Alcohol Reactions: Dehydration Reactions - Online Tutor, Practice Problems & Exam Prep

Alcohol dehydration reactions involve the use of sulfuric acid (H₂SO₄) to transform an alcohol into an alkene through the loss of water (H₂O). This process entails the alcohol carbon losing its hydroxyl (OH) group, while the adjacent carbon sheds a hydrogen atom (H), leading to the formation of a double bond between the two carbons. The reaction follows Zaitsev's rule, which dictates that the hydrogen is removed from the neighboring carbon with fewer hydrogens when the alcohol is unsymmetrical. This rule helps predict the formation of the most stable, and thus, the most likely alkene product. Understanding this mechanism is crucial for predicting the outcomes of dehydration reactions and synthesizing alkenes from alcohols.

Alcohol Reactions: Dehydration Reactions

In this video, we're going to take a look at alcohol reactions and in particular dehydration reactions. Now under dehydration reaction, this type of reaction has sulfuric acid, which is H2SO4 reacting or reacts with an alcohol to form an alkene through the loss of water. Now to form the double bond, the alcohol carbon loses its OH and its neighboring carbon loses an H atom.

If we take a look here at this example question or example reaction, we have our alcohol. Here we're introducing our sulfuric acid. Here goes my alcohol carbon with its OH group, and we have this neighboring carbon and this neighboring carbon, both of which happen to be methyl groups. They're both the same, so we can lose an H from either side. I decide to show it losing from the left side, but could also happen to the right side since they're the same.

So here we're going to lose water. The water we lose is over here. And remember, carbon must continue to make four bonds. They each have lost a bond, one to hydrogen and one to OH. They need to replace that bond they've lost, so their only option is to make a double bond to one another. So that's why we make an alkene as our product.

So remember, in a dehydration reaction, we're just introducing sulfuric acid to our alcohol in order to lose water and form an alkene.

Alcohol Reactions Dehydration Reactions Example

Determine the elimination product form in the following reaction. Now another name for a dehydration reaction is an elimination reaction. Here we have our alcohol reacting with sulfuric acid, so we know we're trying to create an alkene at the end.

Here is our carbon or alcohol carbon with its OH and here are the neighboring carbons. This neighboring carbon here is making two bonds, so it has two hydrogens. This neighboring carbon here has makes 2 bonds, so it has two hydrogens. We don't see in this process we're going to lose water. So lose an OH from the alcohol carbon and lose an H from one of its neighboring carbons.

Here I decide to lose it from this side, but it could equally happen on the other side because those two neighboring carbons are the same. They both have two hydrogens. So here we're going to lose water and when we lose water that's going to force those two carbons to make a double bond with one another in order for them to continue to make four bonds.

O we're al keen would look like this and this would be cyclohexene. This would be our elimination or dehydration product.

Zaitsev’s Rule

Now Zaitsev's rule. The loss of water follows Zaitsev's rule when we undergo an elimination or dehydration reaction of an alcohol. Now it's used when the neighboring carbons have different numbers of hydrogens. Under Zaitsev's rule, we're going to say the OH atom is lost from the alcohol carbon and the H atom is lost from the neighboring carbon with fewer hydrogens.

If we take a look here at this reaction, we have an alcohol that is not symmetrical. Here we have our neighboring carbons. One neighboring carbon has two hydrogens and the other neighboring carbon has three. Following Zaitsev's rule, we would lose an H from the carbon in red since it has fewer hydrogens to lose.

When we do this, we still lose the OH from the alcohol carbon, so it's gone. We'd lose an H from the chapter two group. So now it's just chapter and they need to maintain their four bonds so they'd form a double bond between each other. So again, we utilize Zaitsev's rule when our neighboring carbons next to the alcohol carbon have different numbers of hydrogens. This is just a way of us making sure that we're making the correct alkene at the end.

Alcohol Reactions Dehydration Reactions Example

Here it says determine the elimination product formed in the following reaction. Alright, so we have our alcohol carbon right here with its OH group and we have our neighboring carbons this chapter and this chapter three. They have different number of hydrogens which means we're going to utilize Zitif's rule.

Remember, under ZITIS rule, it's the neighboring carbon with fewer hydrogens that loses an H. So this carbon, the chapter carbon on the left, has only one hydrogen. The methyl carbon has three. So we're going to lose an H from the chapter group.

So now our product will look like this. We still have Chapter 3 here, still connected to this C connected to this Chapter 3. It loses its H in order to make a double bond with the alcohol carbon. The alcohol carbon has lost its OH, and then that alcohol carbon still has its H 'cause it's only losing an OH and it's still connected to this chapter 3. This would be the alkene or elimination product formed within this reaction.

Determine the name of the alkene product formed in the following dehydration reaction.

• A. $cis-1-heptene$
• B. $2-heptene$
• C. $1-heptene$
• D. $trans-1-heptene$

Determine the name of the alkene product formed in the following dehydration reaction.

• A. $1-methylcyclohexene$
• B. $4-methylcyclohexene$
• C. $2-methylcyclohexene$
• D. $3-methylcyclohexene$

Here’s what students ask on this topic:

What type of functional group is produced after a dehydration reaction of an alcohol?

After a dehydration reaction of an alcohol, an alkene functional group is typically produced. Dehydration refers to the removal of a water molecule, H2O, from the starting molecule. In the case of alcohols, which contain a hydroxyl group (-OH), the dehydration process involves the elimination of a water molecule from the alcohol. This usually occurs under the influence of an acid catalyst and heat.

The reaction mechanism generally involves the protonation of the hydroxyl group to make it a better leaving group, followed by the loss of a water molecule to form a carbocation intermediate. A hydrogen atom is then removed from a neighboring carbon atom, leading to the formation of a double bond between two carbon atoms, which is the characteristic feature of alkenes. The resulting alkene is a hydrocarbon with a carbon-carbon double bond, and it is more unsaturated than the original alcohol due to the loss of two hydrogen atoms and one oxygen atom.

Which alcohol would undergo a dehydration reaction the fastest when treated with concentrated H₂SO₄?

When considering dehydration reactions with concentrated sulfuric acid (H₂SO₄), the speed at which an alcohol undergoes this reaction depends on its structure. In general, tertiary alcohols dehydrate the fastest, followed by secondary and then primary alcohols. This is because the carbocation intermediate formed during the dehydration process is more stable when it's tertiary, due to greater alkyl group stabilization.

For example, if you compare tert-butyl alcohol (a tertiary alcohol), isopropyl alcohol (a secondary alcohol), and ethanol (a primary alcohol), tert-butyl alcohol would dehydrate the fastest under the influence of concentrated H₂SO₄. The stability of the carbocation intermediate plays a crucial role in the reaction rate, as a more stable carbocation forms more readily and thus accelerates the dehydration process.

What is the first step in an alcohol dehydration reaction?

The first step in an alcohol dehydration reaction is the protonation of the alcohol. This involves the alcohol molecule reacting with a strong acid, typically sulfuric acid ($H_2{\mathrm{SO}}_4$) or phosphoric acid ($H_3{\mathrm{PO}}_4$), which acts as a catalyst for the reaction. The lone pair of electrons on the oxygen atom of the hydroxyl (-OH) group in the alcohol molecule is donated to a hydrogen ion ($H+$) from the acid, forming an oxonium ion ($R-{\mathrm{OH}}_2+$). This protonation step is crucial because it makes the hydroxyl group a better leaving group, which is necessary for the subsequent steps of the dehydration process, where water ($H_{2}O$) is eventually eliminated, leading to the formation of an alkene. The ease of protonation and subsequent dehydration typically increases with the substitution of the alcohol (tertiary alcohols dehydrate more readily than secondary, which in turn dehydrate more readily than primary alcohols).