Cyclopropane - The little ring that can knock you out cold

Like it's acyclic cousin, cyclopropane is a colourless gas with a notable smell and taste that is both inflammable and potentially explosive and typically stored in metal cylinders as a liquid under pressure.

Cyclopropane is the simplest possible cyclic hydrocarbon, consisting of just three carbon atoms bonded together at 60 degree angles to one another, plus a bunch of hydrogen atoms. Since carbon bonds are normally (and ideally) positioned at a 109.5 degree angle to one another, the bonds in cyclopropane are weakened by angle strain, making the gas more reactive than any other cyclic or acyclic alkane. Molecules of cyclopropane really like to break down to form linear acyclic compounds, and this decomposition can be explosive. Such an explosion can be even more powerful if the gas is mixed with oxygen. Sufficient volumes of pure liquified cyclopropane will self-detonate. The liquid has to be shipped in containers filled with metal or glass wool to improve its stability in order to prevent explosions.

Cyclopropane was first investigated as a potential anaesthetic agent in the early 1930s. Doctors liked it because unlike the other anaesthetic gases/vapours available at the time, it was non-irritating to the respiratory tract, did not significantly depress respiration, heart function or blood pressure, was highly potent (meaning that it could be diluted way down with oxygen), was not appreciably metabolized in the body, and possessed a relatively wide margin of safety between therapeutic and toxic concentrations.

The propensity for cyclopropane to burn/explode and occasionally produce arrhythmias, particularly ventricular tachycardia, led to it being replaced by halothane and it's brethren in the 1950s and 60s (in the Western world).

- Goodman and Gillman
- http://en.wikipedia.org/wiki/Cyclopropane