The reaction works best for forming asymmetrical products if the halide reactants are somehow separate in their relative
chemical reactivities. One way to accomplish this is to form the reactants with halogens of different
periods. Typically the alkyl halide is made more reactive than the aryl halide, increasing the probability that the alkyl halide will form the
organosodium bond first and thus act more effectively as a
nucleophile toward the aryl halide.[4]
Typically the reaction is used for the alkylation of aryl halides. With the use of ultrasound sodium reacts with some
aryl halides to produce biphenyl compounds.[5]
Mechanism
The mechanism of the Wurtz–Fittig reaction has not been the subject of modern investigations. The process was once proposed to involve the combination of an alkyl and aryl radicals.[6][7] Another mechanistic proposal invoked the generation of organosodium intermediates.[8] The reaction of sodium and chlorobenzene produces
triphenylene, which supports a role for radicals.[8] A role for organosodium compounds is supported by indirect evidence.[7][6] For example, addition of carbon dioxide to a mixture of sodium and isobutyl bromide results in the formation of
3-methylbutanoic acid after acid workup.
[9][10]
Use of other metals
The WurtzâFittig reaction can be conducted using metals other than sodium. Some examples include potassium, iron, copper, and lithium.[11] When lithium is used, the reaction occurs with appreciable yield only under ultrasound.[12] Ultrasound is known to cleave halogen atoms from aryl and alkyl halides through a free-radical mechanism[13]
Applications
The WurtzâFittig reaction has limited applicability, because it is plagued by side reactions including rearrangements and eliminations.[11] The reaction has been applied to the laboratory synthesis of some
organosilicon compounds.[14] One example is the conversion of
tetraethyl orthosilicate to the mono-tert-butoxy derivative in 40% yield as summarized in this idealized equation:[15]
^Adam, Waldemar; Richter, Markus J. (1994). "One-Pot Synthesis of α-Trimethylsilyl Enones from Vinylsilanes". Synthesis. 1994 (2): 176â180.
doi:
10.1055/s-1994-25433.
ISSN0039-7881.