Background for this classification summary
- Visit the Classification Index to see other groups.
- The summary on this page follows the classification scheme of Elsner and Hoelzer, 2016. Quantitative Survey and Structural Classification of Hydraulic Fracturing Chemicals Reported in Unconventional Gas Production. Environmental Science and Technology. 3290-3314. DOI: 10.1021/acs.est.5b02818.
- Elsner and Hoelzer’s thorough and influential article provides a wealth of chemical context about fracking materials including health and environmental implications and suggestions for deeper analysis. However, the work was based partially on a early set of FracFocus (through 2013). Subsequent years have added new materials and millions more records. To extend their analysis to the rest of the FracFocus materials, we used Google’s Gemini to assist us in assigning classifications in line with EH’s methodology.
- Analysis from a FracFocus download, Jan 2025.
- Masses are not available for all records, so reported masses below are sums of only the available masses. See documentation.
- To provide useful technical information about these chemical groups, we use generative AI tools to assist with summaries (Google’s Gemini). These tools are prompted with details of the classification scheme (by Elsner and Hoelzer, 2016) and information about reported chemicals in the group. To limit inaccuracies, we feed the resulting output to competing models (OpenAI’s ChatGPT) to check for accuracy and ambiguity. In addition, we consulted Fink, Johannes, “Petroleum Engineer’s Guide to Oil Field Chemicals and Fluids” Third Edition. (2021) Gulf Professional Publishing. 1028pp.
- Functions reported here are summarized from the FracFocus field “Purpose.” There are limitations to this data: not all chemical records have a reported purpose (the “systems approach” explicitly excludes “Purpose” values from chemical records). Furthermore, “Purpose” typically describes the product, not individual chemicals within the product. The individual ingredient may serve a chemical role in the product that is different from the reported “purpose” (for example, synthetic polymers might be labeled “proppant”, but they are not primary proppants, but rather coatings for sand grains to enhance their performance.) In general, consider these “functions” as the larger purpose played by the product that contains the ingredient.
Electrophilic Compounds, as reported to FracFocus, are a class of chemicals characterized by their electron-deficient centers, which make them susceptible to attack by nucleophiles. Between 2014 and 2024, 171,000 records of electrophilic compounds were reported on FracFocus, with a total mass of 465,000,000 pounds.1
This broad group is subdivided into several subgroups, each reflecting the diversity of electrophilic functional groups:
- Aldehydes and Ketones (27 reported chemicals),
- Nitriles (6),
- Quaternary Organic Phosphonium Salt (1),
- N-heterocycles (13),
- Other Halogenated Hydrocarbons (59),
- Substituted Propanols (3).
- Non-specified / Others (7),
The products incorporating these electrophilic compounds are reported to serve a variety of functions in hydraulic fracturing, including:
- Biocides,
- Corrosion inhibitors,
- Iron control agents,
- Crosslinkers,
- Emulsifier controls,
- Tracers,
- Scale control agents, and
- Clay control agents.
Subclass: Aldehydes and Ketones¶
Number of reported materials: 27
The “Aldehydes and Ketones” subgroup within the Electrophilic Compounds category includes molecules containing the carbonyl functional group (C=O). In aldehydes, the carbonyl group is bonded to at least one hydrogen atom, while in ketones, it is bonded to two carbon atoms.
Commonly reported examples in this subgroup include simple aldehydes like Formaldehyde (50-00-0), Acetaldehyde (75-07-0), and Glutaraldehyde (111-30-8), a dialdehyde. Aromatic aldehydes such as Benzaldehyde (100-52-7) and Cinnamaldehyde (104-55-2, also known as 3-Phenylprop-2-enal) are also present. Ketones reported include Acetone (67-64-1), 4-Methyl-2-pentanone (108-10-1, also known as Methyl isobutyl ketone), Acetophenone (98-86-2), and Diisobutyl ketone (108-83-8). Glyoxal (107-22-2), a simple dialdehyde, is also reported.
The electrophilic nature of the carbonyl carbon in both aldehydes and ketones makes these compounds reactive toward nucleophiles. In hydraulic fracturing, some of these compounds—particularly aldehydes—function as biocides due to their ability to react with biological molecules.
Most common Aldehydes and Ketones :
| Material | Name | Records and mass |
|---|---|---|
111-30-8 | Glutaraldehyde | 59,300 records — 307,000,000 pounds |
104-55-2 | 3-Phenylprop-2-enal | 26,800 records — 56,400,000 pounds |
50-00-0 | Formaldehyde | 16,500 records — 2,370,000 pounds |
98-86-2 | Acetophenone | 3,880 records — 128,000 pounds |
75-07-0 | Acetaldehyde | 2,340 records — 81,500 pounds |
Subclass: nitriles¶
Number of reported materials: 6
The “Nitriles” subgroup within the Electrophilic Compounds category includes organic compounds that contain the cyano group (-C≡N). The carbon atom in the cyano group carries a significant partial positive charge due to the electronegativity of nitrogen, making it electrophilic. Many commonly reported nitriles in this subgroup are also halogenated, which further enhances their reactivity.
Examples include 2,2-Dibromo-3-nitrilopropionamide (10222-01-2), Dibromoacetonitrile (3252-43-5), and 1,2-Dibromo-2,4-dicyanobutane (35691-65-7). Acrylonitrile (107-13-1), a simple unsaturated nitrile, is also reported. The presence of bromine atoms in several of these compounds, such as 2-Bromo-3-nitrilopropionamide (1113-55-9) and 2,2-Dibromopropanediamide (73003-80-2), likely contributes to their reported use as biocides, facilitating reactions with nucleophilic sites in biological molecules.
Most common nitriles:
| Material | Name | Records and mass |
|---|---|---|
10222-01-2 | 2,2-Dibromo-3-nitrilopropionamide | 14,100 records — 29,100,000 pounds |
3252-43-5 | Dibromoacetonitrile | 3,310 records — 1,920,000 pounds |
1113-55-9 | 2-Bromo-3-nitrilopropionamide | 1,330 records — 50,900 pounds |
107-13-1 | Acrylonitrile | 458 records — 18,700 pounds |
73003-80-2 | 2,2-Dibromopropanediamide | 272 records — 23,400 pounds |
Subclass: quarternary organic phosphonium salt¶
Number of reported materials: 1
The “Quaternary Organic Phosphonium Salt” subgroup within the Electrophilic Compounds category reported to FracFocus is represented by a single, prominent compound: Tetrakis(hydroxymethyl)phosphonium sulfate (THPS) (55566-30-8). THPS features a central phosphorus atom bonded to four hydroxymethyl groups, with a positive charge on the phosphorus, balanced by a sulfate counterion. Unlike many other electrophilic compounds, where the carbon atom is the electrophilic center, in quaternary phosphonium salts, the positively charged phosphorus acts as the electrophile. However, the primary mode of action for THPS in hydraulic fracturing is often attributed to its biocidal properties, which arise from the release of formaldehyde under certain conditions.
The single reported quarternary organic phosphonium salt:
| Material | Name | Records and mass |
|---|---|---|
55566-30-8 | Tetrakis(hydroxymethyl)phosphonium sulfate | 7,530 records — 30,900,000 pounds |
Subclass: N-heterocycles¶
Number of reported materials: 13
The “N-heterocycles” subgroup within the Electrophilic Compounds category consists of cyclic organic compounds containing at least one nitrogen atom in the ring structure. These compounds exhibit significant structural diversity and varying reactivity. Commonly reported examples include Dazomet (533-74-4), a derivative of tetrahydrothiadiazine, and isothiazolones like 2-Methyl-3(2H)-isothiazolone (2682-20-4), 5-Chloro-2-methyl-3(2H)-isothiazolone (26172-55-4), and 1,2-Benzisothiazolin-3-one (2634-33-5). Oxazolidines, such as 4,4-Dimethyl oxazolidine (51200-87-4) and 3,4,4-Trimethyloxazolidine (75673-43-7), are also represented. Other examples include compounds with an aziridine ring, such as Trimethylolpropane tris(2-methyl-1-aziridinepropionate) (64265-57-2) and 3-{[3-(Aziridin-1-yl)propanoyl]oxy}-2-({[3-(aziridin-1-yl)propanoyl]oxy}methyl)-2-(hydroxymethyl)propyl 3-(aziridin-1-yl)propanoate (57116-45-7). The nitrogen atom, and sometimes additional heteroatoms in the ring, influence the reactivity of these molecules and contribute to their function, often as biocides in hydraulic fracturing operations. Additional entries in this group include less specific compounds such as Tar bases, quinoline derivatives (68513-87-1), Bio-Perge (55965-84-9), Triazinetriethanol (4719-04-4), N-(3-Chloroallyl)hexaminium chloride (4080-31-3), and Isoquinoline, reaction products with 1-(chloromethyl)naphthalene and quinoline (68909-81-9).
Most common N-heterocycles:
| Material | Name | Records and mass |
|---|---|---|
533-74-4 | Dazomet | 5,410 records — 13,600,000 pounds |
| 68513-87-1 | Tar bases, quinoline derivs. | 2,930 records — 24,900 pounds |
2682-20-4 | 2-Methyl-3(2H)-isothiazolone | 2,360 records — 6,930 pounds |
26172-55-4 | 5-Chloro-2-methyl-3(2H)-isothiazolone | 2,360 records — 9,960 pounds |
75673-43-7 | 3,4,4-Trimethyloxazolidine | 2,280 records — 717,000 pounds |
Subclass: other halogenated hydrocarbons¶
Number of reported materials: 59
The “other halogenated hydrocarbons” subgroup within FracFocus’s Electrophilic Compounds category includes a diverse range of organic molecules that contain one or more halogen atoms (F, Cl, Br, I) bonded to carbon. This subgroup excludes compounds already classified as nitriles or N-heterocycles. The reported compounds range from simple, small molecules like Chloromethane (74-87-3), Dichloromethane (75-09-2), Benzyl chloride (100-44-7), and 1,3-Dichloropropene (542-75-6) to more complex, highly fluorinated compounds. The latter group includes several perfluorinated cycloalkanes and alkanes, such as Perfluoroethylcyclohexane (335-21-7), Perfluoroisohexane (355-04-4), Perfluoromethylcyclopentane (1805-22-7), Perfluoro dimethylethylpentane (50285-18-2), and Perfluoro-1,2-dimethylcyclohexane (306-98-9). Additional examples include Undecafluoro(1,1,1,2,3,3,3-heptafluoropropan-2-yl)cyclohexane (423-02-9), Perflunafene (306-94-5), 1H-Indene, 1,1,2,2,3,3,3a,4,4,5,5,6,6,7,7,7a-hexadecafluorooctahydro- (374-80-1), 1,1,2,2,3,4,4,5,5,6-Decafluoro-3,6-bis(trifluoromethyl)cyclohexane (374-77-6), and 1,1,1,2,2,3,4,4,5,5,5-Undecafluoro-3-(1,1,2,2,2-pentafluoroethyl)pentane (2690-05-3). Another compound listed is Silane, dichlorodimethyl-, reaction products with silica (68611-44-9).
The halogen atoms, particularly chlorine and bromine, create polarized C-X bonds, making the carbon atom electrophilic and susceptible to nucleophilic attack. This reactivity contributes to the varied applications of these compounds in hydraulic fracturing, ranging from biocides to potential corrosion inhibitors. The perfluorinated compounds are notably distinct for their high chemical stability – many are classified by the EPA as PFAS materials. It appears that most of these were used as tracers.
Most common other halogenated hydrocarbons:
| Material | Name | Records and mass |
|---|---|---|
100-44-7 | Benzyl chloride | 2,080 records — 80,200 pounds |
74-87-3 | Chloromethane | 467 records — 47 pounds |
75-09-2 | Dichloromethane | 60 records — 1,060 pounds |
| 68611-44-9 | Silane, dichlorodimethyl-, reaction products with silica | 45 records — 16,800 pounds |
542-75-6 | 1,3-Dichloropropene | 32 records — 66,900 pounds |
423-02-9 | Undecafluoro(1,1,1,2,3,3,3-heptafluoropropan-2-yl)cyclohexane | 18 records — 20 pounds |
335-21-7 | Perfluoroethylcyclohexane | 16 records — 19 pounds |
306-94-5 | Perflunafene | 16 records — 18 pounds |
355-04-4 | Perfluoroisohexane | 15 records — 19 pounds |
1805-22-7 | Perfluoromethylcyclopentane | 14 records — 19 pounds |
Subclass: substituted propanols¶
Number of reported materials: 3
The “substituted propanols” subgroup within FracFocus’s Electrophilic Compounds category includes derivatives of propanol (either 1-propanol or 2-propanol) in which one or more hydrogen atoms are replaced by other functional groups. The most commonly reported example is Bronopol (52-51-7), or 2-Bromo-2-nitropropane-1,3-diol. This compound contains two hydroxyl groups, a nitro group, and a bromine atom, the latter two contributing to its electrophilic character and biocidal activity. 2-Amino-2-methylpropan-1-ol (124-68-5) contains an amino group. While it is not directly electrophilic, it has the ability to neutralize acids. Another compound, 2-Methyl-2-nitro-1,3-propanediol (77-49-6), is also present. The specific substituents on the propanol backbone determine the overall reactivity and function of these compounds within hydraulic fracturing fluid systems.
Reported substituted propanols
| Material | Name | Records and mass |
|---|---|---|
52-51-7 | Bronopol | 3,140 records — 673,000 pounds |
124-68-5 | 2-Amino-2-methylpropan-1-ol | 2,360 records — 272,000 pounds |
77-49-6 | 2-Methyl-2-nitro-1,3-propanediol | 374 records — 115,000 pounds |
Subclass: “non-specified / others”¶
Number of reported materials: 7
The “non-specified / others” subgroup within the Electrophilic Compounds category acts as a catch-all for compounds that do not neatly fit into the other, more defined subgroups. This category includes a variety of molecules with differing electrophilic characteristics. Examples include Nitromethane (75-52-5), a simple nitroalkane, and Ampicillin (69-53-4), a beta-lactam antibiotic, where the strained four-membered ring is particularly susceptible to nucleophilic attack. Also included are isocyanates such as Polymethylene polyphenyl polyisocyanate (9016-87-9), 4,4′-Diphenylmethane diisocyanate (101-68-8), and 2,6-Diisopropylphenyl isocyanate (28178-42-9), as well as the more complex compound Benzene, 2,4-diisocyanato-1,3,5-tris(1-methylethyl)-, homopolymer, 2-isocyanato-1,3-bis(1-methylethyl)benzene-blocked (132435-00-8). Isocyanates are well-known for their reactivity with nucleophiles and are commonly used in polymer synthesis. The final example, Poly(oxymethylene), alpha-[(4,4-dimethyl-3-oxazolidinyl)methyl]-omega-hydroxy- (56652-26-7), represents a polymeric structure containing multiple functional groups. The diverse nature of this “non-specified / others” subgroup highlights the complexity of chemical mixtures used in hydraulic fracturing.
Most common “non-specified / others”:
| Material | Name | Records and mass |
|---|---|---|
56652-26-7 | Poly(oxymethylene), alpha-[(4,4-dimethyl-3-oxazolidinyl)methyl]-omega-hydroxy- | 1,140 records — 186,000 pounds |
75-52-5 | Nitromethane | 374 records — 485,000 pounds |
69-53-4 | Ampicillin | 67 records — 318 pounds |
| 9016-87-9 | Polymethylene polyphenyl polyisocyanate | 48 records — 1,480,000 pounds |
101-68-8 | 4,4′-Diphenylmethane diisocyanate | 9 records — 98,900 pounds |
- All molecular structure images courtesy of the US EPA, except where noted. ↩︎