Aromatic Chemistry: Understanding the Benzene Ring

Hello and welcome to one of the most fascinating topics in Organic Chemistry: Aromatic Chemistry!
This chapter focuses primarily on the structure and reactions of benzene, the most important aromatic compound. Why is it so special? Benzene is incredibly stable, and this stability dictates almost all of its chemical behaviour.

Don't worry if the mechanisms look complex at first; we will break them down step-by-step. By the end of these notes, you will understand the unique bonding in benzene and why it prefers to undergo substitution reactions instead of addition reactions.

3.3.10.1 The Special Bonding in Benzene

The Kekulé Model vs. Reality

When chemists first tried to figure out the structure of benzene (\(\text{C}_6\text{H}_6\)), August Kekulé suggested a hexagonal ring with alternating single and double bonds (cyclohexa-1,3,5-triene).

However, this theoretical Kekulé model had two major problems:

  • Bond Lengths: In the Kekulé model, the carbon-carbon bonds should alternate between short double bonds (\(0.134 \text{ nm}\)) and long single bonds (\(0.154 \text{ nm}\)). Experimentally, all six \(\text{C}-\text{C}\) bonds in benzene are exactly the same length, intermediate between single and double bonds (about \(0.139 \text{ nm}\)).
  • Reactivity: Alkenes (which have double bonds) undergo rapid addition reactions. Benzene, despite appearing to have double bonds, is surprisingly unreactive and prefers substitution.

The True Structure: Delocalization

The true structure of benzene involves delocalized bonding.

Each of the six carbon atoms in the ring is \(sp^2\) hybridised and has one electron remaining in a \(p\) orbital that sticks out above and below the plane of the ring.

These six \(p\) electrons overlap sideways across all six carbon atoms, forming two continuous ring-shaped clouds of electron density—one above and one below the plane of the carbon ring.

This shared cloud of electrons is the essence of aromaticity and is why we represent benzene using the structure: a hexagon with a circle inside.

Quick Review: Key Features of Benzene

  • Shape: Planar hexagon.
  • Bond Length: All \(\text{C}-\text{C}\) bonds are intermediate between single and double bonds.
  • Bonding: Due to the delocalization of six \(p\) electrons.

Thermochemical Evidence for Extra Stability

The delocalization of electrons makes benzene much more stable than the theoretical Kekulé structure. We prove this stability using enthalpies of hydrogenation (\(\Delta H\)).

Hydrogenation is the process of adding hydrogen across double bonds.

  1. Hydrogenating one double bond (like in cyclohexene) releases approximately \(-120 \text{ kJ mol}^{-1}\).
  2. If Kekulé's theoretical molecule (cyclohexa-1,3,5-triene) existed, it would have three double bonds. We would expect its enthalpy of hydrogenation to be exactly three times the value for cyclohexene: \(3 \times (-120 \text{ kJ mol}^{-1}) = -360 \text{ kJ mol}^{-1}\).
  3. However, the actual enthalpy of hydrogenation for benzene is only approximately \(-208 \text{ kJ mol}^{-1}\).

The difference: \(-360 \text{ kJ mol}^{-1} - (-208 \text{ kJ mol}^{-1}) = -152 \text{ kJ mol}^{-1}\).

This \(152 \text{ kJ mol}^{-1}\) difference in energy is the delocalization energy (or resonance energy). It proves that benzene is \(152 \text{ kJ mol}^{-1}\) more stable than the structure we would predict based on three normal double bonds.

Key Takeaway: Benzene is significantly more stable because of its delocalized electron cloud.

3.3.10.2 Electrophilic Substitution: Why Benzene is Different

Substitution vs. Addition

Most unsaturated hydrocarbons (like alkenes) undergo electrophilic addition. A reaction where an electrophile attacks, breaking the double bond, and two new atoms attach.
If benzene underwent addition, the stable delocalized ring system would be destroyed.

Analogy: Imagine the delocalization energy is like a very comfortable, stable blanket covering the benzene ring. Addition reactions require ripping this blanket off completely. Substitution reactions only require briefly lifting one corner to swap a hydrogen atom, keeping the blanket mostly intact.

To maintain the high stability provided by the delocalized ring, benzene prefers Electrophilic Substitution. In this type of reaction, a hydrogen atom is replaced by another group, and the ring remains intact.

The mechanism for nearly all benzene reactions is Electrophilic Substitution.

The General Mechanism for Electrophilic Substitution

Benzene is a source of high electron density, making it attractive to electrophiles (\(\text{E}^+\)).

Step 1: Generation of the Electrophile
A strong electrophile (\(\text{E}^+\)) is generated, often using a catalyst.

Step 2: Electrophilic Attack
The delocalized electron cloud in the benzene ring is attacked by the electrophile. This breaks the delocalization temporarily, forming a high-energy carbocation intermediate (sometimes called the arenium ion).

Step 3: Loss of a Proton (\(\text{H}^+\))
The intermediate immediately loses a proton (\(\text{H}^+\)) from the carbon atom that accepted the electrophile. This allows the delocalized ring structure to be fully regenerated, restoring the stability.

Key Takeaway: Substitution reactions are favoured because they preserve the crucial delocalization energy, leading to a more stable product than addition reactions would.

Specific Electrophilic Substitution Reactions (Monosubstitution)

1. Nitration of Benzene

Nitration is crucial for synthesizing explosives (like TNT) and the starting materials for dyes and plastics (amines).

Reagents: Concentrated nitric acid (\(\text{HNO}_3\)) and concentrated sulfuric acid (\(\text{H}_2\text{SO}_4\)).
Conditions: Approximately \(50 \text{°C}\). Heating much higher leads to undesirable side reactions or multiple substitutions.

Step 1: Generation of the Electrophile (\(\text{NO}_2^+\))

The two concentrated acids react to form the powerful electrophile, the nitronium ion. Sulfuric acid acts as a catalyst by protonating the nitric acid.

$$ \text{HNO}_3 + 2\text{H}_2\text{SO}_4 \longrightarrow \text{NO}_2^+ + \text{H}_3\text{O}^+ + 2\text{HSO}_4^- $$

The electrophile is the nitronium ion (\(\text{NO}_2^+\)).

Step 2 & 3: Attack and Regeneration

The \(\text{NO}_2^+\) ion attacks the ring, and a proton is lost, resulting in nitrobenzene.

Products formed in these reactions: Nitrobenzene (\(\text{C}_6\text{H}_5\text{NO}_2\)).

2. Sulfonation of Benzene

Sulfonation introduces the sulfonic acid group (\(\text{SO}_3\text{H}\)). This is important in manufacturing surfactants (detergents) and sulfonamide drugs.

Reagents: Hot, concentrated sulfuric acid, or more commonly, fuming sulfuric acid (which contains dissolved sulfur trioxide, \(\text{SO}_3\)).
Electrophile: Either \(\text{SO}_3\) or \(\text{HSO}_3^+\).

Products formed in these reactions: Benzenesulfonic acid (\(\text{C}_6\text{H}_5\text{SO}_3\text{H}\)).

3. Friedel-Crafts Reactions

These reactions are used to attach alkyl groups (Alkylation) or acyl groups (Acylation) to the benzene ring. Both require a halogen carrier catalyst, usually anhydrous aluminium chloride (\(\text{AlCl}_3\)).

a) Friedel-Crafts Alkylation

Reagents: Haloalkane (e.g., \(\text{CH}_3\text{Cl}\)) + \(\text{AlCl}_3\).

Role of \(\text{AlCl}_3\): It acts as a catalyst by reacting with the chloroalkane to generate a powerful carbocation electrophile. $$ \text{CH}_3\text{Cl} + \text{AlCl}_3 \longrightarrow \text{CH}_3^+ + \text{AlCl}_4^- $$

The \(\text{CH}_3^+\) (methyl carbocation) attacks the benzene ring.
Products formed in these reactions: Alkylbenzene (e.g., methylbenzene, also known as toluene).

Common Mistake to Avoid: Alkylation can sometimes lead to polysubstitution (multiple groups attaching) or rearrangements of the carbocation, making Acylation generally preferred in synthesis.

b) Friedel-Crafts Acylation

Reagents: Acyl chloride (e.g., \(\text{CH}_3\text{COCl}\)) or Acid Anhydride + \(\text{AlCl}_3\).

Electrophile: The acylium ion (\(\text{RCO}^+\)). $$ \text{RCOCl} + \text{AlCl}_3 \longrightarrow \text{RCO}^+ + \text{AlCl}_4^- $$

The acylium ion attacks the ring.
Products formed in these reactions: Phenyl ketones (e.g., phenylethanone).

Key Takeaway: All these essential reactions rely on generating a strong positive electrophile, which then attacks the electron-rich delocalized ring, followed by the loss of \(\text{H}^+\) to restore aromatic stability.

Free-Radical Attack and Chlorine Reactions

The type of reaction chlorine undergoes with an aromatic system depends entirely on the conditions used.

1. Chlorination of Benzene (Ring Substitution)

If we want chlorine to substitute directly onto the benzene ring, we must use an electrophilic substitution mechanism, requiring a halogen carrier catalyst.

Reagents: \(\text{Cl}_2\) and a halogen carrier (e.g., \(\text{AlCl}_3\) or \(\text{FeCl}_3\)).
Mechanism: Electrophilic Substitution.
Product: Chlorobenzene (\(\text{C}_6\text{H}_5\text{Cl}\)).

This process results in a chlorine atom substituted in the ring.

2. Free-Radical Attack on Benzene (Ring Addition)

Under very harsh conditions (UV light/high temperature) where the free radical mechanism dominates, benzene can be forced to undergo addition.

Reagents: \(\text{Cl}_2\)
Conditions: UV light (high energy).
Mechanism: Free Radical Addition.
Product: All six double bonds break, and six chlorine atoms are added. The delocalization is destroyed, forming 1,2,3,4,5,6-hexachlorocyclohexane.

3. Free-Radical Attack on Methylbenzene (Side Chain Substitution)

Methylbenzene (\(\text{C}_6\text{H}_5\text{CH}_3\)) has two distinct parts: the aromatic ring and the aliphatic side chain (\(\text{CH}_3\)).

If chlorine reacts with methylbenzene under UV light/heat, the reaction occurs preferentially on the side chain because the \(\text{C}-\text{H}\) bonds in the aliphatic side chain are easier to break than the stable aromatic ring system.

Reagents: \(\text{Cl}_2\)
Conditions: UV light (heat).
Mechanism: Free Radical Substitution (on the side chain, like an alkane reaction).
Product: Chloromethylbenzene (\(\text{C}_6\text{H}_5\text{CH}_2\text{Cl}\)).

Explaining Relative Reactivities of Chlorinated Products

We must explain the relative reactivities of chlorine substituted in the ring (chlorobenzene) versus chlorine in the side chain (chloromethylbenzene).

  • Chlorine in the Side Chain (\(\text{C}_6\text{H}_5\text{CH}_2\text{Cl}\)): This molecule behaves like a typical halogenoalkane. The \(\text{C}-\text{Cl}\) bond is polar and relatively weak. It readily undergoes nucleophilic substitution reactions (e.g., reaction with \(\text{NaOH}\) solution) because the carbon is susceptible to nucleophilic attack. It is highly reactive.
  • Chlorine in the Ring (\(\text{C}_6\text{H}_5\text{Cl}\)): This molecule is much less reactive towards nucleophilic substitution. This is because a lone pair of electrons from the chlorine atom partially overlaps with the delocalized ring system. This overlap adds partial double-bond character to the \(\text{C}-\text{Cl}\) bond, making it stronger and much harder to break.

Did you know? Chlorobenzene is so unreactive that you need extremely high temperatures and pressures, or very strong bases, to get it to react via substitution—conditions far harsher than those needed for simple halogenoalkanes!

Chapter Key Takeaways

The entire study of Aromatic Chemistry revolves around stability. Benzene's delocalized structure gives it immense stability, forcing it to react via electrophilic substitution to maintain its aromatic character. The type of reagent and conditions used (e.g., catalyst vs. UV light) determines whether substitution occurs on the ring or on an attached side chain.