CHAPTER 16: REACTIONS OF SUBSTITUTED BENZENES
16.1 How Some Substituents on a Benzene Ring Can Be Chemically Changed
Reactions of Alkyl Substituents
A bromine will selectively substitute for a benzylic hydrogen in a radical substitution
Once a halogen has been placed in the benzylic position, it can be replaced by a nucleophile
by means of an S N or S N reaction.
Halo-substitute- alkyl groups can also undergo E2 and E1 reactions. Notice that a bulky
base (tert-BuO ) is used to encourage elimination over substitution. Substituents with double and triple bonds can undergo catalytic hydrogenation. The catalyst
used for hydrogenation of a C≡N to an amine is Raney nickel. Recall that addition of
hydrogen to a double or triple bond is an example of a reduction reaction, a reaction that
increases the number of C–H bonds or decreases the number of C–O, C–N, or C–X bonds in
a compound ( X denotes a halogen).
Because benzene is an unusually stable compound, it can be reduced only at high
temperature and pressure.
An alkyl group bonded to a benzene ring can be oxidized to a carboxyl group. Recall that
when an organic compound is oxidized, either the # of C–O, C–N, or C–X (X denotes a
halogen) bonds increases or the number of C–H bonds decreases.
Commonly used oxidizing agents are potassium permanganate (KMnO ) and aci4ic
solutions of sodium dichromate (H , Na Cr2O 2. 7ecause the benzene ring is so stable, it
will not be oxidized – only the alkyl group is oxidized.
Regardless of the length of the alkyl substituent, it will be oxidized to a COOH group,
provided that a hydrogen is bonded to the benzylic carbon. If the alkyl group lacks a benzylic hydrogen, the oxidation reaction will not occur because
the first step in the oxidation reaction is removal of a hydrogen from the benzylic carbon.
The same reagents that oxidize alkyl substituents will oxidize benzylic alcohols to benzoic
If, however, a mild oxidizing agent such as MnO is2used, benzylic alcohols are oxidized to
aldehydes or ketones, depending on the substituent attached to the benzylic carbon.
Reducing a Nitro Substituent
A nitro substituent can be reduced to an amino substituent. Catalytic hydrogenation is
commonly used to carry out this reaction.
16.2 The Nomenclature of Disubstituted and Polysubstituted Benzenes Naming Disubstituted Benzenes
The relative positions of 2 substituents on a benzene ring can be indicated either by numbers
or by prefixes ortho, meta, and para.
Adjacent substituents are called ortho, substituents separated by one carbon are called meta,
and substituents located opposite one another are designated para.
If the 2 substituents are different, they are listed in alphabetical order. The first mentioned
substituent is given the 1-position, and the ring is numbered in the direction that gives the
second substituent the lowest possible number.
Naming Polysubstituted Benzenes
If the benzene ring has more than two substituents, the substituents are numbered in the
direction that results in the lowest possible numbers. The substituents are listed in
alphabetical order, each preceded by its assignment #. As with disubstituted benzenes, if one of the substituents can be incorporated into a name,
that name is used and the incorporated substituent is given 1-position. The ring is then
numbered in the direction that results in the lowest possible numbers.
16.3 The Effect of Substituents on Reactivity
Like benzene, substituted benzenes undergo the 5 electrophilic aromatic substitution
reactions discussed in Chapter 15:
Substituents that donate electrons to the benzene ring will stabilize the partially positively
charged transition state, thereby increasing the rate of electrophilic aromatic substitution;
these are called activating substituents. Activating Substituents – a substituent that increases the reactivity of an aromatic ring.
Electron-donating substituents activate aromatic rings toward electrophilic attack, and
electron-withdrawing substituents activate aromatic rings toward nucleophilic attack.
In contrast, substituents that withdraw electrons from the benzene ring will destabilize the
transition state, thereby decreasing the rate of electrophilic aromatic substitution; these are
called deactivating substituents.
Deactivating Substituents – a substituent that decreases the reactivity of an aromatic ring.
Electron-withdrawing substituents deactivate aromatic rings toward electrophilic attack, and
electron-donating substituents deactivate aromatic rings toward nucleophilic attack.
Inductive Electron Withdrawal
If a substituent that is bonded to a benzene ring is more electron withdrawing than a
hydrogen, it will draw the σ electrons away from the benzene ring more strongly than a
Withd+awal of electrons through a σ bond is called inductive electron withdrawal.
The NH gro3p is an example of a substituent that withdraws electrons inductively because
it is more electronegative than a hydrogen.
Electron Donation by Hyperconjugation
Alkyl substituents (such as CH )3are more electron donating than a hydrogen because of
Carbon is actually a little more electron withdrawing than hydrogen (because C is more
electronegative than H, but hyperconjugation more than makes up for this inductive effect). Resonance Electron Donation and Withdrawal
If a substituent has a lone pair on the atom directly attached to the benzene ring, the lone
pair can be delocalized into the ring; these substituents are said to donate electrons by
resonance. Substituents such as NH , 2H, OR, and Cl donate electrons by resonance. These
substituents also withdraw electrons inductively because the atom attached to the benzene
ring is more electronegative than a hydrogen.
If a substituent is attached to the benzene ring by an atom that is doubly or triply bonded to a
more electronegative atom, the electrons of the ring can be delocalized onto substituent;
these substituents are said to withdraw electrons by resonance.
Substituents such as C=O, C≡N, SO H, 3nd NO withd2aw electrons by resonance.
Relative Reactivity of Substituted Benzenes
The activating substituents make the benzene ring more reactive toward electrophilic
aromatic substitution; the deactivating substituents make the benzene ring less reactive.
Activating substituents donate electrons to the ring and deactivating substituents withdraw
electrons from the ring.
Electron-donating substituents increase the reactivity of the benzene ring toward
electrophilic aromatic substitution.
Electron-withdrawing substituents decrease the reactivity of the benzene ring toward
electrophilic aromatic substitution.
See Table 16.1 on Pg. 669 All the strongly activating substituents donate electrons to the ring by resonance and
withdraw electrons from the ring inductively.
The moderately activating substituents also both donate electrons to the ring by resonance
and withdraw electrons from the ring inductively.
The moderately activating substituents donate electrons by resonance in 2 competing
directions: to the ring and away from the ring. Overall they donate electrons by resonance
more strongly than they withdraw electrons inductively.
Alkyl, aryl, and CH–CHR groups are weakly activating substituents. The fact that they are
weak activators indicates that they are slightly more electron donating than they are electron
withdrawing. The halogens are weakly deactivating substituents. Like the strongly and moderately
activating substituents, the halogens donate electrons to the rings by resonance and withdraw
electrons from the ring inductively. However, they withdraw electrons inductively more
strongly than they donate electrons by resonance.
The moderately deactivating substituents all have a carbonyl group directly attached
to the benzene ring. A carbonyl group withdraws electrons from a benzene ring both
inductively and by resonance.
16.4 The Effect of Substituents on Orientation
The substituent already attached to the benzene ring determines the location of the new