Page 217 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

P1: GPJ/GLT P2: GQT Final Pages
Encyclopedia of Physical Science and Technology en011-542 July 26, 2001 15:33

Organic Chemistry, Synthesis 503

Sulfonation of arenes is one of the oldest and best- units which can be formed or assembled by known or con-
known reactions for the introduction of sulfur into organic ceivable synthetic transforms. The retrosynthetic analysis
molecules. Treatment with sulfuric acid and oleum or of the carbon skeleton determines which carbon carbon
chlorosulfonic acid leads to the preparation of sulfonated bonds should be formed and in what order they should be
aromatics that are useful as surfactants, dye constituents, assembled. After analyzing the molecule and determining
or chemical intermediates: convenient fragments, the emphasis now focuses on de-
termining which carbon carbon bond forming reactions
SO 3 H
can be employed.
H 2 SO 4
(34) Carbon carbon bond forming reactions can be de-
SO 3
scribed as being ionic processes, in which one fragment
is electron deficient (electrophilic) and the other fragment
Sulfonation under acidic conditions is of little utility in
is electron rich (nucleophilic), or as radical processes in
the preparation of aliphatic sulfonates. Alkyl sulfonates
which each fragment contributes a single electron. Tradi-
are selectively prepared by reaction of alkanes with sulfur
tionally, ionic processes have been the better understood
dioxide and chlorine. When the reaction is conducted in
and more effectively manipulated. However on occasion,
the presence of ultraviolet light, side reactions such as
reactions with radical intermediates have been described
chlorination are effectively suppressed:
and effectively employed as well. Ionic processes will be
h described as resulting from nucleophilic or electrophilic
RCH 3 SO 2 Cl 2 RCH 2 SO 2 Cl (35) components. Such a distinction is somewhat artificial, but
parallels traditional descriptions for ionic carbon carbon
The initial product of these reactions, as well as the prod-
bond forming reactions in which one component must
uct of the reaction of arenes with chlorosulfuric acid, are be electrophilic and the other nucleophilic. Typical
sulfonyl chlorides which may be hydrolyzed to the desired electrophilic reactions such as alkylation and acylation
acids. and typical nucleophilic reagents such as organometallic
compounds, enamines, and deprotonated imines will be
1. Sulfide Formation discussed.
Sulfides may be prepared by the reaction of alkali metal
salts such as sodium sulfide with alkyl halides. Dialkyl sul-
A. Alkylation
fides are prepared by the reaction of the alkali metal salt of
an alkyl sulfide with a second molecule of an alkyl halide: As mentioned earlier alkylations are described as elec-
trophilic from the perspective of the alkylating agent. The
RSNa R CH 2 X R CH 2 SR NaX (36) electrophilic alkylation of alkanes or alkenes is known but
is only of limited utility because of the difficulty of achiev-
Alternatively alkyl halides may be reacted with thiourea
ing selectivity. Nonetheless, the electrophilic alkylation of
to form sulfides upon hydrolysis. Sulfides are also
arenes is a reaction of substantial significance.
formed by the Markovnikov addition of hydrogen
sulfide to alkenes. Alkyl sulfides will add in a conjugate
manner to α,β-unsaturated carbonyl compounds, giving
1. Alkyl Halides
β-ketodialkyl sulfides.
Reports of the reaction of alkyl halides with arenes in
the presence of Lewis acids appeared in the literature
II. CARBON CARBON BOND as early as 1877. Although alkyl fluorides are the most
FORMING REACTIONS reactive of the alkyl halides, alkyl chlorides and bro-
mides are the most widely used. Alkyl iodides are less
The preparation of the functional groups most important commonly used as a result of accompanying side reac-
in organic synthesis has been described, but in the de- tions and decomposition. Tertiary and benzylic halides
sign of a synthesis, the construction of the carbon skele- are the most reactive, with secondary halides less reactive
ton is often the greatest challenge. Frequently the desired but more reactive than primary halides. Dialkyl halonium
functionality is either protected to prevent undesired side ions prepared by treatment of excess alkyl halide with
reactions or is carried through a synthetic sequence in a antimony pentafluoride are especially reactive alkylating
masked form, to be liberated after other transformations reagents. However, the most reactive alkylating agents
have been accomplished. These protected or masked func- are the methyl fluoride–antimony pentafluoride and ethyl
tional groups have been described as synthons, structural fluoride–antimony pentafluoride complexes: