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Phototransformation in air

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phototransformation in air
Type of information:
calculation (if not (Q)SAR)
Adequacy of study:
supporting study
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
accepted calculation method
The result was obtained by the valid application of a well-established predictive method.
Principles of method if other than guideline:
The result was obtained by the valid application of a well-established predictive method.
GLP compliance:
21.2 h
Reaction with:
OH radicals
Rate constant:
0 cm³ molecule-1 s-1
Transformation products:
not specified

Estimated rate constant:  18.2E-12 cm3/molecule.sec 
Half-life:  21.2 hours

Description of key information

Phototransformation in air (dodecan-1-ol): Estimated rate constant: 18.2E-12 cm³/molecule.sec (half-life: 21.2 hours) estimated using a reliable QSAR method (AOPWIN, v1.92).

Key value for chemical safety assessment

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

Additional information

No measured data are available for dodecan-1-ol. Dodecan-1-ol 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 (v1.91) 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)


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 5E+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, 2016).

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 5E+05 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


QSAR estimated photodegradation rate and half-life

Measured photodegradation rate and half-life


Reference for measured data



 9.7E-12 cm3/molecule.sec

Half-life: 39.6 hours

12.5E-12 cm3/molecule.sec

Half-life: 30.8 hours


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



 12.5 E-12 cm3/molecule.sec Half-life: 30.7 hours

14.4 E-12 cm3/molecule.sec

Half-life: 26.7 hours


Atkinson, R., 1994


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

ECHA (2016). European Chemicals Agency. Guidance on information requirements and chemical safety assessment Chapter R.16: Environmental Exposure Estimation. Version: 3.0 February 2016.

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