Question

In this chapter, we studied the mechanism of the acid-catalyzed hydration of an alkene. The reverse of this reaction is the acid-catalyzed dehydration of an alcohol. Propose a mechanism for the acid-catalyzed dehydration of 2 -propanol to propene.

Short answer

Question: Briefly describe the mechanism for the acid-catalyzed dehydration of 2-propanol to propene. Answer: The mechanism for the acid-catalyzed dehydration of 2-propanol to propene involves three main steps: (1) protonation of the alcohol by a strong acid to form an oxonium ion, (2) formation of a carbocation intermediate with release of a water molecule, and (3) elimination of a proton to form the alkene (propene) with the help of a nearby base.

Step-by-step solution

Protonation of alcohol

The first step is the protonation of 2-propanol by a strong acid, typically sulfuric acid (\(\text{H}_2\text{SO}_4\)) or phosphoric acid (\(\text{H}_3\text{PO}_4\)). The lone pair of electrons on the oxygen atom of the hydroxyl group in 2-propanol will attract the hydrogen atom of the acid, forming an \(O\text{-H}\) bond. This step creates an oxonium ion (protonated alcohol) as a result. $$\text{CH}_3\text{CH}\text{(OH)CH}_3 + \text{H}^+ \rightarrow \text{CH}_3\text{CH}\text{(OH}_2^+\text{)CH}_3$$

Formation of a carbocation intermediate

In this step, we will form a carbocation intermediate by breaking the weak and unstable \(O\text{-H}\) bond in the oxonium ion. The bond cleaves, and a water molecule is released, leaving behind a carbocation with a positive charge on the carbon: $$\text{CH}_3\text{CH}\text{(OH}_2^+\text{)CH}_3 \rightarrow \text{CH}_3\text{C}^+\text{HCH}_3 + \text{H}_2\text{O}$$

Elimination of a proton to form the alkene

Finally, the carbocation needs to lose a proton to form the final alkene product. A nearby base (often unchanged conjugate base from the acid used in Step 1) will attract and remove a proton from an adjacent carbon of the carbocation, causing the remaining electrons to form a double bond between the carbons: $$\text{CH}_3\text{C}^+\text{HCH}_3 \rightarrow \text{CH}_3\text{CH}=CH_2 + \text{H}^+$$ Now, we have successfully proposed a mechanism for the acid-catalyzed dehydration of 2-propanol to form propene.