Hydration Reaction - Online Tutor, Practice Problems & Exam Prep

Hydration reactions involve the acid-catalyzed addition of water to alkenes, producing alcohols. In symmetrical alkenes, water adds equally to both double-bonded carbons. For asymmetric alkenes, Markovnikov's rule applies: the hydrogen (H) attaches to the carbon with more hydrogens, while the hydroxyl group (OH) attaches to the carbon with fewer hydrogens. This results in a major product favored by the reaction mechanism, with minor products formed when the rule is not followed. Understanding these principles is crucial for predicting reaction outcomes in organic chemistry.

Symmetric Alkene Hydration Concept 1

In hydration reactions, we have the acid-catalyzed addition of water to an alkene, and it produces an alcohol. In this reaction, we're going to add 1 H and 1 OH, both coming from water, to 1 pi bond. If we take a look at the overview for this reaction, we start with an alkene, and we're going to add to it a water molecule. To do that, we need to utilize sulfuric acid, which acts as an acid catalyst to get the process going. Now, if we look at the alkene, both of these double-bonded carbons are the same. The alkene is symmetrical. They both are connected to 1 carbon and 1 H. That means that the H and OH can go to either of those double-bonded carbons. H goes to one, OH goes to the other one.

Later on, we'll see what happens when we have an alkene that is not symmetrical. Where would the H go in that case, where would the OH go? So, if we take a look, I'm going to add the H onto this double-bonded carbon. We're going to break the double bond in order to add it, so at the end we're not going to have a double bond within our product. And then we have our OH added to the other one. This produces our alcohol. So just remember, a hydration reaction means we're adding water. Here we're adding a water molecule to an alkene in order to produce an alcohol.

Symmetric Alkene Hydration Example 1

Complete the following hydration reaction. So here we have a cyclic alkene, we're having water, and we're using sulfuric acid as our acid catalyst. Now one thing to note before we do this question is that instead of H₂SO₄, sometimes you might see them write H⁺. H⁺ serves the same purpose as H₂SO₄. The whole purpose of this acid catalyst is just to provide an H⁺ ion to help the process go. Right? So you might see H₂O with H⁺, or you might see H₂O with H₂SO₄. Now if we look at our alkene, we have a carbon here and a carbon here. Right now, we see them both making 3 bonds, but carbon needs to make 4 bonds. So there is a hydrogen that is not visible. If we take a look at both alkene carbons, they are both connected to one carbon and one hydrogen. They are essentially the same; this is a symmetrical alkene. That means that the H and OH can go to either one of those alkene carbons. So I can draw this as my product. The H, I say, goes up here, and the OH goes over here. Or you could say that the OH goes up here, and the H goes here. They're the same molecule, I'm just showing you that H and OH could go to either double-bonded carbon in this case because they both have the same type of substitution. They both have the same number of carbons and hydrogens connected to one another. Right? Later on, we'll see what happens when we have non-symmetrical alkenes because then it does matter where the H and OH goes. In this instance, it really doesn't matter. Right? So, either one of these is an answer that you could show, and it would be correct. Right? So we've gone from an alkene to our alcohol product.

Markovniko's Rule Hydration Concept 2

Now the addition of water in asymmetric alkenes follows Markovnikov's rule. Under Markovnikov's rule, we state that the hydrogen atom is added to the alkene carbon that possesses more hydrogens, and the hydroxyl group (OH) is added to the alkene carbon with fewer hydrogens. If we take a look here at our hydration reaction, we're starting out with an alkene that is not symmetrical. It's not symmetrical because the alkene carbons have different numbers of hydrogens attached. This carbon here only has one hydrogen directly attached to it, and this one here has two. Water will attach, and we have either hydrogen ions or sulfuric acid being the catalyst to help this process along. Following Markovnikov's rule, the hydrogen should go to the alkene carbon that has more hydrogens. That would mean it will go to this carbon here. So, we add a hydrogen to that one. And then the OH group will go to the one with fewer hydrogens directly attached, specifically this one. Therefore, OH would add to this one. This would represent our major product because this is the product formed by Markovnikov's rule.

The minor product would be the one that's not following the expected result of Markovnikov's rule. In this instance, that would mean that the hydrogen actually went to the one with fewer hydrogens and the OH group, for some reason, went to the one with more hydrogens. When this reaction happens, the majority of the product is the major product. A high percentage of it is this major product, and you make a very small percentage, sometimes less than a percent, of this minor product. So just remember, when your alkene carbon is not symmetrical, meaning the double bonded carbons do not have the same number of hydrogens, utilize the Markovnikov rule to determine which one will be your major product.

Markovniko's Rule Hydration Example 2

Complete the following hydration reaction. If we take a look here, we have our 2 double bonded carbons, this one and this one. The one on the left, we see it making 1, 2, 3 bonds. Carbon needs to make 4 bonds, so there's a hydrogen that's invisible. The other double bonded carbon, we already see it making 1, 2, 3, 4 bonds, so it has no hydrogens at all. Now, this is an asymmetric alkene because the double bonded carbons have different numbers of hydrogens. So, we're going to follow Markovnikov's rule to determine the alcohol that will form. Under Markovnikov's rule, hydrogen should go to the double bonded carbon with more hydrogens. So the H2 would go here, and it'll be right next to the other H that was already present. And then OH will go to the double bonded carbon that has fewer hydrogens, the one on the right. So OH would add here. So this would represent the alcohol that we formed. Now, if you wanted, you could erase these hydrogens and understand that they're there, they're just not being shown. So this would be the best way to show this particular alcohol when omitting those invisible hydrogens. So, this would be our final answer.

 Draw and name an alkene that would be used to produce the following alcohol through a hydration reaction.

• A.
• B.
• C.
• D.

 Provide major and minor products for the hydration of 4-chloro-3-ethyl-1-hexene.

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

What is a hydration reaction in organic chemistry?

A hydration reaction in organic chemistry involves the addition of water (H₂O) to an alkene, resulting in the formation of an alcohol. This reaction is typically acid-catalyzed, often using sulfuric acid (H₂SO₄) as the catalyst. The process adds a hydrogen atom (H) to one of the double-bonded carbons and a hydroxyl group (OH) to the other, breaking the double bond in the process. This reaction is crucial for converting alkenes into alcohols, which are important functional groups in organic synthesis.

How does Markovnikov's rule apply to hydration reactions?

Markovnikov's rule states that in the addition of HX to an alkene, the hydrogen (H) attaches to the carbon with more hydrogen atoms, and the halide (X) attaches to the carbon with fewer hydrogen atoms. In hydration reactions, this rule helps predict the major product when adding water to asymmetric alkenes. The hydrogen (H) from water will attach to the carbon with more hydrogens, while the hydroxyl group (OH) will attach to the carbon with fewer hydrogens. This results in the formation of the major product, with minor products forming when the rule is not followed.

What is the role of sulfuric acid in hydration reactions?

Sulfuric acid (H₂SO₄) acts as a catalyst in hydration reactions. It helps to protonate the alkene, making it more reactive towards water. The acid donates a proton (H⁺) to the alkene, forming a carbocation intermediate. This intermediate is then attacked by a water molecule, leading to the formation of an alcohol after deprotonation. The sulfuric acid is regenerated at the end of the reaction, allowing it to catalyze multiple hydration reactions.

What are the major and minor products in hydration reactions of asymmetric alkenes?

In hydration reactions of asymmetric alkenes, the major product is determined by Markovnikov's rule. The hydrogen (H) from water attaches to the carbon with more hydrogens, and the hydroxyl group (OH) attaches to the carbon with fewer hydrogens. This results in the major product. The minor product forms when the hydrogen and hydroxyl group add in the opposite manner, not following Markovnikov's rule. Typically, the major product is formed in a much higher yield compared to the minor product, which may constitute less than 1% of the total product mixture.

Why is understanding hydration reactions important in organic chemistry?

Understanding hydration reactions is crucial in organic chemistry because they are fundamental for synthesizing alcohols from alkenes. Alcohols are versatile intermediates and end products in various chemical syntheses, including pharmaceuticals, polymers, and fine chemicals. Knowledge of hydration reactions, including the role of catalysts and the application of Markovnikov's rule, allows chemists to predict and control the outcomes of these reactions, leading to more efficient and targeted synthesis processes.