The compound 2-chlorobenzalmalononitrile (also called o-chlorobenzylidene malononitrile; chemical formula: C10H5ClN2), a cyanocarbon, is the defining component of tear gas commonly referred to as CS gas, which is used as a riot control agent.

CS gas
Skeletal formula of CS gas
Space-filling model of CS gas
Preferred IUPAC name
Other names
o-Chlorobenzylidene malononitrile
Tear gas
3D model (JSmol)
ECHA InfoCard 100.018.435 Edit this at Wikidata
EC Number
  • 220-278-9
RTECS number
  • OO3675000
UN number 2810, 3276, 2811
  • InChI=1S/C10H5ClN2/c11-10-4-2-1-3-9(10)5-8(6-12)7-13/h1-5H checkY
  • InChI=1/C10H5ClN2/c11-10-4-2-1-3-9(10)5-8(6-12)7-13/h1-5H
  • Clc1ccccc1\C=C(/C#N)C#N
Molar mass 188.6 g/mol[2]
Appearance White crystalline powder
Colourless gas when burned
Odor Pepper-like[3]
Density 1.04 g/cm3
Melting point 93 °C (199 °F; 366 K)
Boiling point 310 °C (590 °F; 583 K)[4]
Vapor pressure 3.4×10−5 mmHg at 20 °C
GHS labelling:
GHS05: CorrosiveGHS06: ToxicGHS07: Exclamation markGHS08: Health hazardGHS09: Environmental hazard
H302, H314, H330, H335, H372, H410
P260, P261, P264, P270, P271, P273, P280, P284, P301+P312, P301+P330+P331, P303+P361+P353, P304+P340, P305+P351+P338, P310, P312, P314, P320, P321, P330, P363, P391, P403+P233, P405, P501
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformFlammability 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
Lethal dose or concentration (LD, LC):
  • 1806 mg/m3 (rat, 45 min)
  • 2753 mg/m3 (mouse, 20 min)
  • 1802 mg/m3 (rabbit, 10 min)
  • 2326 mg/m3 (guinea pig, 10 min)[5]
NIOSH (US health exposure limits):
PEL (Permissible)
TWA 0.05 ppm (0.4 mg/m3)[3]
REL (Recommended)
C 0.05 ppm (0.4 mg/m3) [skin][3]
IDLH (Immediate danger)
2 mg/m3[3]
Related compounds
Related compounds


Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Exposure causes a burning sensation and tearing of the eyes to the extent that the subject cannot keep their eyes open, and a burning irritation of the mucous membranes of the nose, mouth and throat, resulting in profuse coughing, nasal mucus discharge, disorientation, and difficulty breathing, partially incapacitating the subject. CS gas is an aerosol of a volatile solvent (a substance that dissolves other active substances and that easily evaporates) and 2-chlorobenzalmalononitrile, which is a solid compound at room temperature. CS gas is generally accepted as being non-lethal. It was first synthesized by two Americans, Ben Corson and Roger Stoughton,[6] at Middlebury College in 1928, and the chemical's name is derived from the first letters of the scientists' surnames.[7][8]

CS was developed and tested secretly at Porton Down in Wiltshire, UK, in the 1950s and '60s. CS was used first on animals, and subsequently on British Army servicemen volunteers. CS has less effect on animals because they have different tear ducts and, in the case of non-human mammals, their fur inhibits the free entry of the gas.[9]


CS is synthesized by the reaction of 2-chlorobenzaldehyde and malononitrile via the Knoevenagel condensation:

Preparation of CS
ClC6H4CHO + H2C(CN)2 → ClC6H4CHC(CN)2 + H2O

The reaction is catalysed with a weak base like piperidine or pyridine. The production method has not changed since the substance was discovered by Corson and Stoughton.[10] Other bases, solvent free methods and microwave promotion have been suggested to improve the production of the substance.[11]

The physiological properties had been discovered already by the chemists first synthesising the compound in 1928: "Physiological Properties. Certain of these dinitriles have the effect of sneeze and tear gases. They are harmless when wet but to handle the dry powder is disastrous."[10]

Use as an aerosol

As 2-chlorobenzalmalononitrile is a solid at room temperature, not a gas, a variety of techniques have been used to make this solid usable as an aerosol:

In the Waco Siege in the United States, CS was dissolved in the organic solvent dichloromethane (also known as methylene chloride). The solution was dispersed as an aerosol via explosive force and when the highly volatile dichloromethane evaporated, CS crystals precipitated and formed a fine dispersion in the air.[2]


CS Gas used on 1 May 2013 in Istanbul
Tear gas shells used in Istanbul in 2013
CS gas shells used in Taksim Gezi Park, Istanbul, May 2013

Many types of tear gas and other riot control agents have been produced with effects ranging from mild tearing of the eyes to immediate vomiting and prostration. CN and CS are the most widely used and known, but around 15 different types of tear gas have been developed worldwide, e.g. adamsite or bromoacetone, CNB, and CNC. CS has become the most popular due to its strong effect. The effect of CS on a person will depend on whether it is packaged as a solution or used as an aerosol. The size of solution droplets and the size of the CS particulates after evaporation are factors determining its effect on the human body.[12]

The chemical reacts with moisture on the skin and in the eyes, causing a burning sensation and the immediate forceful and uncontrollable shutting of the eyes. Effects usually include tears streaming from the eyes, profuse coughing, exceptional nasal discharge that is full of mucus, burning in the eyes, eyelids, nose and throat areas, disorientation, dizziness and restricted breathing. It will also burn the skin where sweaty or sunburned. In highly concentrated doses, it can also induce severe coughing and vomiting. Most of the immediate effects wear off within a few hours (such as exceptional nasal discharge and profuse coughing), although respiratory, gastrointestinal, and oral symptoms may persist for months.[13][14] Excessive exposure can cause chemical burns resulting in permanent scarring.[15]

Adults exposed to tear gas during the 2020 protests in Portland Oregon (USA) also reported menstrual changes (899; 54.5% of 1650 female respondents). Exposure to tear gas is associated with avoidable healthcare utilization.[16]

Secondary effects

People or objects contaminated with CS gas can cause secondary exposure to others, including healthcare professionals and police. In addition, repeated exposure may cause sensitisation.[17]


TRPA1 (Transient Receptor Potential-Ankyrin 1) ion channel expressed on nociceptors (especially trigeminal) has been implicated as the site of action for CS gas in rodent models.[18][19]

Although described as a non-lethal weapon for crowd control, studies have raised doubts about this classification. CS can cause severe pulmonary damage and can also significantly damage the heart and liver.[20]

On 28 September 2000, Prof. Dr. Uwe Heinrich released a study commissioned by John C. Danforth, of the United States Office of Special Counsel, to investigate the use of CS by the FBI at the Branch Davidians' Mount Carmel compound. He said no human deaths had been reported, but concluded that the lethality of CS used would have been determined mainly by two factors: whether gas masks were used and whether the occupants were trapped in a room. He suggests that if no gas masks were used and the occupants were trapped, then, "there is a distinct possibility that this kind of CS exposure can significantly contribute to or even cause lethal effects".[2]

CS gas can have a clastogenic effect (abnormal chromosome change) on mammalian cells, but no studies have linked it to miscarriages or stillbirths.[20] In Egypt, CS gas was reported to be the cause of death of several protesters in Mohamed Mahmoud Street near Tahrir square during the November 2011 protests. The solvent in which CS is dissolved, methyl isobutyl ketone (MIBK), is classified as harmful by inhalation; irritating to the eyes and respiratory system; and repeated exposure may cause skin dryness or cracking.[21]

See also


  1. ^ Williams KE. "Detailed Facts About Tear Agent O-Chlorobenzylidene Malononitrile (CS)]" (PDF). U.S. Army Center for Health Promotion and Preventive Medicine. Archived from the original (PDF) on 26 September 2007.
  2. ^ a b c d Heinrich U (September 2000). "Possible lethal effects of CS tear gas on Branch Davidians during the FBI raid on the Mount Carmel compound near Waco, Texas" (PDF). Archived (PDF) from the original on 25 December 2014. Retrieved 23 September 2007.
  3. ^ a b c d NIOSH Pocket Guide to Chemical Hazards. "#0122". National Institute for Occupational Safety and Health (NIOSH).
  4. ^ Hoenig, Steven L. (2006). Compendium of Chemical Warfare Agents. Springer. p. 138. ISBN 978-0-387-34626-7.
  5. ^ "o-Chlorobenzylidene malononitrile". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  6. ^ Corson BB, Stoughton RW (1928). "Reactions of Alpha, Beta-Unsaturated Dinitriles". Journal of the American Chemical Society. 50 (10): 2825–2837. doi:10.1021/ja01397a037.
  7. ^ "CS". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
  8. ^ "CS, chemical compound Archived 19 December 2005 at the Wayback Machine". Archived 29 July 2005 at the Wayback Machine. Retrieved on 23 September 2007.
  9. ^ "Orthochlorobenzylidenemalononitrile ClC6H4CHCCN(CN)2 Archived 28 November 2006 at the Wayback Machine". Zarc International. Retrieved on 23 September 2007
  10. ^ a b Corson BB, Stoughton RW (1928). "Reactions of Alpha, Betha-Unsaturated Dinitriles". J Am Chem Soc. 50 (10): 2825–2837. doi:10.1021/ja01397a037.
  11. ^ Pande A, Ganesan K, Jain AK, Gupta PK, Malhotr RC (2005). "Novel Eco-Friendly Process for the Synthesis of 2-Chlorobenzylidenemalononitrile and ITS Analogues Using Water As a Solvent". Org Proc Res Develop. 9 (2): 133–136. doi:10.1021/op0498262.
  12. ^ "Safer Restraint: A report of the conference held in April 2002 at Church House, Westminster." Police Complaints Authority. Retrieved on 23 September 2007
  13. ^ Karagama YG, Newton JR, Newbegin CJ (April 2003). "Short-term and long-term physical effects of exposure to CS spray". Journal of the Royal Society of Medicine. 96 (4): 172–4. doi:10.1177/014107680309600404. PMC 539444. PMID 12668703.
  14. ^ Torgrimson-Ojerio BN, Mularski KS, Peyton MR, Keast EM, Hassan A, Ivlev I (April 2021). "Health issues and healthcare utilization among adults who reported exposure to tear gas during 2020 Portland (OR) protests: a cross-sectional survey". BMC Public Health. 21 (1): 803. doi:10.1186/s12889-021-10859-w. PMC 8074355. PMID 33902512.
  15. ^ "CS spray man 'scarred for life'" Archived 5 March 2016 at the Wayback Machine. BBC News. 2 February 2006. Retrieved on 23 September 2007
  16. ^ Torgrimson-Ojerio BN, Mularski KS, Peyton MR, Keast EM, Hassan A, Ivlev I (April 2021). "Health issues and healthcare utilization among adults who reported exposure to tear gas during 2020 Portland (OR) protests: a cross-sectional survey". BMC Public Health. 21 (1): 803. doi:10.1186/s12889-021-10859-w. PMC 8074355. PMID 33902512.
  17. ^ Carron PN, Yersin B (June 2009). "Management of the effects of exposure to tear gas". BMJ. 338: b2283. doi:10.1136/bmj.b2283. PMID 19542106. S2CID 7870564.
  18. ^ Bessac BF, Sivula M, von Hehn CA, Caceres AI, Escalera J, Jordt SE (April 2009). "Transient receptor potential ankyrin 1 antagonists block the noxious effects of toxic industrial isocyanates and tear gases". FASEB Journal. 23 (4): 1102–14. doi:10.1096/fj.08-117812. PMC 2660642. PMID 19036859.
  19. ^ Brône B, Peeters PJ, Marrannes R, Mercken M, Nuydens R, Meert T, Gijsen HJ (September 2008). "Tear gasses CN, CR, and CS are potent activators of the human TRPA1 receptor". Toxicology and Applied Pharmacology. 231 (2): 150–6. doi:10.1016/j.taap.2008.04.005. PMID 18501939.
  20. ^ a b Hu H, Fine J, Epstein P, Kelsey K, Reynolds P, Walker B (August 1989). "Tear gas--harassing agent or toxic chemical weapon?". JAMA. 262 (5): 660–3. doi:10.1001/jama.1989.03430050076030. PMID 2501523.
  21. ^ "MSDS for 99% 4-Methyl-2-pentanone (MIBK)" (PDF). Alfa Aesar. Archived from the original (PDF) on 16 August 2018. Retrieved 7 January 2013.