4. benzena electrophilic aromatic subst_2012

Senyawa Aromatis

BenzenaBenzena

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Teori ResonansiTeori Resonansi1. Bentuk Resonansi merupakan imaginer

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

Struktur ResonansiBentuk “canonical”

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Struktu Hibrid

StrukturStrukturTeoriTeori Resonance Resonance benzenabenzena• Semua ikatan : equivalen• Elektron S : terdelokalisasi mengelilingig g

cincin

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St kt i K k lé• Struktur resonansi Kekulé• Bentuk/Struktuk canonical / resonansi• Strukur Hibrid / Resonansi

HH H

C CCH H

C CC HH

C CC HH

CC

CH H

H

CC

CH

HH

CC

CH

H

H

4

HH H

Teori ResonansiTeori Resonansi

• Bentuk Resonansi hanya berbeda pada posisi elektron S

• Posisi atom tidak berubah• Makin banyak Struktur ResonansiMakin banyak Struktur Resonansi,

molekulnya semakin stabil.

M l k l di t bilk l h i5

Molekule distabilkan oleh resonansi

AromatisitasAromatisitas• Sistem stabilisasi resonansi mempunyai

karakter yang harus dipenuhi

K it iK it i titi

karakter yang harus dipenuhi

KriteriaKriteria aromatisaromatis ::• Cincin• planar• Terkonjugasij

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

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AromatisitasAromatisitasContohContoh : : BenzenaBenzena

• Cincin 99

• Planar 99

• Terkonjugasi 99

Elektron 6 elektron 99• Elektron S = 6 elektron 99

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AromatisitasAromatisitas

Contoh lain : Senyawa berikut aromatis bukan ?bukan ?

cyclic 99

99

cyclic 99

l 99 planar 99

conjugated 99

planar 99

conjugated 99

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j g6S electrons XX6S electrons XX

Tata Nama (Nomenclature)• Substituent diikuti benzena

• Ada penamaan umum

CH3 Cl NO2

Metilbenzena( ) OH

O

NitrobenzenaKlorobenzena(Toluena)

OH NH2

OH

OMe Asam BenzoatOH NH2 Asam Benzoat

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Metoksibenzena(Anisol)

Amino benzenaAnilina

Hidroksi benzenaFenol

Tata Nama (Nomenclature)• Substituen yang sama diberi awalan di- (2), tri-

(3), tetra- (4) dst.

CH315

6 CH315

6CH3

6

CH3

1

2

3

4

5 1

2

3

4

5

CH3

1

2

3

4

5

3

CH3

3 3 3

1,2-Dimetilbenzena

1,4-Dimetilbenzenap-DimetilbenzenaDimetilbenzena

o-Dimetilbenzenao-Xilena

1,3-Dimetilbenzenam-Dimetilbenzena

p Dimetilbenzenap-Xilena (p: para)

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(o: ortho) m-Dimetilbenzenam-Xilena (m: meta)

A. Substitusi Electrofilik Aromatis

1 R k i U1. Reaksi Umum :

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A. Substitusi Electrofilik AromatisGeneral mechanism - two steps :General mechanism two steps :

+ E B

H E H E H EStep 1:

+ E B

+ B Resonansi menstabilkaResonansi menstabilkakarbokation

H E E

Step 2:

H E

+ B + H-B

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2. Reaction with strong acids (H2SO4, HBr, etc)

E+ = H+

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3. Sulfonationa. overall reaction - fuming sulfuric acid

b. mechanism

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4. A horse of a different color - nitrationa. overall reaction - note use of H2SO4!

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b. mechanism - setting up th electrophile

(from H2SO4)

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5. Halogenationa. overall reaction: X

+ X2

FeX3 when X = ClAlCl3 is also used

X Cl BX = Cl, Brb. mechanism

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6. Alkilasi Friedel - Craftsa. overall reaction: X Cl Ba. overall reaction: X = Cl, Brb. mechanism

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7. Asilasi Friedal - Craftsa overall reaction O C

O RAlCl3a. overall reaction

R C ClAlCl3

+ HCl+

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b. mechanism

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Directing effects in electrophilic aromatic substitution reactionsFor di-substituted arenes recall that there are three isomers:

So, for anisole:

22The methoxy group directs ortho, para.

Substitent EffectsSubstitent EffectsSubstitent EffectsSubstitent EffectsSummarySummarySummarySummary

HNH2 OCH3 Ar COOR COR NO2X

Strongest activators Strongest deactivators

OH R CHO COOH CN

ORTHO /PARA METADirecting: ORTHO /PARAORTHO-/PARA-ACTIVATORS

META-DEACTIVATORS

Directing:Act/Deact:

ORTHO-/PARA-DEACTIVATORS

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

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OCH3 OCH3H+OCH3H OCH3HE+

OCH3E

ortho substitution

E+

E+

HE

+

E E

OCH3H

E

+

particularily stable

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O CH3 NH2 NH CO

R

S - donating

O H NR2

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+ f t tt k l t i h b

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E+ prefers to attack electron rich carbons

2. S-electron withdrawing

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

The alternative argument goes like this:

32(same thing happens at ortho attack)

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C. Polysubstitution of aromatic compounds.•A general rule of thumb:g1. Activating o,p directors are stronger than the meta dire

deactivators. 2 The alkyl groups and halides are in-between2. 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 Wh t th f l f ti it ?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.

OHOH

CH

+ Br2major product

CH3

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

Cl

OAlCl3

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O

Tutorial QuestionsNO2NO2

HNO3 / H2SO4 HNO3 / H2SO4

isomeric dinitrobenzenes

A + B + C

85%

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 withon 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 Questions1. Draw the chemical structure of A, and give a full reaction mechanism for the formation of

N2CH3

Cl_+

yellow azo-compound B (Scheme 1).

N N NCH3

3

+ A NaOAc, 0 ºC, H2O

BScheme 1

NO2

A

NH2HNO3 / H2SO4

Br2 / FeBr2

Creagents X

A

BrBr

C

D

Br

D Ereagents & conditions, Y CuBr

Scheme 2

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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).