Acid-Catalyzed Hydration

This is the first of three ways to add alcohol to a double bond. Similar to hydrohalogenation in terms of mechanism, however it will require a protonation and deprotonation step since it is acid-catalyzed. 

Concept: General properties of acid-catalyzed hydration.    

Video Transcript

Guys, now we're going to discuss what might be the most important addition reaction of this entire section and that's called acid catalyzed hydration. Keep in mind that of the three methods to make alcohol that we're going to learn in this section, acid catalyzed hydration is considered the first of three ways to make alcohols by addition. So let's go ahead and get started.
So, guys, acid catalyzed hydration is more similar than it is different to the addition reactions that we learned so far. The intermediate is the same as the ones that we've seen so far, which is that it is a carbocation. Now if the intermediate is a carbocation, what do you think that says about its ability to do shifts or rearrangements? Totally. This thing is going to rearrange all the time.
Now that could actually be a bad thing because remember that rearrangements lead to unpredictable outcomes for our reactions. It could lead to a reaction or a product that we weren't expecting. So we may actually see that the ability to rearrange is a drawback that we have to fix later on, but we'll get there later.
Now, one thing about carbocations and their stereochemistry is that they're very unpredictable. Carbocations, remember that they're trigonal planar, so they can be attacked either from the front or from the back. What that means is that the stereochemistry of our products is really still going to be unknown. Unknown because we don't know where it's hitting. There may be several chiral centers produced. So I don't want to say anything more than just unknown. There's usually a big mixture of chiral products in these reactions.
As we said earlier, acid catalyzed hydration or simply known as hydration, is the first of three methods to make what? Alcohol. So we know that my product is going to be an alcohol. Now is this going to be a Markovnikov alcohol or an anti-Markovnikov alcohol? Well, it turns out that in this section we're going to learn how to do both. We're going to learn how to do Mark and anti-Mark alcohols. But hydration is definitely a Markovnikov reaction. It's a Markovnikov alcohol because your carbocation is always going to want to form in its most stable location.
So before we go to the general reaction, let's just read off a few bullet points because they will be helpful for us. Now, notice that our reagents are H2O and H2SO4. Now, put more generally, that's the most likely way to see it, but it's simply going to be H2O with some form of acid or HA. Recall that when I talked about dehydration. I said dehydration was H2O and HA, so actually the reagents haven't changed at all since dehydration. They're the same.
So you might be wondering, “Johnny, how can I tell, if the reagents are exactly the same, how can I tell if it's going to be a hydration or a dehydration? How do you know?” Well, you look at what you're starting with. Since you're starting with a double bond that means that a double bond is going to hydrate to an alcohol. So it's the same reagents as acid catalyzed dehydration except starting with a double bond instead of starting with an alcohol. Cool.
So you look not at the reagents, but you look at the starting molecule to know if you're going to go to hydration or dehydration. Just think about hydration means you're adding water and look at what we just did. We're adding a water. We're going to add an H and we're going to add an OH so that means we're hydrating the double bond. We're adding a water to it.
Now in terms of the general mechanism, it's also going to be very predictable. It's going to be the same as hydrohalogenation, which you guys might remember is just the simple reaction of a double bond with HX, which is really our example reaction for addition. It's the same general mechanism as that except that we're going to use water as the nucleophile. Instead of using X- and attacking X-, we're going to attack water instead.
Then guys, remember, I stated this when we talked about dehydration, but remember that every acid catalyzed mechanism always is very predictable. It always starts with something and it always ends with something. If the name is acid catalyzed, that means it always begins with protonation and it ends with deprotonation. That's because if it's acid catalyzed, that means you're always going to put a proton on something and at the end, since it's a catalyst, you have to take that proton off so you can regenerate your acid.
So let's just look at the general reaction here. The general reaction states that a double bond with water and acid is going to do what? Well, notice that this is my Markovnikov site and this is what I would call my anti-Markovnikov site. So this is my Mark site. So even without knowing the mechanism, because I told you guys all the things about the properties of the reaction, I can predict that my product is going to look like what? I'm going to have a Markovnikov alcohol. An alcohol in the most substituted place and then the H of the water will be in the least substituted place. So I'm adding water. I'm adding H2O, but the OH forms Mark and the H forms on the other position.
Now the last thing you might be wondering – not wondering about me, is wondering what's up with the squiggly line because it looks super weird. You may have not seen that before. You may have forgotten what that was. Just remember guys, if you see a squiggly line like that, it means that it's unknown stereochemistry. It means that it could be towards the front. It could be towards the back.
You're not expected to draw all the stereoisomers because that could be a huge burden. All you need to know is that that H went somewhere. And that's why I put the stereochemistry unknown because you're not – there could be up to four different stereoisomers on these things, so I don't think you're expected to draw all of them, just to know that several could form.
Awesome guys. So that's it for this. So then let's move on to the next problem where we're going to draw the mechanism at length. 

This reaction uses the same reagents as acid-catalyzed dehydration, so how do you know which reaction to use? Just look at what you are starting with:

General Reaction:

Note: The squiggly line on the product just means “indeterminate stereochemistry”. We aren’t sure where that –H will add, so we’ll just draw it on a squiggly line. 

Concept: A worked-example of the acid-catalyzed hydration mechanism.     


Problem: Predict the product of the following reaction.


Acid-Catalyzed Hydration Additional Practice Problems

Give three examples of acid-catalyzed reactions. For each example clearly indicate the starting material, reagents and products.

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The acid catalyzed addition of water to alkenes is a common method to prepare alcohols. For the example below, show the mechanism of this reaction. Include all flows of electrons, charges and intermediates. The mechanism MUST show how the acid is catalytic. 

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Please provide the full arrow-pushing mechanism for this reaction. Include stereochemistry for the final product if applicable. 

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Which of the following molecules would be produced from the reaction below?

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The following problems are a new format. We turn the tables and give you the product. In the space provided show the starting material required to make that product using the given reagents. When more than one starting material would work, you must draw both.

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Create the full arrow pushing mechanism which shows all intermediates and all products that result from these reactions and label minor and major products if they are not equal in potential energy.

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Draw the starting material that under the given reaction conditions, result in the following product.

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Fill in the box with the product of the following reaction of alkenes. Draw only the PREDOMINATE REGIOISOMER and indicate stereochemistry by drawing dashes and wedges where appropriate. When a racemic mixture is formed you must draw both enantiomers and write RACEMIC in the box. 

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Follows Markovnikov’s Rule

a)  Hydrogen goes to the double bonded carbon with _______ hydrogens. ( ______ substituted alkene carbon) 

b)    Hydroxide ion goes to the ___________ stable carbocation


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Write a mechanism for the following reaction.

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Predict the product(s) or reagents of the following reactions.

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The acid catalyzed addition of water to alkenes is a common method to prepare alcohols. For the example below, show the mechanism of this reaction. Include all flows of electrons, charges and intermediates. The mechanism MUST show how the acid is catalytic.

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How many atoms and electrons are directly involved in the bond-making and bond-breaking of the reaction given below?  Choose the right answer. 

The first step of hydronium ion-catalyzed addition of water to an alkene

a) four atoms, four electrons

b) four atoms, three electrons

c) four atoms, two electrons

d) three atoms, four electrons

e) three atoms, three electrons

f) three atoms, two electrons 

g) two atoms, three electrons

h) two atoms, two electrons

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Provide a detailed mechanism for the following reaction using a curved arrow notation.

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Provide a detailed mechanism for the following reaction using a curved arrow notation. 

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