A Level Chemistry (9701) Study Notes: Arenes (Topic 30.1)

Welcome to the fascinating world of **Arenes**! This chapter introduces you to a special class of hydrocarbons known as aromatic compounds, with the star molecule being benzene. Unlike the alkenes you studied at AS Level, arenes are incredibly stable. Understanding this stability and how it dictates their unique reactions is key to mastering this topic.


Don't worry if this feels like a big step up from aliphatic chemistry (alkanes and alkenes). We will break down the structure and reactions of benzene using simple steps, focusing on the concepts that explain why benzene is so different!


1. Structure, Bonding, and Stability of Benzene

Arenes are hydrocarbons containing one or more six-membered carbon rings that exhibit **aromaticity**. The simplest and most important arene is benzene ($\text{C}_6\text{H}_6$).


1.1 The Benzene Ring Structure
  • Benzene is a planar, cyclic molecule with all six carbon atoms arranged in a regular hexagon.
  • Each carbon atom is bonded to two other carbon atoms and one hydrogen atom.
  • All bond angles are exactly $\text{120}^\circ$.
  • All carbon-carbon bond lengths are identical (0.139 nm), intermediate between a $\text{C-C}$ single bond (0.154 nm) and a $\text{C=C}$ double bond (0.134 nm).

Key Concept: $\text{sp}^2$ Hybridisation and Delocalisation

  1. Each carbon atom uses three $\text{sp}^2$ hybrid orbitals to form three sigma ($\sigma$) bonds: one to hydrogen and two to neighbouring carbon atoms.
  2. One unhybridised $\text{p}$ orbital remains on each carbon atom, lying perpendicular to the plane of the ring.
  3. These six $\text{p}$ orbitals overlap sideways above and below the ring to form a continuous, doughnut-shaped system called the **delocalised $\pi$ electron system**.

Analogy: Imagine the benzene ring as a six-sided motorway. The $\sigma$ bonds are the solid structure of the roads. The delocalised $\pi$ system is the *traffic* (the electrons) that is free to flow continuously around the entire ring, instead of being stuck between two specific junctions (like in a normal $\text{C=C}$ double bond).


1.2 Aromatic Stability (Stabilisation Energy)

The **delocalisation** of the $\pi$ electrons across the whole ring makes benzene incredibly stable. This stability is quantified by its aromatic stabilisation energy.

  • The observed enthalpy change of hydrogenation for benzene is significantly less exothermic (less energy is released) than expected if it were just cyclohexatriene (three isolated double bonds).
  • This extra stability (about $152 \text{ kJ mol}^{-1}$) explains benzene’s characteristic chemical behaviour.

Key Takeaway (Aromatic Stability): Benzene's delocalised electron cloud acts like a protective shield. Because breaking this shield means losing the huge stabilisation energy, benzene strongly resists reactions that would break the ring (like addition reactions). Instead, it prefers **substitution reactions** where the stable ring system is preserved.


2. Characteristic Reactions: Electrophilic Substitution

Because the $\pi$ electron system is electron-rich, benzene reacts primarily with **electrophiles** (electron-seeking species). To preserve stability, the reaction type is always **Electrophilic Substitution**.


2.1 Nitration of Benzene (30.1(1)(b))

This reaction introduces a $-\text{NO}_2$ group onto the ring.

  • Reagents: Concentrated nitric acid ($\text{conc. HNO}_3$) and concentrated sulfuric acid ($\text{conc. H}_2\text{SO}_4$).
  • Conditions: Temperature controlled between $\text{25}^\circ\text{C}$ and $\text{60}^\circ\text{C}$. (If the temperature goes too high, further substitution occurs, or oxidation happens.)
  • Product: Nitrobenzene.

The Electrophile: The two acids react to generate the potent electrophile, the **nitronium ion** (\(\text{NO}_2^{+}\)).
$$\text{HNO}_3 + 2\text{H}_2\text{SO}_4 \longrightarrow \text{NO}_2^{+} + \text{H}_3\text{O}^{+} + 2\text{HSO}_4^{-}$$


2.2 Halogenation (Bromination/Chlorination) (30.1(1)(a))

This reaction introduces a halogen atom onto the ring.

  • Reagents: Halogen ($\text{Cl}_2$ or $\text{Br}_2$) and a **halogen carrier catalyst** (Lewis Acids) such as $\text{AlCl}_3$, $\text{AlBr}_3$, or $\text{FeBr}_3$.
  • Conditions: Room temperature.
  • Products: Halogenoarenes (e.g., Bromobenzene).

The Electrophile: The catalyst helps generate a highly polarised or ionic electrophile (e.g., \(\text{Br}^{+}\)) by reacting with the halogen molecule.
$$\text{Br}_2 + \text{FeBr}_3 \longrightarrow \text{Br}^{+}[\text{FeBr}_4]^{-}$$


2.3 Friedel-Crafts Reactions (30.1(1)(c) & 30.1(1)(d))

These reactions are used to add alkyl ($\text{-R}$) or acyl ($\text{-COR}$) chains to the ring. Both require $\text{AlCl}_3$ as a catalyst and heat.

(i) Friedel-Crafts Alkylation (30.1(1)(c))

  • Reagents: Haloalkane (e.g., $\text{CH}_3\text{Cl}$) and $\text{AlCl}_3$.
  • Conditions: Heat.
  • Product (e.g., with $\text{CH}_3\text{Cl}$): Methylbenzene (Toluene).

(ii) Friedel-Crafts Acylation (30.1(1)(d))

  • Reagents: Acyl chloride (e.g., $\text{CH}_3\text{COCl}$) and $\text{AlCl}_3$.
  • Conditions: Heat.
  • Product (e.g., with $\text{CH}_3\text{COCl}$): Phenylethanone (an aromatic ketone).

Quick Review Box: The General Mechanism (30.1(2))

The mechanism of electrophilic substitution is crucial. It consists of three steps, ensuring the ring’s stability is regained.

  1. Generation of the Electrophile (E$^+$): Catalyst and reagent react to make a strong electrophile (e.g., \(\text{NO}_2^{+}\)).
  2. Electrophilic Attack: The $\pi$ electron cloud attacks the electrophile. This breaks the aromaticity, forming a temporary, unstable, positive intermediate (a carbocation, sometimes called a **sigma complex**).
  3. Loss of $\text{H}^+$ (Rearomatisation): A proton ($\text{H}^+$) is rapidly lost from the adjacent carbon atom. This step restores the delocalised $\pi$ system, regenerating the highly stable aromatic ring.

The predominance of substitution over addition (30.1(2)(b)) is entirely due to this final step—regaining the aromatic stability is the driving force. If addition occurred, the ring would be broken permanently.


3. Reactions of Methylbenzene (Toluene)

Methylbenzene ($\text{C}_6\text{H}_5\text{CH}_3$) has a methyl group attached to the ring. This alters its reactivity significantly.


3.1 Side-Chain Oxidation (30.1(1)(e))

The alkyl side chain of methylbenzene (or any alkylbenzene) is susceptible to strong oxidation, regardless of the chain length.

  • Reagents: Hot, alkaline potassium manganate(VII) ($\text{hot alkaline KMnO}_4$).
  • Conditions: Refluxing, followed by acidification with dilute acid (e.g., $\text{H}_2\text{SO}_4$).
  • Product: The side chain is completely oxidized to a **carboxylic acid** group ($\text{-COOH}$). Methylbenzene yields benzoic acid.

Did you know? This reaction is a useful diagnostic test. If an arene has an alkyl side chain, no matter how long, it will be converted to benzoic acid.


3.2 Halogenation: Ring vs. Side-Chain (30.1(3))

Whether halogenation occurs on the ring (substitution) or the side chain (free-radical substitution, like alkanes) depends entirely on the **conditions** used:

(i) Ring Halogenation (Electrophilic Substitution)

  • Conditions: Halogen ($\text{Cl}_2$ or $\text{Br}_2$) in the presence of a **halogen carrier catalyst** ($\text{AlCl}_3$ or $\text{AlBr}_3$).
  • Result: Halogenation occurs on the ring. The $\text{-CH}_3$ group is an *activating, ortho/para directing* group (see Section 4).
  • Example: $\text{Methylbenzene} + \text{Cl}_2 + \text{AlCl}_3 \longrightarrow \text{2-chloromethylbenzene and 4-chloromethylbenzene}$ (31.1(1))

(ii) Side-Chain Halogenation (Free-Radical Substitution)

  • Conditions: Halogen ($\text{Cl}_2$ or $\text{Br}_2$) in the presence of **UV light** (no catalyst).
  • Result: The hydrogen atoms on the side chain are substituted.
  • Example: $\text{Methylbenzene} + \text{Cl}_2$ (UV light) $\longrightarrow \text{Chloromethylbenzene}$ ($\text{C}_6\text{H}_5\text{CH}_2\text{Cl}$)

Memory Aid: C-A-R is for the ring (Catalyst, Aromatic, Ring). UV is for the chain (UV, free-radical, side-chain).


4. Directing Effects of Substituents (30.1(4))

If a benzene ring already has a group attached, that group (the **substituent**) influences two things:

  1. Reactivity: Does it make the ring react faster (activate) or slower (deactivate)?
  2. Position: Where does the next incoming electrophile attack? (It directs to the **2- (ortho), 4- (para), or 3- (meta)** positions).

The syllabus requires knowledge of the directing effects of specific groups:

4.1 Ortho/Para Directing Groups (Activating)

These groups donate electron density to the ring, making the ring more reactive (activating it). They direct incoming electrophiles primarily to the 2- (ortho) and 4- (para) positions.

  • Groups to know:
    • $-\text{NH}_2$ (Amino)
    • $-\text{OH}$ (Hydroxyl)
    • $-\text{R}$ (Alkyl, e.g., $-\text{CH}_3$)

Example: Bromination of phenol ($\text{C}_6\text{H}_5\text{OH}$) with $\text{Br}_2(\text{aq})$ is so fast because the $-\text{OH}$ group is a powerful activator, directing to 2, 4, and 6 positions simultaneously (forming 2,4,6-tribromophenol).


4.2 Meta Directing Groups (Deactivating)

These groups withdraw electron density from the ring, making the ring less reactive (deactivating it). They direct incoming electrophiles primarily to the 3- (meta) position.

  • Groups to know:
    • $-\text{NO}_2$ (Nitro)
    • $-\text{COOH}$ (Carboxyl)
    • $-\text{COR}$ (Acyl, e.g., $-\text{COCH}_3$)

Encouragement Tip: The deactivating groups usually contain a $\text{C}$ or $\text{N}$ atom double-bonded to an electronegative atom like $\text{O}$ (e.g., $\text{C=O}$ in carboxylic acids or $\text{C=O}$ in acyl groups, or $\text{N=O}$ in nitro groups), pulling electrons away from the ring.


5. Other Key Reactions of Arenes

5.1 Hydrogenation (Addition Reaction) (30.1(1)(f))

While benzene usually resists addition, forcing conditions can overcome the aromatic stability.

  • Reaction: Reduction (Addition of hydrogen gas).
  • Reagents: Hydrogen gas ($\text{H}_2$).
  • Conditions: Platinum ($\text{Pt}$) or Nickel ($\text{Ni}$) catalyst and heat.
  • Product: Cyclohexane (the ring loses its aromaticity and becomes saturated).

Note: This is an *addition* reaction, which is only possible because the high heat and catalytic conditions provide enough energy to overcome the massive energy barrier posed by the aromatic stability.


Key Takeaway (Summary):

Arenes are defined by the stability of their delocalised $\pi$ system, which means they prefer **electrophilic substitution** over addition. Their reactivity and the position of the second substitution depend entirely on the nature of the first substituent group.


You have now covered all the core reactions and concepts for the Arenes chapter! Focus on understanding the stability derived from delocalisation and memorizing the reagents needed for the specific substitution reactions.