Alcohol Alkylation: Definition, Reaction & Mechanism

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

The primary focal point of this lesson is alcohol alkylation. We'll discuss the definition and specific reactions and then take a look at how the reaction occurs via its mechanism.

An Old Anesthetic

Have you ever had to go under general anesthesia for a surgical procedure? Ether has been around for centuries and was actually used as a general anesthetic as early as the 1840s to put surgical patients into unconscious states for their procedures. You see, if a patient was undergoing surgery, the doctors would make them inhale ether vapors until they lost consciousness.

Compounds like diethyl ether are made through a process known as alcohol alkylation. Our discussion of this process will start with defining what alcohol alkylation is. Then, we'll take a look at a few specific alkylation reactions before examining how the process occurs in terms of its mechanism. Let's dive in!

Definition of Alcohol Alkylation

Alcohol alkylation occurs when we take an organic alcohol and add an alkyl group to it through a chemical reaction. Alcohols are organic compounds (meaning they're carbon-hydrogen based) that contain the hydroxyl group (-OH). An alkyl group is defined as any substituent (group of atoms) that is an alkane, which is simply a group composed of only carbon and hydrogen atoms.

For example, an isopropyl group is classified as an alkyl group because it only contains carbon and hydrogen atoms. In this particular picture, the squiggly line simply represents the attachment point of the group to another molecule.

An isopropyl group is a specific example of an alkyl group

Essentially, what we're talking about is taking an alcohol and bonding an alkyl group, like an isopropyl group, to the oxygen atom of the alcohol itself. When this happens, the end result is that the hydrogen on the hydroxyl group is replaced by our alkyl substituent.

Alcohol Alkylation Reactions

Now that you're familiar with alcohol alkylation, let's look at some specific examples to get a better understanding of the reaction. In general, alcohols can be alkylated by a name reaction known as the Williamson ether synthesis. In this reaction, an alcohol is reacted with an alkyl halide (an alkyl group bonded to a halogen) in the presence of a base, and the final product is an ether. An ether is just an alcohol that's been alkylated.

As an example, let's consider the reaction of ethanol with ethyl bromide in the presence of sodium hydride (NaH, a base). When these two molecules react the end product is actually diethyl ether, the compound we talked about previously that was once used as a general anesthetic.

Williamson ether synthesis of diethyl ether

Another example of an alcohol alkylation is the reaction of benzyl alcohol with isopropyl bromide in the presence of sodium hydride. This reaction is also classified as a Williamson ether synthesis. The product is benzyl isopropyl ether.

Benzyl alcohol reacts with isopropyl bromide in the presence of sodium hydride to produce benzyl isopropyl ether

Notice that in both cases of our Williamson ether examples, the hydrogen on the alcohol is being replaced with the alkyl group, which is what makes these reactions alcohol alkylations.

The Mechanism of Alcohol Alkylation

Let's now get a detailed look at how an alcohol is actually alkylated by examining how the reaction happens in terms of the mechanism. We already know that sodium hydride (a base) is needed for the reaction, but what's its exact role? In the first step of the reaction, the hydride ion from sodium hydride acts as a base and pulls off the hydrogen atom from the alcohol. When this reaction happens, a negative charge results on the oxygen atom of the alcohol. This type of reaction intermediate is called an alkoxide ion.

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