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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5574_Библиотеки_им_академика_М_И_Перельмана.pdf
HNO3 + 2H2SO
4
NO2 + 2HSO
4−
+ H3O
+
+
+ NO
2
Slow
H
NO
2
H
NO
2
H
NO
2
+
+
H
NO
2
+
+
+
H
NO
2
+ H2SO
4
NO
2
+ HSO
4
Cl
Cl + FeCl
3
Cl..........Cl.....FeCl
3
Cl + FeCl
4
Chloronium
ion
⊕
δ+ δ−
+ Cl
Slow
H
Cl
H
Cl
H
Cl
+
+
H
Cl
+
+
+
9.7.2 Mechanisms of Some Electrophilic Substitution Reactions of Benzene
1. mechanism of nitration:
+ 2
2 serves as the acid and
3
341
) to
2. mechanism of halogenation: The commonly accepted mechanism for aromatic halogenation,
+
+
342
H
—
Cl
+ FeCl
3
+ HCl
Cl + FeCl
4
Slow
δ+ δ−
+ Cl..........Cl.....FeCl
3
H
Cl + FeCl
4
+
HSO4 + H3O+ + SO
3
2H2SO
4
Slow
H+SO
3
+ SO
3
H⊕SO
3
⊕
HSO
3
⊕
HSO
3
l
3. mechanism of sulphonation: The commonly accepted mechanism for sulphonation of aromatic
3
sulphonic acid.
Slow
SO3 + HSO
4
SO
3
+ H2SO
4
⊕
H
SO
3
SO3H
Benzene sulphonic acid
+ H
2
O
+ H
3
O
⊕
RCl + AlCl
3
alkylchloride
[R --- Cl --- AlCl
3
]
R
+
+ AlCl
4
+δ −δ
Slow
R+H
+ R
+
R⊕H
⊕
RH
⊕
RH
+ AlCl
4
+ AlCl3 + HCl
R
H
R
3+
acid.
mechanism of Friedel–crafts alkylation reaction:
3
Friedel–Crafts alkylation reaction.
3
343
344
RC
O
RCCl: + AlCl3
O:
:
::
RCCl AlCl
3
O
—
+
RC⊕ + AlCl
4
O:
:
+ AlCl3
AlCl
3
AlCl
3
O:
:
O
C
O
C
O
C
O
C
O
⊕
R
R
C⊕ + RC
O
O
O
R
R
R
CO :
+
RCO :
+
C
Slow
H
⊕
C
O
R
R
+
O ⊕
C
Slow
H
⊕
C
O
R
R
+
O
⊕
5. Friedel–crafts acylation: The
group is called an acyl group
an acyl group is introduced into a compound is called an acylating agent
H
+
C
O
R
R
H
C
O
⊕
H
C
O
R
⊕
O
+
H
C
R
Fast
H
C
O
+
R + AlCl
4
COR + HCl + AlCl
3
RCCl + AlCl
3
O
RC --- Cl --- AlCl
3
O
+δ −δ
+δ −δ
[R ... Cl ... AlCl3]
R – Cl + AlCl
3
Ar–H
+δ
Ar
H
−δ
R ... Cl ... AlCl
3
+
Ar
H
R
Ar – R + HCl + AlCl
3
+ AlCl
4
345
electrophile in acylation reactions is actually the positive
Learning Plus
Latest researches have revealed that the formation of a free carbonium from alkyl halide is not
general at least for primary alkyl halides.
The metal halide catalyst function as a source
(real or potential) of a positive substituting agent
and the reaction involves the formation of the
complex:
on this account that this reaction is often employed for the
9.8 THEORY OF REACTIVITY
1. Determining the time required for reaction to occur under identical conditions.
period of time.
conditions.
346
NH
2
:
OH
::
OCH
3
::
CH
3
1.52 1.201.45 0.40
Activating substituents
Deactivating substituents
NO
2
CO2CH
3
Cl
3.97 1.91
3.90 1.56
CN
2, 3 and 3
and
2, N, 3
-
p
A group that releases electrons activates the ring, while a group that withdraws electrons deactivates the
ring.
than the meta position.
A deactivationg group, on the other hand, deactivates all positions including the meta position. But the
+
HY
(I)
(II)
+
HY
CH
3
(III)
NO
2
+
HY
Transition state
developing positive
charge
Carbonium ion
full positive charge
Y
⊕
+
HY
HY
δ+
δ+
formed more rapidly.
and thus leads to a faster reaction.
347
2 -
9.9 DIRECTIVE INFLUENCE OF GROUPS IN ELECTROPHILIC SUBSTITUTION
directive inuence of the group.
348
CH
3
Toluene o-Nitrotoluene p-Nitrotoluene
HNO
3
NO
2
NO
2
+
H
2SO4
CH
3
CH
3
COOH
Benzoic acid m-Nitrobenzoic acid
HNO
3
NO
2
H2SO
4
COOH
ortho- and
para-directing groups.
3, 2H5, Br, 2, 2, 3, 3, etc.
op
increases.
meta-directing
groups.
2, 3H, etc.
m
y in c6H5–y reaction % ortho product % meta product % Para product
3
3
2
3
3
3
Br
Br
Sulphonation
- -2 -
5- -5 -
5-8 - -5
55-65 1-5 35-
-35 5- -65
-15 2-8 85-
35- - 55-65
- 5- -
9.9.1 Orientation
349
9.9.2 Theory of Orientation Based on Stability of Carbonium Ion
1. electron release or withdrawal through inductive effect:
than the meta.
deactivates the ortho and para positions even more than the meta.
strongest at the ortho and para positions.
a. Ortho- and para-directing groups with +1 effect (carbonium ions formed by attack at para, ortho
and meta positions in toluene
3 is attached. Although 3 releases electrons to all positions
positions ortho and para to it.
350
Stable charge on carbon-carrying substituent
Para attac
k
(i) (iii)(ii)
⊕
⊕
HY H
CH
3
CH
3
CH
3
YHY
⊕
Ortho attack
(V)
⊕
H
Y
CH
3
⊕
(VI)
H
Y
CH
3
⊕
CH
3
H
Y
(IV)
Meta attack
⊕
CH
3
(VII)
H
Y
⊕
(IX)
CH
3
H
Y
⊕
H
Y
CH
3
(VIII)
Meta-directing groups with –I and –M effects (carbonium ions formed by attack at para, ortho and
meta positions of nitrobenzene):
2
2 group is
attached. Although
2
atoms.