Have you ever wondered how a match catches fire when you strike it? What causes the head of the match to spontaneously burn and what gives it that characteristic smell? Have you wondered what causes certain chemical weapons to be so deadly? What about smoke grenades used to generate a smoke screen? Well, if you have, then what you were thinking about are chemicals that contain phosphorus.

It turns out that matches ignite due to the presence of two chemicals; red phosphorus and potassium perchlorate. In a typical match, the head contains potassium perchlorate (an oxidizing agent) along with fillers, sulfur, and glass powder while the side of the box contains red phosphorus and binder. When a match is rubbed across the striking surface, the friction causes the red phosphorus to be converted to white phosphorus. White phosphorus ignites spontaneously in air (this is used in munitions as smoke grenades and other purposes) and begins the decomposition of potassium perchlorate which eventually ignites sulfur and the wood. You can see the reaction of phosphorus with the potassium perchlorate below.

What about chemical weapons? These are chemical compounds with devastating effects on mammals. One such compound is commonly known as sarin. Sarin has been in the news recently in regards to the Syrian attack on rebels using this potent nerve agent. Sarin works by inhibiting acetocholinesterase. This enzyme degrades acetylcholine after it is released into the synaptic cleft. In short, acetylcholine is used to transfer signals between neurons and muscle fibers. Once the necessary signal has been transmitted, the acetylcholine is no longer needed and must be removed to avoid a continual release of signal in an unwanted fashion. This is the action of acetylcholinesterase.

It turns out that sarin is particularly active in lung tissue. The inability to breakdown acetylcholine in the lungs causes a loss of control over those muscles and leads to death typically by asphyxiation. Sarin can be lethal in only 1 to 10 minutes and is 81 times more toxic than cyanide according to the US Army Field Manual published in 2005. Below, you can see the structure of sarin. Oh, and by the way, this action is similar to how some insecticides work as well.

So, what about the use of phosphorus in a typical organic chemistry course? Well, perhaps one of the most important reactions to discuss is the Wittig (pronounced "Vittig") reaction. This reaction uses a phosphorus ylide (a neutral structure with adjacent opposite charges) to react with an aldehyde or ketone and eventually generate an alkene. Let's break down how all of this works!

The first step is to make the organophosphorus compound. In a typical Wittig reaction, this is accomplished by the SN2 reaction of triphenylphosphine with an alkyl halide as shown below.

Once the phosphorus has been added, there is now a positive formal charge that remains on the phosphorus (remember that P likes to have a either 3 or 5 bonds). Next, the ylide needs to be formed. The use of a strong base like n-butyllithium, sodium hydride, or sometimes sodium hydroxide can accomplish this next step.

Notice that the ylide is actually one part of a resonance structure involving the phosphorus and the adjacent carbon. Once the ylide has been formed, the compound can now react with an aldehyde or ketone. Note, to allow the following pictures to show the reaction mechanism more clearly, the aromatic rings attached to the phosphorus are represented as Ph.

The negative portion of the phosphorus ylide attacks the carbonyl carbon forcing the electrons in the carbonyl to move to the oxygen. This forms what is known a betaine, pronounced "beta-ene," and allows for the formation of the oxaphosphetane (the cyclic intermediate). Due to the nature of the bond between phosphorus and oxygen, the oxaphosphetane spontaneously decomposes into triphenylphosphine oxide and the desired alkene.

The new double bond formed in this fashion is typically the Z-isomer when relevant. There have been numerous variations of the Wittig reaction that allow for the formation of the E alkene as well.

Thanks for reading! Check out the following video to further explain the Wittig reaction!