Aromatic chemistry

In a nutshell

Benzene, C₆H₆, is the parent arene. Its six carbons form a flat ring, and six electrons are delocalised right around it. That delocalised ring is unusually stable, and it controls everything benzene does.

This subtopic is two linked ideas: the bonding in benzene (why it is planar, why all its C-C bonds are the same, and the thermochemical evidence that it is extra stable), and its electrophilic substitution reactions (nitration and Friedel-Crafts acylation). Mechanism marks here appear on almost every A-level organic paper.

Assumed knowledge: Introduction to organic chemistry, Bonding, Energetics, Alkenes.

Core content

Bonding in the benzene ring

Benzene is a flat, regular hexagon of six carbon atoms, each bonded to one hydrogen.

Build the bonding up one layer at a time:

• Each carbon forms three σ (sigma) bonds: to two neighbouring carbons and to one hydrogen.
• That leaves one spare electron on each carbon, in a p orbital that points at right angles to the plane of the ring (one lobe above, one below).
• The six p orbitals overlap sideways all the way round, merging into a ring of delocalised π electrons above and below the plane of the carbon atoms.

Delocalised means the electrons are spread over the whole ring and not held between any two particular atoms.

Two consequences you must be able to state:

• Benzene is planar (all the atoms lie in one plane), with 120° bond angles.
• All six C-C bonds are identical, each with a length intermediate between a C-C single bond and a C=C double bond (about 0.139 nm, between a single bond at roughly 0.154 nm and a double bond at roughly 0.134 nm). There are no separate single and double bonds.

This is why the old "cyclohexa-1,3,5-triene" picture (three single and three double bonds, alternating) is wrong: real benzene has one kind of bond, not two.

Thermochemical evidence for extra stability

The delocalised ring makes benzene more stable (lower in energy) than the imaginary molecule cyclohexa-1,3,5-triene would be. Enthalpies of hydrogenation prove it.

Adding hydrogen across one C=C double bond releases a fixed amount of energy:

The logic:

• If benzene really had three ordinary double bonds, hydrogenating it should release 3 × (−120) = −360 kJ mol^-1.
• The measured value is only −208 kJ mol^-1, which is 152 kJ mol^-1 less exothermic than expected.
• Less energy is released because benzene started lower in energy (more stable) than the hypothetical triene. That 152 kJ mol^-1 is the extra stability given by delocalisation.

Still don't get it? · why "less exothermic" means "more stable"

Think of hydrogenation as rolling each molecule downhill to the same valley floor (cyclohexane). The energy released is how far it rolls.

The hypothetical triene sits high on the hill, so it rolls a long way: 360 kJ/mol released. Real benzene sits part-way down the hill already, so it has less distance left to roll: only 208 kJ/mol released. Both finish in the same valley.

The height benzene has "already lost", 360 minus 208 = 152 kJ/mol, is exactly how much more stable the delocalised ring made it before the reaction even started.

Exam version: hydrogenation of benzene is 152 kJ mol-1 less exothermic than expected for cyclohexa-1,3,5-triene, so benzene is 152 kJ mol-1 more stable, and that extra stability is due to delocalisation of the pi electrons. Watch the arithmetic: it is the difference between the magnitudes (360 - 208 = 152), and benzene is more stable, not less.

Why benzene reacts by substitution, not addition

An alkene has a localised, electron-rich C=C, so it undergoes electrophilic addition (see Alkenes). Benzene behaves differently.

The causal chain the mark scheme wants:

1. The delocalised ring gives benzene its extra stability (the 152 kJ mol^-1 above).
2. An addition reaction would break up the delocalised system (it would use up the ring electrons and leave a localised, saturated product), so that stability would be lost. This is energetically unfavourable.
3. A substitution reaction swaps one H for a new group but keeps the delocalised ring intact, so the stability is retained.
4. Therefore benzene reacts by substitution in preference to addition.

Because the ring is a region of high electron density, the species it attracts and reacts with is an electrophile (an electron-pair acceptor). Benzene's characteristic reaction is therefore electrophilic substitution, and only monosubstitution is required at A-level.

Nitration of benzene

Reaction: benzene ⟶ nitrobenzene (C₆H₅NO₂).

Reagents: concentrated nitric acid with concentrated sulfuric acid (catalyst).

Conditions: warm, about 50 °C (keep below about 55 °C, or a second NO₂ group is substituted).

Mechanism: electrophilic substitution. Electrophile: the nitronium ion, NO₂⁺.

Step 1 - generating the electrophile. Sulfuric acid is the stronger acid, so it donates a proton to (protonates) the nitric acid; the nitric acid therefore acts as a base. The protonated nitric acid then loses water to give NO₂⁺:

• HNO₃ + H₂SO₄ ⟶ H₂NO₃⁺ + HSO₄^-
• H₂NO₃⁺ ⟶ NO₂⁺ + H₂O

Or as a single overall equation:

• HNO₃ + 2H₂SO₄ ⟶ NO₂⁺ + 2HSO₄^- + H₃O⁺

Step 2 - the substitution.

1. Two electrons from the delocalised ring attack the N of NO₂⁺, forming a C-N bond and an unstable, positively charged intermediate (the delocalised system is now broken and spread over only five carbons, shown as a horseshoe).
2. The intermediate loses H⁺ from the carbon that was attacked; those electrons drop back into the ring, restoring the stable delocalised system and giving nitrobenzene.

Step 3 - regenerating the catalyst: H⁺ + HSO₄^- ⟶ H₂SO₄. The sulfuric acid is reformed, so it is a catalyst.

Why nitration matters in synthesis: it is a key first step. Nitroarenes are reduced to aromatic amines (used to make dyes and pharmaceuticals, covered in Amines, 3.3.11), and repeated nitration makes explosives such as TNT.

Friedel-Crafts acylation

Reaction: benzene ⟶ a phenyl ketone. With ethanoyl chloride the product is phenylethanone, C₆H₅COCH₃.

Reagents: an acyl chloride (e.g. CH₃COCl) with anhydrous aluminium chloride, AlCl₃ (catalyst).

Conditions: heat under reflux.

Mechanism: electrophilic substitution. Electrophile: the acylium ion, RCO⁺ (here CH₃CO⁺).

Step 1 - generating the electrophile. AlCl₃ is a Lewis acid (an electron-pair acceptor): it takes a chloride ion from the acyl chloride, creating the acylium ion:

• CH₃COCl + AlCl₃ ⟶ CH₃CO⁺ + AlCl₄^-

Step 2 - the substitution.

1. Two electrons from the delocalised ring attack the positively charged carbon of CH₃CO⁺, forming the unstable positive intermediate (horseshoe over five carbons).
2. Loss of H⁺ restores the delocalised ring, giving phenylethanone.

Step 3 - regenerating the catalyst: H⁺ + AlCl₄^- ⟶ AlCl₃ + HCl. The AlCl₃ is reformed, so it is a catalyst, not a reagent.

Why Friedel-Crafts acylation matters in synthesis: it forms a new carbon-carbon bond, joining a carbon side chain (a reactive carbonyl group) directly onto the ring. That is how you build larger, more useful aromatic molecules from benzene.

Required practical skill: nitration of methyl benzoate

This is a skills-development opportunity for this subtopic (it is not one of the twelve numbered required practicals).

The preparation of methyl 3-nitrobenzoate by nitration of methyl benzoate, then purification by recrystallisation and measurement of the melting point, practises:

• making and handling a nitrating mixture of concentrated acids safely, keeping the temperature low in an ice bath;
• recrystallisation: dissolve the crude solid in the minimum volume of hot solvent, cool to crystallise, filter under reduced pressure, wash and dry;
• measuring a melting point and judging purity (a pure solid melts sharply at the expected temperature; impurities lower and broaden the melting point).

Worked examples

Example 1 - delocalisation (extra) stability from enthalpies of hydrogenation.

The enthalpy of hydrogenation of cyclohexene is −120 kJ mol^-1. The measured enthalpy of hydrogenation of benzene is −208 kJ mol^-1. Use the data to find how much more stable benzene is than cyclohexa-1,3,5-triene.

Expected value for the hypothetical triene (three isolated C=C, each like cyclohexene):

$\Delta H_{expected} = 3 \times (-120) = -360 \text{ kJ mol}^{-1}$

Extra stability = how much less exothermic the real value is than the expected value:

$360 - 208 = 152 \text{ kJ mol}^{-1}$

Benzene is 152 kJ mol^-1 more stable than cyclohexa-1,3,5-triene, because its p electrons are delocalised.

Mark-scheme points in that answer: showing −360 (or 3 × −120); the value 152 (the mark scheme states "152 NOT 150"); and attributing it to delocalisation. You must use the data, not just quote 152.

Example 2 - predicting a hydrogenation enthalpy (the stretch question).

Predict the enthalpy of hydrogenation of cyclohexa-1,4-diene, then say how cyclohexa-1,3-diene would compare.

Cyclohexa-1,4-diene has two isolated C=C (the double bonds are too far apart to interact), so:

$\Delta H = 2 \times (-120) = -240 \text{ kJ mol}^{-1}$

Cyclohexa-1,3-diene has its two double bonds adjacent (separated by one single bond, a conjugated system), so the p electrons can partially overlap/delocalise. This gives it a little extra stability, so its enthalpy of hydrogenation is less exothermic than −240 kJ mol^-1 (i.e. a smaller amount of energy released).

Common exam mistakes

• Not actually using the data in a "use the data" question: writing "152 kJ mol^-1" without showing both the expected −360 and the actual −208 it comes from.
• Treating the enthalpy of hydrogenation as energy required/absorbed rather than energy released. Benzene releasing less energy means it was more stable to start with.
• Botching the arithmetic or the conclusion: the stability gap is 360 − 208 = 152 kJ mol^-1 and benzene is more stable; it is not 150, and it is not less stable.
• Writing "hydration" when you mean "hydrogenation".
• Nitration reagents: forgetting that both acids must be concentrated. "Nitric acid and sulfuric acid" without "concentrated" loses the mark.
• Leaving the positive charge off H₂NO₃⁺ (or off NO₂⁺) when writing the electrophile-forming equations.
• Naming the mechanism after the reaction: it is electrophilic substitution, not "nitration" and not "Friedel-Crafts acylation", when a mechanism name is asked for (and never "electrophilic addition" or "reduction").
• In the nitration mechanism, drawing the first curly arrow to the oxygen of NO₂⁺: it must go to the nitrogen.
• Drawing the intermediate carelessly: the + charge must not sit on (or too close to) the carbon that gained the group (C1); it belongs on the horseshoe spread over carbons 2 to 6.
• Drawing the second curly arrow (loss of H⁺) the wrong way round: it starts at the C-H bond and goes into the ring.
• In Friedel-Crafts, omitting the equation for forming the electrophile and/or for regenerating AlCl₃, and giving AlCl₄^- as the electrophile instead of the acylium ion CH₃CO⁺ (with the + on the carbonyl carbon).

Key definitions

• Delocalised electrons - electrons that are shared between (spread over) more than two atoms, not fixed in a bond between two particular atoms.
• Electrophile - an electron-pair acceptor.
• Electrophilic substitution - a reaction in which an electrophile replaces an atom (a hydrogen) on a molecule.
• Delocalisation energy (extra stability) - the difference in energy between benzene and the hypothetical cyclohexa-1,3,5-triene; the energy by which delocalisation makes benzene more stable (152 kJ mol^-1).
• Nitronium ion - NO₂⁺, the electrophile in nitration.
• Acylium ion - RCO⁺ (e.g. CH₃CO⁺), the electrophile in Friedel-Crafts acylation.
• Lewis acid - an electron-pair acceptor (AlCl₃ in Friedel-Crafts acylation).
• Catalyst - a substance that speeds up a reaction and is regenerated (reformed) at the end, so it is not used up.

Specification

• I can describe the bonding in benzene: a planar ring, p electrons delocalised above and below the plane, and C-C bonds all equal, intermediate in length between a single and a double bond.
• I can use enthalpies of hydrogenation to show benzene is more stable than cyclohexa-1,3,5-triene, and state the extra stability is due to delocalisation.
• I can explain why benzene undergoes substitution in preference to addition (addition would destroy the stable delocalised system).
• I can outline the electrophilic substitution mechanism for nitration, including the generation of the nitronium ion NO₂⁺.
• I can outline the electrophilic substitution mechanism for Friedel-Crafts acylation using AlCl₃ as the catalyst, including forming the electrophile and regenerating the catalyst.
• I can state that nitration and Friedel-Crafts acylation are important steps in synthesis (explosives, amines, building carbon side chains).
• I can prepare methyl 3-nitrobenzoate by nitration of methyl benzoate, purify it by recrystallisation and determine its melting point.

Related notes

• Introduction to organic chemistry (naming benzene derivatives and the phenyl group)
• Bonding (sigma and pi bonds, delocalisation)
• Energetics (enthalpy changes and Hess's law behind the hydrogenation argument)
• Alkenes (electrophilic addition, the contrast with benzene's substitution)
• Amines (3.3.11): reduction of nitrobenzene to phenylamine, the step that follows nitration
• Organic synthesis (3.3.14): using nitration and acylation to build aromatic targets