Arenes

Arenes are aromatic hydrocarbons containing one or more benzene rings. The simplest arene is benzene, C₆H₆. Its structure was a long-standing puzzle. The historical Kekulé structure proposed a flat ring of six carbons with alternating single and double bonds. However, this model is incorrect. Evidence shows that benzene is more stable and has a different geometry.

1. Bond Lengths: X-ray diffraction shows all carbon-carbon bonds in benzene are identical in length (0.139 nm). This is intermediate between a typical C-C single bond (0.154 nm) and a C=C double bond (0.134 nm). The Kekulé structure would have alternating long and short bonds.

1. Enthalpy of Hydrogenation: Hydrogenating one C=C bond in cyclohexene releases -120 kJ/mol. For Kekulé's structure with three C=C bonds, we would expect an enthalpy of hydrogenation of 3 x (-120) = -360 kJ/mol. The actual experimental value for benzene is only -208 kJ/mol. Benzene is 152 kJ/mol more stable than the Kekulé structure would suggest. This extra stability is called delocalisation energy.

1. Reactivity: Alkenes readily undergo electrophilic addition reactions, which break the C=C double bond. Benzene resists addition reactions because this would destroy the stable delocalised system. Instead, it undergoes substitution reactions.

The modern model describes benzene as a planar, hexagonal molecule where each carbon atom is sp² hybridised. Each carbon forms three σ-bonds (one to a hydrogen atom, two to adjacent carbon atoms), creating a 120° bond angle. The remaining p-orbital on each carbon atom overlaps sideways with its neighbours, forming a continuous ring of electron density above and below the plane of the molecule. This forms a delocalised π-system containing 6 electrons.

The delocalised π-system in benzene is an area of high electron density, making it susceptible to attack by electrophiles (electron-loving species). However, unlike alkenes which undergo addition, benzene undergoes substitution. This is because substitution allows the highly stable delocalised ring to be preserved in the final product. The general mechanism for electrophilic substitution occurs in two main steps:

Step 1: Electrophilic Attack A curly arrow is drawn from the delocalised π-ring of benzene to the electrophile (E⁺). This shows the π-system providing a pair of electrons to form a new C-E bond. This step is slow (rate-determining) as it disrupts the stable aromatic system. It forms a highly unstable, positively charged intermediate called a carbocation or Wheland intermediate. In this intermediate, the positive charge is delocalised over five of the six carbon atoms, which is often represented by an incomplete circle or 'horseshoe' within the hexagon.

Step 2: Restoration of Aromaticity The C-H bond on the same carbon atom that the electrophile attacked then breaks. A curly arrow is drawn from this C-H bond back into the ring. The pair of electrons from the C-H bond reforms the delocalised π-system, restoring the aromatic stability. The hydrogen leaves as a proton (H⁺), which is typically removed by the conjugate base of the catalyst used to generate the electrophile. This second step is very fast.

Halogenation and nitration are classic examples of electrophilic substitution. Both require specific conditions to generate a strong enough electrophile to attack the stable benzene ring.

Halogenation (e.g., Bromination) Benzene does not react with bromine water. To brominate benzene, you need pure bromine and a halogen carrier catalyst, such as iron(III) bromide (FeBr₃) or iron filings (which react with bromine to form FeBr₃ in situ). The catalyst's job is to polarise the Br-Br bond, making one bromine atom sufficiently positive to act as an electrophile (Br⁺). Reagents: Br₂ Catalyst: FeBr₃ (or Fe) Conditions: Room temperature, in the dark (to prevent free-radical reactions).

Nitration Nitration introduces a nitro group (-NO₂) onto the ring. This requires a 'nitrating mixture' of concentrated nitric acid and concentrated sulfuric acid. The sulfuric acid is the catalyst; it is a stronger acid than nitric acid and protonates it, causing it to lose a water molecule and generate the powerful nitronium ion electrophile (NO₂⁺). Reagents: Concentrated HNO₃ and concentrated H₂SO₄ Conditions: Maintain temperature at 50°C. If the temperature rises above this, multiple substitutions can occur, forming dinitrobenzene.

Friedel-Crafts reactions are important methods for forming new carbon-carbon bonds to a benzene ring, allowing for the synthesis of more complex molecules.

Friedel-Crafts Alkylation This reaction attaches an alkyl group (like -CH₃ or -CH₂CH₃) to the benzene ring. It is achieved by reacting benzene with a haloalkane (R-X) in the presence of a Lewis acid catalyst, typically anhydrous aluminium chloride (AlCl₃). The catalyst helps generate a carbocation electrophile (R⁺) from the haloalkane. Example: Benzene reacts with chloroethane to form ethylbenzene. `C₆H₆ + CH₃CH₂Cl --(AlCl₃)--> C₆H₅CH₂CH₃ + HCl` Alkylation has some drawbacks: the alkyl group product is more reactive than benzene, leading to polysubstitution (multiple alkyl groups adding). Also, the carbocation intermediate can rearrange to form a more stable carbocation, leading to unexpected products.

Friedel-Crafts Acylation This reaction attaches an acyl group (R-C=O) to the benzene ring, forming a phenylketone. It is achieved by reacting benzene with an acyl chloride (RCOCl) in the presence of anhydrous AlCl₃. The catalyst helps generate a resonance-stabilised acylium ion (R-C≡O⁺), which is the electrophile. Example: Benzene reacts with ethanoyl chloride to form phenylethanone. `C₆H₆ + CH₃COCl --(AlCl₃)--> C₆H₅COCH₃ + HCl` Acylation is often preferred over alkylation because the acylium ion does not rearrange, and the ketone product is less reactive than benzene, preventing polysubstitution. The resulting ketone can then be reduced to an alkyl group if needed, providing a more controlled way to make alkylbenzenes.

Alkylbenzenes, such as methylbenzene (toluene), have two regions of reactivity: the aromatic ring and the alkyl side-chain. The reaction that occurs depends entirely on the conditions used.

1. Reactions of the Aromatic Ring The alkyl group is an 'activating' group, meaning it donates electron density into the benzene ring. This makes the ring more electron-rich and therefore more reactive towards electrophiles than benzene itself. For example, methylbenzene reacts faster than benzene in nitration and halogenation. The reaction conditions are often milder. The alkyl group directs incoming electrophiles to the 2- (ortho) and 4- (para) positions. Example: Halogenation of methylbenzene with Br₂ and an FeBr₃ catalyst gives a mixture of 2-bromomethylbenzene and 4-bromomethylbenzene.

2. Reactions of the Side-Chain Under different conditions, the alkyl side-chain reacts just like an alkane.

a) Free-Radical Substitution: In the presence of ultraviolet (UV) light, halogens react with the side-chain via a free-radical mechanism. No catalyst is needed. The aromatic ring is unaffected. Example: Methylbenzene reacts with chlorine in UV light to form (chloromethyl)benzene. `C₆H₅CH₃ + Cl₂ --(UV light)--> C₆H₅CH₂Cl + HCl`

b) Oxidation: Alkyl side-chains are readily oxidised to a carboxylic acid group (-COOH) by strong oxidising agents. The standard reagent is hot, alkaline potassium manganate(VII) (KMnO₄), followed by acidification with a dilute acid (e.g., H₂SO₄). A key point is that any alkyl side-chain, regardless of its length, is oxidised back to a single -COOH group attached to the ring. For example, both methylbenzene and ethylbenzene are oxidised to benzoic acid. `C₆H₅CH₂CH₃ + 6[O] → C₆H₅COOH + CO₂ + 2H₂O`

When a benzene ring already has a substituent, that group influences where a second substituent will add. It also affects the overall rate of reaction. This is known as the directing effect.

Activating Groups (2,4-directing) These groups donate electron density to the benzene ring, making it more nucleophilic and thus more reactive towards electrophiles. The reaction is faster than with benzene itself. They direct the incoming electrophile to the 2- (ortho) and 4- (para) positions. The 4-isomer is often the major product due to less steric hindrance. Examples of activating groups: `-OH`, `-NH₂`, `-OR` (e.g., -OCH₃), `-R` (alkyl groups, e.g., -CH₃). Phenol (-OH) and aniline (-NH₂) are so strongly activated that they react with bromine water at room temperature without a catalyst, often substituting at all available 2, 4, and 6 positions.

Deactivating Groups (3-directing) These groups withdraw electron density from the benzene ring, making it less nucleophilic and less reactive towards electrophiles. The reaction is slower than with benzene. They direct the incoming electrophile to the 3- (meta) position. Examples of deactivating groups: `-NO₂`, `-CN`, `-COOH`, `-CHO`, `-COR`. For example, nitrating nitrobenzene requires harsher conditions (higher temperature) than nitrating benzene, and the product is 1,3-dinitrobenzene.

Halogens: The Exception Halogens (`-F`, `-Cl`, `-Br`, `-I`) are an anomaly. They are deactivating because their high electronegativity withdraws electron density from the ring through the sigma bond (the inductive effect). This makes the ring less reactive than benzene. However, they have lone pairs of electrons which can be donated into the ring through resonance. This donation increases electron density at the 2- and 4- positions, so they are 2,4-directing. So, chlorobenzene reacts more slowly than benzene, but it forms a mixture of 1,4-dichlorobenzene and 1,2-dichlorobenzene.