Aromatic compounds — OCR A-Level Chemistry

This row is a section heading for OCR A Level Chemistry H432, module 6.1.1, so treat it as a map of the topic rather than a single examinable sentence. Read it as covering the structure and bonding of benzene, the evidence that supports a delocalised model, the meaning of the term aromatic, and the characteristic reactions of benzene and related aromatic compounds such as electrophilic substitution. Work through your specification and class notes in that order: first secure the shape, bond lengths and bonding description of benzene, then the Kekulé comparison, then named reactions and their mechanisms. Build a one-page summary linking each idea to a diagram, and check each claim against the specification wording rather than memorising isolated facts.

Compare two models of benzene. The Kekulé model draws alternating single and double bonds in a six-membered ring, which implies three localised π bonds and two different C–C bond lengths. The delocalised model keeps the planar hexagonal ring of six carbon atoms, each bonded to one hydrogen and to two neighbouring carbons by σ bonds, and gives each carbon a p orbital perpendicular to the ring plane. Side-by-side overlap of all six p orbitals forms one continuous delocalised π-system above and below the ring, so all six C–C bonds are equivalent at about 0.139 nm, between a single and a double bond. Explain that this model accounts for benzene's resistance to addition and for the single environment seen in its proton NMR spectrum.

Benzene is planar with all six carbon–carbon bonds equal in length, between typical single and double bond lengths, which supports a delocalised model rather than alternating single and double bonds. The enthalpy change of hydrogenation of benzene is less exothermic than the value predicted for a hypothetical cyclohexatriene with three isolated double bonds, so benzene is more stable than the Kekulé structure implies. Benzene also resists addition reactions: it does not readily decolourise bromine water without a catalyst, because delocalisation stabilises the ring. Together these three pieces of evidence support delocalisation.

IUPAC naming of substituted benzenes follows a set of rules. Identify the parent ring as benzene and name substituents as prefixes (methyl, chloro, nitro, hydroxy) or, for some groups, as a suffix that changes the parent name (for example, –COOH gives benzoic acid, –OH gives phenol, –NH₂ gives phenylamine). Number the ring so substituents receive the lowest possible locants, and use di-, tri- and so on with commas between numbers and hyphens before letters (for example, 1,2-dimethylbenzene). When a substituent can be treated as the principal functional group, that group takes priority for numbering. For monosubstituted benzene the substituent name is simply prefixed to benzene (chlorobenzene, methylbenzene). Recognise that the phenyl group C₆H₅– is used when benzene is a substituent. Apply these rules systematically to build or interpret a name.

This row is a course and text-selection guide, not an assessed answer target. It tells you that the mechanism you must be able to describe and apply is electrophilic substitution of aromatic compounds. Benzene's delocalised ring of six π electrons, formed from overlapping p orbitals above and below the plane, attracts electrophiles rather than nucleophiles, so the characteristic reaction replaces one hydrogen with an electrophile. Learn the general two-stage route: the electrophile accepts a pair of π electrons to give a positively charged intermediate that retains some delocalisation, then a base removes the hydrogen from the same carbon to restore the aromatic ring. Use this guidance to select worked examples such as nitration, halogenation and Friedel–Crafts acylation, and to check each example against the general mechanism.

This criterion requires you to know the electrophilic substitution reactions of aromatic compounds, including the reagents and conditions that generate each electrophile and the organic product formed. For nitration, concentrated nitric acid with concentrated sulfuric acid generates the nitronium ion, NO₂⁺, which substitutes a hydrogen to give nitrobenzene. For halogenation, a halogen carrier such as a iron(III) halide or aluminium chloride polarises the halogen and produces the electrophile, giving a halogenoarene. For Friedel–Crafts acylation, an acyl chloride with aluminium chloride generates an acylium ion, RCO⁺, giving a ketone such as phenylethanone. In each case the aromatic ring is regenerated, so the reaction is substitution rather than addition.

This row names the nitrating mixture used to introduce a nitro group into a benzene ring. Concentrated nitric acid supplies the nitro group and concentrated sulfuric acid acts as the catalyst and dehydrating agent; together they generate the nitronium ion, NO₂⁺, which is the electrophile. The learner should recognise the reagent pair, identify the electrophile and the role of each acid, and write the overall equation for the formation of nitrobenzene from benzene. A typical question asks which species attacks the ring or what the sulfuric acid does. The reaction is electrophilic substitution, carried out under reflux at about 50–60 °C, and the product is nitrobenzene. Recognising that the sulfuric acid is regenerated, not consumed, is part of the teaching point.

This row names the reagents for halogenation of benzene: a halogen such as bromine together with a halogen carrier such as iron or aluminium chloride, which acts as a catalyst. The halogen carrier polarises the halogen molecule and helps generate a halogen electrophile, for example Br⁺, which attacks the electron-rich ring. The learner should recognise the reagent combination, name the electrophile and the catalyst, and write the overall equation for the substitution of a hydrogen atom by a halogen. A typical question asks why a halogen carrier is needed or which species is the electrophile. The reaction is electrophilic substitution, and the product is a halogenobenzene such as bromobenzene.

Friedel–Crafts chemistry attaches a carbon chain to a benzene ring. A haloalkane such as CH₃CH₂Cl with a halogen carrier such as AlCl₃ generates an electrophile, here CH₃CH₂⁺, which attacks the electron-rich ring; the carrier is a Lewis acid that polarises the C–Cl bond and later removes Cl⁻. An acyl chloride such as CH₃COCl with AlCl₃ gives an acylium ion CH₃CO⁺ and forms a ketone, useful because the C=O can be reduced to an alkyl chain. In both routes a new C–C bond joins the substituent to the aromatic ring, so the reaction extends a carbon skeleton, which is why it matters in synthesis. The ring keeps its delocalised stability after the intermediate loses H⁺.

Nitration and halogenation of benzene both proceed by electrophilic substitution. In nitration, concentrated HNO₃ with concentrated H₂SO₄ generates the nitronium ion NO₂⁺; the electrophile attacks the delocalised ring to form a positively charged intermediate, which loses H⁺ to restore aromaticity and give nitrobenzene. In halogenation, a halogen such as Br₂ needs a halogen carrier such as FeBr₃ or AlCl₃ to polarise the molecule and generate Br⁺; the same attack, intermediate and deprotonation steps follow. Curly-arrow mechanisms must show the electrophile attacking a ring carbon, the positive charge delocalised over the intermediate, and loss of H⁺ to the carrier anion. Both reactions substitute one hydrogen, keeping the ring intact.

Benzene resists bromination compared with an alkene because its six π electrons are delocalised over the whole ring, so the π electron density at any one carbon is lower than the localised density of the π bond in an alkene such as ethene. Lower electron density means the ring is less able to induce a dipole in, and attract, a bromine molecule, so benzene needs a halogen carrier such as FeBr₃ or AlBr₃ to generate a strong electrophile (Br⁺-type). An alkene's localised π bond polarises Br₂ directly and decolourises bromine water at room temperature without a catalyst.

When an unfamiliar aromatic compound reacts with an electrophile, you can predict the mechanism by applying the general electrophilic substitution pattern. First identify the electrophile, often generated from a reagent by a catalyst such as FeBr₃, AlCl₃ or concentrated H₂SO₄. Then draw the ring with its delocalised π-system, show the electrophile attacking a ring carbon to form a positively charged intermediate (a Wheland-type intermediate) in which the delocalisation is temporarily broken, and show a proton being lost to restore the aromatic ring. The product keeps the ring and gains the substituent. Use curly arrows from the π-system to the electrophile and from the C–H bond to reform the π-system. Apply the same logic to unfamiliar substituents and reagents, checking charges and lone pairs at each step.

This row is a topic heading, not an assessed statement, so treat it as a reading guide for the phenol content of the aromatic compounds section. Read the specification text that follows the heading and identify what you must be able to do: describe the structure of phenol as a benzene ring bearing an –OH group directly bonded to a ring carbon, explain why the –OH group is not on a side chain, and connect that structure to the reactions listed under the heading. Build a short comparison table of phenol against ethanol and against benzene so you can see how the ring changes the behaviour of the –OH group. Check each later sub-statement against your table and note where the specification cross-refers to another section. Use the heading to organise revision, not as a fact to memorise.

Phenol is a weak acid: the aromatic ring withdraws electron density and stabilises the phenoxide ion, so the O–H bond releases a proton more readily than in ethanol but far less readily than in a carboxylic acid. This weak acidity is shown by two contrasting observations. Phenol reacts with aqueous sodium hydroxide in a neutralisation, forming sodium phenoxide and water, because hydroxide is a strong enough base to remove the phenolic proton. Phenol does not react with sodium carbonate or sodium hydrogencarbonate, so no effervescence of carbon dioxide is seen, because the hydrogencarbonate and carbonate ions are not strong enough bases to deprotonate so weak an acid. The cross-reference to 5.1.3 b links this to the general test that carboxylic acids liberate carbon dioxide from carbonates.

Phenol has an –OH group attached directly to a benzene ring. The oxygen's lone pairs overlap with the ring's delocalised π system, so the ring becomes more electron-rich than benzene and attracts electrophiles more strongly. This makes phenol more reactive than benzene and directs substitution to the 2- and 4-positions (ortho and para). In an electrophilic substitution, the electrophile attacks the ring, a delocalised intermediate (a carbocation) forms, and a proton is lost to restore aromaticity. Aqueous bromine at room temperature gives 2,4,6-tribromophenol without a catalyst, and dilute nitric acid gives a mixture of 2-nitrophenol and 4-nitrophenol. Recognise that the –OH group is activating and 2,4-directing, and that the reaction is substitution, not addition.

When phenol reacts with bromine water at room temperature, substitution occurs at all three available 2-, 4- and 6-positions, giving 2,4,6-tribromophenol as a white precipitate and decolourising the bromine water. No halogen carrier catalyst is needed because the –OH group makes the ring sufficiently electron-rich. The equation is C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr. Each substitution replaces one ring hydrogen with one bromine atom, so three bromine molecules are consumed per phenol molecule. The observation of decolourisation and a white precipitate is a test for phenol. Distinguish this from benzene, which needs a halogen carrier and gives only monosubstitution.

Phenol reacts with dilute nitric acid at room temperature to give a mixture of two substitution products: 2-nitrophenol and 4-nitrophenol. The –OH group directs incoming electrophiles to the 2- and 4-positions because its oxygen lone pair donates electron density into the ring by resonance, raising electron density at the ortho and para positions and making those positions more attractive to the nitrating electrophile. Inductively the electronegative oxygen withdraws electron density, but resonance donation dominates the directing effect. The electrophile is the nitronium ion, NO₂⁺, generated from nitric acid. Because two positions are activated, both 2-nitrophenol and 4-nitrophenol form together, so the product is a mixture rather than a single compound.

Phenol undergoes electrophilic substitution more readily than benzene. The oxygen atom of the –OH group has a lone pair in a p-orbital that overlaps with the delocalised π-system of the ring. Donating this electron pair into the π-system increases the electron density around the ring, especially at the 2- and 4-positions. A higher electron density attracts electrophiles more strongly, so the ring reacts faster and under milder conditions than benzene. Benzene has no such electron-donating group, so its π-system is less electron-rich and it needs stronger reagents and conditions. A question may ask you to compare reactivity, to explain the role of the oxygen p-orbital, or to link electron density to the rate of substitution.

A substituent already on a benzene ring controls where the next electrophile attacks. An electron-donating group such as OH or NH₂ pushes electron density into the ring, especially at the 2- and 4-positions relative to itself, so the next electrophile arrives mainly there; the 2-position is the same as the 6-position by symmetry. An electron-withdrawing group such as NO₂ pulls electron density out, leaving the 2- and 4-positions relatively electron-poor and the 3-position relatively favoured, so substitution occurs mainly at the 3-position. The mechanism is still electrophilic substitution: the ring attacks the electrophile, an unstable arenium intermediate forms, then a proton is lost to restore the delocalised ring. Directing effects follow from how the existing group stabilises or destabilises the positive charge in that intermediate at each position.

Directing effects let a chemist predict which substitution product will dominate when an electrophile attacks a substituted benzene ring. Read the existing group first: an electron-donating group such as OH or NH₂ sends the next electrophile mainly to the 2- and 4-positions, while an electron-withdrawing group such as NO₂ sends it mainly to the 3-position. Apply that rule to a named reaction, for example nitration of phenol, and state the major product with its position. This predictive control matters in organic synthesis because it allows a target molecule to be built by choosing the order in which groups are introduced, so a desired substituent can be placed where it is needed rather than relying on chance. The section cross-refers to 6.2.5 Organic Synthesis, where such routes are planned.