The correct option is **B. alkenes**. ### Detailed Explanation **Understanding the Reaction:** Bromine water is a solution of bromine (Br₂) in water, which is often used as a test for unsaturation in organic compounds. When bromine water is added to an alkene, a rapid reaction occurs, resulting in the addition of bromine across the double bond of the alkene. This reaction is known as electrophilic addition. **Why Alkenes React Rapidly:** 1. **Structure of Alkenes:** Alkenes contain a carbon-carbon double bond (C=C), which is a region of high electron density. This makes alkenes more reactive than many other organic compounds. 2. **Electrophilic Addition Mechanism:** When bromine water is added to an alkene, the double bond attacks the bromine molecule, leading to the formation of a cyclic bromonium ion intermediate. This intermediate is then attacked by a bromide ion (Br⁻) from the bromine water, resulting in a vicinal dibromide (a compound with two bromine atoms added to adjacent carbon atoms). 3. **Visualizing the Reaction:** Imagine the double bond as a "door" that opens up to allow bromine to come in and attach to the carbon atoms. This rapid reaction is what makes alkenes easily identifiable when tested with bromine water. **Why Other Options Are Incorrect:** - **A. Alcohols:** - Alcohols (R-OH) do not undergo rapid oxidation with bromine water. Instead, they can react with bromine in the presence of an acid to form alkyl bromides, but this is not a direct oxidation reaction. The reaction is slower and requires specific conditions. - **C. Carboxylic Acids:** - Carboxylic acids (R-COOH) are already oxidized forms of alcohols and do not react with bromine water in a way that would indicate rapid oxidation. They can react with bromine under certain conditions, but this is not a typical reaction for identifying them. - **D. Esters:** - Esters (R-COOR') do not react with bromine water in a way that indicates rapid oxidation. They are relatively stable and do not have the double bond that facilitates the rapid addition reaction seen with alkenes. ### Summary of Key Points: - **Alkenes react rapidly with bromine water** due to their carbon-carbon double bonds, which allow for electrophilic addition. - **Alcohols, carboxylic acids, and esters do not undergo rapid oxidation** with bromine water, making them less reactive in this context. - **Bromine water is a useful test for unsaturation**, as it changes color (from brown to colorless) when it reacts with alkenes. ### Revision Summary: - Alkenes undergo rapid oxidation with bromine water due to their double bonds. - The reaction involves electrophilic addition, forming vicinal dibromides. - Alcohols, carboxylic acids, and esters do not react rapidly with bromine water. - Bromine water is a key reagent for testing unsaturation in organic compounds.