechanism* 15.13 Nucleophilic Aromatic Substitution Reactions: An Elimination-Addition Mechanism That Forms a Benzyne Intermediate 15.14 Polycyclic Benzenoid Hydrocarbons # * Sections that will be focused # Sections that will be skipped Recommended additional problems 15.34 15.69 1
Class Note 15.1 Nomenclature of Disubstituted and Polysubstituted Benzenes H 2 N CH 3 Cl CH 3 NH 2 Cl Cl Cl 2
15.2 Some Substituents Increase the Reactivity of a Benzene Ring and Some Decrease Its Reactivity* and 15.3 The Effect of Substituents on Orientation (Regioselectivity)* electron donation group (EDG) electron withdrawing group (EWG) EDG EWG A. Relative rate of electrophilic aromatic substitution Examples: OCH 3 Rate-determining step (r.d.s) Resonance effect and inductive effect 3
B. Relative reactivity and regioselectivity of substituted benzenes Strong activating groups (substituents) NH 2 OR Moderate activating groups (substituents) O O HN R O R 4
Weak activating groups (substituents) R Weak deactivating groups (substituents) F Cl I 5
Moderate deactivating groups (substituents) O H O R O OR O H N R 6
Strong deactivating groups (substituents) SO 3 H CN NR 3 7
C. Examples (combined with 15.5 The Ortho-Para ratio and 15.6 Additional Considerations Regarding Substituent Effects) (i) CH 3 2, FeCl 3 (ii) CH 3 HNO 3 H 2 SO 4 CH 2 CH 3 HNO 3 H 2 SO 4 C(CH 3 ) 3 HNO 3 H 2 SO 4 8
(iii) Cl 2, FeCl 3 (iv) OR 2 (1 equivalent) FeCl 3 OR 2 (excess) FeCl 3 9
(v) NH 2 CH 3 COCl AlCl 3 O HN R CH 3 COCl AlCl 3 (vi) O R HNO 3 H 2 SO 4 HNO 3 H 2 SO 4 10
(vii) Synthesis of trinitrotoluene CH 3 HNO 3 H 2 SO 4 11
15.4 The Effect Substituent on pka I II III IV V VI OCH 3 CH 3 Cl CHO I II III IV V VI CO CO 2 H CO 2 H CO 2 H 2 H CO 2 H CO 2 H OCH 3 CH 3 CHO I II III IV V VI NH 3 NH 3 NH 3 NH 3 NH 3 NH 3 OCH 3 CH 3 CHO 12
15.7 Designing a Synthesis III: Synthesis of monosubstituted and Disubstituted Benzenes* and 15.8 Synthesis of Trisubstituted Benzenes Design multiple-step synthesis: * Selectivity: chemoselectivity, regioselectivity, and stereoselectivity * Retrosynthetic analysis: breaking and formation of chemical bonds 13
A. Examples (i) from 14
(ii) O CH 3 from O 2 N (iii) O CH 3 from 15
15.9 Synthesis of Substituted Benzenes Using Arenediazonium Salts and 15.11 Mechanism for the Reaction of Amines with Nitrous Acid* A. Formation of diazonium salt R NH 2 0 o C Na, HCl R N N Cl diazonium salt Mechanism: 16
B. Reaction of arenediazonium salt with nucleophiles Sandmeyer reation (Cu, CuCl, and CuCN) Reaction with KI, HBF 4, H 3 O +, and H 3 PO 2 17
15.10 The Arenediazonium Ion as an Electrophile* N N Nu * Nucleophile better be bulky * Terminal nitrogen reacts with nucleophile 18
15.12 Nucleophilic Aromatic Substitution Reactions (S N Ar): An Addition-Elimination Mechanism* A. Comparison of S N Ar, S N 1, and S N 2 B. General mechanism 19
C. Examples (i) Cl Cl Cl O 2 N O 2 N (ii) NHCH 2 CH 3 CH 3 CH 2 NH 2 then 20
15.13 Nucleophilic Aromatic Substitution Reactions: An Elimination-Addition Mechanism That Forms a Benzyne Intermediate 21