We Dream of Alkenes

Alkenes are also simple hydrocarbons, following the formula C_nH_2n. Alkenes differ from alkanes in that they contain at least one double bond. They can also contain many double bonds, but all they need is one to qualify as an alkene. 



Naming alkenes follows the same rules as alkanes. However, we need to specify where the double bond is placed, and, similar to how side-chains are named, we want the double bond to be in between the two lowest carbons. The above example is called 1-butene.

Remember the shell game? What if we move the double bond around the alkene like the ball was moved around under the cups?



Just remember you want the double bond in between the lowest numbered carbons. So, this example is named 2-pentene.

What happens when things get complicated?



Side chains are always named first, making this hydrocarbon go by the name of 3-methyl, 2-heptene.

Like alkanes, alkenes are relatively stable compounds. Their double bonds, however, make them more prone to chemical action than their sister alkanes are. The double bonds act almost like a box; they can open up and allow more molecules to join the hydrocarbon.

Hydrogenation reactions happen when hydrogen atoms are added to the double bond, changing it into a single bond, and therefore turning it into an alkane.



Halogenation reactions occur when halogens, such as bromine or chlorine, are added to the double bond, again changing it into a single bond (with a halogen in place of the usual hydrogen), and therefore becomes an alkane.



Naming halogens follows the same rules as side-chains and double bonds. The product of this halogenation is called 1,2-dibromobutane.

This is a good place to introduce benzene. While benzene does contain double bonds, it has the formula C₆H₆, so it is not technically an alkene. It is considered to be an aromatic compound, which means that its six carbons form a ring. The double bonds contained within the ring are in constant flux. Basically, these two forms of benzene are constantly flipping back and forth. They are your typical flip-floppers. Chemists call this flipping resonance. (At least that's what they call it where they're in a good mood.)



Resonance happens with other molecules, too. Take NO₂, for example. The double bond found here is able to flip between oxygen atoms, like a ninja.

Brain Snack

Alkene reactions: there is more to alkenes than meets the eye.