This document provides detailed notes on the properties and reactions of arenes, a key class of organic compounds in chemistry. These notes are designed for Cambridge A-Level Chemistry 9701 students.
What are Arenes?
Arenes are cyclic hydrocarbons containing a six-membered ring with alternating single and double bonds. The most common arene is benzene.
Suggested diagram: Structure of benzene showing resonance delocalization of π electrons.
Properties of Arenes
Physical Properties
Typically liquids or solids at room temperature, depending on the substituents.
Generally have a characteristic aromatic odor.
Poorly soluble in water, but soluble in organic solvents.
Chemical Properties
**Stability:** Arenes are remarkably stable due to the delocalization of π electrons in the ring. This stability is known as aromaticity.
**Resistance to Addition Reactions:** Unlike alkenes, arenes do not readily undergo addition reactions.
**Electrophilic Aromatic Substitution:** Arenes undergo electrophilic aromatic substitution reactions, which are the most important reactions of arenes.
Aromaticity and the Benzene Ring
Benzene's aromaticity is a consequence of the following:
The ring has 6 π electrons (3 double bonds).
According to Hückel's rule, a cyclic, planar, fully conjugated system with 4n+2 π electrons is aromatic. Benzene (n=1) satisfies this rule.
The π electrons are delocalized over the entire ring, resulting in a stable, symmetrical structure.
The delocalization leads to equal bond lengths between carbon atoms in the ring.
Bond Lengths and Bond Strengths
The bond lengths in benzene are intermediate between single and double bonds. All C-C bond lengths are approximately 1.39 ?
.
The C-C bonds in benzene are stronger than typical C-C single bonds but weaker than typical C=C double bonds.
Electrophilic Aromatic Substitution (EAS)
Electrophilic aromatic substitution is the primary way arenes react. An electrophile (E+) attacks the π system of the arene ring, leading to the substitution of a hydrogen atom.
Mechanism of EAS
**Electrophile Generation:** An electrophile is generated, often by reacting a reagent with a catalyst (e.g., a Lewis acid like AlCl3).
**Electrophilic Attack:** The electrophile attacks the π system, forming a carbocation intermediate (σ-complex).
**Deprotonation:** A base removes a proton from the carbon atom bearing the electrophile, restoring aromaticity and forming the substituted arene.
Activating and Deactivating Groups
Substituents on the benzene ring can influence the rate and position of EAS reactions.
Substituent
Effect on Rate
Effect on Position
-CH3 (Methyl)
Activating
Ortho/Para-directing
-OH (Hydroxyl)
Activating
Ortho/Para-directing
-NH2 (Amino)
Activating
Ortho/Para-directing
-COOH (Carboxylic Acid)
Deactivating
Meta-directing
-NO2 (Nitro)
Deactivating
Meta-directing
-X (Halogens)
Deactivating
Ortho/Para-directing (weakly deactivating)
Examples of EAS Reactions
**Halogenation:** Reaction with a halogen (e.g., Cl2) in the presence of a Lewis acid catalyst (e.g., FeCl3).
**Nitration:** Reaction with a mixture of concentrated nitric acid and sulfuric acid.
**Sulfonation:** Reaction with concentrated sulfuric acid or oleum (fuming sulfuric acid).
**Friedel-Crafts Alkylation:** Reaction with an alkyl halide in the presence of a Lewis acid catalyst (e.g., AlCl3).
**Friedel-Crafts Acylation:** Reaction with an acyl halide or anhydride in the presence of a Lewis acid catalyst (e.g., AlCl3).
Reactivity Factors
The rate of EAS reactions is influenced by several factors:
**Nature of the Electrophile:** Stronger electrophiles react faster.
**Nature of the Aromatic Compound:** Electron-donating groups increase the rate, while electron-withdrawing groups decrease the rate.
**Nature of the Catalyst:** A strong Lewis acid catalyst is required to generate a strong electrophile.
**Temperature:** Higher temperatures generally increase the rate of reaction.
Applications of Arenes
Arenes are important building blocks in organic chemistry and have numerous applications, including:
**Pharmaceuticals:** Many drugs contain aromatic rings.
**Dyes and Pigments:** Aromatic compounds are often used in the synthesis of dyes and pigments.
**Polymers:** Aromatic monomers are used to create strong and heat-resistant polymers (e.g., polystyrene).
**Fuels:** Benzene, toluene, and xylene are important components of gasoline.