What is Acid Catalyzed Hydration?

Instructor: Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

Acid catalyzed hydration reactions are very useful at removing double and triple bonds in the presence of an acid catalyst. Continue reading to learn more about and recognize the steps required to perform this reaction.

Review of Chemical Reactions and Catalysts

Before we begin exploring acid catalyzed hydration, let's review some of the key terms that can help to put this topic in context, beginning with a chemical reaction. A chemical reaction is the process of transforming chemical substances into new substances. These processes sometimes involve an acid, which is a substance that can either donate protons or accept electron pairs in reactions. Depending on an acid's pH value it can be classified as strongly acidic (pH range of 1-3), such as the ones we'll focus on this lesson, or weakly acidic (pH range 4-6).

A catalyst is a molecule speeds up the rate of a chemical reaction, without being consumed in the process. Acids, functioning as catalysts, are required for an acid catalyzed hydration reaction to take place. Finally, hydration refers to the process of adding water. During an acid catalyzed hydration, water is used to facilitate the transformation of an unsaturated compound to a saturated one.

What Is An Acid Catalyzed Hydration Reaction?

An acid catalyzed hydration reaction is a reaction where an unsaturated compound is reduced to a saturated compound in the presence of an acid catalyst. Saturated compounds are those that contain single bonds in their structures (Figure 1a). Unsaturated compounds are the exact opposite: they are compounds that contain multiple bonds in their structures (Figure 1b). Unsaturated compounds, used as reactants for this reaction, include alkenes and alkynes.

Saturated and Unsaturated Compounds: (a) Pentane and (b) Pentene
compounds

Why would an alkene and alkyne functional group be the reactant of choice for this reaction? Well, because both are unsaturated compounds that contain double and triple bonds in their structure. Specifically, an alkene is a functional group containing organic compounds with double bonds. An alkyne is a functional group containing organic compounds with triple bonds.

Acid Catalyzed Reaction: Step-by-Step

The following example involves the acid catalyzed hydration of the alkene molecule, ethene. Take note that there are three steps involved in this reaction, as well as three key ingredients: (1) acid catalyst, (2) water and (3) an unsaturated compound. As with all chemical reactions, the arrows tell you in which direction the electrons are moving. Keep an eye on these arrows, and use them as a guide while going through the steps of this reaction.

Step 1: Acid catalyst sulfuric acid reacts with water to form a hydronium ion (H30). A hydronium ion is a very strong acid that can exist in an aqueous solution.

Step 1 of an Acid Catalyzed Hydration
step 1

Step 2: The double bond on the ethene molecule is protonated using the hydronium ion. Protonation refers to a reaction where a proton (H) is added to a molecule or ion. The hydrogen atom from the hydronium ion has a very strong attraction to the double bond on ethene (a). This causes the bond to break and allows the electrons to form a new bond with the hydrogen atom (b). Remember that each bond consists of two electrons. This new bond forms an intermediate called a carbocation, or a molecule whose carbon atom is positively charged.

Step 2 of an Acid Catalyzed Hydration
step 2

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