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Elimination Reactions In Organic Chemistry

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson, we'll explore organic chemistry elimination reactions. Learn how elimination reactions create alkenes from alkanes, and why their creation is difficult unless alcohols and alkyl halides are there to help.

Elimination Reactions

An elimination reaction does just what it says it does: it eliminates atoms on a molecule. This is done in order to create a carbon-carbon double bond. This can be done by using a base or an acid. Elimination reactions that use a base use an alkyl halide as the starting material. Elimination reactions that use an acid use alcohols as the starting material.

Both types of reactions create an alkene, a hydrocarbon with a carbon-carbon double bond. Many elimination reactions involve alkanes, which are carbon chains that are fully saturated (typically with hydrogen atoms). To go from an alkane to an alkene (whose carbon chain is not fully saturated) can be quite difficult, but we'll see how elimination reactions make this change possible.

Alcohols in Elimination Reactions

Ultimately, we want to remove a hydrogen from an alkane to create an alkene, but the pKa of a typical alkane is about 60. At this pKa, it is impossible to remove a hydrogen from an alkane. The pKa is one method used to measure the acidity of a compound. The higher the pKa, the weaker the acid. Strong acids will easily give up a hydrogen, while a weak acid (or base) will not give up a hydrogen.

So, how do you create an alkene from an alkane? This can be done by making the alkane have a carbocation. A carbocation is a carbon that has a positive charge on it. The carbocation is more willing to give up a hydrogen because its pKa is -3, much lower than the pKa of an alkane.

In order to make a carbocation, we use an alcohol. The alcohol can be easily protonated, which is done by simply putting an extra hydrogen on the alcohol. Common acids, such as hydrochloric acid and sulfuric acid, can protonate the alcohol. This protonated alcohol is a great leaving group; its oxygen is more electronegative than carbon, so it's able to just leave the carbon. The protonated alcohol then leaves as water, leaving behind a carbocation.

Now, we have a carbocation formed. So the hydrogen can be removed by just about any base (even a weak base) and create a double bond. Frequently, the deprotonated acid (from the step with the alcohol being protonated) can act as this weak base.

The arrows show the flow of electrons in an elimination reaction that uses alcohol
Alcohol elimination reaction

In this example, we see that the red hydrogen from the alkane ends up on the acid, while the purple hydrogen from the acid ends up on the water. The alkene's carbon-carbon double bond is formed using the electrons that were attached to the hydrogen. And the electrons from the carbon-oxygen bond are now on the water.

Alkyl Halides in Elimination Reactions

The reaction mechanism for alkyl halides in an elimination reaction is different from the alcohol elimination reactions. An alkyl halide is a carbon chain where one or more of the carbons are connected to a halogen, such as bromine.

Since halogens are a lot more electronegative than carbon, a lot of the electron density resides with the halogen instead of with the carbons. This makes the carbons electrophilic, or electron-loving. So, the carbons need more electrons.

The blue region represents an electron deficient region around the carbons while the red region represents an electron rich region around the halogen Bromine
Electron density

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