Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 47

impede solvent reactions with alkali metals. The examples in Table 1.21 are ordered by chemical
species and year cited.

1.2.6.2 Antioxidants
Antioxidants reduce the solvent's potential to form oxidation products (Archer, 1984). The degree to
which a solvent can undergo oxidation depends on its chemical structure, its exposure to ultraviolet
light, and, if used in a vapor degreaser, its boiling point. Solvents with lower boiling points—
dichloromethane (40°C) and methyl chloroform (74°C)—are less susceptible to oxidation than
solvents with higher boiling points, such as perchloroethylene (121°C). The ethylene bond in TCE
and perchloroethylene increase their vulnerability to oxidative attack.
Antioxidants usually fall into three chemical groups: phenols, amines, and amino-phenols, all of
which contain an unsaturated benzene ring with either an amine group or a phenol group.
Antioxidants suppress the free radical chain reaction that decomposes unsaturated solvents by form-
ing stable resonance hybrids after losing a hydrogen atom to an oxidation-free radical and slowing
the propagation step of auto-oxidation (Joshi et al., 1989).
Table 1.22 provides a list of commonly cited antioxidant compounds added to the major chlori-
nated solvents.
Although some sources list antioxidants in methyl chloroform, they are largely unnecessary.
Methyl chloroform is not prone to oxidation reactions because it does not have a double bond
(Archer, 1984). The use of phenolic compounds as antioxidants for TCE has caused problems for
extracting essential oils from l avorings and for making decaffeinated coffee. For example, p - t -amyl
phenol is unsuitable because of toxicity reasons.
A 1957 patent mentions that n -methyl pyrrole is a widely used antioxidant for TCE and is effec-
tive against normal air, light, and heat decomposition (Starks, 1957). However, contemporaneous
patents note that pyrroles used as antioxidants have a tendency to form a dark purple sludge and that
TCE stabilized with pyrrole becomes discolored over time when stored for several months in drums
(Kauder, 1960; Willis and Christian, 1957). One proposed remedy to prevent the decomposition of
pyrrole-stabilized TCE is the addition of diisopropylamine (Ferri and Patron, 1959). Furthermore,
n -methyl pyrrole itself was shown to have stability problems, and an organometallic chelate com-
pound was required to stabilize it (Starks, 1957).
Motorola's industrial chemists capitalized on the propensity for ethylene compounds to undergo
oxidative attack by proposing that these compounds be used as stabilizers for methyl chloroform.
Motorola's patent notes that Chlorothene VG, a widely used grade of methyl chloroform sold by
Dow Chemical, will withstand semiconductor degreasing conditions for only one or two hours. For
methyl chloroform, Motorola's chemists proposed a stabilizer package that uses TCE, perchloroeth-
ylene, and l uorine as free radical scavengers to meet the stringent demands of degreasing semicon-
ductor devices while also satisfying EPA regulations. Free radical scavengers combine with
trichloromethyl radicals as follows (Goodner et al., 1977):

R 3 CH + Cl 3 C → R 3 C + Cl 3 CH. (1.13)

Use of TCE, perchloroethylene, and l uorine as free radical scavengers in methyl chloroform will
trap trichloromethyl radicals as intermediates before the acidic decomposition of these radicals
takes place.

1.2.6.3 Light Inhibitors
A variety of chemicals have been patented to stabilize perchloroethylene against the oxidative attack
of ultraviolet light. “Light inhibitors” claimed in patents have included n -methyl morpholine, isoeu-
genol (4-hydroxy-3-methoxy-1-propenylbenzene), alkyl cyanide compounds such as alkylaminoal-
kylcyanide, and nitrile compounds such as -methoxyacetonitrile (Stevens,

1955; Strain and De Witt, 1956; Dial, 1957; Starks, 1957; Skeeters, 1959, 1960b). The mechanism

β -ethoxyacetonitrile and β