Exhaustive Digestion as a Simple Prelude to High Yield Crystallization

 

This article is about the technique I call ‘exhaustive digestion’. This is a phrase that I used in my blog entitled, Getting Better Recovery from Crystallization. In this blog, I make the point that chemists scaling up a laboratory process often fail to do adequate preliminary purification of a solid and as a consequence get a lower recovery from their crystallization than is possible if a preliminary treatment were done. I proposed digestion or trituration as pretreatments before crystallization. I don’t know whether there are any widely accepted differences in meaning between the words digestion and trituration but, when used in my blogs, ‘trituration’ can be understood to apply equally to oils or solids while ‘digestion’ will only be applied to solids. Second, ‘trituration’ can be done with either hot or cold solvent while ‘digestion’ and particularly ‘exhaustive digestion’ as described hereunder is to be done with hot, boiling liquid, usually a rather poor solvent for the desired product. 

Exhaustive digestion is related to swish trituration about which I have written earlier. The difference is that the swish trituration employs its anti-solvent system at ambient temperature and does not measure the boiling point depression of the liquid to assess the extent of purification. 

In the old literature, the progress of purification by digestion on the laboratory scale was followed by observing the change in the boiling point of the digesting solvent or solvent mixture using a Beckmann thermometer, which is a thermometer that can measure differences as small as one-tenth of one Centigrade degree. I don’t know how long it has been since I have seen anyone use a Beckmann thermometer but, at least in my undergraduate Phys. Chem., (over 50 years ago) it was done quite often. The scale on a Beckmann thermometer is only 10-20 degrees but its range can be adjusted by removing or adding mercury to the column of mercury used for measurement by moving mercury back and forth from an attached mercury reservoir. Today a digital thermometer is more likely to easily achieve the same or better precision of measurement without managing liquid mercury.

I will illustrate exhaustive digestion by quoting from an experiment taken from Laboratory Technique in Organic Chemistry by Avery Adrian Morton, First Edition, McGraw-Hill, 1938,  pg. 128-229.

“The apparatus consists of a tube or flask of such size that a large part of the bulb is covered by the solid material being extracted, a thermometer graduated in tenths of a degree, a reflux condenser, and a water or oil bath. Place the sample in the container, insert the thermometer so the bulb is immersed in or covered by the sample, and add solvent to cover the solid and the thermometer bulb. The solvent is usually one that will dissolve impurities without appreciable quantities of the desired material. Petroleum ether or ligroin is often suitable. A mixture of solvents may be employed without affecting their utility in this experiment, as long as the composition of the mixture is not varied. Reflux the solution [I think slurry is intended] using a water or oil bath as a source of heat. After about 20 minutes the temperature has reached a constant level. Usually, no trouble will be experienced from superheating as long as a solid phase is present, although a little agitation with the thermometer is sometimes needed. Filter either by pouring into a Buchner funnel or by using a filter stick. Add more of the same solvent, reflux once more, and observe the temperature after equilibrium conditions have been reached. Continue the operation until the temperature of two or more successive operations is identical. The more soluble impurities have now been removed, and the solution contains only the pure compound or the compound with [less soluble type] impurities that have not been entirely removed…. The operation may be continued if desired until all the product has been dissolved. A change of solvent [may be implemented at some point for further purification]….. Constancy in the boiling points of successive portions constitutes further evidence of the purity of the material, whereas a drop in the boiling point is evidence of the exhaustion of still another component. Usually, the final portions of exhaustive digestion are pure materials.”

Continuously measuring and recording the changes, in real-time, of the boiling point without any actual sampling makes this an early application of Process Analytical  Technology (PAT). The improvements in the size and sensitivity of temperature measuring devices for following small changes in boiling point under conditions where superheating does not occur deserve more frequent consideration.

Exhaustive digestion if used at scale would require a more rugged method for measuring small changes in the boiling point of the slurry than the old Beckmann thermometer. Fortunately, modern digital thermometers meet that need.

Exhaustive digestion, similar to swish trituration, may profit from using either water-rich or hydrocarbon-rich binary azeotropes,  as the anti-solvents, as the desired product’s solubility dictates. Some are suggested in the following list:

97.0% water 3.0% acetic acid azeotrope bp 76.6ºC

91.0% water 9.0% benzyl alcohol azeotrope bp 99.9ºC

87.1% heptane 12.9% water azeotrope bp 79.2ºC

94.4% hexane 5.6% water azeotrope bp 61.6ºC

95.5% hexane 4.5% allyl alcohol azeotrope bp 65.5ºC

97.0% hexane 3.0% 1-butanol azeotrope bp 67.0ºC

91.5% cyclohexane 8.5% water azeotrope bp 69.8ºC

83.7% acetonitrile 16.3% water azeotrope bp 76.5 C

72.9% allyl alcohol 27.1% water azeotrope bp 88.2ºC

66.0% allyl cyanide 34.0% water azeotrope bp 89.4ºC

77.5% formic acid 22.5% water azeotrope bp 107.1ºC

The Context of a Chemical Process Development Training Blog like KiloMentor

 

If the KiloMentor blog were being written twenty years ago it would be different.  If it had been written thirty years ago it would have been different again.  The progress of a technical art, such as process development, creates dramatic changes. The step which had been a bottle-neck in the creation of a process becomes less demanding and another aspect of the art becomes the chief challenge to the scientist.  Thus, if any of these documents were revised in another dozen years, the relative difficulties of different aspects of the challenge will have again changed and the reasons for the proposals made here may have evaporated and the advice provided may become completely wrong-headed.  With this in mind, an author should at the outset state what the status quo is in his field at the time of writing so that future readers can decide for themselves whether the same state of affairs still exists and if not what logical changes should be inferred in the recommendations being offered.

To illustrate this point, forty-three years ago, when I was just beginning my activities as a process chemist, devising the sequence of organic reactions that could lead from commercially available starting materials to the target product was the great challenge.  Dependable reactions were limited.  HPLC did not exist.  IR and UV measurements were just being replaced by NMR methods for following reactions and identifying products.  But the more important difference was that on-line database searching did not exist.  Most significant of all, electronic substructure searching did not exist.  A close analog of the relevant portion of a molecule you wanted to synthesize might be present in the literature, but there was no dependable way to find it.  

To-day, although many would disagree about the degree of change, creating a realistic synthesis scheme for substances of moderate complexity is no longer the most challenging step simply because we have replaced our memories and our punch cards with computer memories professionally indexed available on our desktops.

In this earlier period, when creating the process step flow sheet was the dominant challenge, we synthetic chemists got into the habit of measuring excellence in synthesis by measuring the number of sequential steps in the longest arm of the contemplated process or the total number of reaction steps.  We also calculated the overall yield although this could only be determined after the proposed sequence was converted to a real process.

Because this challenge strained our ability to approximate we made some rough assumptions in inaccuracies of which we hoped would balance out between competing processes and still allow us to make a valid judgment of which was the most promising. For example, we assumed that the amount of crude impure product which one got out from a reaction step was proportional to the quantity of product, which was present in crude form in the mixture after the reaction was complete.  Why do I say we made that assumption? Because we looked at the yield of model reactions for simple substrates with just one or two functional groups and assumed that the recovery of pure product from these would be about the same when we used complicated substrates with multiple functional groups.  In the terminology I will use herein, we assumed that the isolation yield- the yield of pure product as a percent of the assay yield (the amount of product in the reaction mixture) is consistent for a reaction type.  The corollary of this which we also adopted as a simplifying assumption was that isolation although it might be tedious was routine and could be taken for granted as generally of similar difficult when integrated over all the steps of a process.  That is isolations can be ignored so long as the total reaction steps are minimized.

Today, the focus of attention has shifted.  Scientists can gather large amounts of relevant data about the likely properties not just of substances that have been reported somewhere in the vast chemical literature, but even predicted properties of unknown molecules.  What the electronic databases have not been able to do is help us select the simplest and most rugged purification methods to use with reactions at scale.  What we as scientists have not tried to do is ask ourselves the question, “What kinds of functional groups do I want in my intermediates because they will simplify the isolation and purification of that step?”

An axiom of the approach to process development that will be found here is that there is an advantage in ranking the degree of difficulty of isolation/purification of each process step and using it as an additional criterion of selection of the most preferable paper chemistry route along with the traditional criteria: number of steps, number of steps in the longest branch of a convergent process, and the known approximate yields for the reaction type.  The result of this I predict will be that preferred processes may on average contain more process steps but the speed with which these steps can be carried out will be much higher, the overall purity will be much higher, and the cost will be much lower because the time spent in the isolation purification is typically much more than the actual reaction time.

Methods for Forming and Crystallizing Organic Salts particularly Pharmaceutical Salts

 

Most Common Method for Forming Salts

Mixing  stoichiometric proportions of acid and base in a suitable solvent; then

1. Cooling to a lower temperature
2. Adding a miscible anti-solvent or liquefied gas
3. Adding an immiscible  or partially miscible anti-solvent
4. Drowning out in a miscible anti-solvent
5. Slowly adjusting the pH
6. Use of the common ion effect to decrease the salt’s solubility

Other Methods

1. Exchange of Ammonium Salt with Nonvolatile Base

An exchange between ammonium salt and another non-volatile cation to give a more insoluble salt of an anionic drug.

2. Exchange of formate or acetate or thiocyanate salt with a non-volatile acid

3. Double Decomposition Reactions

Metathetical reactions between a salt solubilize by the presence of a particular cation and a second salt solubilized by the presence of a particular anion giving one insoluble salt and one soluble salt from which the insoluble salt is recovered by filtration and washing.  

The use of metal salts of 2-ethyl hexanoic acid for the basification of organic acids is an example.

Methods for 

1. Direct addition

Addition of a solution of the salt-forming acid or base slowly into a solution or slurry of the pharmaceutical product whose salt is sought.

2. Inverse addition

Addition of the pharmaceutical salt capable species, either as a solid or as a solution into at least a full equivalent quantity of the salt-forming reagent.

3. Slow addition of poorly soluble neutral species by extraction

Extraction of the pharmaceutical salt capable species from a Soxhlet extractor by hot solvent and quench of the extracted species by an excess of the salt-forming reagent in the boiler of the extraction apparatus.

4. Impinging Streams of Salt Solution and Anti-solvent

5. Impinging Streams of Acid and Base
   

Methods for Precipitating Pharmaceutical Salts

Crystallization by Diffusion of an Ant-iSolvent

Dissolution of the salt in a mixture of solvents followed by the addition of an immiscible third solvent creating two phases in both of which the salt is insoluble.

Partial Evaporation of a Single Volatile Solvent

Partial Evaporation of a Mixed Solvent System

      Dissolving the pharmaceutical salt in a mixed solvent of a less volatile poorer solvent and a more volatile better solvent and then removing the better solvent by distillation of evaporation..

Lyophilization/Inorganic Salt Removal

Lyophilisation (freeze-drying) of a solution. Dissolution in methanol and filtration to remove inorganic salts.

Slurry to Slurry

Transformation from a slurry of the slightly soluble pharmaceutical acid or base candidate into a slurry of the desired salt form until a method of solution analysis shows equilibrium.

Precipitation by pH Adjustment

Dissolving of the pharmaceutical candidate in a partially aqueous solution followed by adjustment of the pH gradually by the hydrolysis of a solution component. For example: methyl acetate and base giving acetate and methanol; ethyl carbamate and acid giving ammonium carbon dioxide and ethanol.

Solvent Expansion

Dissolving the pharmaceutical salt or making the pharmaceutical salt in solution and then exposing the solution to a volatile anti-solvent so that the composition slowly becomes more insoluble

Persilylation and its Uses

 

The KiloMentor blog emphasizes the usefulness of simple, robust, scalable methods for work-ups and isolations in organic chemical process development.

Biphasic organic solvent systems such as methanol/hexane can in principle be very useful for the simple extractive separation of components of a reaction mixture. The trick for success is to get partition ratios that are neither too small (<0.2) or too large (>5).

The idea being explored in this blog is whether persilylation of a mixture of solutes from a completed reaction could give a modified mixture that could be separated by liquid-liquid extraction between two immiscible aprotic solvents.

While it is true that most biphasic organic solvent systems comprise at least one protic component and such a solvent would use up all the silylating agent and prevent substrate silylation, there are aprotic solvents that can be mixed and retain two liquid phases. Cyclohexane forms two liquid phases with any one of acetonitrile, propionitrile, nitromethane, nitropropane, dimethylsulfoxide, dimethylformamide or dimethylacetamide. Hexane and heptane would likely behave similar to cyclohexane.  Sulfolane and t-butyl methyl ether are both aprotic and only partially miscible. 

KiloMentor proposes that silylation of all the components of a mixture to be separated should either decrease some and retain unaltered some of their polarities and so perhaps cause their partitioning between the component phases of a two-phase solvent pair to become more competitive. Smaller partition ratios could make a couple of stages of counter-current extraction feasible for separation.

Disclaimer

Please be warned that this methodology has not been experimentally verified in any situation that I know about.  What I can say is it is simple enough to work and I cannot see any particular difficulty.

I have always urged my coworkers to make a clear distinction between facts and theory and this is my effort to do the same.

Making the Silylation Facile

To proceed in this way, a practical method to persilylate all the applicable functional groups in all the components in a reaction mixture is necessary. A further practical consideration is that such silylation procedure must be inexpensive; otherwise, the additional reagent cost will make the procedure uncompetitive with alternative separation means.  Fortunately, it has long been known that there are catalysts for silylation, which allow chemists to use the convenient and inexpensive hexamethyldisilazane reagent for all functional groups.  Although this has been in the literature for many years, it is infrequently used and seems to have today vanished from our chemical toolboxes.

Cornelis A. Bruynes and Theodorus K. Jurriens, then scientists at Gist-Brocades in Delft Netherlands, published a paper called Catalysts for Silylations with 1,1,1,3,3,3-hexamethyldisilazane in J. Org. Chem. 47, 3966-3969 1982.  They reported that the following compound types could be trimethylsilylated using the title reagent and an appropriate one of their catalysts with yields of typically more than 90%:

Alcohols, phenols, carboxylic acids, hydroxamic acids, carboxylic amides, and thioamides, sulfonamides, phosphoric amides, mono and dialkyl phosphates, mercaptans, hydrazines, amines, NH groups in heterocyclic rings, and enolizable β-diketones

The silylation times were in all cases no more than two hours and the catalyst concentration is typically from 0.001-10.0 mole percent.

Silylation Catalyst Structures

Although many catalysts are claimed (there is a corresponding patent  EP81200771.4 now expired), five were used in most examples:

• Saccharin [81-07-2]
• Sodium saccharin [128-44-9]
• Bis(4-nitrophenyl)N-(4-toluenesulfonyl)phosphoramidate [81589-21-`]
• Tetraphenylimidodiphasphate [3848-53-1]
• Bis-(4-nitrophenyl)N-trichloroacetyl)phosphoramidate [38187-67-6]

The registry numbers for these catalysts are given in square brackets.

Methods of Application of this Idea

There are two variants of this idea. In one, all the solutes in a reaction mixture are persilylated and allowed to partition between the two immiscible solvents. In the second, all the solutes in a reaction mixture are mixed with the two immiscible solvents and the silylating reagents are added and the mixture is analyzed as the competitive silylation proceeds and the partitioning of unsilylated, partially silylated, and completely silylated materials accumulate in the two different phases. This second is a kinetic silylation with simultaneous partitioning. 

To use this either strategy all that ought to be necessary would be to

• make a solvent change into acetonitrile, propionitrile, dimethylacetamide, dimethylformamide, nitromethane or nitroethane, whichever is appropriate for the separation trial
• add the minimum necessary amount of a catalyst
• add the calculated amount of hexamethyldisilazane
• heat for the requisite time to get either a complete or another requisite degree of silylation of the mixture with the expulsion of the co-product ammonia
• adjust the solvent volumes so that the biphasic mixture will be produced at the appropriate temperature
• cool to that temperature if necessary
• separate the phases
• repeat extraction if necessary
• hydrolyze the silyl derivatives and recover the products from their respective phases

Potential Problems

It will only be determined by actual experiment with a particular mixture of solutes  how high a relative concentration of the solutes can be worked with before the biphasic solvent mixture goes homogeneous. Obviously, there is some point where the concentration of the solutes will wreck the balance of solvent properties that allows the two phases to coexist.

As is always the case if one adds something to promote a separation that facilitating agent must itself be separated in the end. So it is with the catalyst, which must remain in one or the other phase along with some elements of the mixture being separated.

Another Possible Approach

Consider the possibility of partitioning a reaction mixture between two of these partially immiscible solvents and then with mild stirring adding a silylation catalyst followed by an insufficient amount of a silylating agent such as hexamethyldisilazane.

What would happen?

I would think that whichever solute silylates faster will be partitioned into the less polar hydrocarbon layer where it would be protected from further reaction. The reagent trimethylsilyldiethylamine is probably the most sterically demanding silylating agent one could try to get kinetically controlled silylation.

Genotoxic Impurity Considerations

Genotoxicity is the capacity of a substance to damage genetic material such as DNA, and thus cause mutations or possibly cancer. Genotoxic impurities need to be avoided in a pharmaceutical product because they can possibly cause serious health consequences even at levels less than standard analytical methods can detect.

It has been argued that most genotoxic reagents/intermediates present in syntheses that have four or more stages prior to API isolation/purification are likely to be deactivated by reaction with other reagents or purged by dissolution in solvents, removed by vacuum distillation or other phase switching operations; however, regulatory reviewers who have a vested interest in emphasizing the seriousness of genotoxins show little sympathy for such arguments. Government reviewers tend predominantly to request quantitative analytical information on the API and/or intermediates possibly combined with the results of spiking experiments (impurity fate analysis) and could well require these in order to demonstrate the absence of carryover of potential genotoxic impurities (PGIs), at a suitably low (TTC) level. Such investigations can be highly resource-intensive and challenging, particularly in respect of developing validated analytical methods for low levels of multiple PGIs. Moreover, limits for certain PGIs may become part of the API specification and so become an ongoing quality-control commitment.

However, concerns usually relate to late-stage operations that involve potential genotoxic impurities PGIs. One EU review raised as a major objection, potential residues of mesityl oxide (4-methyl-3-penten-2-one) in a drug substance crystallized from acetone.  Another cited possible traces of alkyl mesylates (methyl, ethyl and isopropyl mesylates) in a mesylate salt drug substance.

Unless the threatened concern about the PGI is identified at the early planning stage, modifying a route that is otherwise the best proposal should be the last resort.  Nonetheless, modification of the API synthesis in a way that minimizes PGI levels in the drug substance has been undertaken in some cases; recent published examples pertain to formaldehyde, chloroalkanes and acetamide.

Given the difficulties in obtaining reliable predictions, particularly for aromatic amines, it may be prudent to undertake an Ames assay on any PGI containing an aromatic amine structural alert, if data are not already available in the public domain. Aromatic amines that otherwise make admirable process intermediates because of the multiple options for their purification, should where it is feasible be transformed to more acceptable functionalities early in a process route.

The potential action of small alkyl alkylating agents presents a particularly frequent and acute problem for pharmaceutical products because so many of them in their API salt form are hydrochlorides, methanesulfonates, tosylates and besylates as well as to a much smaller extent, hydrobromides.

These pharmaceutical salts are most often prepared in the final chemical transformation of a sequence and so any PGI has the highest probability of lingering behind in the API.

The best evidence that these concerns about PGIs arising from making the aforementioned pharmaceutical salts are without any foundation is the epidemiological evidence of the health of medicinal and process chemists who have been making and using these salts for years and years without any relative health damage.

The apparent lack of health damage is readily explained since the reactivity of halo compounds in biological systems can be predicted to a significant extent on the basis of their relevant chemical properties such as alkylating potential and susceptibility to hydrolysis. Thus, bromo compounds are expected to be more reactive than chloro compounds. SN1 and SN2 characteristics will determine the nature and the extent of reactivity towards nucleophiles, and steric factors may also play a part in some cases. In the 4-(p-nitrobenzyl)pyridine alkylation assay, alkyl halides generally show negligible activity, methyl methanesulfonate (MMS) being at least 40 times more active than ethyl, propyl or butyl bromide. Allyl bromide appears to be more active (around one-eighth of the activity of MMS although allyl chloride shows minimal activity. Benzyl chloride, while rather active for a chloro compound, is around 20-fold less active than allyl bromide. Owing to their volatility and/or hydrophobicity many alkyl halides show negative results in conventional Ames Salmonella assays, and it is often necessary to employ vapour phase exposure in a closed system (using a desiccator for example) in order to obtain positive results. Most alkyl halides, especially bromides, are Ames positive (using a closed test system if necessary), although 1-chloropropane, 1-chlorobutane and neopentyl bromide are all Ames-negative. As expected, based on their lack of alkylating activity, both chloro- and bromobenzene are Ames-negative. Some unsaturated halo compounds have the potential to be metabolized to form quite active mutagenic molecular species. For example, evidence suggests that oxidative biotransformation of vinyl chloride produces chloroethylene oxide and 2-chloroacetaldehyde as active metabolites. Binding of bromobenzene-3,4-oxide to liver proteins is thought to account for the hepatotoxicity of bromobenzene. The predominant metabolic pathway for simple alkyl halides is halide displacement by glutathione, although some C-hydroxylation reactions may occur.

Rodent bioassay data on alkyl halides strongly suggest that these compounds are either non-carcinogens (1- chlorobutane, bromomethane) or low-potency carcinogens (chloroethane, bromoethane). Both chloroethane and bromoethane produced an increased incidence of a rare type of endometrial tumour in female mice and it seems highly plausible that the carcinogenic effect is caused by a species-/ gender-specific stress-related adrenal overstimulation and excessive corticosteroid production. The rodent carcinogenicity profile for chloroethane (increased incidence of a rare tumour type at an extremely high concentration of 15,000 ppm in one species/gender) is thus much closer to that for a non-genotoxic carcinogen than for a genotoxic carcinogen. Thus, the (feeble) alkylating activity of chloroethane seems largely incidental to its carcinogenic activity, a scenario likely to apply to many other similar alkyl halides. This prediction is strongly supported by the fact that benzyl, ethyl, isopropyl, and trityl bromides were inactive as carcinogens at doses up to 0.83, 12.5, 8.3, and 0.25 mmol/kg, respectively, when administered by single subcutaneous injection to female rats. A number of independent expert assessments are available on halo compounds.

Acceptable/tolerable exposures are expressed in various ways, for example as minimal risk levels (MRLs) by the Agency for Toxic Substances and Disease Registry, or as reference concentrations/doses (RfCs/RfDs) by the U.S. Environmental Protection Agency. There is a clear consensus that chloroethane is less hazardous than the more reactive chloromethane, although recommended safe exposures for the former range from the highly conservative OEHHA value of 150 μg/day to 200 mg/day (10 mg/m³ at an average air intake of 20 m³/day⁴³) based on the EPA IRIS assessment.

 Acceptable exposures in the context of genotoxic impurities can also be calculated on the basis of the TD50 values as described above, resulting in PDEs of 1810 and 149 μg/day for chloroethane and bromoethane respectively. A PDE for non-carcinogenic 1-chlorobutane could be determined using ICH Q3C (R3) methodology on the basis of the most appropriate NOAEL in lifetime studies.

Quotes from Org. Process Res. Dev. 2011, 15, 1243–1246

[Compounds can be grouped together that have] strikingly different reactivities, isopropyl chloride and isopropyl mesylate for example. Isopropyl chloride is Ames-negative in assays using standard conditions; testing has to be carried out in a desiccator to obtain a (feebly) positive result; on the other hand, isopropyl mesylate (Swain-Scott s = 0.29) is a potent mutagen in the standard Ames test and gives positive results in several in vivo assays. Hydrolysis half-lives at pH 7.0 and 25 C are 38 days (the same as for chloroethane; TD50 1810 mg/kg/day) and 4.5 h, respectively. Unfortunately, no rodent bioassay data appear to be available for either compound, although 1,2- dichloropropane is Ames-positive in standard assays and has a mouse TD50 of 276 mg/kg/day (negative in the rat). A Risk Specific Dose (RSD) for isopropyl chloride of approximately 37 μg/day (for a 50 kg patient) has been determined by Bercu et al. and Contrera based on QSAR (quantitative structure- activity relationship) techniques using regression analysis of “training sets” of TD50 data. [The choice of compounds in the isopropyl chloride training set could be questioned in that it contained several non-genotoxic polychloro compounds but not 1,2-dichloropropane or chloroethane.] Use of QSAR models to predict genotoxic/carcinogenic potency relies on rule-based techniques (such as DEREK) or statistical techniques (using regression analysis of training data sets), and particularly in relation to carcinogenicity, the latter approach is considered to provide more reliable results. Since halo compounds in particular and other compounds containing “Ashby alerts” are often identified as PGIs, it is interesting to note that in the determination of the TTC limit Kroes et al. classified only two such compounds (5% of the total in the data set) in the lowest potency category (equivalent to 1.5 μg/day), but since the data set is non-transparent it is not possible to identify these two key compounds. [In a prior publication using essentially the same data set, the TD50 for methyl methanesulfonate is listed as 0.178 mg/kg/day, 179 times lower than the true value, and it is unclear whether this miss-transcription was carried forward to the data set used by Kroes et al. in the more “definitive” publication.] In the Kroes publication, no carbamates were classified in the lowest potency category, suggesting that the standard TTC of 1.5 μg/day may be unnecessarily conservative for this structural class.  

Diethyl Phosphite: A Solvent you wouldn’t easily think of -Miscible both with Water and Ordinary Organic Solvents

 

Diethyl Phosphite is variously called diethyl phosphonate, diethyl hydrogen phosphite, diethyl phosphonite. 

According to Fieser & Fieser Organic Reagents Vol. 1 pg. 252 “Diethyl phosphonate {diethyl phosphite/diethyl phosphonite/(EtO)₂P(=O)H} is miscible with water and with ordinary organic solvents and has remarkable solvent power.” Diethyl phosphite may be just the trick when all the ingredients for a reaction cannot be dissolved together!

 It is also recommended for the preparation of phenyl hydrazones and 2,4-dinitrophenylhydrazones in the short communication: Maynard, J.A. Australian Journal of Chemistry, 1962, vol. 15, p. 867 - 868. These latter reactions drive the equilibrium by removing water by reaction while generating a useful strong acid as catalyst. It is also important that diethyl phosphite readily dissolves both the reagent and substrate. Dr. Maynard points out that the addition of an acid catalyst to the mixture is unnecessary because the solvent reacts with any water initially present and all that is formed in the reaction to produce the strong acid ethyl hydrogen phosphate. Thus the needed acid is produced and the by-product water is scavenged. Many of the phenyl hydrazones made using this solvent precipitated directly; others required mixing with water equal in volume with the solvent. Recrystallizations were from methanol, ethanol, benzene, or acetone as the solubilities directed.

This trick of binding up by-product water and generating useful acid may apply to other reactions.