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EPA-560/11-79-012

August 1979

STRUCTURE REACTIVITY CORRELATION S

FOR ENVIRONMENTAL REACTIONS

by

Theodore Mill

Physical Organic Chemistry Department

Physical Sciences Divisio n

7

Contract 68-01-41-09

Project Officer

Asa Leifer

OFFICE OF TECHNICAL EVALUATION

OFFICE OF TOXIC SUBSTANCES

U .S . ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D . C . 20460

Qllnttematior

63980000352
p. 2

The review of Mabey and Mill (1978) is a useful point of departure for

summarizing the SRC for hydrolysis in pure or natural water where only H20HOand H+ are important catalytic agents . Organic chemicals can hydrolyze by a

variety of processes including both solvolytic and substitution mechanisms

catalyzed by H30+ and OH- . We can narrow the scope of the enquiry by restricting

our interests to reactions proceeding at pH 7 and 25*C

. '

General and detailed

effects of structure on hydrolytic reactivity, as measured by the half-life

at pH 7, are summarized in Tables 4 through 11 . In some cases half-lives are

expressed only as > or < 1 year or some multiple thereof .* By this

classification, one can quickly differentiate the reactive from the unreactive

chemical structures . For some classes, the information on LFER can be use d

to make detailed estimates of reactivity based on structure .

Table 12 provides an summary of hydrolyzable chemical structures that

will be expected to persist in water for significantly more than a year at pH

7 and 25*C . These classes of chemicals probably need not be screened in the

laboratory for hydrolysis using the current screening tests because these tests

are designed to estimate reliably only those half-lives of a few hours to a

few months ; longer half-lives are not well-defined by the test method (Federal

Reg ., 1979) and, indeed, current assessment procedures generally focus onl y

on chemicals that exhibit loss rates of days to weeks .

Photochemistry

Measurement Methods

The cutoff for the solar spectrum by the upper atmosphere is at about

290 nm; only absorptTon of photons-b~ K emcailat this or longer wavelengths

can result in direct photochemical transformations . Direct absorption of sunlight may result in cleavage of bonds, dimerization, oxidation, hydrolysis ,

or rearrangement . No simple selection rules are available to predict the

specific chemical process that may occur, although some .useful generalizations

have been found (Calvert and Pitts, 1967 ; Turro, 1978) .

Quantitative aspects of direct photolysis in water, on soil surfaces,

or in the atmosphere have the same general kinetic relationships . The rat e

of absorption of light, IA (rate constant ka), by a chemical at one wavelength

is determined by eX, the molar absorbance ; IX, the intensity of the incident

light at wavelength X ; and [C], the concentration of chemical . At low concentrations of C where only a small percentage of the light is absorbe d

I

A(X) ' ' I X [C] = ka(X)ICI (17)

The rate of direct photolysis of a chemical at wavelength X is obtained by

multiplying ka(X) by the quantum yield ~X, which is the efficiency fo r

This time limit is somewhat arbitrary but corresponds to the limits on

estimation of rate constants using EPA screening test protocols .

15

C),
p. 3

TABLE 13. APPROXIMATE ABSORPTION REGIONS FO R

ORGANIC MOLECULES

Region

Clas~__ Example nm Reference

Aliphatics

c

hydrocarbons n-C4H10 120-170 a

fluorides, chlorides CHBr3, CHC13 180-260 a, b

bromides

-"~Iodides C3H,I 180-320 b

ethers, alcohols C,H,OH, C2H1OC2H5 150-200 a

aldehydes CH3CHO 240-340 a.

%o ketones C2HsC(O)CH3 240-320 a, b

acids CH'C(0)O 200-230 a, b

esters CH,C(O)OCH, 200-240 a

amides C2H,C(O)NHt2 180-220 a, b

amines (C2H!1)3N 180-240 a, b

azines (CH3)2C=N-N=C(CH3)2 200-29G b

azo CH3-N=N-CH3 200-230, 320-400 a

nitro CH3NO2 200-220, 260-320 a, b

nitroso (CH3)3CNO 200-300, 540-740, a

nitrite (CH3)3CONO 200-240, 320-400 a, b

nitrate C2H,ON02 200-300 a

sulfide C2H5SC2Hs 200-220 a, b

disulfide CH3SSCH3 200-240 a, b

Non-conjugated

c

Olefinics and Acetylene s

hydrocarbons (CH3)2C=C(CH3)2 170-240 a, b

aldehydes CH2=CHCHO 200-250, 290-400 b

ketones CH,CH=CH-C(O)CH3 200-240, 260-330 b

nitro CH,C(N02)=C(CH3)2 180-380 b

a Calvert and Pitts (1967) .

b

Perkampus (1971) .

c

From Hendry and Kenley (1979) .

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