Senyawa Aromatis Benzena
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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
Struktur Teori Resonance benzena • Semua ikatan : equivalen • Elektron : terdelokalisasi mengelilingi cincin
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• Struktur resonansi Kekulé • Bentuk/Struktuk canonical / resonansi • Strukur Hibrid / Resonansi
H H
H
H H
C C
C
C
C C 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 4
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
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
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Aromatisitas Contoh : Benzena • Cincin • Planar • Terkonjugasi • Elektron = 6 elektron
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Aromatisitas Contoh lain : Senyawa berikut aromatis bukan ?
cyclic
cyclic
planar
planar
conjugated
conjugated
6 electrons X
6 electrons X 8
Tata Nama (Nomenclature) • Substituent diikuti benzena • Ada penamaan umum CH3
Metilbenzena (Toluena) NH2
Klorobenzena
OH
NO2
Cl
Nitrobenzena
OMe
Hidroksi benzena Fenol O OH
Amino benzena Anilina
Metoksibenzena (Anisol)
Asam Benzena Karboksilat Asam Benzoat
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Tata Nama (Nomenclature) Substituen yang sama diberi awalan di- (2), tri- (3), tetra(4) dst. 6
6
CH3
6
CH3
CH3
5
1
5
1
5
1
4
2
4
2
4
2
3
CH3
1,2-Dimetilbenzena o-Dimetilbenzena o-Xilena (o: ortho)
3
CH3
1,3-Dimetilbenzena m-Dimetilbenzena m-Xilena (m: meta)
CH3
3
1,4-Dimetilbenzena p-Dimetilbenzena p-Xilena (p: para)
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Reaksi Reaksi yang terjadi pada Benzena : substitusi, bukan adisi Br
Br2
Br
Br Br
Br2
never observed
Substitusi Elektrofilik pada Aromatis H
Br2
Br
Substitusi Elektrofilik pada Aromatis Adisis 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 : H
Step 1:
E
H
H
E
E
+ E B
Resonansi menstabilkan karbokation
+ B
Step 2: H
E
E
+ B
+ H-B
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2. Reaksi dengan asam kuat (H2SO4, HBr, dll) E+ = H+
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3. Halogenasi X
a. Reaksi : FeX3 + X2
when X = Cl AlCl3 is also used
X = Cl, Br
b. Mekanisme
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3. Halogenasi Brominasi
Klorinasi
Iodinasi
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4. Nitrasi
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4. Nitrasi a. Reaksi menggunakan H2SO4! (from H2SO4)
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5. Sulfonasi Mekanisme reaksi
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6. Alkilasi Friedel - Crafts a. Reaksi:
X = Cl, Br
b. mekanisme
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7. Asilasi Friedal - Crafts a. Reaksi
O O
+ R C Cl
AlCl3
C
R
+ HCl
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b. mekanisme
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Pengaruh gugus pengarah Substitusi Elektrofilik pada Aromatis
Benzena dua substituen ada 3 isomer :
Reaksi pada anisol :
Gugus metoksi pengarah ortho, para.
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Substitent Effects Summary Strongest deactivators
Strongest activators NH2
OCH3
OH
Directing: Act/Deact:
R
ORTHO-/PARAACTIVATORS
Ar
H
COOR
X
CHO
ORTHO-/PARADEACTIVATORS
COR
COOH
NO2
CN
METADEACTIVATORS
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There are two ways to look at these patterns. 1. -donating substituents - let’s use E+ as the electrophile:
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ortho substitution OCH3 E+
OCH3 H +
OCH3 H E
E +
OCH3 H E
OCH3 E
+
+OCH3
H
particularily stable
E
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- donating O CH3
NH2
O NH C R
etc. O H
NR2
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31
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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 33
2. -electron withdrawing
E+ prefers to attack electron rich carbons - so meta substitution is preferred! 34
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 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 +
major product
Br2
CH3 5. Provide a brief synthesis for TNT starting from toluene 6. Provide a mechanism for the following reaction; O AlCl3 Cl O
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Tutorial Questions HNO3 / H2SO4
NO2 HNO3 / H2SO4
A + B +C isomeric dinitrobenzenes
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 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 1. Draw the chemical structure of A, and give a full reaction mechanism for the formation of yellow azo-compound B (Scheme 1). +
N2 Cl
_
+ A
CH3
NaOAc, 0 ºC, H2O
N
N
N CH3
B Scheme 1
NO2 HNO3 / H2SO4
NH2
Br2 / FeBr2
reagents X
C
A
Br reagents & conditions, Y
D
E
Scheme 2
D
Br
CuBr
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
