Ester hydrolysis mechanism can be acid or base catalyzed. Base-catalyzed ester hydrolysis has the advantage of being irreversible. For amide hydrolysis, the situation is complicated by the fact that another, different, mechanism takes over in more strongly acidic media, according to the. Amides contain carbonyl (C=O) and ether (N-C) dipoles arising from covalent Amide hydrolysis is also similar to ester hydrolysis in that the rates of. 700000 BTC
Much of the data on hydrolytic transformations in the environment comes from studies on the environmental fate of agricultural chemicals. For pesticide registration, studies characterizing the hydrolysis rates and degradation products of the pesticide in water are required in most Organisation for Economic Co-Operation and Development OECD member countries 5 , 6. These hydrolysis studies are conducted in sterile water in the absence of light to distinguish hydrolytic transformations from other environmental transformation processes, including biodegradation and phototransformation.
Hydrolysis reactions can occur even in dark environments with limited microbial activity, such as may be found in the deep subsurface; therefore; hydrolysis rates provide a lower limit on the transformation rates that may be expected to occur when a chemical is introduced into the environment.
These models are also capable of simulating the fate of a limited number of transformation products; however, the fate of transformation products is generally not modeled unless a particular product is of toxicological concern. Software tools that predict likely transformation products in environmental and biological systems can support chemical exposure and risk assessment by identifying potential products that should be considered in the assessment.
A number of software tools have been developed in recent years to predict transformation products of organic chemicals resulting from mammalian metabolism e. There are also a few examples of software tools to predict abiotic transformation processes, such as the META expert system, which has been expanded to predict phototransformation products Additionally, the Zeneth 14 software package predicts degradation of pharmaceuticals under the extreme conditions used in stability tests.
At the core of these tools for predicting transformation products is a library of reaction schemes also known as rules , which define how a particular structural fragment within a molecule will be modified by the specified transformation process. For example, the reaction scheme for hydrolysis of an anhydride structural fragment will show cleavage of the C — O bond of the molecule and formation of two carboxylic acids.
The reaction schemes are encoded and implemented using cheminformatics software tools. For a chemical of interest, these tools search the molecule for each structural fragment in the reaction library and then modify the fragment according to the associated reaction scheme to predict the molecular structure of potential transformation products.
This paper presents the development of an abiotic hydrolysis reaction library which will be implemented in the Chemical Transformation Simulator 15 , a web-based software tool under development in the United States Environmental Protection Agency U. For each chemical class that is known to undergo abiotic hydrolysis under environmentally relevant conditions, one or more reaction schemes have been encoded to define how the structural fragment that is susceptible to hydrolysis may be modified by reaction with water.
These schemes have been ranked using reported hydrolysis rates to enable qualitative prediction of the most likely transformation route when more than one structural fragment susceptible to hydrolysis is present in the molecule of interest. METHODS Development of the hydrolysis reaction library began with an initial set of reaction schemes that were obtained from published reviews on hydrolysis reactions in the environment 1 , 2 , 3 , 4 , For each scheme, scientific journals and government regulatory documents were searched for examples of specific molecules observed to be transformed according to the scheme.
The goal was to find at least 10 example chemicals with a diversity of molecular structure to assess whether the reaction scheme was representative of the transformation process. The entry for each reaction scheme includes the scheme itself, example transformations for molecules that have been observed to be transformed according to the scheme, citations for both the reaction scheme and examples of the transformation pathway, and the rank assigned to the scheme.
The rank is essentially a relative reaction rate, defined on a scale of one to six, with six being assigned to the fastest reaction schemes. The purpose of a reactivity rule is to constrain the reaction to occur only if the structure of the molecule meets specified conditions e. The purpose of a selectivity rule is to indicate a preferred reaction site when more than one structural fragment within the molecule matches the reaction center for the scheme. First, potential transformation pathways are identified on the basis of the presence of molecular structural fragments that are susceptible to a transformation scheme within the library and the application of any reactivity or selectivity rules associated with the scheme.
An explanation of the Metabolizer algorithm used to estimate the percent production of each product is provided in the Supporting Information. The possible persistence of these chemicals and accompanying risk of exposure to humans and other species of concern has resulted in a demand on regulators to provide effective techniques for quantifying their mobility and fate.
As part of the effort to evaluate potential mobility and fate associated with chemical constituents of wastes under consideration for land disposal, EPA's Office of Solid Waste OSW uses a relatively simple model to estimate potential groundwater contamination at specified withdrawal points in proximity to a landfill. This model calculates horizontal chemical movement in the aquifer based on advection, dispersion, sorption and transformation.
Hydrolysis is the only transformation process specifically considered at this time. To apply this model to chemicals of interest to OSW, hydrolysis rate constants for 98 chemicals were previously obtained either from literature sources or laboratory determinations using protocols developed at ERL-Athens 5. The objective of this report was to examine the rate data presented in references and organize them by compound class, with the goal of either enhancing existing property-reactivity correlations PRCs or developing new PRCs if sufficient data have been generated for a particular class of chemicals.
These correlations then can be applied to new chemicals in wastes being considered by OSW for regulation. Any chemical released to the environment is subjected to a wide variety of conditions that can transform it to a different product. PRCs offer a means for estimating kinetic constants for important transformation processes such as hydrolysis, photolysis, and redox reactions. Several SARs correlated molecular structure with toxicity to aquatic organisms.
Two approaches that are less dependent on measured kinetic data are being developed at ERL-Athens. Collette 8 is developing a method for predicting environmental fate constants of chemicals based on their infrared spectra. Even though many reactivity parameters may be amenable to this approach, to date, only alkaline hydrolysis of organic esters has been considered in depth.
Karickhoff et al. This allows estimation of values for a broad variety of reactivity parameters both kinetic and equilibrium: Uv light absorption, pKa, and various reaction rate constants, or any parameters that depend on molecular structure. The agreement of the alkaline hydrolysis rate constants for carboxylate esters calculated by all of the above methods as well as determinedE im the laboratory will be discussed in more detail later. The former is referred to as specific acid catalysis and the latter as base catalysis.
These two processes together with the neviititaJ! Functional groups that are potentially susceptible to hydrolysis are: 1. Aliphatic and aromatic carboxylate esters 2. Alkyl and aromatic halides 3. Amides 4. Carbamates 5. Epoxides 6. Nitriles 7. Phosphate esters 8. Alkylating agents 9. Halogenated ethers The corresponding figures contain structures of selected chemicals in these classes. The data in Tables 1 through 10 were either reported in references or extracted from the indicated references for comparison purposes.
Mabey and Mill 10 completed a critical review of the hydrolysis of organic compounds in water under environmental conditions. Hydrolysis rate constant values from Mabey and Mill are herein used, where available, for comparison purposes. For these compounds, hydrolysis at pH 7 is dominated by hydroxide ion for simple alkyl and aromatic esters Thus, the values of kg yield a reliable calculation of k,, and half-lives at pH 7.
For more structurally complex esters and esters with substitutents that reduce the electron density of the carbonyl carbon a-halogenated neutral hydrolysis k,, will be competitive at pH 7. The general rule is that methyl esters are twice as reactive as other n-alkyl esters and branching on the a-carbon on either side of the carboxylate group will retard hydrolysis by factors of three to ten 7.
The above effects i. The 2,4-D methyl ester was used as a standard reference compound and the reported rate constant is the mean of 27 determinations. Lasiocarpine and reserpine are structurally complex molecules that have two hydrolyzable groups labeled 1 and 2 in Figure 1.
Site 1 in lasiocarpine is structurally similar to methyl methacrylate and site 2 is structurally similar to ethyl glycolate. Branching on the o-carbon at site 2 potentially the most easily hydrolyzed retards hydrolysis to the extent that disappearance is dominated by neutral hydrolysis.
Similarly, in reserpine, branching on the acyl carbon at site 1 compare with methyl acetate retards hydrolysis by hydroxide ion to the extent that neutral hydrolysis is again dominant at pH 7. Addition of a methyl group to the a- carbon of the acyl group of methyl acrylate 11 to form methyl methacrylate decreases the kg by a factor of two. The small kg for di-n-octylphthalate is consistent with increasing chain length in the alcohol group 8. Aliphatic and Aromatic Carboxylate Esters 10 4.
All have in common the potential hydroxide ion or water mediated cleavage of the carbon-halogen bond to give alcohols. E2 elimination to give olefins and hydrogen halides, although not true hydrolysis, can occur in water and may be enhanced by increasing hydroxide ion concentration and temperature.
Elimination is treated as hydrolysis in the present text. Multifunctional halogenated chemicals such as halogenated ethers and nitriles are also included in Table 2 because the point of attack for these compounds is also the carbon-halogen bond. Table 2 is subdivided into the four groups: chlorinated alkyls, brominated alkyls, bifunctional chloroalkanes, and polycyclic and aromatic hydrocarbons. Chlorinated Alkyls Table 2. Figure 2 Allyl halides such as allyl chloride are known to hydrolyze rapidly by a neutral mechanism.
Therefore, because this compound has two equally reactive groups per molecule, the disappearance rate constant for the DCBs should be larger than that for allyl chloride. It is, in fact, larger by a factor of Pentachlorocyclohexene PCCH has a chlorine on the gamma carbon of the allyl fragment. Jeffers reported that a gem-dihalide with a H-atom on the gem-substituted carbon 1,1-DCA undergoes slow neutral hydrolysis in comparison with the perhalogenated gem- dichloride 2,2-DCP.
The methyl group in 1,2-dichloropropane 1,2 DCP increases the reactivity by a factor of five versus 1,2- dichloroethane 1,2 DCA. The third chlorine in 1,2,3- trichloropropane decreases the hydrolysis rate by a factor of three versus versus that of 1,2 DCP. The fully chlorinated hexachloroethane is almost totally resistant to hydrolysis.
Brominated Alkyls The reactivity of the vicinal bromines in the three brominated alkyls studied should determine the hydrolysis disappearance rate constants. The factor of nine greater reactivity of ethylene dibromide EDB over 1,2-dichloroethane Table 2 Section a illustrates the increased susceptibility of brominated compounds to hydrolysis. Statistically, tris 2,3- dibromopropyl phosphate Tris with three vicinal bromine centers should have a larger k,, than ethylene dibromide.
The two k,, values are almost identical, however, which leaves alkaline hydrolysis kg of the phosphate ester as the determining factor for the shorter half-life 4. The standard rate constant value in Table 2 is the average of 28 determinations and agrees well with the value of 4.
Hydrolysis of benzyl chloride is similar to that of the allyl chlorides in that the rate is enhanced by the ease of formation of a reactive carbonium ion. Jeffers 12 and Queen and Robertson 13 independently determined that gem-dihalides are unreactive to nucleophilic displacement by hydroxide ion but both measured a neutral hydrolysis rate constant attack by water and activation energy for 2,2-dichloropropane. Jeffers1 and Queen and Robertson's kN values are approximately a factor of 2 lower than our value 4.
If Jeffers' average value of The average of neutral activation energies reported by Jeffers was Thus, for chlorinated alkanes when the activation energy is unknown, use of Jeffers1 values to extrapolate rates to other temperatures would not only be 13 elimination as evidenced by the kg term, whereas for another similarly substituted trihaloalkyl, 1,2,3 trichloropropane Table 2, Section a , disappearance is dominated by reaction with water k,,.
Bifunctional Chloroalkanes The halogenated alkanes in this group contain either hydroxyl, cyano, or the ether linkage as the second functional group. The enhanced reactivity of 1,3-DCA can be attributed to the 2-hydroxyl group. Additionally, the acid character of the hydroxyl is enhanced by the two chlorines. The oxyanion resulting from ionization of the weak acid can, through intramolecular cyclization, displace either adjacent chlorine to form epichlorohydrin, a reactive intermediate.
Similar ionization and cyclization enhances the reactivity of 2-chloroethanol 2-CE versus 1,2-dichloroethane 1,2-DCA. The 2,3-DCA contains vicinal chlorines but the hydroxyl substituent enhances reactivity by only one order of magnitude over 1,2-DCA. The bis-dichloroethyl ether DCE reactivity, as expected, was similar to that of ethyl chloride.
The 2- 2-chloroethoxy ethanol CEE with one less reactive chlorine than DCE has a 14 slightly enhanced rate over DCE, again due to intramolecular oxyanion displacement of the chlorine to form the cyclic dioxane. The oxygen substituent on the a-carbon of chloromethyl methyl ether enhances reactivity by six orders of magnitude over bis- dichloroethyl ether.
The ease of formation and stability of the methoxymethyl carbonium ion has been proposed to explain the enhanced reactivity of chloromethyl ethers 20 over chlorethyl ethers. Polycylic and Aromatic Halogenated Hydrocarbons The halogenated polycyclic and aromatic hydrocarbons comprise a very persistent group of chemicals in the environment. The three most reactive polycyclic compounds studied Aldrin, Dieldrin, and Isodrin all contain a perchlorinated gem-dichloro carbon.
Dieldrin Aldrin epoxide also contains an epoxide oxygen, and, with two reactive centers, would be expected to have the shortest half-life. Apparently hydrolysis of the epoxide is stericly hindered and the orientation of the cyclic ring is such that hydrolysis of the gem-dihalide also is retarded in comparison to 1,2-DCP. Structural orientation is also the reason that the cis isomer of chlordane is more susceptible to hydroxide-ion-mediated dehydrohalogenation.
The 1-exo, 2-exo orientation of the chlorine atoms in cis-chlordane facilitates the E2 elimination of HC1. No disappearance of trans-chlordane was observed under the same conditions. All the chlorinated aromatics studied were very stable to hydrolysis. Half-life pH 7, 25'C 8y 14 4. Alkyl and Aromatic Halides 19 4. When the ratos determined at pH's 3 and 7 or pH's 11 and 7 were the snmo, the rate was reported as the neutral rate constant.
Thioacetamide Illustrates tho instability introduced when sulfur ia substituted for oxygon. Acetamide is the oxygen analog. Electronegative substltuents fluoro- and ohloroaoetamide or subutitutientu that doloc. Pronamide hydrolysis nlso is enhanced by tho delocalization of electrons into the dichlorophony1 substituont. OE-2 2-Fluoroacetamide 2-Chloroacetamide 4. OE-2 Acrylamide 38y Ref. Half-lives of carbamates as illustrated in Table 4 vary from seconds to centuries Mitomycin C contains among its functional groups a carbamate, an aziridine, and a labile methoxy group.
The rate limiting step in disappearance of mitomycin C is expulsion of the methoxide prior to opening of the aziridine ring 22, The half-life of mitomycin C is comparable to 2- methylaziridine in Table Ethyl carbamate, contrary to most carbamates, is not N-substituted.
The reactivity of ethyl carbamate is comparable to the simple alkyl amides. Carbamates 25 4. Epoxides are reactive compounds with half-lives generally minutes to less than 15 days 10 , see Table 5. Aromatic or conjugated epoxides tend to be more reactive than strictly aliphatic epoxides. Acid- catalyzed hydrolysis is sensitive to the stability of the transient carbonium ion formed by protonation of the oxirane ring and subsequent breaking of one of the carbon-oxygen bonds.
This enhanced reactivity is reflected in the hydrolysis rate constant of D. L-transchlorostilbene oxide CSO. The stability of Dieldrin is possibly due to shielding of the oxirane group from attack by water and to lower reactivity of alkyl epoxides.
Neutral rate from Reference Epoxides 28 4. In both cases, amides are the first reaction products, but amides cannot be isolated as intermediates unless their rate of hydrolysis is lower than that of the parent nitrile. Nitrile data, summarized in Table 6, confirms the stability of the monofunctional alkyl nitriles acrylonitrile and acetonitrile under environmental conditions.
Malonitrile 1,1-dicyanomethane , by comparison, is very labile to degradation even under neutral conditions. This can be attributed to the activation ionization of the a- hydrogens. The anion formed is unstable and some form of cleavage or hydrolysis occurs when malonitrile loses a proton to form the monoanion. Half-life -S! Under environmental conditions, only the neutral and hydroxide-ion-mediated hydrolyses are relevant as illustrated in Table 7.
In breaking the P-O S single bond, the mercaptide anion is a better leaving group than the alkoxide. Rates of hydrolysis are therefore accelerated when S is substituted for O in this 31 position. Using the m and c values from Wolfe 18 , and conjugate acid pKa values calculated 9 by SPARC 2-hydroxypyrazine or Perrin [ 19 p-N,N-dimethylsulfamoyl phenol], we calculated second-order alkaline hydrolysis rate constants for O,O-diethyl-O-pyrazinyl phosphorothioate and Famphur.
The calculated values in Table 7 agree within a factor of two with the laboratory-determined values. The hydrolysis of the trialkylthioester is an order of magnitude slower than the above thioesters with at least one aromatic substituent.
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The parent carboxylic acid can be obtained from all its derivatives by this mechanism through a hydrolysis process. We shall just consider the mechanism of hydrolysis of esters and amides although similar reaction pathways are followed with other carboxylic acid derivatives. Hydrolysis of Methyl Esters Unlike acid chlorides, which are strong electrophiles and react readily with mild nucleophiles such as water, esters are much less electrophilic and do not react with water at pH 7 to any appreciable extent.
However, the rate of ester hydrolysis can be substantially increased by carrying out the reaction under acidic or basic conditions. Acid-Catalysed Hydrolysis of Methyl Esters Since water is such a poor nucleophile , one method for increasing the rate of nucleophilic addition with an ester is to increase the electrophilicity of the ester. This can be achieved in a number of ways. When carrying out hydrolysis reactions, a large excess of water often used as a co-solvent , is used to drive the reaction over to the carboxylic acid product.
The reaction is again an equilibrium process so to drive the forward reaction, an excess of the alcohol often used as the solvent that will provide the new ester is used: Base-Mediated Hydrolysis of Methyl Esters An alternative method for increasing the rate of ester hydrolysis is to increase the reactivity of the nucleophile. Negatively charged species are generally more nucleophilic than electronically neutral species they are also more basic. On these grounds hydroxide should be, and is, a better nucleophile than water.
This step is reversible. If we consider the loss of methoxide, the carbonyl-containing product is a carboxylic acid. Methoxide is a relatively good base and can abstract the proton of the carboxylic acid in an acid-base process. As a result of this, the base- mediated hydrolysis reaction is a more efficient method for forming a carboxylic acid from an ester than the corresponding acid-catalysed process.
Example from R. Other types of alkyl esters undergo hydrolysis via different reaction mechanisms. For example when the alkyl group can form a relatively stable carbocation, the hydrolysis proceeds via a so- called AAL1 reaction mechanism. Hydrolysis of tert-butyl esters proceeds via this type of mechanistic pathway: Hydrolysis of Amides Owing to resonance stabilisation effects, amides are one of the least electrophilic carboxylic acid derivatives.
Hydrolysis of an amide to the carboxylic acid therefore requires quite forcing reaction conditions. Acid-Mediated Hydrolysis of Amides. The pH is one of the most important factors because an acid or a base acts as a catalyst, that is, they greatly accelerate the degradation process.
These factors depend on the polarity of the polymer. A lower polarity tends to decrease the reaction rate because both the water content in the polymer as well as the water permebability decrease with decreasing polarity of the polymer. Thus, the hydrolytic stability increases in the same order as the hydrophobicity. In the case of aliphatic polyesters, the hydrolytical stability increases with the lengths of the aliphatic portion in the chain.
Hydrolyis reactions may be also catalyzed by certain enzymes known as hydrolases. These biological catalysts accelerate the reaction rate in living organisms bacteria, fungi, etc without undergoing themself any permanent change. The enzymatic hydrolysis of polymeric materials is a heterogenous process that is affected by both the physiochemical properties of the plastic MW, crosslinking, chemical composition, surface area, porosity, crystallinity etc.
Important extrinsic factors include pH, humidity, temperature, oxygen, and nutrient availability etc. Synthetic polymers particularly susceptible to enzyme, acid, or base catalyzed hydrolysis include aliphatic polyanhydrides like poly sebacic anhydride and polyesters with short midblocks such as poly lactic acid PLA , poly glycolic acid PGA , polycaprolactone PCL , and poly hydroxybutyrate PHB.
These biodegradable materials find many uses in pharamceutical and medical products such as resorbable surgical sutures, resorbable orthopedic devices and controlled-release coatings for drug delivery systems. Besides water, many other polar solvents such as alcohols, amines, and acids may cause cleavage of C-O or C-N bonds. This process is known as solvolysis and if the solvent is an alcohol as alcoholysis.
The reaction rate depends on the type of solvolytic agent and its solubility in the polymer as well as on the pH and temperature. Summary Hydrolysis is the cleavage of bonds in functional groups in the polymer backbone by reaction with water. Many synthetic polymers with water-sensitive groups in the backbone as well as most natural polymers undergo hydrolytic bond cleavage to form water soluble fragments. The process is known as polymer erosion.
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