81 Redox
81.1 Birch reduction.
TheBirch reduction^1 is the reduction of aromatic compounds by sodium in liquid am-
monia. It is attributed to the chemist Arthur Birch^2. The reaction product is a 1,4-
cyclohexadiene. The metal can also be lithium or potassium and the hydrogen atoms are
supplied by an alcohol such as ethanol or tert-butanol.
Figure 170 Birch reduction of aromatic rings
The first step of a Birch reduction is a one-electron reduction of the aromatic ring to a
radical anion. Sodium is oxidized to the sodium ion Na+. This intermediate is able to
dimerize to the dianion. In the presence of an alcohol the second intermediate is a free
radical which takes up another electron to form the carbanion. This carbanion abstracts a
proton from the alcohol to form the cyclohexadiene.
Figure 171 Birch reduction reaction mechanism
In the presence of an alkyl halide, the carbanion can also engage in nucleophilic substitu-
tion with carbon-carbon bond formation. In substituted aromatics an electron withdraw-
ing substituent such as a carboxylic acid stabilizes a carbanion and the least-substituted
alkene is generated. With an electron donating substituent, the opposite effect occurs. The
non-conjugated 1,4-addition product is preferred over the conjugated 1,3-diene which is ex-
plicable by the principle of least motion^3. Experimental alkali metal alternatives that are
safer to handle, such as the M-SG reducing agent^4 , also exist.
81.2 Oxidation of Benzene in the Human Body.
Because benzene is nonpolar, it cannot be passed in urine, and will remain in the body until
oxidized. Benzene itself is not dangerous to health, but in order to be passed, it is oxidized
by cytochrome P-450 in the liver. This produces benzene oxide, a highly teratogenic and
carcinogenic compound. Benzene has been replaced by toluene as an industrial solvent,
because toluene can be oxidized to benzoic acid, which is mostly harmless to health, and
1 * A. J. Birch, J. Chem. Soc. 1944 , 430.
2 https://en.wikipedia.org/wiki/Arthur%20John%20Birch
3 https://en.wikipedia.org/wiki/Principle%20of%20least%20motion
4 https://en.wikipedia.org/wiki/M-SG%20reducing%20agent