Aromatic Hydrocarbons I: Structure, Nomenclature, and Reactions of Benzene

Aromatic Hydrocarbons I

Introduction

Aromatic hydrocarbons, also known as arenes, are a class of organic compounds characterized by the presence of one or more benzene rings. Benzene (C6H6) is the simplest aromatic hydrocarbon and serves as the prototype for understanding aromaticity, nomenclature, and reactivity in organic chemistry.

Structure of Benzene

Kekulé and Resonance Structures

The structure of benzene must account for its six-membered ring, three additional degrees of unsaturation, planarity, and equal C–C bond lengths. The Kekulé structures, proposed in 1866, depict benzene as alternating single and double bonds, but this does not explain the observed equal bond lengths.

• Kekulé Structures: Show two resonance forms in equilibrium, with alternating single and double bonds.

• Resonance Hybrid: Benzene is best described as a resonance hybrid, where six π electrons are delocalized over the ring, resulting in equal bond lengths (1.40 Å).

• sp2 Hybridization: Each carbon in benzene is sp2 hybridized, with an unhybridized p orbital perpendicular to the ring, allowing for cyclic conjugation.

Example: All C–C bonds in benzene are 1.40 Å, intermediate between typical single (1.54 Å) and double (1.34 Å) bonds.

Properties of Aromatic Compounds

Stability and Reactivity

The delocalization of π electrons above and below the plane of the benzene ring provides exceptional stability, known as aromatic stabilization. Benzene is electron-rich and reacts readily with electrophiles via substitution rather than addition, preserving aromaticity.

• Bond Lengths: All C–C bonds in benzene are equal due to delocalization.

• Reactivity: Benzene undergoes electrophilic aromatic substitution (EAS) rather than addition reactions typical of alkenes and alkynes.

Example: Benzene does not react with Br2 in CH2Cl2, unlike alkenes.

Criteria for Aromaticity – Hückel’s Rule

Four Criteria for Aromaticity

For a molecule to be aromatic, it must satisfy the following:

1. Cyclic: The molecule must be cyclic, allowing p orbitals to overlap continuously.

2. Planar: All atoms in the ring must be in the same plane for effective π electron delocalization.

3. Completely Conjugated: Every atom in the ring must have a p orbital (no sp3 carbons).

4. Hückel’s Rule: The molecule must have (4n+2) π electrons, where n = 0, 1, 2, ...

Cyclic, planar, and completely conjugated compounds with 4n π electrons are antiaromatic, while those lacking one or more criteria are not aromatic.

Classification	Criteria	Example
Aromatic	Cyclic, planar, conjugated, (4n+2) π electrons	Benzene (6 π electrons)
Antiaromatic	Cyclic, planar, conjugated, 4n π electrons	Cyclobutadiene (4 π electrons)
Not aromatic	Lacks cyclic, planar, or conjugated structure	Cyclooctatetraene

Nomenclature of Benzene Derivatives

Monosubstituted Benzenes

To name a benzene ring with one substituent, name the substituent and add the word 'benzene'. Carbon substituents are named as alkyl groups.

• Examples: Ethylbenzene, tert-butylbenzene, fluorobenzene, chlorobenzene, bromobenzene, nitrobenzene.

• Some substituents result in a new parent name (common name): Toluene (methylbenzene), Phenol (hydroxybenzene), Aniline (aminobenzene), Benzenesulfonic acid, Benzoic acid, Acetophenone.

Disubstituted Benzenes

When two substituents are present, their relative positions are indicated by prefixes:

• Ortho (o-): 1,2-positions

• Meta (m-): 1,3-positions

• Para (p-): 1,4-positions

If the substituents are different, alphabetize their names. If one is part of a common root, name the molecule as a derivative of that monosubstituted benzene.

Polysubstituted Benzenes

For three or more substituents:

• Number the ring to give the lowest possible numbers.

• Alphabetize the substituent names.

• If substituents are part of a common root, name as a derivative of that monosubstituted benzene, with the common root at C1.

Example: 4-chloro-1-ethyl-2-propylbenzene; 2,5-dichloroaniline.

Naming Aromatic Rings as Substituents

The C6H5– group is called phenyl when it is a substituent (abbreviated as Ph–). If the chain is unsaturated, it is the parent and benzene is a phenyl substituent.

• Example: Butylbenzene, (Z)-2-Phenyl-2-butene.

Stability and Reactivity of Benzene Ring

Electrophilic Aromatic Substitution (EAS)

Benzene acts as a nucleophile due to its π electrons. Electrophiles are attracted to the electron-rich ring, forming a carbocation intermediate. The reaction proceeds in two steps:

1. Addition of the electrophile (Y+) to form a carbocation.

2. Loss of a proton to re-form the aromatic ring.

Substitution preserves aromaticity, while addition would destroy it.

Electrophilic Aromatic Substitution: General Mechanism

Mechanism Steps

• Step 1: Benzene reacts with an electrophile (Y+), forming a carbocation intermediate (arenium ion). This step is slow due to loss of aromaticity.

• Step 2: A base removes a proton from the carbocation intermediate, restoring aromaticity. This step is fast.

Example: Substitution of a hydrogen atom by an electrophile (Y+).

Types of Electrophilic Aromatic Substitution Reactions

Halogenation

Halogenation of benzene requires a Lewis acid (e.g., FeBr3) to polarize the halogen molecule, making it a better electrophile.

• Mechanism: Formation of a polarized Br–Br bond, addition to benzene, and removal of a proton by FeBr4–.

• Analogous reactions with I2 and F2 are less useful due to reactivity issues.

Nitration

Nitration requires strong acid to generate the nitronium ion (NO2+), the active electrophile.

• Mechanism: Sulfuric acid protonates nitric acid, loss of water forms NO2+.

• Reaction: Benzene + HNO3 (with H2SO4) → Nitrobenzene.

Sulfonation

Sulfonation involves protonation of sulfur trioxide (SO3), forming a positively charged sulfur species (SO3H+).

• Reaction: Benzene + SO3 (with H2SO4) → Benzenesulfonic acid.

Friedel-Crafts Alkylation

Alkylation of benzene is achieved by treating it with an alkyl halide and a Lewis acid (AlCl3), forming an alkylbenzene.

• Mechanism: Formation of a carbocation or alkyl complex as the electrophile, followed by substitution.

• Carbocation rearrangement may occur for more stable intermediates.

Example: Benzene + CH3Cl (with AlCl3) → Toluene.

Friedel-Crafts Acylation

Acylation involves treating benzene with an acid chloride (RCOCl) and AlCl3, forming a ketone (acylbenzene).

• Mechanism: Formation of an acylium ion (RCO+), which is resonance stabilized, followed by substitution.

Example: Benzene + CH3COCl (with AlCl3) → Acetophenone.

Summary Table: Major Electrophilic Aromatic Substitution Reactions

Reaction	Electrophile	Product
Halogenation	Br+, Cl+	Halobenzene
Nitration	NO2+	Nitrobenzene
Sulfonation	SO3H+	Benzenesulfonic acid
Friedel-Crafts Alkylation	R+ (alkyl carbocation)	Alkylbenzene
Friedel-Crafts Acylation	RCO+ (acylium ion)	Acylbenzene (ketone)

Key Equations

• Hückel’s Rule: $4n+2$ π electrons (n = 0, 1, 2, ...)

• General EAS Mechanism: $\text{Benzene} + \text{Electrophile} \rightarrow \text{Arenium ion} \rightarrow \text{Substituted benzene} + \text{H}^+$

• Halogenation: $\text{C}_6\text{H}_6 + \text{Br}_2 \xrightarrow{\text{FeBr}_3} \text{C}_6\text{H}_5\text{Br} + \text{HBr}$

• Nitration: $\text{C}_6\text{H}_6 + \text{HNO}_3 \xrightarrow{\text{H}_2\text{SO}_4} \text{C}_6\text{H}_5\text{NO}_2 + \text{H}_2\text{O}$

• Sulfonation: $\text{C}_6\text{H}_6 + \text{SO}_3 \xrightarrow{\text{H}_2\text{SO}_4} \text{C}_6\text{H}_5\text{SO}_3\text{H}$

• Friedel-Crafts Alkylation: $\text{C}_6\text{H}_6 + \text{RCl} \xrightarrow{\text{AlCl}_3} \text{C}_6\text{H}_5\text{R} + \text{HCl}$

• Friedel-Crafts Acylation: $\text{C}_6\text{H}_6 + \text{RCOCl} \xrightarrow{\text{AlCl}_3} \text{C}_6\text{H}_5\text{COR} + \text{HCl}$

References

• Carey, F.A. (2008) Organic Chemistry, 7th ed. McGraw Hill

• McMurry, J. (2008) Organic Chemistry, 7th ed. Thomson Brooks Cole

• Bruice, P.Y. (2017) Organic Chemistry, 8th ed. Prentice Hall International

• Brown W.H., Poon, T. (2016) Introduction to Organic Chemistry, 6th ed. Wiley