Senyawa Aromatis. Benzena

Transkripsi

1 Senyawa Aromatis Benzena 1

2 Teori Resonansi 1. Bentuk Resonansi merupakan imaginer benzena mempunyai suatu struktur hibrid yang merupakan gabungan dari semua bentuk struktur resonansi. Struktur Resonansi Bentuk canonical Struktu Hibrid 2

3 Struktur Teori Resonance benzena Semua ikatan : equivalen Elektron : terdelokalisasi mengelilingi cincin 3

4 4 Struktur resonansi Kekulé Bentuk/Struktuk canonical / resonansi Strukur Hibrid / Resonansi C C C C C C H H H H H H C C C C C C H H H H H H C C C C C C H H H H H H

5 Teori Resonansi Bentuk Resonansi hanya berbeda pada posisi elektron Posisi atom tidak berubah Makin banyak Struktur Resonansi, molekulnya semakin stabil. Molekule distabilkan oleh resonansi 5

6 Aromatisitas Sistem stabilisasi resonansi mempunyai karakter yang harus dipenuhi Kriteria aromatis : Cincin planar Terkonjugasi Overlaping orbital p terjadi pada seluruh atom Elektron = (4n + 2) elektron 6

7 Contoh : Benzena Aromatisitas Cincin Planar Terkonjugasi Elektron = 6 elektron 7

8 Aromatisitas Contoh lain : Senyawa berikut aromatis bukan? cyclic planar conjugated 6 electrons X cyclic planar conjugated 6 electrons X 8

9 Tata Nama (Nomenclature) Substituent diikuti benzena Ada penamaan umum CH 3 Cl NO 2 OH Metilbenzena (Toluena) Klorobenzena Nitrobenzena Hidroksi benzena Fenol NH 2 OMe O OH Amino benzena Anilina Metoksibenzena (Anisol) Asam Benzena Karboksilat Asam Benzoat 9

10 Tata Nama (Nomenclature) Substituen yang sama diberi awalan di- (2), tri- (3), tetra- (4) dst. 6 CH 3 6 CH 3 6 CH CH 3 3 CH 3 3 1,2-Dimetilbenzena o-dimetilbenzena o-xilena (o: ortho) CH 3 1,3-Dimetilbenzena m-dimetilbenzena m-xilena (m: meta) 1,4-Dimetilbenzena p-dimetilbenzena p-xilena (p: para) 10

11 Reaksi Reaksi yang terjadi pada Benzena : substitusi, bukan adisi Br 2 Br Br 2 Br Br Br never observed Substitusi Elektrofilik pada Aromatis H Br 2 Br

12 Substitusi Elektrofilik pada Aromatis Adisis Elektrofilik tidak terjadi karena akan merusak aromatisitas. 12

13 A. Substitusi Electrofilik Aromatis 1. Reaksi Umum : 13

14 A. Substitusi Electrofilik Aromatis Mekanisme reaksi - two steps : Step 1: H E H E H E + E B Step 2: + B Resonansi menstabilkan karbokation H E E + B + H-B 14

15 2. Reaksi dengan asam kuat (H 2 SO 4, HBr, dll) E + = H + 15

16 3. Halogenasi a. Reaksi : + X 2 FeX 3 X when X = Cl AlCl 3 is also used b. Mekanisme X = Cl, Br 16

17 3. Halogenasi Brominasi Klorinasi Iodinasi 17

18 4. Nitrasi 18

19 4. Nitrasi a. Reaksi menggunakan H 2 SO 4! (from H 2 SO 4 ) 19

20 5. Sulfonasi Mekanisme reaksi 20

21 6. Alkilasi Friedel - Crafts a. Reaksi: X = Cl, Br b. mekanisme 21

22 7. Asilasi Friedal - Crafts O a. Reaksi + R C Cl AlCl 3 O C R + HCl 22

23 b. mekanisme 23

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25 Pengaruh gugus pengarah Substitusi Elektrofilik pada Aromatis Benzena dua substituen ada 3 isomer : Reaksi pada anisol : Gugus metoksi pengarah ortho, para. 25

26 Substitent Effects Summary Strongest activators Strongest deactivators NH 2 OCH 3 Ar H X COOR COR NO 2 OH R CHO COOH CN Directing: Act/Deact: ORTHO-/PARA- ACTIVATORS ORTHO-/PARA- DEACTIVATORS META- DEACTIVATORS 26

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28 There are two ways to look at these patterns. 1. -donating substituents - let s use E + as the electrophile: 28

29 ortho substitution OCH 3 E + OCH 3H + E + OCH 3H E OCH 3H + E OCH 3 E + OCH 3H E particularily stable 29

30 - donating O CH 3 O H NH 2 NR 2 O NH C R etc. 30

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33 But -Cl, -Br, -I slow down electrophilic substitution This brings up the other way to look at o, p activating groups: E + prefers to attack electron rich carbons 33

34 2. -electron withdrawing E + prefers to attack electron rich carbons - so meta substitution is preferred! 34

35 The alternative argument goes like this: (same thing happens at ortho attack) 35

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41 C. Polysubstitution of aromatic compounds. A general rule of thumb: 1. Activating o,p directors are stronger than the meta directing deactivators. 2. The alkyl groups and halides are in-between. 3. Steric effects can be important. Some simple examples - the directors work together: 41

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44 Some not - so - simple examples and tricks! 44

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46 junk, tar 46

47 Tutorial Questions 1. What are the four rules of aromaticity? 2. Draw one aromatic molecule, which does not contain a benzene ring. 3. Draw the following molecules; ortho-dibromobenzene, para-nitroaniline, phenol, meta-nitroanisole and 2,4,6-trinitrotoluene (TNT) 4. Predict the major product for the following reaction, and name the product. OH + Br 2 major product CH 3 5. Provide a brief synthesis for TNT starting from toluene 6. Provide a mechanism for the following reaction; O Cl AlCl 3 O 47

48 Tutorial Questions NO 2 HNO 3 / H 2 SO 4 HNO 3 / H 2 SO 4 A + B + C isomeric dinitrobenzenes 85% 1. Write a mechanism for the generation of the nitronium ion (NO 2+ ) from concentrated nitric and sulfuric acid. 2. Write a mechanism for the nitration of benzene. 3. Draw dinitrobenzenes A-C. 4. Predict the ratio of isomeric products A-C from the nitration of nitrobenzene, and comment on the rate of nitration of benzene compared to nitrobenzene. Rationalize your answers with resonance structures. 5. Draw the structure of an aromatic that undergoes electrophilic aromatic substitution faster than benzene. Briefly give reasons for your answer. 48

49 Tutorial Questions 1. Draw the chemical structure of A, and give a full reaction mechanism for the formation of yellow azo-compound B (Scheme 1). N 2 + Cl _ + A NaOAc, 0 ºC, H 2 O N N N CH 3 B CH 3 Scheme 1 A NO 2 HNO 3 / H 2 SO 4 Br 2 / FeBr 2 C reagents X NH 2 Br D Br D reagents & conditions, Y E CuBr Scheme 2 Br 2. Draw structures A, C and E, and give reagents & brief conditions X and Y. 3. Give a mechanism for the formation of C (Scheme 2). 49