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CHEM 221: Alkenes

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image by Sean MacEntee

Alkenes and Double Bonds
Alkenes are hydrocarbons that contains double bonds. Each double contains two different types of bonds - a sigma bond, which is created as a result of the overlap between two sp2 orbitals, and a pi-bond, which is created as a result of overlap between two pure p orbitals. Notice that because the sigma bond contains more s-character, it is lower in energy and more stable than the pi-bond. Therefore, whenever energy is input to break a double bond, the pi-bond will the bond to break first.

Naming of Alkenes
Whenever naming alkenes, several important steps must be followed. Firstly, you must determine the longest possible carbon backbone that contains all the double bonds. Secondly, you most determine the double bond that contains the lowest possible numerical value. Thirdly, an alkene that contains more than one double bond is given the appropriate name (diene, triene, etc). Lastly, if the hydrocarbon is a ring structure, it is given the name of cycloalkene.

Stability of Alkenes
Typically speaking, any given alkene can contain a multitude of different isomers and these isomers will differ in stability. As one particular example, let us consider the hexene molecule that contains five different isomers. If we compare the enthalpy of formation of each one of these isomers, we see that the isomers with the more negative enthalpy of formation will be more stable and lower in energy. That is because those isomers contain bonds that have more s-character. Remember, bonds with more s-character contain electrons which are closer to the nucleus and therefore more stable.

Degree of Unsaturation
It is usually very difficult to determine the structure of any molecule using simply the molecular formula of that compound. We can however use something known as the degree of unsaturation of that compound to determine what the structure is. The degree of unsaturation can be found using the molecular formula itself and it tells us the number of pi-bonds and or ring structures found with that compound.