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 )



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.

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.

ppt mekanisme reaksi_elektrofilik

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