In each of the following questions, a statement of statement I is given followed by a corresponding statement of statement II just below it of the statements; mark the correct answer.
Statement I: \(C_6H_6\) decolourises \(Br_2\) water.
Statement II: All the six carbon atoms have delocalized p-electrons.
Answer & explanation
Correct answer: option 4
The correct answer is option 4. If statement I is false but statement II is true.
Let us delve into the statements provided and their implications regarding benzene (\(C_6H_6\)):
Statement I: \(C_6H_6\) decolourises \(Br_2\) water.
Benzene (\(C_6H_6\)) is known not to react with bromine (Br₂) in water under normal conditions. This lack of reaction is due to benzene's aromatic nature, which provides it with exceptional stability compared to alkenes or other unsaturated hydrocarbons. Benzene does not readily undergo addition reactions typical of alkenes, where bromine can add across a double bond to form a dibromo derivative. Instead, benzene's π-electron cloud is stable and does not open up to react with bromine.
Statement II: All the six carbon atoms have delocalized p-electrons.
Benzene (\(C_6H_6\)) is a planar molecule with a hexagonal ring structure. Each carbon atom in the benzene ring is sp² hybridized, meaning it has one unhybridized p-orbital perpendicular to the plane of the ring. These p-orbitals overlap sideways to form a delocalized π-electron cloud above and below the plane of the ring.
Analysis of the Statements
Statement I is false: Benzene does not decolorize bromine water because it does not undergo addition reactions with bromine. This is due to the stability provided by its aromatic structure, where the delocalized π-electron cloud is maintained without disruption by electrophilic attack.
Statement II is true: All six carbon atoms in benzene participate in the delocalized π-electron system. Each carbon contributes one p-orbital to the π-system, creating a continuous ring of electron density above and below the plane of the benzene ring. This delocalization of electrons is a key feature of aromatic compounds like benzene.
Explanation
Aromatic Stability: Benzene's stability arises from its aromaticity, which is characterized by the delocalization of π-electrons over the entire ring. This delocalization results in a lower overall energy compared to the hypothetical structure of alternating single and double bonds (which would not be aromatic and less stable).
Reaction with Bromine: Bromine water (Br₂ in water) is typically used to test for unsaturation in organic compounds, where alkenes and alkynes readily react by addition of bromine across their double or triple bonds, resulting in a color change from orange/red to colorless due to the formation of a dibromo derivative. Benzene, being aromatic, does not react in this way because its aromatic stabilization prevents the opening of the ring structure to accommodate bromine atoms.
Conclusion
Given the above analysis, the correct answer to the question is: 4. If statement I is false but statement II is true.
This choice correctly reflects that benzene does not decolorize bromine water due to its aromatic stability (Statement I is false), while all six carbon atoms in benzene do have delocalized p-electrons (Statement II is true), contributing to its aromatic character.