Hydrates: Videos & Practice Problems

Water acts as a solvent that reacts with carbonyls to form hydrates, specifically geminal diols. The mechanism involves the nucleophilic attack of water on the electrophilic carbon, leading to a tetrahedral intermediate and subsequent proton transfer. Larger R groups in carbonyl compounds hinder hydrate formation, shifting equilibrium towards the carbonyl. For small carbonyls like formaldehyde, hydrates are favored. This reaction is relevant in biological contexts, such as the preservation of specimens in formalin, a hydrate of formaldehyde, which is often misidentified by its characteristic smell.

Mechanism

Let's talk about a solvent that likes to react with carbonyls and that's water. Water loves reacting with carbonyls to make a molecule called a hydrate. But remember that a hydrate is just a gem diol. Geminaldiol, they're both attached to the same carbon. Now the mechanism is pretty straightforward, guys. What winds up happening is that the lone pairs on the oxygen are attracted to the electrophilic carbon and you get the formation of a tetrahedral intermediate, TI for short. What that's going to give us is a negatively charged oxygen and a positively charged water. Notice that the reason that this oxygen is positively charged is because this is the water that came from here and now it has one extra bond. Well then what you get is a proton transfer. This step is called a proton transfer where the oxygen literally just grabs an H, plucks an H off of another part of the tetrahedral intermediate. Now if this looks unfamiliar to you, you haven't done a lot of proton transfers yet, get used to it. A lot of these solvents that attack carbonyls, some of them are going to have proton transfers. It's something that you should be aware of. This is pretty interesting. As I mentioned before, this means that if you're in lab and you mix, you have a 50% solution of 2-butanone. That 50%, that means that if the other 50% is water, then it's not just going to be that you have 50% water and 50% 2-butanone. It's actually going to be that you're going to have some percentage in there is going to be a hydrate where the water is interacting with the ketone to make a gem diol. You guys actually already might have experienced this because in your biology lab, if you guys have taken Bio 1 or 2 and if you ever smell those like animals that they bring out for you to like cut open and look at, sometimes you'll have to like maybe cut open like an earthworm or like I don't know like a bunny. I don't know depending on animal cruelty. Regardless, they're always soaked in what we usually call formaldehyde. And formaldehyde, we think has that nasty smell of like a dead thing that they're preserving. But actually guys, when you're in lab cutting open that animal, it's actually not the formaldehyde that you're smelling. It's formalin. Formalin is the specific aldehyde, the specific hydrate that's made from formaldehyde. When it reacts with water, it makes formalin. Formalin is what gives off that smell. It turns out that you've actually already experienced the hydrate in your life possibly or you will. If you take Bio 1 or 2, you're going to smell these dead animals that are being preserved and that is the smell of formalin which is a hydrate, not the smell of formaldehyde by itself. It turns out that this reaction is not really synthetically useful because the larger the R groups get, the more bulky that tetrahedral intermediate is going to be and the less favored it is. The equilibrium is going to be greatly shifted to the left the bigger that the R groups are. As your R groups get bigger and bigger, you're going to have more and more original carbonyl and less and less hydrate. You can imagine that if you have a 10 carbon chain on both sides, it's going to be very difficult in terms of sterics to form a hydrate and it's going to be much easier to keep it as a carbonyl. The only time that you would actually get a predominance of the hydrate is with an extremely small carbonyl like formaldehyde where formaldehyde and water actually gives a majority of formalin. But it's because you have the smallest R groups possible which is just H's. In that case, it's favored. But if you have larger R groups, then you're going to usually shift towards the carbonyl in terms of your equilibrium. Let's do a mechanism, show the whole mechanism and then predict the equilibrium for the product and then I'll show you the answer.

Show the mechanism, predict the equilibrium

The mechanism was pretty straightforward. You would take your water, your oxygen. You would attack the carbonyl carbon. You're going to get a tetrahedral intermediate that has an O^- and an OH2⁺ with an isopropyl and a phenyl on either side, then we know we're going to do a proton transfer. That's going to give us our OH, OH, benzene, and isopropyl. Not that hard. Now I also ask for equilibrium. Notice that I kind of messed up because I drew a forward arrow. It's not a forward arrow. It's an equilibrium arrow. Let's draw those in. The equilibrium arrows for both of these steps would be shifted towards you think the right or to the left. What do you think is more favored, the hydrate or the original ketone? These R groups are definitely bigger than hydrogen. They're pretty bulky. The equilibrium is going to be greatly shifted to the left, and only a tiny bit is going to go forward. In fact, it might be on the order of less than 1% hydrate. That's why hydrates, they're interesting to understand in terms of the theory of solvents attacking carbonyls. But synthetically, we don't really use these because they're so unfavored to form that really you can't really get a stable gem diol out of it. The gem diol is going to eventually go back towards being a carbonyl. Anyway, that's the end of this reaction. Let's move on to the next topic.

Here’s what students ask on this topic:

What is a hydrate in organic chemistry?

In organic chemistry, a hydrate is a compound formed when water reacts with a carbonyl group (C=O) to produce a geminal diol, also known as a gem diol. This means that two hydroxyl groups (OH) are attached to the same carbon atom. The general reaction involves the nucleophilic attack of water on the electrophilic carbon of the carbonyl group, leading to the formation of a tetrahedral intermediate, followed by a proton transfer. Hydrates are more commonly formed with smaller carbonyl compounds like formaldehyde.

How does water react with carbonyl compounds to form hydrates?

Water reacts with carbonyl compounds through a nucleophilic addition mechanism. The lone pairs on the oxygen atom of water are attracted to the electrophilic carbon of the carbonyl group, forming a tetrahedral intermediate. This intermediate has a negatively charged oxygen and a positively charged water molecule. A proton transfer then occurs, where the oxygen atom plucks a hydrogen atom from another part of the intermediate, resulting in the formation of a geminal diol (hydrate). This process is more favorable for smaller carbonyl compounds like formaldehyde.

Why are hydrates less favored with larger R groups in carbonyl compounds?

Hydrates are less favored with larger R groups in carbonyl compounds due to steric hindrance. As the size of the R groups increases, the tetrahedral intermediate formed during the reaction becomes more bulky and less stable. This steric hindrance shifts the equilibrium towards the original carbonyl compound rather than the hydrate. Therefore, larger carbonyl compounds are less likely to form hydrates, while smaller ones like formaldehyde are more likely to do so.

What is the difference between formaldehyde and formalin?

Formaldehyde is a simple aldehyde with the chemical formula HCHO. When formaldehyde reacts with water, it forms a hydrate known as formalin. Formalin is a solution of formaldehyde in water, typically containing 37-40% formaldehyde by volume. It is commonly used as a preservative for biological specimens. The characteristic smell often associated with formaldehyde in labs is actually due to formalin, not formaldehyde itself.

What is the mechanism for the formation of a hydrate from a carbonyl compound?

The mechanism for the formation of a hydrate from a carbonyl compound involves several steps:

1. Nucleophilic attack: The lone pairs on the oxygen atom of water attack the electrophilic carbon of the carbonyl group, forming a tetrahedral intermediate.
2. Proton transfer: The oxygen atom in the intermediate plucks a hydrogen atom from another part of the intermediate, resulting in the formation of a geminal diol (hydrate).

This mechanism is more favorable for smaller carbonyl compounds like formaldehyde due to less steric hindrance.