Dehydration Reaction: Study with Video Lessons, Practice Problems & Examples

Acid-catalyzed dehydration transforms alcohols into alkenes through elimination reactions. Primary alcohols typically undergo the E2 mechanism, while secondary and tertiary alcohols follow the E1 mechanism, involving carbocation formation. Protonation of the alcohol enhances its leaving group ability, converting it to water. The stability of carbocations influences the reaction pathway, with tertiary carbocations being the most stable. Understanding these mechanisms is crucial for predicting reaction outcomes and applying concepts like Zaitsev's rule in organic synthesis.

General features of acid-catalyzed dehydration.

Now we're going to talk about a way that we can make double bonds out of alcohols. And this is through a mechanism called acid catalyzed dehydration. Okay? So how does this work? Well, remember back to when I've talked about leaving groups in the past. Okay? And a leaving group is even a concept from acid-base chemistry. All it means is that it's something that really doesn't want to leave, something that really doesn't want to get a negative charge. So another word for leaving group is just conjugate base. Okay? So if you maybe don't really remember exactly what a leaving group is, just think conjugate base, back to acids and bases. And basically, alcohols are really bad leaving groups or they're really bad conjugate bases. They hate to become OH^-. Why? Because OH^- is actually a really strong base. Remember that you always want to go from stronger to weaker? If we're making OH^- hydroxide, that's a really strong base. Okay? So this is not going to be very favored to just leave a molecule. But it turns out that there is one thing we can do to make alcohol a better leaving group and that is to use. And if we can use some kind of acid, we can actually convert alcohol into an awesome leaving group. That awesome leaving group would just be that we add an H to it, we protonate it, so it turns into water. And water is an awesome leaving group because it's neutral. It loves to leave and it loves to just be by itself in solution. Okay? So here's the general formula. I'm not going to show you the mechanism just yet. But basically what we have is we have some kind of alcohol and we have some kind of acid over water. Now in this case, I just put the general H₈. I mean, really any acid, but your common acids are going to be sorry sorry about that. Any of the strong acids. So H₂SO₄ is seen all the time. Any of the hydrogen halide strong acids. I've also seen phosphoric acid. This is not really a strong acid, but it's still strong enough to make the reaction go. These are all very frequently used acids for acid catalyzed dehydration. Okay? And what we're basically doing is we're taking an alcohol. We're going to be removing 2 sigma bonds. We're going to be removing the alcohol and a beta hydrogen. Okay. So this is my alpha. This is my beta carbon. Alpha and beta relative to the alcohol. We're going to be taking away 2 different sigma bonds and we're going to be making 1 pi bond instead. Now that general reaction of taking away 2 sigmas and making 1 pi is an elimination. So this is actually going to be an elimination reaction. Okay? Hope that's making sense so far. Now there's another reaction that you may have already learned or that you will learn soon and that's actually the opposite. It's called acid catalyzed hydration. Acid catalyzed hydration is a reaction where we go from the double bond and we go back to the alcohol. Okay? This is actually what's called an addition reaction. Okay? So if you remember back to the general types of reactions we talked about, that when you take one bond and you make it into 2, that's actually addition. When you take 2 bonds and you make it into 1, that's elimination. So hopefully that makes sense. So basically, the addition part is gonna be the hydration. The elimination part is the dehydration. Okay? How do you know which one it is? How do you know if it's going to be a hydration or a dehydration? You just look at what you're starting with. So in this case, since I have my alcohol, I know that I'm starting with an alcohol and I'm going to try to eliminate it with an acid to become a double bond. However, if I was starting with a double bond and I used acid, then I could add water to it and that would become a hydration. In this step, in this video, we're just gonna learn about dehydration, but I'm just asking you to keep this in the back of your mind later on for when you have to do practice problems with hydration. That this is the way that we tell the two reactions apart. Okay? So let's just talk more in-depth about dehydration. There's a few facts I want you to know. First of all, the more R groups on that alcohol, the easier it's gonna be to dehydrate. This is just a fact that might come up on maybe a conceptual part of your exam or your professor might even ask you, give you 4 different alcohols. Which one's the easiest to dehydrate? Tertiary would be the easiest. Secondary, primary, and then is the worst and then, Oops, sorry. I don't know what I'm trying to draw here. But basically, the methyl can't even happen because if it's a methyl alcohol then or methanol, that can't even eliminate cause it's only got one carbon. Okay? So basically, the easiest one is tertiary, the worst one is primary. That's the first thing. The second thing is that the specific elimination mechanism that we use is going to depend on how easily the molecule is going to form a carbocation. Okay? And the understanding of carbocations is kind of essential to these two mechanisms. All that means is that if you guys remember back to the trend of carbocation stability, that tertiary carbocations are the most stable and primary carbocations are the worst. Okay? And then obviously, methyl is even worse than that, but like I said, methyl doesn't even get counted because we can't use it.

An extra note of caution with 1° alcohols.

Let's do some cumulative practice based on everything we've learned from acid-catalyzed dehydration and make sure to be mindful of all the different details I taught you about which mechanism you would use and what would happen in terms of how many steps they would have and stuff like that. Now there is one instruction that I want to give you, and that's remember that I told you that primaries would do an E2 reaction or mechanism, and then secondaries and tertiaries would perform an E1. Okay? Now that is almost always true, but there is one exception to that. That is going to be if you have a primary alcohol that can rearrange to a tertiary carbocation through a shift, then it's actually going to do a carbocation mediated E1 mechanism instead. Okay? So you're going to have to use that information to maybe determine this first one. Notice that it is primary. Figure out, okay, this would usually be E2. If it could shift to a tertiary position, then you should actually use E1. But I'm going to let you guys figure that out.

The second one, same thing. You have to figure out what mechanism and everything that would happen in between. Alright? So go ahead and get started on this first one and then I'll show you guys how to do the second.

Predict the major product of the reaction

• A.
• B.
• C.
• D. No reaction

Predict the major product of the reaction

• A.
• B.
• C.
• D. No reaction

Here’s what students ask on this topic:

What is the mechanism of acid-catalyzed dehydration of alcohols?

Acid-catalyzed dehydration of alcohols involves converting an alcohol into an alkene through an elimination reaction. The mechanism depends on the type of alcohol. Primary alcohols typically undergo the E2 mechanism, which is a concerted process where protonation of the alcohol enhances its leaving group ability, converting it to water. Secondary and tertiary alcohols follow the E1 mechanism, involving carbocation formation. The steps include protonation of the alcohol, formation of a carbocation (for secondary and tertiary alcohols), and beta elimination to form the double bond. The stability of the carbocation influences the reaction pathway, with tertiary carbocations being the most stable.

Why are tertiary alcohols easier to dehydrate than primary alcohols?

Tertiary alcohols are easier to dehydrate than primary alcohols because they form more stable carbocations. In the E1 mechanism, which is common for secondary and tertiary alcohols, the formation of a carbocation intermediate is a key step. Tertiary carbocations are more stable due to hyperconjugation and inductive effects from the surrounding alkyl groups. This stability lowers the activation energy required for the reaction, making the dehydration process more favorable. In contrast, primary alcohols form less stable carbocations and typically follow the E2 mechanism, which is less favorable for dehydration.

What role does the acid play in the dehydration of alcohols?

The acid in the dehydration of alcohols serves as a catalyst that protonates the hydroxyl group of the alcohol, converting it into a better leaving group, water. This protonation step is crucial as it transforms the poor leaving group (OH-) into a neutral and stable molecule (H2O), facilitating its departure from the molecule. The acid also helps stabilize the carbocation intermediate in the E1 mechanism for secondary and tertiary alcohols. Common acids used include sulfuric acid (H2SO4), phosphoric acid (H3PO4), and hydrochloric acid (HCl).

How does the E1 mechanism differ from the E2 mechanism in acid-catalyzed dehydration?

The E1 and E2 mechanisms differ primarily in their steps and the types of alcohols they involve. The E1 mechanism, common for secondary and tertiary alcohols, involves two steps: protonation of the alcohol to form a better leaving group, followed by the formation of a carbocation intermediate and subsequent beta elimination to form the double bond. The E2 mechanism, typical for primary alcohols, is a concerted process where protonation and elimination occur simultaneously without forming a carbocation intermediate. The E1 mechanism relies on carbocation stability, while the E2 mechanism does not.

What is Zaitsev's rule and how does it apply to dehydration reactions?

Zaitsev's rule states that in an elimination reaction, the more substituted alkene is usually the major product. This rule applies to dehydration reactions of alcohols, where the formation of the more stable, more substituted alkene is favored. During the beta elimination step, the hydrogen is typically removed from the beta carbon with the fewest hydrogens, leading to the formation of the more substituted double bond. This results in the major product being the alkene with the greater number of alkyl groups attached to the double-bonded carbons.