Ppt mekanisme reaksi_elektrofilik
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Transcript of Ppt mekanisme reaksi_elektrofilik
Mekanisme Reaksi Substitusi Elektrofilik (SE)reaksi substitusi elektrofilik, pada umumnya terjadi pada senyawa aromatik, dan penyerangnya adalah sebuah elektrofil (E+)
Khususnya pada senyawa benzena
Mekanisme reaksinya:
E + Katalis E+ + katalis
Dua tahapan yaitu;
1. pembentukan elektrofil dengan bantuan katalis
Elektrofil (E+)
2. Penyerangan elektrofil pada cincin benzena
E+
H
E
E
H+H
E
H
Elambat cepat
Beberapa contoh pembentukan elektrofil:
dengan bantuan katalis
RCl + AlCl3 R+ + AlCl4-
Br2 + FeBr3 Br+ + FeBr4-
HNO3 + H2SO4 NO2+ + H2O
H2SO4 (berasap) SO3H+ + HSO4-
Elektrofil (E+)
HNO3 + H2SO4 NO2+ + H2O
1. Beberapa reaksi SE pada cincin benzenaR
O R R
[ H ]
RCOCl, RCOOCOR/AlCl3
RCl AlCl3
NO2
HNO3H2SO4
SO3H
X
SO3 / H2SO4
X2{Cl2, Br2, I2)
halogenasi
Sulfonasi
alkilasi asilasi
nitrasi
Contoh:
asilasi Friedel-Crafts O
Cl
AlCl3
O
+ AlCl3 + HCl
ClAlCl3
+ AlCl3 + HCl
O
O
Mekanisme reaksinya.................
Mekanisme reaksinya.................
O
Bagaimana membuat senyawa tersebut dari
benzena....?
O
asilasi Friedel-Crafts dalam industri pembuatan bensaldehid
O
H
1. CO + HCl +tekanan2. Benzena3. AlCl3/CuCl
O
H
?
Contoh:
alkilasi Friedel-Crafts
+Cl AlCl3 ?
+AlCl3 ?Cl
Notice the stabilization of
formed carbocation as
electrophile
Benzene alkylation with akene
+ HF
Benzene alkylation with an alcohol
+ H2SO4
OH
?
Obtain the mayor product and its reaction mechanism
when an alkylhalide is reacted with benzene by Friedel-
Crafts alkylation
1. chloroetane
2.chloropropane
3. 2-chlorobutane
4. 1-chloro-2,2-dimetihylpropane
5. 1-chloro-2-methylpropane
6. 1-chloro-2-propene
2. Fungsionalization og subtituted benzene tersubstitusi (Second substituen)
GE+
Orto /Para Atau Meta
If G is electron or releasing activated benzene ring
If G is electron withdrawing or deactivated benzene ring
G is first subtituen and direct the second subtituen (E+) and effect reaction rate
Three factors effect aromatic SE reactivity:
(1) Kinetics(2) Electron density(3) Stability of formed complex (related to the present of
resonance)
Kinetis,Second substituen orientation can be determined by subtituent constanta formula
k = equilibrium constante with substituentko = equilibrium constante without substituent β = RX factorσ = substituent constante
Electron density,
Substract electron density is effected by first subtituen. It can be noticed from their effect: mesomery (M) and inductive effect (I).
Examples:Subtituen –NH2 (M) = + dan (I) = - o,p directingSubtituen –CH3 (M) = + dan (I) = + o,p directing (Ekstrim)
Subtituen –Cl dan –OH (M) = + dan (I) = - o,p directingSubtituen –NO2 (M) = - dan (I) = - m directing (Ekstrim)
Complex stability
Resonance structure showed stability grade. Second subtituen orientation on aromatics (benzene): 0,p or m, is depend on stabilisation resonance
Examples: Draw the resonance structure of monosubtituted benzene when react with electrophile as second subtituent on otho, para, dan meta position. Find the most stable carbocationl
OCH3
?
Cl
?
NO2
?
NH2
?
Several Substituents
G Orientation Rate
R- / Ar- orto , para faster-OH / -OR orto , para Faster- NH2, NHR, NR2 orto , para Faster
- X (halogen) orto , para Slower-C=O meta Slower - CN meta Slower - NO2 meta Slower
- SO3H meta Slower
Both subtituen can undergo sinergism or antagonis effetc for third substituen involve sterical effect
3. Third Substituted Benzene
NO2
NO2
CH3
NO2
H3C NO2
• A methoxy group is ortho, para-directing, and a carbonyl group is meta-directing. The open positions of the ring that are activated by the methoxy group in p-methoxyacetophenone are also those that are meta to the carbonyl, so the directing effects of the two substituents reinforce each other. Nitration of p-methoxyacetophenone yields 4-methoxy-3-nitroacetophenone.
• All the substituents in 2,6-dibromoanisole are ortho, para-directing, and their effects are felt at
• different positions. The methoxy group, however, is a far more powerful activating substituent
• than bromine, so it controls the regioselectivity of nitration.
There are several factors effect the third substitue to
H3C NO2 H3C NO2
Br2
FeBr3
Br
1.
Observe and give your conclution:
...............................................
..............................................
2.
H3C OH H3C OHBr2
FeBr3
Br
What is your conclution:
...............................................
..............................................
3. OCH3
OCH3
Br2
FeBr3
H2N
H2N
Br+
OCH3
H2N
Br
tidaktidak
78%22%
What is your conclution
...............................................
..............................................
Dalam sintesis turunan senyawa benzena, pengarah ini sangat penting diperhatikan. Misalnya pada sinteis bromonitrobenzena, kedudukan kedua substituen saling menentukan. Faktor substituen mana yang didahulukan
Br
NO2
Br
NO2
Br
NO2
?
?
?
For Piridin compound occurs at C3
4. Electrophilic substitution on heterocyclic aromatic compound
N
Br2
H2SO4
N
Br
H2SO4
NaNO3KNO3
N
O2N
H2SO4
HgSO4
N
HO3S
Furan,pirol, tiofen occurs at C 2 dan C3 (mainly)
S
H2SO4
SSO3H
S
NBS
SBr
S
Al2O
SNO2
HNO3
AcOH+
S
NO2
S
Ac2O
SCOCH3
SnCl4
S
HCHO
SCH2Cl
HCl
Br
NO2
Br
NO2
(i) (ii)
How to synthesize compound below from benzene
NO2
OCH3
Br
Br
Br
OH
( i ) ( ii ) ( iii )
How to synthesize compound below from benzene
How to synthesize compound below from benzene
CO2H
BrO2N
OH
CHO
O
(i) (ii) (iii)
CH3
NO2O3N
NO2
OH
(iv) (v)
36
Disubstituted Benzenes
1. When the directing effects of two groups reinforce, the new substituent is located on the position directed by both groups.
37
2. If the directing effects of two groups oppose each other, the more powerful activator “wins out.”
38
3. No substitution occurs between two meta substituents because of crowding.
39
Synthesis of Benzene Derivatives
In a disubstituted benzene, the directing effects indicate which substituent must be added to the ring first.
Let us consider the consequences of bromination first followed by nitration, and nitration first, followed by bromination.
40
Pathway I, in which bromination precedes nitration, yields the desired product. Pathway II yields the undesired meta isomer.
41
Halogenation of Alkyl Benzenes
Benzylic C—H bonds are weaker than most other sp3 hybridized C—H bonds, because homolysis forms a resonance-stabilized benzylic radical.
As a result, alkyl benzenes undergo selective bromination at the weak benzylic C—H bond under radical conditions to form the benzylic halide.
42
43
Note that alkyl benzenes undergo two different reactions depending on the reaction conditions:
• With Br2 and FeBr3 (ionic conditions), electrophilic aromatic substitution occurs, resulting in replacement of H by Br on the aromatic ring to form ortho and para isomers.
• With Br2 and light or heat (radical conditions), substitution of H by Br occurs at the benzylic carbon of the alkyl group.