What type of reaction is involved in the formation of alkanols from alkenes?

Analysis

The question asks for the classification of the chemical reaction that converts an alkene (a hydrocarbon with a carbon-carbon double bond) into an alkanol (an alcohol).

Concept Explanation

To understand this, we look at the functional group transformation:

• Alkenes contain a double bond ($C=C$). This double bond is a site of high electron density, making it reactive.
• Alkanols contain a hydroxyl group ($-OH$).
• The process of converting an alkene to an alkanol is typically hydration, where water ($H_2O$) is added across the double bond in the presence of an acid catalyst (like $H_3PO_4$).

Because the double bond is "broken" to allow the atoms of the water molecule ($H$ and $OH$) to attach to the carbon atoms, the reaction is classified as an addition reaction. In an addition reaction, two or more molecules combine to form a single, larger molecule without the loss of any atoms.

Step-by-Step Solution

1. Identify the reactants: An alkene (e.g., ethene, $CH_2=CH_2$) and water ($H_2O$).
2. Observe the change: $$CH_2=CH_2 + H_2O \xrightarrow{H^+} CH_3CH_2OH$$
3. Analyze the mechanism: The double bond between the carbons is converted into a single bond. The hydrogen atom ($H$) from the water attaches to one carbon, and the hydroxyl group ($OH$) attaches to the other.
4. Classify the reaction: Since atoms are being added to the molecule to saturate the double bond, this is definitively an addition reaction.

• Elimination is the reverse (removing atoms to form a double bond).
• Substitution involves replacing one atom with another (common in alkanes).
• Redox involves electron transfer, which is not the primary classification for hydration.

Conclusion

The formation of alkanols from alkenes involves the addition of water across the double bond. Therefore, it is an addition reaction.

Exam Tip

Whenever you see a reaction where a double or triple bond is converted into a single bond, it is almost always an addition reaction. Think of it as "adding" atoms to the "open" spaces created by breaking the extra bonds!