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CS gas a super strong irritant

OK now you get to look a real super strong irritant in the eye, without tears !   

The substance in question is CS gas, this is an example of an alpha beta unsaturated nitrile which is similar to an alpha beta unsaturated carbonyl compound. In general compounds which have unsaturation which is conjugated to a group which can withdraw electron density by resonance tend to be irritants or be toxic. A short list of the nasties from this class (just the ones off the top of by head) are.   

1. 4-vinyl pyridine   

2. Methyl vinyl ketone   

3. Methyl methacrylate   

4. Acrylonitrile   

5. CS gas   

Some examples of this class are not so bad such as cinnamaldehyde (Ph-CH=CH-CHO) but where there is a rule in chemistry there is almost always a compound which breaks the rule. But back to CS gas, firstly it is not a gas the boiling point of CS is very high. Here is a picture of a molecule of CS to show you what it looks like.   

A molecule of CS


The CS molecule is likely to be able to exert its effects through a Micheal reaction, as part of my consideration of the molecule I then calculated the shape using molecular mechanics (a hi tech POSH system of balls and springs) to give me the following shape of the molecule.   

You might wonder why the top nitrile is so close to the green chlorine atom, this is because the steric interaction of the hydrogen bonded to the alkene is more important, the conformation which I have shown is a higher energy conformation. The hydrogen is as far away from the big chlorine at as possible. But the nitrile group is close to the chlorine. Here is a plot of the energy as a function of the torsion angle for atoms 1,2,3 and 4.   

Numbers added to make it more clear for you.


Now here is the graph of energy. For those not blessed with super eyesight or a magnifying glass the peak energy is 49.78 thousand calories per mole, while when the torsion angle is zero it is 22.36 thousand calories per mole. When the torsion angle is 180 degrees (pi radians) then it is 19.48 thousand calories per mole.   

Energy as a function of the torsion angle for CS


Next I searched the Cambridge crystallography database for CS, and I found the crystals of the substance have the molecule in the other conformation. So based on this and the MM result I choose to change the conformation of the molecule in the model. This should be a good lesson to you all that in computational chemistry it is possible for a molecule to be in a local minimum rather than the global minimum. Here is our new lower energy form of CS.   

The lower energy conformation of CS.


Now we have the arrangement of the atoms it is time to try some quantum mechanics, Here is a picture of the molecule showing the atomic charges. The more red an atom is the more positive it is. It is clear that the most positive atoms are the carbons within the nitrile (cyanide) groups, and the most negative atoms are the nitrile nitrogens.   

CS with the atomic charges shown


The more important feature is that the alkene carbon bearing the hydrogen has a charge of + 0.13 while the other alkene carbon (bearing two nitriles) has a charge of – 0.03. The positive charge on the alkene carbon is what makes the CS molecule able to do a Micheal reaction. Thus as a result the CS molecule is able to bind to biomolecules and make your eyes water. I repeated the calculation with isophorone (3,5,5-trimethyl-2-cyclohexen-1-one) and I got similar results. I might get back to you soon about other tear gases, in the mean time be good, behave yourselves and try not to provoke/irk the police into spraying you with CS or other irritant sprays.


2 Responses

  1. Dear Dr.Foreman,

    I was reading about CS gas or tear gas online and get curious about how the CS gas causes irritation when the gas is exposed to our body. I did a little research and came across some information. A molecule of CS possibly reacts with water to form 2-chlorobenzaldehyde that could be turned into 2 chlorobenzoic acid. I assume that the acid would cause skin irritation. What I don’t quite get is how the CS gas gets turned into 2-chlorobenzaldehyde in the first place. I guess that cyanide behaves like carbonyl. Could you shed some light on the mechanism regarding the CS gas turning into 2-chlorobenzaldehyde and also the possible mechanism that oxidizes 2-chlorobenzaldehyde to 2-chlorobenzoic acid?

    Thank you very much.

    • I will check the literature and get back to you. But I think that the way in which CS works is by acting as a micheal acceptor

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