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

Dehydration reactions involve the conversion of alcohols to alkenes through the loss of water, facilitated by sulfuric acid (H₂SO₄). The alcohol carbon loses an -OH group, while a neighboring carbon loses an H atom, forming a double bond. Zaitiff's rule applies when neighboring carbons have different hydrogen counts, guiding the loss of H from the carbon with fewer hydrogens. This ensures the correct alkene is produced, highlighting the importance of understanding reaction mechanisms in organic chemistry.

Alcohol Reactions: Dehydration Reactions Concept 1

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 involves sulfuric acid, which is H2SO4, reacting with an alcohol to form an alkene through the loss of water. 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 it could also happen on 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 4 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. That's why we make an alkene as our product.

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 1

Determine the elimination product form in the following reaction. Another name for a dehydration reaction is an elimination reaction. Here, we have alcohol reacting with sulfuric acid, so we know we're trying to create an alkene at the end. Here is our carbon, our alcohol carbon with its OH, and here are the neighboring carbons. This neighboring carbon here is making 2 bonds, so it has 2 hydrogens. This neighboring carbon here makes 2 bonds, so it has 2 hydrogens we don't see.

In this process, we're going to lose water, losing an OH from the alcohol carbon, and losing 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 2 hydrogens. So, when we lose water, that's going to force those 2 carbons to make a double bond with one another to continue to make 4 bonds. Our alkene would look like this, and this would be cyclohexene. This would be our elimination or dehydration product.

Zaitsev’s Rule Concept 2

Now Zaitsev's rule, the loss of water follows Zaitsev's rule when we undergo an elimination or dehydration reaction of an alcohol. 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 2 hydrogens, and the other neighboring carbon has 3. 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'd still lose the OH from the alcohol carbon, so it's gone. We'd lose an H from this CH₂ group so now it's just CH, and they need to maintain their 4 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 2

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 CH and this CH₃. They have a different number of hydrogens, which means we're going to utilize Zaitsev's rule. Remember, under Zaitsev's rule, it's the neighboring carbon with fewer hydrogens that loses an H. So this carbon, the CH carbon on the left has only 1 hydrogen, the methyl carbon has 3. So we're going to lose an H from the CH group. So now our product will look like this. We'd still have CH₃ here, still connected to this C, connected to this CH₃. 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 because it's only losing an OH, and it's still connected to this CH₃. 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 is a dehydration reaction in organic chemistry?

A dehydration reaction in organic chemistry involves the removal of a water molecule (H₂O) from a compound. Specifically, in the context of alcohols, it refers to the conversion of an alcohol to an alkene. This reaction is typically facilitated by a strong acid, such as sulfuric acid (H₂SO₄). The alcohol carbon loses an -OH group, while a neighboring carbon loses a hydrogen atom (H), resulting in the formation of a double bond between the two carbons. This process is crucial for understanding reaction mechanisms in organic chemistry.

How does sulfuric acid facilitate dehydration reactions of alcohols?

Sulfuric acid (H₂SO₄) acts as a catalyst in dehydration reactions of alcohols by protonating the -OH group of the alcohol, making it a better leaving group. The protonated -OH group (now H₂O) leaves, forming a carbocation intermediate. A neighboring carbon then loses a hydrogen atom (H), and a double bond forms between the two carbons, resulting in an alkene. The overall reaction involves the loss of a water molecule (H₂O) and the formation of a double bond, converting the alcohol into an alkene.

What is Zaitsev's rule in the context of dehydration reactions?

Zaitsev's rule states that in elimination reactions, including dehydration reactions, the more substituted alkene is favored. When an alcohol undergoes dehydration, the -OH group is lost from the alcohol carbon, and a hydrogen atom is lost from a neighboring carbon. According to Zaitsev's rule, the hydrogen is lost from the neighboring carbon with fewer hydrogen atoms. This ensures the formation of the more stable, more substituted alkene. This rule helps predict the major product in dehydration reactions when neighboring carbons have different numbers of hydrogens.

Can you provide an example of a dehydration reaction of an alcohol?

Sure! Let's consider the dehydration of ethanol (CH₃CH₂OH) using sulfuric acid (H₂SO₄). The reaction proceeds as follows:

1. Ethanol is protonated by sulfuric acid, forming a protonated alcohol (CH₃CH₂OH₂⁺).

2. The protonated alcohol loses a water molecule (H₂O), forming a carbocation (CH₃CH₂⁺).

3. A hydrogen atom is lost from the neighboring carbon, resulting in the formation of ethene (CH₂=CH₂).

The overall reaction is:

CH₃CH₂OH → CH₂=CH₂ + H₂O

Why is understanding dehydration reactions important in organic chemistry?

Understanding dehydration reactions is crucial in organic chemistry because they are fundamental transformations that convert alcohols to alkenes, which are key intermediates in many synthetic pathways. These reactions illustrate important concepts such as reaction mechanisms, the role of catalysts, and the application of rules like Zaitsev's rule. Mastery of dehydration reactions enables chemists to predict and control the outcomes of organic syntheses, facilitating the design and production of a wide range of chemical compounds, including pharmaceuticals, polymers, and other industrial chemicals.