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Electrophilic Aromatic Substitution (Aromatic compounds) Ar-H = aromatic compound 1. Nitration Ar-H...

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Slide 2 Electrophilic Aromatic Substitution (Aromatic compounds) Ar-H = aromatic compound 1. Nitration Ar-H + HNO 3, H 2 SO 4 Ar-NO 2 + H 2 O 2.Sulfonation Ar-H + H 2 SO 4, SO 3 Ar-SO 3 H + H 2 O 3.Halogenation Ar-H + X 2, Fe Ar-X + HX 4.Friedel-Crafts alkylation Ar-H + R-X, AlCl 3 Ar-R + HX Slide 3 Friedel-Crafts alkylation (variations) a)Ar-H + R-X, AlCl 3 Ar-R + HX b)Ar-H + R-OH, H + Ar-R + H 2 O c) Ar-H + Alkene, H + Ar-R Slide 4 Slide 5 toluene faster than the same reactions with benzene Slide 6 nitrobenzene slower than the same reactions with benzene Slide 7 Substituent groups on a benzene ring affect electrophilic aromatic substitution reactions in two ways: 1)reactivity activate (faster than benzene) or deactivate (slower than benzene) 2)orientation ortho- + para- direction or meta- direction Slide 8 -CH 3 activates the benzene ring towards EAS directs substitution to the ortho- & para- positions -NO 2 deactivates the benzene ring towards EAS directs substitution to the meta- position Slide 9 Common substituent groups and their effect on EAS: -NH 2, -NHR, -NR 2 -OH -OR -NHCOCH 3 -C 6 H 5 -R -H -X -CHO, -COR -SO 3 H -COOH, -COOR -CN -NR 3 + -NO 2 increasing reactivity ortho/para directors meta directors Slide 10 Slide 11 If there is more than one group on the benzene ring: 1.The group that is more activating (higher on the list) will direct the next substitution. 2.You will get little or no substitution between groups that are meta- to each other. Slide 12 Slide 13 Orientation and synthesis. Order is important! synthesis of m-bromonitrobenzene from benzene: synthesis of p-bromonitrobenzene from benzene: You may assume that you can separate a pure para- isomer from an ortho-/para- mixture. Slide 14 note: the assumption that you can separate a pure para isomer from an ortho/para mixture does not apply to any other mixtures. Slide 15 synthesis of benzoic acids by oxidation of CH 3 Slide 16 Links to problem sets on the web involving EAS: http://chemistry2.csudh.edu/organic/aromatics/reactions.html Reactivity and sites on monosubstituted benzene Reaction Sties on disubstituted benzenes Synthesis of disubstituted benzenes Synthesis of trisubstituited benzenes Slide 17 + HO-NO 2 + H 2 SO 4 H 2 O-NO 2 + HSO 4 - + H 2 O-NO 2 H 2 O + NO 2 H 2 SO 4 + H 2 O HSO 4 - + H 3 O + HNO 3 + 2 H 2 SO 4 H 3 O + + 2 HSO 4 - + NO 2 + nitration Slide 18 nitration: electrophile Slide 19 resonance Slide 20 Mechanism for nitration: Slide 21 Mechanism for sulfonation: Slide 22 Mechanism for halogenation: Slide 23 Mechanism for Friedel-Crafts alkylation: Slide 24 Mechanism for Friedel-Crafts with an alcohol & acid Slide 25 Mechanism for Friedel-Crafts with alkene & acid: electrophile in Friedel-Crafts alkylation = carbocation Slide 26 Generic Electrophilic Aromatic Substitution mechanism: Slide 27 Why do substituent groups on a benzene ring affect the reactivity and orientation in the way they do? electronic effects, pushing or pulling electrons by the substituent. Electrons can be donated (pushed) or withdrawn (pulled) by atoms or groups of atoms via: Induction due to differences in electronegativities Resonance delocalization via resonance Slide 28 Slide 29 Slide 30 X Slide 31 Slide 32 Slide 33 Common substituent groups and their effect on reactivity in EAS: -NH 2, -NHR, -NR 2 -OH -OR -NHCOCH 3 electron donating -C 6 H 5 -R -H -X -CHO, -COR -SO 3 H -COOH, -COOR electron withdrawing -CN -NR 3 + -NO 2 increasing reactivity Slide 34 Electron donating groups activate the benzene ring to electrophilic aromatic substitution. 1.electron donating groups increase the electron density in the ring and make it more reactive with electrophiles. 2.electron donation stabilizes the intermediate carbocation, lowers the Eact and increases the rate. Slide 35 Electron withdrawing groups deactivate the benzene ring to electrophilic aromatic substitution. 1.electron withdrawing groups decrease the electron density in the ring and make it less reactive with electrophiles. 2.electron withdrawal destabilizes the intermediate carbocation, raising the Eact and slowing the rate. Slide 36 Slide 37 Slide 38 Slide 39 Slide 40 If G is an electron donating group, these structures are especially stable. Slide 41 Slide 42 Electron donating groups stabilize the intermediate carbocations for ortho- and para- in EAS more than for meta-. The Eacts for ortho-/para- are lower and the rates are faster. Electron donating groups direct ortho-/para- in EAS Slide 43 If G is an electron withdrawing group, these structures are especially unstable. Slide 44 Slide 45 Electron withdrawing groups destabilize the intermediate carbocations for ortho- and para- in EAS more than for meta-. The Eacts for ortho-/para- are higher and the rates are slower. Electron withdrawing groups direct meta- in EAS Slide 46 Halogens are electron withdrawing but are ortho/para directing in EAS. The halogen atom is unusual in that it is highly electronegative but also has unshared pairs of electrons that can be resonance donated to the carbocation. Slide 47 Slide 48 Common substituent groups and their effect on EAS: -NH 2, -NHR, -NR 2 -OH -OR -NHCOCH 3 -C 6 H 5 -R -H -X -CHO, -COR -SO 3 H -COOH, -COOR -CN -NR 3 + -NO 2 increasing reactivity ortho/para directors meta directors

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