Since, substitution is only a minor component, most of the rate acceleration comes
from reaction at the. Recall
that stabilization of the reactant tends to increase the activation energy for
a reaction and to decrease the rate. Incidentally, the electrophile could be
any of the electrophiles which we have studied in this unit (nitration,
bromination, F-C alkylation and acylation, etc.). the rate. Hence we call this reaction is an electrophilic addition reaction. addition reaction, alkyl The halogen
substituents essentially are rule-breakers. Alkynes also react with bromine liquid and give alkyl halides as products. Next, one carbon atom in the brominium ion is TS’s generate extensive carbocation character on the ring carbon directly
attached to the methyl group, which can then exert a strong stabilizing effect
on the TS. In the example which follows we are substituting a CH3CO- group into the ring, but you could equally well use any other alkyl group instead of the CH3. and especially
strongly so when the carbocation character resides on the ring carbon to which
the EWG is directly attached. Consider, first, an
electrophilic subsitution reaction (nitration) of toluene as a specific
example. First, a bromine atom is added to the propene through double bond. The result is that substitution at both the ortho and para positions
is accelerated relative to benzene, and also relative to the meta position. q
In contrast, both the para and ortho
TS’s generate extensive carbocation character on the ring carbon directly
attached to the methyl group, which can then exert a strong stabilizing effect
on the TS. Some substituents, the rate in comparison to benzene ( in comparison to
hydrogen as the “substituent”) and others. You could go through the
same TS arguments for the methoxy or the dimethylamino substituent. A dramatic example is the reaction (or
rather the non-reaction) with bromine. Legal. In next step, Br- or Cl- or I- can connect to give three different products if both Cl- and I- are in the presence with Br2. Due to predence of water, there are hydroxyl ions with bromide ion. The six electrons are shared equally by all six carbon atoms. The arenium ion does not do this
because the reaction path involving proton elimination regenerates the
aromaticity of the ring, while reaction at the ortho position would have given
a product which is not at all aromatic, and would have lost even the resonance
stabilization of the arenium ion.. We have just seen
that the qualitative sense of the
reactivity of benzene (and other aromatics) is quite different from the
reactivity mode preferred in simple alkenes. q
The rationalization of
the positional selectivity and relative rate effects is parallel to that
discussed for EDG’s, except that now the EWG’s destabilize
carbocation character and especially
strongly so when the carbocation character resides on the ring carbon to which
the EWG is directly attached. q
Using this predominant
character, we can now reason about the effect that a specific substituent would
have on the energy of this TS and therefore upon the rate and selectivity of
the relevant reaction of a monosubstituted arene having this substituent
present on the ring. Also it adds to through the pi bond. Strictly speaking iron isn't a catalyst, because it gets permanently changed during the reaction. Substitution Reactions of Benzene and Other Aromatic Compounds. Mechanism of Alkylation of Benzene with Methyl Halides. In this case, we have to go to the mechanism of the reaction to understand what will happen and what are the products can be given. We have also seen that. q
To compare the relative
rate of nitration of toluene with that of benzene, we should use the Method of
Competing TS’s. Ethylbenzene is produced in on a large scale by combining benzene and ethylene in an acid-catalyzed chemical reaction. The electrophilic substitution mechanism. q
Also as before, the
strongest base available to accept the proton from the arenium ion is the
negatively charged tetravalent aluminate species. (Brδ+) and other bromine atom gets negatively charged (Brδ-). Benzene alkylation with ethene over zeolite H-ZSM-5 has been investigated using density functional theory.