arah reaksi, gugus pengarah
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ARAH REAKSI, GUGUS ARAH REAKSI, GUGUS AKTIVASI DAN DEAKTIVASIAKTIVASI DAN DEAKTIVASI
O CH3
Nitration of AnisoleNitration of Anisole
NO2
O CH3
NO2
O CH3
Reacts faster than benzene
+
ortho para
= “ACTIVATED”
The -OCH3 group when it preexists on the ring gives only ortho and para products, and no meta.
Substituents that cause this result are called o,p directors
HNO3
H2SO4
and they usually activate the ring.
anisole
ACTIVATED RING
CO
OMe
Nitration of Methyl BenzoateNitration of Methyl Benzoate
CO
NO2
OMe
Reacts slower than benzene
meta
HNO3
H2SO4
= “DEACTIVATED”
methyl benzoate
The -COOMe group when it preexists on the ring gives onlymeta, and no ortho or para products.
Substituents that cause this result are called m directors
and they usually deactivate the ring.
DEACTIVATED RING
Most ring substituents fall into one of these two categories:
o,p - directors m- directors
activate the ring deactivate the ring
SUBSTITUENT CATEGORIESSUBSTITUENT CATEGORIES
We will look at one of each kind in order tounderstand the difference…..
G
NITRATION OF ANISOLE
H
H NO2
O CH3
+H
H
NO2
O CH3
+
H NO2
H
O CH3
+
O CH3
+ N
O
O
+
Nitration of AnisoleNitration of Anisole
NO2
O CH3
NO2
O CH3
BENZENIUM ION INTERMEDIATES
actualproducts
activatedring
ortho meta para
ortho para+
H NO2
H
O CH3
+
H NO2
H
O CH3
+
H NO2
H
O CH3+
H NO2
H
O CH3
+
H
H
NO2
O CH3
+
H
H
NO2
O CH3
+
H
H
NO2
O CH3
+H
H
NO2
O CH3+
H
H NO2
O CH3
+H
H NO2
O CH3
+H
H NO2
O CH3
+
ortho
meta
para :
:
EXTRA!
EXTRA!
Energy ProfilesEnergy Profiles
meta
ortho
para
NITRATION OF ANISOLEbenzeniumintermediateRECALL:
HAMMONDPOSTULATE
Ea
benzeniumintermediateshave moreresonance
ortho-paradirector
H
HNO2
O CH3
+
:B elimination_
H
NO2
O CH3
H
HNO2
O CH3
+:B
addition
_
H
HNO2
B
O CH3
doesn’t happenresonance would be lost
restores aromatic ringresonance
ADDITION REACTION
ELIMINATION REACTION
BENZENIUM IONS GIVE ELIMINATION INSTEAD OF ADDITION
( 36 Kcal / mole )
X
NITRATION OF METHYL BENZOATE
H
H
CO
NO2
OMe
+
HC
O
H
OMe
NO2+
CO
OMe
H NO2
H+
CO
OMe
+ N
O
O
+
Nitration of Methyl BenzoateNitration of Methyl BenzoateBENZENIUM ION INTERMEDIATES
CO
NO2
OMe
actualproduct
deactivatedring
ortho meta para
meta
H
H
CO
NO2
OMe
+
H
H
CO
NO2
OMe
+
H
H
CO
NO2
OMe
+
ortho
meta
para
HC
O
H
OMe
NO2+
HC
O
H
OMe
NO2+
HC
O
H
OMe
NO2+
C
OOMe
H NO2
H+
CO
OMe
H NO2
H
+
CO
OMe
H NO2
H+
BAD!
BAD!
ortho
meta
para
Energy ProfilesEnergy ProfilesNITRATION OF METHYL BENZOATE
somebenzeniumresonancestructureshave a badsituation
metadirector
DIRECTIVITY OF SINGLE GROUPSDIRECTIVITY OF SINGLE GROUPS
orthoortho, , parapara - Directing Groups - Directing Groups
X
Groups that donateelectron density to the ring.
XX :
+I Substituent +R Substituent
CH3-
R-
CH3-O-
CH3-N-
-NH2
-O-H
These groups also“activate” the ring, ormake it more reactive.
E+
The +R groups activatethe ring more stronglythan +I groups.
..
..
..
..
..
..
increasedreactivity
PROFILE:
X YY
metameta - Directing Groups - Directing Groups
X
Groups that withdrawelectron density fromthe ring.
These groups also“deactivate” the ring,or make it less reactive.
E+
-I Substituent -R Substituent
C
O
R
C
O
OR
C
O
OH
C N
NO
O
N
R
R
R
CCl3
-SO3H
+
decreasedreactivity
+
-
PROFILE:
Halides - Halides - o,po,p Directors / Deactivating Directors / Deactivating
X
E+
: :..
Halides represent a special case:
They are o,p directors (+R effect )
They are deactivating ( -I effect )
Most other other substituents fall into one of these four categories:
1) +R / o,p / activating
2) +I / o,p / activating
3) -R / m / deactivating
4) -I / m / deactivating
+R / -I / o,p / deactivating
They are o,p directing groups that are deactivating
-F-Cl-Br-I
THE EXCEPTION
CH3
O CH3
NO2
CO
O CH3
PREDICT !PREDICT !
o,p m
o,p m
DIRECTIVITY OF MULTIPLE GROUPSDIRECTIVITY OF MULTIPLE GROUPS
GROUPS ACTING IN CONCERTGROUPS ACTING IN CONCERT
O CH3
NO2
m-director
o,p director
HNO3
H2SO4 O CH3
NO2
NO2 majorproduct
verylittleformed
O CH3
NO2O2N
stericcrowding
When groups direct to thesame positions it is easy topredict the product.
GROUPS COMPETINGGROUPS COMPETING
o,p-directing groups winover m-directing groups
HNO3
H2SO4
O CH3
NO2
NO2
O CH3
NO2
O2N
O CH3
NO2
toocrowded
X+
HNO3
H2SO4
RESONANCE VERSUS INDUCTIVE EFFECTRESONANCE VERSUS INDUCTIVE EFFECT
O CH3
CH3
NO2
O CH3
CH3
+R
+I
resonance effects are moreimportant than inductive effects
majorproduct
SOME GENERAL RULESSOME GENERAL RULES
1) Activating (o,p) groups (+R, +I) win over deactivating (m) groups (-R,-I).
2) Resonance groups (+R) win over inductive (+I) groups.
3) 1,2,3-Trisubstituted products rarely form due to excessive steric crowding.
4) With bulky directing groups, there will usually be more p-substitution than o-substitution.
5) The incoming group replaces a hydrogen, it will not usually displace a substituent already in place.
PR : HOW CAN YOU MAKE ...PR : HOW CAN YOU MAKE ...
CO
O CH3
NO2
CH3
NO2
NO2
NO2
O2N
CH2CH2CH2CH3
only,no para