4. benzena electrophilic aromatic subst...
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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
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