CN, or chloracetophenon, is a 
gas used as a 
riot control agent.  It has the molecular form C
8H
13ClO.  It was investigated, but not used, during the 
First and 
Second World Wars, and was used by United States forces in 
Vietnam.  As its toxicity is apparently greater than that of 
CS gas, it has largely been supplanted by the latter agent.
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