Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 35

TABLE 1.17
Acidity Generation by Boiling Trichloroethylene and Perchloroethylene

Test Condition Trichloroethylene Perchloroethylene

Dry solvent under nitrogen 10.2 a 10.5 a

Wet solvent under nitrogen 7.7 12.5

H 2 O-saturated solvent exposed to soft steel under nitrogen 11.0 11.6

Wet solvent exposed to soft steel under nitrogen 5.6 —

Dry solvent in oxygen 820 782

Note: Reported as cubic centimeters of 0.01 N hydrochloric acid per 25 mL solvent (from Shepherd, 1962).
a

Air leakage introduced oxygen.

In the absence of air and metals, solvents will undergo hydrolysis at slow rates and form only minor
quantities of acids. TCE and perchloroethylene both hydrolyze at a rate of 0.01 mg/L per day in the
absence of air and metals (6.8 × 10 −7 mol% and 6.2 × 10 −7 mol%), respectively (Carlisle and Levine,
1932). Citations of Carlisle and Levine's work record the rates of solvent decomposition listed in
Table 1.17 when boiling TCE and perchloroethylene are subjected to the specii c conditions given.
Through hydrolysis, methyl chloroform reacts abiotically to form 1,1-dichloroethylene:

Cl H H Cl

⎜ ⎜ \

/

H +

CC

Cl ⎯ C ⎯C ⎯H + HCl +

/

⎜ ⎜ \

(1.3)

Cl H H Cl

Hydrogen

Methyl
chloroform

Hydrogen 1,1-dichloroethylene

chloride

Abiotic degradation of methyl chloroform has also been described as follows (Cornell Dubilier,
2003):

CO 2 + H 2 O+C - CH 3 CCl 3 CH 2 CCl 2 +CH 3 COOH

Methyl
chloroform

1,1-dichloroethylene Acetic acid Mineralization

(1.4)

1.2.4.5 Solvent Deterioration Catalyzed by Metals and Metal Salts
Methyl chloroform is most vulnerable to reaction with alkali metals, a problem also faced by dichlo-
romethane and carbon tetrachloride when those two compounds are in the vapor phase. TCE and
perchloroethylene are comparatively less vulnerable to reaction with alkali metals (Smallwood,
1993). The absence of hydrogen atoms in perchloroethylene makes it less sensitive to the action of
bases and light metals (Solvay SA, 2002a). Chlorinated organic products in contact with a light or
alkali metal such as aluminum or magnesium produce some hydrochloric acid and a Lewis acid
catalyst, for example, anhydrous aluminum trichloride (AlCl 3 ). AlCl 3 will catalyze condensation of
chlorinated solvents onto themselves or onto other organic substances, especially chloride ions or
other nucleophiles. * The products of this reaction include a brownish-black, tarry condensate and
more hydrochloric acid, which in turn reacts further with aluminum. Such a reaction increases the
amount of AlCl 3 present; hence, the reaction is perpetuated. In some cases, the reaction can be

* A nucleophile, or nucleus-seeking compound , is typically negatively charged with unshared electron pairs that can be
used to form a covalent bond with a carbo-cation (Carey, 1987).