Question

Which of the following is not correct? (1) The bond angle in benzenc is \(120^{\circ}\) (2) The \(\mathrm{C} \mathrm{C}\) bond length in benzene is \(1.39 \AA\) (3) Benzene was discovered by Raraday (4) Benzene is an unsaturated hydrocarbon and participate in addition reactions casily

Short answer

The incorrect statement is (4).

Step-by-step solution

Review Bond Angle in Benzene

Benzene has a hexagonal ring structure where each internal bond angle is exactly \(120^{\circ}\). Thus, statement (1) is correct.

Review \( \text{C}-\text{C} \) Bond Length in Benzene

The \( \text{C}-\text{C} \) bond length in benzene is \(1.39 \mathring{A}\). This distance results from the resonance structure of benzene, making statement (2) correct.

Check the Discoverer of Benzene

Benzene was indeed discovered by Michael Faraday in 1825. Therefore, statement (3) is also correct.

Review the Reactivity of Benzene

Benzene is known for its stability and does not readily participate in addition reactions due to its resonance stability. Instead, benzene typically participates in substitution reactions. Thus, statement (4) is incorrect.

Key concepts

Bond angle

In benzene, all the carbon atoms are arranged in a perfect hexagonal ring. This unique structure leads to each internal bond angle measuring precisely \(120^{\text{°}}\). These equal bond angles occur because benzene is a planar molecule, and the electrons are shared evenly across the ring. Such a consistent bond angle helps contribute to benzene's stability. When studying benzene or similar aromatic compounds, always remember that their bond angles play a crucial role in their chemical properties and reactivity.

C-C bond length

The carbon-carbon (C-C) bond length in benzene is notably shorter than in typical single C-C bonds but longer than in typical double bonds. This bond length is precisely \(1.39 \text{\r{A}}\).
This intermediate bond length is due to the resonance structure of benzene. Resonance involves the delocalization of electrons within the ring structure, making all the C-C bonds equivalently long. This delocalization stabilizes benzene and gives it unique chemical properties distinct from those of alkenes and alkanes.

Discovery of benzene

Benzene was discovered by Michael Faraday in 1825. He first isolated it from the oily residue left behind when illuminating gas was produced from whale oil.
This compound quickly became important in the growing field of organic chemistry. Faraday's discovery paved the way for more research, eventually leading to an in-depth understanding of aromatic compounds and their unique properties. Knowing about the discovery and historical context of benzene can give us insights into the evolution of organic chemistry and its methodologies.

Benzene reactions

Benzene, unlike alkenes, does not typically partake in addition reactions. This is because of its resonance stability; the delocalized electrons make the molecule very stable, which dissuades addition reactions.
Instead, benzene undergoes substitution reactions, where one of the hydrogen atoms bonded to the carbon ring is replaced with another substituent. Examples include nitration, halogenation, and sulfonation. Understanding the types of reactions benzene engages in is critical for students studying aromatic chemistry and helps explain why benzene doesn't act like typical unsaturated hydrocarbons.