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Environmental fate & pathways

Phototransformation in air

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Description of key information

Phototransformation in air ((Z)-octadec-9-enol): Estimated rate constant: 79.7E-12 cm3/molecule.sec (half-life: 4.8 hours) estimated using a reliable QSAR method (AOPWIN, v1.92). 

Key value for chemical safety assessment

Half-life in air:
4.8 h
Degradation rate constant with OH radicals:
0 cm³ molecule-1 s-1

Additional information

No measured data are available for (Z)-octadec-9-enol. (Z)-Octadec-9-enol contains no chromophores that would absorb visible or UV radiation, so direct photolysis is not likely to be significant. Indirect photolysis resulting from gas-phase reaction with photochemically-produced hydroxyl radicals may occur.

The AOPWIN program (v1.92) has been used to obtain values of the rate constant kOH. The overall half-life in air under default conditions of hydroxyl radical concentration was calculated using the following expressions:

kdegair(d-1) = kOH(cm3/molecule.sec) x OH Concair(molecules/cm3) x 24 x 3600

DT50(d) = ln 2/ kdegair(d-1)

Where:

kdegair= total rate constant for degradation in air

kOH= rate constant for reaction with hydroxyl radicals

OH Concair= concentration of hydroxyl radicals in air = 5 E+05 OH molecules/ cm3

DT50= half-life

The concentration of hydroxyl radicals in air of 5 E+05 OH molecules/ cm3, and the 24 hour photoperiod, are the values specified in ECHA Guidance on Information requirements and chemical safety assessment, Part R.16 Environmental exposure estimation (ECHA, 2012).

Discussion of trends in the Category of C6-24 linear and essentially-linear aliphatic alcohols:

Two measurements of atmospheric degradation rate are available for registration substances, for hexan-1-ol and octan-1-ol. Comparison of these measured rates with the predicted rates suggests that the estimated half-lives are realistic and slightly conservative.

To fill data gaps for this endpoint, which is not a REACH requirement but is helpful in making a realistic assessment of degradation processes in the environment, the rate of degradation by hydroxyl radicals in the atmosphere has been based upon estimates made using the SRC AOPWIN v1.92 program (part of the EPIWeb suite v4.01). The nomenclature for the equivalent substances used in the HPV SIAR and published information is also included for clarity.

The half-life for photo-oxidation in air, based on a hydroxyl radical concentration of 5 x 105 molecules/cm3, varies between 12 hours (estimated for 1-docosanol) to approximately 30 hours (measured for 1-hexanol).  

 

2-Alkyl branched structures and unsaturated structures are both predicted to be photodegraded at a faster rate than saturated, linear structures of equivalent carbon number.

 

Table: Available measured atmospheric photodegradation data

Chemical Name

CAS

QSAR estimated photodegradation rate and half-life

Measured photodegradation rate and half-life

Reliability

Reference for measured data

1-Hexanol

111-27-3

 9.7 x 10-12

cm3/molecule.sec

Half-life: 39.6 hours

12.5 x 10-12cm3/molecule.sec

Half-life: 30.8 hours

2

Kwok, E.S.C. and Atkinson, R., 1994

 1-Octanol

111-87-5

 12.5 x 10-12cm3/molecule.sec

Half-life: 30.7 hours

14.4 x 10-12cm3/molecule.sec

Half-life: 26.7 hours

2

Atkinson, R., 1994

References:

Atkinson, R., 1994, Gas-Phase Tropospheric Chemistry of Organic Compounds.   J. Phys. Chem. Ref. Data Monograph No. 2

ECHA (2012). European Chemicals Agency. Guidance on information requirements and chemical safety assessment Chapter R.16: Environmental Exposure Estimation. Version: 2.1 October 2012.

Kwok, E.S.C. and Atkinson, R., 1994 cited in the AOPWIN v 1.91 Program Database (USEPA, 2000) (full original reference not available)