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Environmental fate & pathways

Hydrolysis

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Administrative data

Endpoint:
hydrolysis
Type of information:
other: handbook
Adequacy of study:
supporting study
Study period:
2000
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data

Data source

Reference
Reference Type:
publication
Title:
Handbook of Property Estimation Methods for Chemicals.
Author:
Boethling R and Mackay D
Year:
2000
Bibliographic source:
CRC Press, Boca Raton, FL, USA. cited in Neoacids C5 to C28 Category Analysis Report-IUCLID datasheet

Materials and methods

Principles of method if other than guideline:
Hydrolysis is a bond-making, bond-breaking process in which a molecule, RX, reacts with water, forming a new R-O bond with the oxygen atom from the water and breaking the R-X bond in the substance’s molecule (March, 1977). One possible pathway is the direct displacement of X with OH.
R-X + H2O -> R-OH + HX
Methods to predict the hydrolysis rates of organic compounds for use in the environmental assessment of pollutants have not advanced significantly since the early 80s of the 20th century. Two approaches have been used extensively to obtain estimates of hydrolytic rate constants for use in environmental systems. The first and potentially more precise method is to apply quantitative structure/activity relationships (QSARs). To develop such predictive methods, one needs a set of rate constants for a series of compounds that have systematic variations in structure and a database of molecular descriptors related to the substituents on the reactant molecule. The second and more widely used method is to compare the target compound with an analogous compound or compounds containing similar functional groups and structure, to obtain a less quantitative estimate of the rate constant.
Some preliminary examples of hydrolysis reactions illustrate the very wide range of reactivity of organic compounds. For example, triesters of phosphoric acid hydrolyze in near-neutral solution at ambient temperatures with half-lives ranging from several days to several years (Wolfe, 1980), whereas the halogenated alkanes pentachloroethane, carbon tetrachloride, and hexachloroethane have "environmental" (pH = 7; 25 °C) half-lives of about 2 hr, 50 yr, and 1000 millennia, respectively (Mabey and Mill, 1978; Jeffers et al., 1989). On the other hand, pure hydrocarbons from methane through the PAHs are not hydrolyzed under any circumstances that are environmentally relevant.
GLP compliance:
no

Test material

Radiolabelling:
no
Remarks:
not applicable, theoretical evaluation only

Study design

Analytical monitoring:
no
Remarks:
not applicable, theoretical evaluation only
Positive controls:
no
Negative controls:
no

Results and discussion

Transformation products:
not measured
Remarks:
not applicable, theoretical evaluation only
Dissipation DT50 of parent compound
Remarks on result:
not measured/tested
Remarks:
not applicable, theoretical evaluation only
Details on results:
Estimation of Hydrolysis Rate Constants Based on Analogy
Because of the large number of organic compounds and the diversity of their structures and reactivity, it often is not possible to use the more precise and reliable QSARs to estimate hydrolysis rates (Karickhoff et al., 1991). However, even for compounds for which no data or QSARs exist, one often can estimate hydrolytic activity by structural analogy to related compounds for which kinetic data exist. An EPA report (Köllig et al., 1993) used this approach extensively in assessing hydrolysis rate constants and reaction pathways. In that report, the authors assigned chemicals to one of three categories, NHFG, NLFG, and HG.
1. No hydrolysable functional groups (NHFG)
NHFG compounds are those that do not have any heteroatoms that can undergo hydrolysis over the pH range of 5 to 9 at 25 °C. Examples include xylenes, carboxylic acids, and polycyclic aromatic hydrocarbons (PAHs),
2. No labile functional groups (NLFG)
NLFG compounds contain one or more heteroatoms that can react, but they react so slowly over the pH range of 5 to 9 at 25 °C that their half-lives will be greater than 50 years, if they react at all. Examples of these compounds include anilines/amines, halogenated aromatics, and ethers.
3. Hydrolysable groups (HG)
HG compounds have functional groups more labile to hydrolysis. For compounds that can be deduced to be reactive but for which no measured or calculated rate constants can be obtained, rate constants can often be estimated semi-quantitatively by comparison to compounds for which hydrolysis data are available.
The general approach is straightforward. First, the reaction pathway(s) is outlined, based on fundamental reaction chemistry. Often this can be done by comparison to the known reactions of similar compounds in the same class (i.e., having the same functional groups). Second, a literature search is performed to collect hydrolysis rate constants for this class of compounds or other compounds with similar structure. Third, the compound of interest and its analogs are examined for similarity in structure and substituents, and an estimate of the rate constant(s) for the untested compound is made by interpolation from the analog data.

Any other information on results incl. tables

Table 1. Classes of compounds undergoing hydrolysis

Compounds

Remarks

Carboxylic acid esters

They hydrolyse by base-promoted reactions at pH 5-6.

Amides

Less hydrolytically reactive than esters. Typical half-lives under environmental conditions – hundreds to thousands of years.

Halocarbons

In fresh waters they hydrolyse to the corresponding alcohol. For polyhalogenated alkanes, a 1,2 or 1,1 elimination is the most general elimination reaction.

Epoxides

Hydrolyses occurs by neutral, acid- or base- mediated reactions. Acid and neutral processes generally dominate over the range of environmental pH.

Nitriles

They undergo acid and alkaline hydrolysis to the corresponding amide first, and then to carboxylic acid and ammonia.

Carbamates

They can undergo hydrolysis depending on the substituents on the N atom. When an alkyl substituent is present on the N atom, hydrolysis is much slower.

Sulfonylureas

The hydrolysis reaction is highly pH dependend. The principal cleavage occurs at the sulfonylurea bridge.

Organophosphate esters

Hydrolysis can occur by direct nucleophilic attack at the P atom, without the formation of a pentavalent intermediate.

Applicant's summary and conclusion

Validity criteria fulfilled:
not applicable
Remarks:
theoretical evaluation only
Conclusions:
Recent years have seen limited advances in formulating quantitative prediction correlations for hydrolysis rate constants. Fortunately, numerous experimental studies provide pH-dependent hydrolysis rate constants for one or more compounds in most classes of organics that might be of environmental concern. Estimation of reactivity by comparison with structural analogs within a given class is often the fastest and most reliable approach.
Consideration of the benchmark chemicals illustrates this approach. Anthracene and 2,6-di-tert-butylphenol have no hydrolysable functional groups (i.e., are NLFG compounds), hence they cannot undergo hydrolysis. Trichloroethylene hydrolysis has been reported (Jeffers et aL, 1989; Jeffers and Wolfe, 1996), but the measured rate constants imply an environmental half-life at pH 7 and 25 °C of 100,000 years. Similarly long half-lives have been calculated for other halogenated ethenes, so that, as a class, hydrolysis can be disregarded for these compounds.