Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Structure of Benzene: Benzene is a cyclic hydrocarbon with the formula . All six carbon atoms are hybridized, forming a planar hexagonal ring. Each carbon is bonded to one hydrogen atom and two other carbons via bonds, with a delocalized electron cloud above and below the plane.
Resonance and Stability: Benzene is a resonance hybrid of two Kekulé structures. The delocalization of electrons (resonance) provides extra stability called resonance energy, making it less reactive than typical alkenes.
Hückel's Rule of Aromaticity: For a compound to be aromatic, it must be cyclic, planar, completely conjugated, and possess electrons, where is an integer ().
Electrophilic Substitution Reactions (ESR): Benzene undergoes substitution rather than addition to maintain its aromatic stability. Key reactions include Nitration, Sulphonation, Halogenation, and Friedel-Crafts Alkylation/Acylation.
Directive Influence: Functional groups already present on the benzene ring determine the position of the incoming electrophile. Activating groups (e.g., ) are and directing, while deactivating groups (e.g., ) are directing (except halogens, which are deactivating but directing).
Toxicity: Benzene and polynuclear hydrocarbons (like Benzo[a]pyrene) produced by incomplete combustion of organic matter are carcinogenic and toxic.
📐Formulae
💡Examples
Problem 1:
Explain why the Cyclooctatetraene () molecule is not aromatic even though it is cyclic and contains conjugated double bonds.
Solution:
Cyclooctatetraene has electrons. According to Hückel's Rule, electrons are required for aromaticity. For , gives , which is not an integer. Furthermore, the molecule adopts a non-planar 'tub' shape to minimize strain.
Explanation:
To be aromatic, a compound must strictly satisfy all four criteria: cyclic, planar, fully conjugated, and the electron count. fails the electron count and planarity tests.
Problem 2:
Predict the major product when Toluene () undergoes nitration with a mixture of and .
Solution:
The major products are -nitrotoluene and -nitrotoluene.
Explanation:
The methyl group () is an electron-releasing group due to inductive effect () and hyperconjugation. This increases the electron density at the and positions, making them more reactive towards the electrophile . Thus, it is an -directing activator.
Problem 3:
What is the catalyst used in Friedel-Crafts reaction and what is its role?
Solution:
The catalyst is Anhydrous Aluminium Chloride ().
Explanation:
acts as a Lewis acid. It reacts with the alkyl or acyl halide to generate a powerful electrophile (carbocation, ) by accepting a lone pair from the halogen atom: . This electrophile then attacks the benzene ring.