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Senyawa Aromatis

Benzena

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Teori ResonansiTeori Resonansi1. Bentuk Resonansi merupakan imaginer

– benzena mempunyai suatu struktur hibridyang merupakan gabungan dari semua bentuk struktur resonansi.

Struktur ResonansiBentuk “canonical”

Struktu Hibrid

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StrukturStrukturTeori Resonance benzena• Semua ikatan : equivalen• Elektron : terdelokalisasi mengelilingi

cincin

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• Struktur resonansi Kekulé• Bentuk/Struktuk canonical / resonansi• Strukur Hibrid / Resonansi

C

CC

C

CCH

H

H

H

H

H

C

CC

C

CC

H

H

H

HH

HC

CC

C

CC

H

H

H

H

H

H

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• Bentuk Resonansi hanya berbeda pada posisi elektron

• Posisi atom tidak berubah• Makin banyak Struktur Resonansi,

molekulnya semakin stabil.

Molekule distabilkan oleh resonansi

Teori ResonansiTeori Resonansi

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AromatisitasAromatisitas

Kriteria aromatis :• Cincin• planar• Terkonjugasi

– Overlaping orbital p terjadi pada seluruh atom• Elektron = (4n + 2) elektron

• Sistem stabilisasi resonansi mempunyai karakter yang harus dipenuhi

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AromatisitasAromatisitasContoh : Benzena

• Cincin

• Planar

• Terkonjugasi

• Elektron = 6 elektron

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AromatisitasAromatisitas

Contoh lain : Senyawa berikut aromatis bukan ?

cyclic

planar

conjugated 6 electrons X

cyclic

planar

conjugated 6 electrons X

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Tata Nama (Nomenclature)• Substituent diikuti benzena

• Ada penamaan umum

CH3

Metilbenzena(Toluena)

Cl NO2 OH

NH2

OH

OOMe

Nitrobenzena

Metoksibenzena(Anisol)

Amino benzenaAnilina

Hidroksi benzenaFenol

Klorobenzena

Asam Benzena KarboksilatAsam Benzoat

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Tata Nama (Nomenclature)Substituen yang sama diberi awalan di- (2), tri- (3), tetra-(4) dst.

CH3

CH3

1

2

3

4

5

6 CH3

CH3

1

2

3

4

5

6CH3

CH3

1

2

3

4

5

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1,2-Dimetilbenzenao-Dimetilbenzenao-Xilena (o: ortho)

1,3-Dimetilbenzenam-Dimetilbenzenam-Xilena (m: meta)

1,4-Dimetilbenzenap-Dimetilbenzenap-Xilena (p: para)

ReaksiReaksi

Substitusi Elektrofilik pada Aromatis

Reaksi yang terjadi pada Benzena : substitusi, bukan adisi

Br

BrBr2

BrBr

Br2never observed

H BrBr2

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Substitusi Elektrofilik pada AromatisAdisis Elektrofilik tidak terjadi karena akan merusak aromatisitas.

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A. Substitusi Electrofilik Aromatis

1. Reaksi Umum :

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A. Substitusi Electrofilik Aromatis

Mekanisme reaksi - two steps :

+ E B

H E

+ B

H E H EStep 1:

Resonansi menstabilkankarbokation

H E

+ B

E

+ H-B

Step 2:

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2. Reaksi dengan asam kuat (H2SO4, HBr, dll)

E+ = H+

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3. Halogenasi

a. Reaksi :+ X2

X

FeX3 when X = ClAlCl3 is also used

X = Cl, Brb. Mekanisme

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3. Halogenasi

Iodinasi

Klorinasi

Brominasi

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4. Nitrasi

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4. Nitrasia. Reaksi menggunakan H2SO4!

(from H2SO4)

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5. Sulfonasi

Mekanisme reaksi

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6. Alkilasi Friedel - Craftsa. Reaksi:

X = Cl, Br

b. mekanisme

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7. Asilasi Friedal - Craftsa. Reaksi

R C

O

Cl

CO R

AlCl3+ HCl

+

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b. mekanisme

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Pengaruh gugus pengarah Substitusi Elektrofilik pada Aromatis

Reaksi pada anisol :

Gugus metoksi pengarah ortho, para.

Benzena dua substituen ada3 isomer :

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Substitent EffectsSubstitent EffectsSummary

HNH2 OCH3 Ar

OH R

ORTHO-/PARA-ACTIVATORS

CHO

COOR

COOH

COR

CN

NO2

META-DEACTIVATORS

Directing:Act/Deact:

X

ORTHO-/PARA-DEACTIVATORS

Strongest activators Strongest deactivators

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

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OCH3 OCH3HE

+

OCH3HE

+

OCH3HE

+

OCH3HE

+

E+OCH3

E

particularily stable

ortho substitution

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O CH3

O H

NH2

NR2

NH CO

R

- donating

etc.

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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

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2. -electron withdrawing

E+ prefers to attack electron rich carbons - so metasubstitution is preferred!

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The alternative argument goes like this:

(same thing happens at ortho attack)

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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:

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Some not - so - simple examples and tricks!

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junk, tar…

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Tutorial Questions1. 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

CH3

+ Br2major product

5. Provide a brief synthesis for TNT starting from toluene6. Provide a mechanism for the following reaction;

Cl

O

O

AlCl3

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Tutorial QuestionsNO2

HNO3 / H2SO4 HNO3 / H2SO4

isomeric dinitrobenzenes

85%

A + B + C

1. Write a mechanism for the generation of the nitronium ion (NO2+) 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.

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Tutorial Questions

N2

N N NCH3

CH3

Cl_

+ A NaOAc, 0 ºC, H2O

B

+

1. Draw the chemical structure of A, and give a full reaction mechanism for the formation of yellow azo-compound B (Scheme 1).

NO2

A

NH2

BrBr

Br

HNO3 / H2SO4Br2 / FeBr2

Creagents X

D

D Ereagents & conditions, Y CuBr

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).

Scheme 1

Scheme 2