Summary 617The Suzuki reactioncouples an aryl halide with an organoborane.
The required organoborane is obtained from the reaction of an alkene with catecholbo-
rane. Because alkenes are readily available, this method can be used to prepare a wide
variety of alkyl benzenes.
PROBLEM 24Describe how the following compounds could be prepared from benzene:a. CHCH 2 CH 2 CH 3 b. CH 2 CH 2 CH 2 CH 2 CH 3CH 3catecholboraneCH 3 CH CH 2OOHCHB 3 CH 2 CH 2OO+ BSummary
To be classified as aromatic, a compound must have an un-
interrupted cyclic cloud of electrons that contains an odd
number of pairsof electrons. An antiaromaticcom-
pound has an uninterrupted cyclic cloud of electrons with
an even number of pairsof electrons. Molecular orbital
theory shows that aromatic compounds are stable because
their bonding orbitals are completely filled, with no elec-
trons in either nonbonding or antibonding orbitals; in con-
trast, antiaromatic compounds are unstable because they
either are unable to fill their bonding orbitals or they have a
pair of electrons left over after the bonding orbitals are
filled. As a result of their aromaticity, the cyclopentadienyl
anion and the cycloheptatrienyl cation are unusually stable.
An annuleneis a monocyclic hydrocarbon with alternat-
ing single and double bonds. A heterocyclic compoundis a
cyclic compound in which one or more of the ring atoms is a
heteroatom—an atom other than carbon. Pyridine, pyrrole,
furan, and thiophene are aromatic heterocyclic compounds.
Benzene’s aromaticity causes it to undergo electrophilic
aromatic substitution reactions. The electrophilic addition
reactions characteristic of alkenes and dienes would lead to
much less stable nonaromatic addition products. The most
common electrophilic aromatic substitution reactions are
halogenation, nitration, sulfonation, and Friedel–Crafts acy-
lation and alkylation. Once the electrophile is generated, all
electrophilic aromatic substitution reactions take place by
the same two-step mechanism: (1) The aromatic compound
reacts with an electrophile, forming a carbocation intermedi-
ate; and (2) a base pulls off a proton from the carbon that
pppppformed the bond with the electrophile. The first step is rela-
tively slow and endergonic because an aromatic compound
is being converted into a much less stable nonaromatic inter-
mediate; the second step is fast and strongly exergonic be-
cause the stability-enhancing aromaticity is being restored.
Some monosubstituted benzenes are named as substituted
benzenes (e.g., bromobenzene, nitrobenzene); some have
names that incorporate the name of the substituent (e.g.,
toluene, phenol, aniline). Bromination or chlorination re-
quires a Lewis acid catalyst; iodination requires an oxidizing
agent. Nitrationwith nitric acid requires sulfuric acid as a
catalyst. Either an acyl halide or an acid anhydride can be
used for Friedel–Crafts acylation, a reaction that places an
acyl group on a benzene ring. If the carbocation formed from
the alkyl halide used in a Friedel–Crafts alkylationreaction
can rearrange, the major product will be the product with the
rearranged alkyl group. A straight-chain alkyl group can be
placed on a benzene ring via a Friedel–Crafts acylation reac-
tion, followed by reduction of the carbonyl group by catalytic
hydrogenation, a Clemmensen reduction, or a Wolff–
Kishner reduction. Alkylbenzenes with straight-chain alkyl
groups can also be prepared by means of coupling reactions.
A benzene ring can be sulfonated with fuming or con-
centrated sulfuric acid. Sulfonationis a reversible reac-
tion; heating benzenesulfonic acid in dilute acid removes
the sulfonic acid group. The principle of microscopic
reversibilitystates that the mechanism of a reaction in the
reverse direction must retrace each step of the mechanism
in the forward direction in microscopic detail.an organoborane propylbenzenePd(PPh 3 ) 4
NaOHCH 3 CH 2 CH 2OO++B + NaCl CH 2 CH 2 CH 3
HOOOB