mecoprop-P (ISO); (R)-2-(4-chloro-2-methylphenoxy)propionic acid

Phototransformation in air

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

phototransformation in air

Type of information:

calculation (if not (Q)SAR)

Remarks:

Estimated according to the method described in the OECD Environment Monograph No. 61.

Adequacy of study:

key study

Study period:

07 April 1994 to 16 April 1994

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

accepted calculation method

Qualifier:

according to guideline

Guideline:

other: OECD Environment Monographs No. 61: The rate of photochemical transformation of gaseous organic compounds in air under tropospheric conditions.

Version / remarks:

1992

Deviations:

no

GLP compliance:

yes

Estimation method (if used):

PHOTOCHEMICAL TRANSFORMATION OF GASEOUS ORGANIC COMPOUNDS IN AIR
Numerous chemicals, both natural and anthropogenic, are emitted to the troposhere from a variety of sources and may be removed by wet deposition or by three important transformation processes. These three transformation processes are:
(1) Direct photoreaction which involves the absorption of sunlight followed by transformation;
(2) Indirect photoreaction which involves the reaction of a chemical with hydroxyl radicals (OH); and
(3) Oxidation which involves the reaction of a chemical with ozone (O3).
There is another indirect photoreaction which involves the reaction of a chemical with nitrate radicals (NO3) during the night. However, this transformation process only occurs for a few types of chemicals (e.g., olefins, phenols and cresols); hence, it will not be considered when determining the rate of transformation in the troposphere.
Of the direct and indirect phototransformation processes possible in the troposphere, reaction with OH-Radicals is generally the most important. This is because reaction with OH-Radicals is the most rapid phototransformation process for the majority of organic chemicals (Atkinson R 1985 - Atkinson R 1988a).
Reaction with ozone is generally of secondary importance. Only for low molecular weight unsaturated aliphatics may reaction with ozone in the troposphere be more rapid than reaction with OH-Radicals.
Direct phototransformation reactions may also be very rapid but only for a limited number of organic chemicals. The rate of a direct phototransformation reaction depends on the overlap between the solar light emission spectrum under tropospheric conditions and the light absorption spectrum of the compound, and on the quantum yield, i.e. the fraction of the molecules of the organic chemical that is transformed after absorption of a photon. The quantum yield can be as high as 1 but for most organic compounds lies in the range of 0.1 to 0.001.
The test material is not a low molecular unsaturated aliphatic, it does not absorb light and its quantum yield was estimated to be less than 0.23 mol x Einstein^-1 (Klöpffer W 1991), consequently reaction with the OH-Radicals during daylight hours is expected to be the dominant atmospheric removal process for this compound.
The kinetics of the reaction of a chemical with OH-Radicals in the gas phase, at a constant temperature, can be treated mathematically in the following manner:
C + OH → Products (with kOH being the second order rate constant determined during the reaction)
and
-(dC/dt) = kOH x C x [OH]

where kOH is the second-order rate constant in cm^3 x molecule^-1 x s^-1 and C and [OH] represent the concentration of the chemical and hydroxyl radicals, respectively. In gas-phase chemistry, the OH-Radicals and chemical concentration are usually expressed in molecule x cm^-3.
Since the chemical is usually present in the troposphere at very low concentration at a given time t and a steady-state concentration of OH-Radicals is produced by sunlight, the hydroxyl radical concentration can be treated as a constant so that equation becomes a pseudo first-order equation:
-(dC/dt) = k x C
Where:
k = kOH x [OH]

The pseudo first-order rate constant k is in the units’ reciprocal time, usually s^-1. Integrating the equation -(dC/dt) = k x C under the boundary conditions (t=0, Co) and (t, Ct) yields:
ln(C0/Ct) = k x t

The half-life t½ in the troposphere is defined as the time for the chemical concentration to reach one-half its initial concentration. Therefore, under boundary condition (Ct= C0/2, t=t½) the equation above yields:
t½ = In2 / (kOH x [OH])
with [OH] = 5 x 10^5 molecules x cm^-3 in the northern hemisphere.

Four techniques are available for estimating OH-Radical rate constant (kOH).
One described by Hendry and Kenley (1979) and updated by Davenport et al. (1986) is based on structure-activity relationships (SAR).
The second technique, proposed by Heicklen (1981) for abstraction reactions, is based on bond energies.
The third method was proposed by Zetzsch (1982), who showed that a good correlation exists between the electrophilic substituent constant (σi+) and kOH for mono- and polysubstituted benzenes.
The fourth and most recent method of Atkinson (1985, 1986, 1987 and 1988b) is conceptually similar to that of Hendry and Kenley (1979).
Atkinson critically analysed the hydroxyl radical rate data (kOH) for a large number of organic chemicals and developed a number of SAR based solely on the molecular structure of these chemicals. In developing these SAR, he assumed that a number of OH-Radical reaction pathways exist and the various OH-Radical reaction pathways could be separated and treated individually. Therefore, he calculated rate constants for each of these reaction pathways. The reaction pathways and rate constants for each of these pathways were:
kabstr = k(H-atom abstraction from C-H and O-H bonds)
kadd = k(OH-Radical addition to C=C and C≡C bonds)
karom = k(OH-Radical addition to aromatic rings)
kinter = k(OH-Radical interaction with N-, S- and P- containing groups)
Atkinson postulated that the overall OH-Radical rate constant kOH is equal to the sum of the rate constant for each of these reaction pathways. Therefore, the OH-Radicals rate constant is given by the equation:
kOH = kabstr + kadd + karom + kinter

kabstr, kadd, karom and kinter are expressed in cm^3 x molecule^-1 x s^-1

DT50:

21 h

Test condition:

Estimation

Reaction with:

OH radicals

Rate constant:

0 cm³ molecule-1 s-1

Transformation products:

not specified

ESTIMATION RATE OF REACTION AT 298 K OF THE TEST MATERIAL WITH OH-RADICALS

- H-Atom abstraction rate constant kabstr

kabstr = k1(CH3-X) + k2(CH3-X) + k(X-CH(Y)-Z) + k(HO -X)

= 9.28884 x 10^-12 cm^3 x molecule^-1 x s^-1

 

- OH-Radicals addition to unsaturated carbon-carbon bonds rate constant kadd

There are no unsaturated carbon-carbon bonds in the test material, consequently kadd = 0

 

- OH-Radicals addition to aromatic rings constant karom

karom = 9.000 x 10^-12 cm^3 x molecule^-1 x s^-1

Calculation performed with PCGEMS Software - (General Sciences Corporation).

 

- Overall OH-Radical rate constant kOH and half-life

kOH = kabstr + kadd + karom

with:

kabstr = 9.28884 x 10^-12

kadd = 0

karom = 9.000 X 10^-12

 

Hence:

kOH = 18.28884 x 10^-12 cm^3 x molecule^-1 x s^-1

k = kOH x [OH] = 18.28884 x 10^-12 x 5 x 10^5 ≈ 9.1444 x 10^-6 s^-1

 

And

t½ = ln2 / (kOH x [OH]) = 0.693 / 9.1444 x 10^-6

= 75 784 s = 1263 min ≈ 21 daylight hours.

Discussion

If the test material is emitted into the troposphere it will be removed rapidly by chemical reaction with OH-Radicals. The estimated reaction rate constant at 298 K is 9.1444 x 10^-6 s^-1 which corresponds to a half-life of about 21 daylight hours. Nevertheless, the upper layers of the troposphere (10 to 12 km altitude) have temperatures of 233 to 213 K; this causes a reduction in rate constants derived at room temperature which should be kept in mind.

The main reactions will be probably abstraction of OH-Radicals from -CH3 and -CH groups and addition of OH-Radicals to the aromatic ring.

Validity criteria fulfilled:

not applicable

Conclusions:

Under the conditions of this calculation, the test material is removed rapidly from the troposphere by photoreaction with OH-Radicals. The estimated reaction rate constant at 298 K is 9.1444 x 10^-6 s^-1 which corresponds to a half-life of about 21 daylight hours.

Executive summary:

The rate of photochemical transformation of the test material in the atmosphere under tropospheric conditions was estimated according to the method described in the OECD Environment Monographs No. 61: The rate of photochemical transformation of gaseous organic compounds in air under tropospheric conditions (1992) and in compliance with GLP.

Under the conditions of this calculation, the test material is removed rapidly from the troposphere by photoreaction with OH-Radicals. The estimated reaction rate constant at 298 K is 9.1444 x 10^-6 s^-1 which corresponds to a half-life of about 21 daylight hours.

The main reactions will probably be abstraction of OH-Radicals from -CH3 and -CH groups and addition of OH-Radicals to the aromatic ring.

Reference 2

Endpoint:

phototransformation in air

Type of information:

calculation (if not (Q)SAR)

Adequacy of study:

supporting study

Study period:

02 February 1999 to 22 June 1999

Reliability:

4 (not assignable)

Rationale for reliability incl. deficiencies:

documentation insufficient for assessment

Qualifier:

no guideline followed

Principles of method if other than guideline:

Estimates of the rate constant and half-life for the atmospheric gas-phase reaction between the test material and hydroxyl radicals were obtained using a computer program, ‘Atmospheric Oxidation Program, Version 1.51, Syracuse Research Corporation’.

GLP compliance:

yes

Estimation method (if used):

Estimates of the rate constant and half-life for the atmospheric gas-phase reaction between the test material and hydroxyl radicals were obtained using a computer program, ‘Atmospheric Oxidation Program, Version 1.51, Syracuse Research Corporation’. The program uses estimation methods based upon the structure-activity relationships developed by Atkinson and co-workers.
The Atmospheric Oxidation Program estimated the rate constant for the atmospheric, gas-phase reaction between photochemically produced hydroxyl radicals and organic chemicals. It also estimated the rate constant for the gas-phase reaction between ozone and olefinic/acetylenic compounds. The rate constants estimated by the program are then used to calculate atmospheric half-lives for the organic compounds based upon average atmospheric concentrations of hydroxyl radicals and ozone.
The Atmospheric Oxidation Program is supplied with a table containing experimental and calculated hydroxyl radical rate constants for 479 chemicals. Each time the program is used to estimate a rate constant for a new substance, a reference chemical is selected from this table and used to show the program is functioning correctly. The reference is selected to be as close in structure as possible to the substance under test. In cases where the test substance is a complex multi-functional molecule the use of several reference substances may be required to cover all functionalities.

DT50:

7.4 h

Reaction with:

OH radicals

Rate constant:

0 cm³ molecule-1 s-1

Transformation products:

not measured

The test material has a hydroxyl radical rate constant of 17.4 x 10^-12 cm^3/molecule-sec.

Assuming a hydroxyl radical concentration of 1.5 x 10^6 cm^3, a half-life of 7.4 hours was obtained.

Validity criteria fulfilled:

not specified

Conclusions:

Under the conditions of the study the test material has a hydroxyl radical rate constant of 17.4 x 10^-12 cm^3/molecule-sec. Assuming a hydroxyl radical concentration of 1.5 x 10^6 cm^3, a half-life of 7.4 hours was obtained.

Executive summary:

The rate constant and half-life for the atmospheric gas-phase reaction between the test material and hydroxyl radicals were obtained using a computer program, ‘Atmospheric Oxidation Program, Version 1.51, Syracuse Research Corporation’.

Under the conditions of the study the test material has a hydroxyl radical rate constant of 17.4 x 10^-12 cm^3/molecule-sec.

Assuming a hydroxyl radical concentration of 1.5 x 10^6 cm^3, a half-life of 7.4 hours was obtained.

Description of key information

Maestracci (1994)

Under the conditions of this calculation, the test material is removed rapidly from the troposphere by photoreaction with OH-Radicals. The estimated reaction rate constant at 298 K is 9.1444 x 10^-6 s^-1 which corresponds to a half-life of about 21 daylight hours. On the assumption that a day has 12 daylight hours the DT50 would be 21/12 =  1.75 days.

Supporting Study: Comb (2000b)

Under the conditions of the study the test material has a hydroxyl radical rate constant of 17.4 x 10^-12 cm^3/molecule-sec. Assuming a hydroxyl radical concentration of 1.5 x 10^6 cm^3, a half-life of 7.4 hours was obtained.

Key value for chemical safety assessment

Half-life in air:

1.75 d

Degradation rate constant with OH radicals:

0 cm³ molecule-1 s-1

Additional information

Maestracci (1994)

The rate of photochemical transformation of the test material in the atmosphere under tropospheric conditions was estimated according to the method described in the OECD Environment Monographs No. 61: The rate of photochemical transformation of gaseous organic compounds in air under tropospheric conditions (1992) and in compliance with GLP. The study was awarded a reliability score of 2 in accordance with the criteria set forth by Klimisch et al. (1997).

Under the conditions of this calculation, the test material is removed rapidly from the troposphere by photoreaction with OH-Radicals. The estimated reaction rate constant at 298 K is 9.1444 x 10^-6 s^-1 which corresponds to a half-life of about 21 daylight hours.

The main reactions will probably be abstraction of OH-Radicals from -CH3 and -CH groups and addition of OH-Radicals to the aromatic ring.

Supporting Study: Comb (2000b)

The rate constant and half-life for the atmospheric gas-phase reaction between the test material and hydroxyl radicals were obtained using a computer program, ‘Atmospheric Oxidation Program, Version 1.51, Syracuse Research Corporation’. The study was awarded a reliability score of 4 in accordance with the criteria set forth by Klimisch et al. (1997).

Under the conditions of the study the test material has a hydroxyl radical rate constant of 17.4 x 10^-12 cm^3/molecule-sec. Assuming a hydroxyl radical concentration of 1.5 x 10^6 cm^3, a half-life of 7.4 hours was obtained.