Chloroethane

Hydrolysis

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Acceptable, well documented publication which meets basic scientific principles (peer reviewed in OECD-SIDS)

Qualifier:

no guideline followed

Principles of method if other than guideline:

Experimental data

GLP compliance:

no

Radiolabelling:

no

Analytical monitoring:

yes

Details on sampling:

GC-MS

Transformation products:

not measured

pH:

7

Temp.:

25 °C

Hydrolysis rate constant:

ca. 0 min-1

DT50:

2.6 yr

Type:

(pseudo-)first order (= half-life)

Remarks on result:

other: r2 >=0.9

1.   The Arrhenius parameters for both the neutral and alkaline reactions are presented in the table below:

Table: Arrhenius parameters for ethyl chloride for the neutral and alkaline reactions

	A (L mol^-1 min-1)	E (kJ/mol)	K (25°C, pH 7)	t1/2**
Arrhenius parameters for the neutral reaction*	(1.4 ± 0.3) x 10^13	110.8 ± 3.8	Kn= 5.1 x 10^-7	2.6 years
Arrhenius parameters for the alkaline reaction**	(3.7 ± 0.8) x 10^13	101.2 ± 10.2	Kb= 6.7 x 10^-12	-

*  ‘‘Neutral’’ hydrolysis carried out in solutions of 0.01 M HCl 

** Alkaline hydrolysis measured in NaOH solutions ranging from 0.1 to 0.001 M NaOH. The alkaline hydrolysis half-life is determined by the “neutral” reaction.

 

Based on the results, a brominated compound is more reactive than its chlorinated analogue by a factor of 10 to 100, by both neutral and alkaline reaction mechanisms. Greater reactivity of Br versus Cl compounds is due to lower activation energy and entropy effects that also contribute in the same direction. Within a series of halogenated compounds, the order of reactivity is identical for brominated and chlorinated analogues. Substitution of F for Cl in the ethanes causes an enormous drop in neutral hydrolysis.

Validity criteria fulfilled:

not applicable

Conclusions:

The neutral reaction rate constant for the hydrolysis of chloroethane is 5.1 x 10-7, resulting in an estimated half-life of 2.6 years.

Executive summary:

Jeffers and Wolfe (1996) determined both the neutral and alkaline reaction rates for the hydrolysis of chloroethane. The neutral reaction measured in 0.01 M hydrochloric acid at 25 °C was found to predominate, with a rate constant of 5.1 x 10-7 min-1, resulting in an estimated half-life for chloroethane of 2.6 years.

Reference 2

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Acceptable, well documented publication which meets basic scientific principles (peer reviewed in OECD-SIDS)

Qualifier:

no guideline followed

Principles of method if other than guideline:

Literature review

GLP compliance:

no

Radiolabelling:

no

Analytical monitoring:

no

Estimation method (if used):

Kinetic based calculations

Transformation products:

not specified

pH:

7

Temp.:

25 °C

Hydrolysis rate constant:

0 s-1

DT50:

38 d

Type:

(pseudo-)first order (= half-life)

Remarks on result:

other: 38 d also at pH 4 and 9

1. Hydrolysis kinetic

Overall hydrolysis is a transformation of an organic chemical with water, resulting in the formation of a new covalent bond with OH. The overall reaction may be summarized as

R-X + H₂O → R-OH + HX

X represents a good leaving group (an atom much more electronegative that carbon, e.g. Cl). The detailed mechanism may involve a protonated or anionic intermediate or a carbonium ion, or any combination of these intermediates. But whatever the mechanism, the rate law for hydrolysis of substrate RX usually can be put in the form

-d[RX]/dt = K_H [RX] = K_B [OH-][RX] + K_A [H+][RX] + K_N’ [H2O] [RX] (1)

Where K_A, K_B and K_N are the second-order rate constants for acid and base catalysed and neutral processes, respectively. In water is the K_N’ [H2O] a constant (KN). KH is the observed or estimated rate constant for hydrolysis at constant pH. Equation (1) assumes that the individual rate processes for the acid, base and neutral hydrolysis obey first-order kinetics with the hydrolysable chemical, RX, it is possible to write the following equation for K_H:

K_H = K_B [OH-] + K_A [H+] + K_N (2)

From the equilibrium term for the ionisation of water, K_W:

K_W = [H+] [OH-] = 1 x 10 -14 (3)

It is possible to substitute for [OH-] in the equation (2), giving

K_H = K_B ∙ K_W / [H+] + K_A [H+] + K_N (4)

Because K_H is a pseudo first-order rate constant at a fixed pH, the half-life of hydrolysis can be calculated as below:

t1/2 = ln2 / K_H = 0.693 / K_H (5)

 

pH Dependence:

From the equation (5) it is evident how pH affects the overall rate constant for hydrolysis, K_H: at high or low pH (high OH- or H+) either base or acid catalysed term is usually dominant, while at pH 7 the neutral processes can often be the most important. However, the detailed relationship of pH and rate depends on the specific values of K_A, K_B and K_N.

The typical log K_H vs. pH can be plotted to demonstrate for the chemicals which undergo acid, water and base catalysed hydrolysis at a specific pH with a curve composited of three straight lines with slops +1, 0 and -1 respectively as below:

(a) log K_H = log (K_B ∙ K_W ) + pH

(b) log K_H = log K_N

(c) log K_H = log K_A – pH

Most log KH vs. pH curves are found to have one or two intercepts corresponding to pH values where two kinds of rate processes contribute equally to the overall process. Thus in Figure 1 of the publication (see Mabey and Mill, 1978) the intercept I_AN corresponds to a pH value where K_A [H+] = K_N, similarly, I_BN corresponds to K_B [OH-] = K_N. In cases where K_A, K_B or KN = 0, only one intercept is observed. Values of pH corresponding to the I may be calculated readily from the values of K_A, K_B and KN as below:

I_AN = - log (K_N / K_A) (6)

I_NB = - log (K_B ∙ K_W / K_N) (7)

I_AB = - [log (K_B ∙ K_W / K_A)] / 2 (8)

Use of intercepts in tabulating dat on rates of hydrolysis as a function of pH greatly simplifies the task of estimating the effect of pH on the rate constant K_H for a specific compound.

 

2. Rate constants for Chloroethane

Chloraethane hydrolyses in water by neutral and base catalysed reaction to given ethanol and hydrogen chloride. The hydrochloric acid formed dissociates at the neutral pH of most natural waters and forms a chloride salt. No acid catalysed processes observed.

The rate constant of 1.x 10-4 by neutral and base catalysed reaction was reported for chloraethane at 100.2 °C. Rate constants of K_N for hydrolysis reaction at 25 °C and pH was extrapolated based on the experimental rate constant and the temperature coefficient.

 

3. Hydrolysis as a function of pH for Chloroethane

The intercepts I, corresponding to pH values where two rate processes contribute equally to the observed rate constants, K_N and K_B for several primary alkyl halides are listed in the table below:

Organic halides K_N s-1        K_B [OH-] s-1              I_NB*

MeF              7.44 x 10-10       5.82 x 10-14          11.1

MeCl              2.37 x 10-8       6.18 x 10-13           11.6

MeBr              4.09 x 10-7       1.41 x 10-11           11.5

MeI               7.28 x 10-8       6.47 x 10-12             11.0

CH2CHCH2Cl   1.16 x 10-7    6.42 x 10-12          11.3

C6H5CH2Cl       1.28 x 10-5                                > 13

*These values were calculated from the equations (6), (7) and (8).

The intercepts I_NB lying above pH 11 indicate that neutral hydrolysis rate term for these compounds will dominate over typical environmental pH values of 4 to 9. Only above pH 11 will the base catalysed rate term contribute to the overall hydrolysis rate term. Likewise, hydrolysis of chloroethane is considerd not to be affected by pH values ranged from 4 to 9.

Validity criteria fulfilled:

not applicable

Conclusions:

The hydrolytic half-life in water at 25 °C and pH 4, 7 and 9 was estimated to be 38 days.

Executive summary:

In their critical review of hydrolysis of organic compounds in water under environmental conditions Mabey and Mill (1978) examined the rate constants for hydrolysis in water of 12 classes of organic compounds. The objective of their work was to use available data to estimate the persistence of these compounds in freshwater systems. For ethylchloride experimental data at 100 °C are available, from which the authors extrapolated the hydrolytic half-life of 38 days in water at 25 °C and pH 4 - 9. The hydrolysis rate constant is 2.10e-07 s-1.

Reference 3

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Acceptable, well documented publication which meets basic scientific principles (peer reviewed)

Qualifier:

no guideline followed

Principles of method if other than guideline:

Literature review

GLP compliance:

no

Radiolabelling:

no

Estimation method (if used):

Literature review

Transformation products:

not measured

pH:

5.6

Temp.:

25 °C

Hydrolysis rate constant:

0 min-1

DT50:

2.54 yr

Type:

(pseudo-)first order (= half-life)

pH:

7

Temp.:

25 °C

Hydrolysis rate constant:

0 min-1

DT50:

2.54 yr

Type:

(pseudo-)first order (= half-life)

pH:

5.6

Temp.:

10 °C

Hydrolysis rate constant:

0 min-1

DT50:

27.2 yr

pH:

7

Temp.:

10 °C

Hydrolysis rate constant:

0 min-1

DT50:

27.2 yr

Kinetic parameters for hydrolysis of chloroethane based on the data of Jeffers and Wolfe (1996) and Kollig (1990)

Arrhenius Parameters:

Kb: A= 3.72E+13 [L/(mol*min)]; EA=101200 (J/mol)

Kn: A= 1.37E+13 [L/(mol*min)], EA = 110800 (J/mol)

 

25 °C Data:

pH 7: Kobs = 5.18E-07; t1/2= 2.54 yr

pH 5.6: Kobs = 5.18E-07; t1/2= 2.54 yr

 

10 °C Data:

pH 7: Kobs = 4.84E-08; t1/2= 27.2 yr

pH 5.6: Kobs = 4.84E-08; t1/2= 27.2 yr

 

Dominat hydrolysis products are ethanol and HCl.

The neutral hydrolysis reaction is favored over the alkaline for chloroethane.

Validity criteria fulfilled:

not applicable

Conclusions:

For chloroethane the rate constant in water at 25 °C and pH 7 is 5.18 x 10-7, and the hydrolytic half-lie is 2.54 years.

Executive summary:

Washington (1995) reports in his review Arrhenius parameters for chloroethane for pseudo-first order rate constants for base (K_b) and neutral (K_n) reactions (data provided under “Any other information on results incl. tables”). Further, the review shows pseudo-first order hydrolysis rate constants describing the observed rate of substrate consumption (Kobs) at pH 7 and pH 5.6, each at 25°C (“environmentally relevant value”) and at 10°C (lower bound temperature for deep ground water):

25 °C Data:

pH 7: K_obs = 5.18E-07; t1/2= 2.54 yr

pH 5.6: K_obs = 5.18E-07; t1/2= 2.54 yr

 

10 °C Data:

pH 7: K_obs = 4.84E-08; t1/2= 27.2 yr

pH 5.6: K_obs = 4.84E-08; t1/2= 27.2 yr

Description of key information

Chloroethane hydrolyses in water. The estimated half-life was reported between 38 d and 2.6 years at 25 °C, pH 4, 7 and 9.

Additional information

Chloroethane (CAS 75-00-3) is not expected to be present in natural waters due to the high vapour pressure of the substance. Thus hydrolysis is not expected to be an important environmental fate process. If present in water, chloroethane is susceptible to slow chemical hydrolysis and forms ethanol and hydrochloric acid as reaction products. The hydrochloric acid formed dissociates at the neutral pH of most natural waters and forms a chloride salt. In water ß-elimination to given olefin and hydrogen halides is rare except at high temperature (ATSDR, 1998).

The hydrolytic half-life of chloroethane is not known with certainty.

Mabey and Mill (1978) reported an estimated hydrolytic half-life in water at 25 °C and pH 7 of 38 days based on a reaction rate constant extrapolated from experimental data at 100 °C.

 

Jeffers and Wolfe (1996) determined both the neutral and alkaline reaction rates for the hydrolysis of chloroethane. The neutral reaction measured in 0.01 M hydrochloric acid at 25 °C was found to predominate, with a rate constant of 5.1 x 10-7 min-1, resulting in an estimated half-life for chloroethane of 2.6 years.

 

Chloroethane undergoes neutral and base catalysed hydrolysis to given alcohol. The estimated intercept INB for chloroethane is above pH 11, meaning base catalysed reactions are not important at pH is lower than 11. As no acid catalysed reaction observed, pH conditions in natural waters (pH 4 – 9) are not to have a significant effect on hydrolysis rate constant and half-life for chloroethane (Mabey and Mill, 1978).

 

Reference

ATSDR (1998). Toxicological profiles for Chloroethane. U.S. Department of Health and human services, Public Health Service, Agency for Toxic Substances and Disease Registry, Division of Toxicology, Atlanta, GA