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Alcohols, Elimination Reactions & Alkenes

Instructor: Laura Foist

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

Alkenes are important in forming several compounds such as plastic. In this lesson we will learn how alkenes can be formed through alcohol elimination reactions.

Plastics All Around

Plastic is such a big part of our everyday lives. Look around the room and you will likely find several things made out of plastic. But having so much plastic is a concern for many reasons. One reason is that the way we typically make plastic is by drilling oil and polymerizing it into plastics. The process of drilling and processing oil can be very disruptive for the environment.

So, researchers have tried finding alternatives. One such alternative method to making plastic involves creating plastics out of alcohols using elimination reactions.

Elimination Reactions

Elimination reactions are reactions that take off a part of a compound (eliminating it). This often results in the formation of a double bond. So, we often form an alkene, a carbon-carbon double bond.

In order for a double bond to form, two molecules need to be removed from two neighboring carbon atoms. Often one of the two molecules is a hydrogen atom. This reaction can occur as an E1 reaction, in two steps, or as an E2 reaction, in a single concerted step.

Alcohol Elimination Reaction

An alcohol is a carbon with an OH group attached. The alcohol can be primary (the carbon is only attached to one other carbon atom and two hydrogen atoms), secondary (the carbon is attached to two other carbon atoms and one hydrogen atom), or tertiary (the carbon is attached to three other carbon atoms and no hydrogen atoms).

Primary alcohols react with an E2 reaction, forming a double bond
Primary alcohol

Alcohol elimination reactions occur when an alcohol reacts with an acid. The acid needs to be phosphoric acid, sulfuric acid, or tosic acid. Acids such as hydrochloric acid do not work because the resulting anion is highly nucleophilic, so it will actually replace the alcohol with the resulting anion instead of removing the alcohol to form an alkene.

Secondary alcohols undergo an E1 reaction. There are two possible products, but the major product, circled in red, is the one with the more substituted alkene
Secondary alcohol elimination

With primary alcohols an E2 reaction occurs, while secondary and tertiary alcohols undergo an E1 reaction. With secondary and tertiary alcohols there are two options for where the alkene can form; the major product is the one on the most substituted carbon (as long as that carbon still has a hydrogen to be removed).

The tertiary alcohol undergoes an E1 reaction. This typically results in two possible products, but the first product cannot form in this example because that carbon atom had no available hydrogen to be removed
Tertiary alcohol elimination

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