chemistry

Hydration reaction

Hydration reaction is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Hydration reaction rather than just read about it. In short: In chemistry, a hydration reaction is a chemical reaction in which a substance combines with water. In organic chemistry, water is added to an unsaturated substrate, which is usually an alkene or an alkyne.

Key takeaways

Hydration reaction belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Hydration reaction to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Hydration reaction from memory before moving on to harder problems.

Reference excerpt

In chemistry, a hydration reaction is a chemical reaction in which a substance combines with water. In organic chemistry, water is added to an unsaturated substrate, which is usually an alkene or an alkyne. This type of reaction is employed industrially to produce ethanol, isopropanol, and butan-2-ol.

Organic chemistry Any unsaturated organic compound is susceptible to hydration.

Epoxides to glycol Several million tons of ethylene glycol are produced annually by the hydration of oxirane, a cyclic compound also known as ethylene oxide:

C2H4O + H2O → HO–CH2CH2–OH Acid catalysts are typically used.

Alkenes The general chemical equation for the hydration of alkenes is the following:

RRC=CH2 + H2O → RRC(OH)-CH3 A hydroxyl group (OH−) attaches to one carbon of the double bond, and a proton (H+) adds to the other. The reaction is highly exothermic. In the first step, the alkene acts as a nucleophile and attacks the proton, following Markovnikov's rule. In the second step an H2O molecule bonds to the other, more highly substituted carbon. The oxygen atom at this point has three bonds and carries a positive charge (i.e., the molecule is an oxonium). Another water molecule comes along and takes up the extra proton. This reaction tends to yield many undesirable side products, (for example diethyl ether in the process of creating ethanol) and in its simple form described here is not considered very useful for the production of alcohol. Two approaches are taken. Traditionally the alkene is treated with sulfuric acid to give alkyl sulphate esters. In the case of ethanol production, this step can be written:

H2SO4 + C2H4 → C2H5-O-SO3H Subsequently, this sulphate ester is hydrolyzed to regenerate sulphuric acid and release ethanol:

C2H5-O-SO3H + H2O → H2SO4 + C2H5OH This two step route is called the "indirect process". In the "direct process," the acid protonates the alkene, and water reacts with this incipient carbocation to give the alcohol. The direct process is more popular because it is simpler. The acid catalysts include phosphoric acid and several solid acids. Here an example reaction mechanism of the hydration of 1-methylcyclohexene to 1-methylcyclohexanol:

Many alternative routes are available for producing alcohols, including the hydroboration–oxidation reaction, the oxymercuration–reduction reaction, the Mukaiyama hydration, the reduction of ketones and aldehydes and as a biological method fermentation.

Alkynes Acetylene hydrates to give acetaldehyde: The process typically relies on mercury catalysts and has been discontinued in the West but is still practiced in China. The Hg2+ center binds to a C≡C bond, which is then attacked by water. The reaction is

H2O + C2H2 → CH3CHO

Aldehydes and ketones Aldehydes and to some extent even ketones, hydrate to geminal diols. The reaction is especially dominant for formaldehyde, which, in the presence of water, exists significantly as dihydroxymethane. Conceptually similar reactions include hydroamination and hydroalkoxylation, which involve adding amines and alcohols to alkenes.

Nitriles Nitriles are susceptible to hydration to amides: RCN + H2O → RC(O)NH2 This reaction requires catalysts. Enzymes are used for the commercial production of acrylamide from acrylonitrile.

Inorganic and materials chemistry Hydration is an important process in many other applications; one example is the production of Portland cement by the crosslinking of calcium oxides and silicates that is induced by water. Hydration is the process by which desiccants function.

Illustrations

Worked examples

Example 1 — a first encounter with Hydration reaction

Start with the simplest possible case. Write down what Hydration reaction claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Hydration reaction before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Hydration reaction ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Hydration reaction

In research: Hydration reaction appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Hydration reaction in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Hydration reaction is common in secondary-school and first-year university syllabi. It links to neighbouring topics Addition reactions, General chemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Hydration reaction outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Hydration reaction in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Hydration reaction means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Hydration reaction out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Hydration reaction in simple terms?

Why does Hydration reaction matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Hydration reaction?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Hydration reaction.