Hydrocarbons

Unit 8Aromatic hydrocarbons: an introduction

Hydrocarbons are compounds of carbon and hydrogen only. They form two big families. Aliphatic hydrocarbons (alkanes, alkenes, alkynes and their ring forms) contain no benzene ring; the word comes from the Greek aleiphar, meaning oil, because the first ones were obtained by breaking down fats. Aromatic hydrocarbons contain at least one benzene ring.

The modern systematic name for aromatic hydrocarbons is arenes. The ar stands for aromatic, and -ene comes from the last three letters of benzene, which shows that the molecule is unsaturated.

Discovery of benzene

Michael Faraday isolated an oily hydrocarbon from the residue that collected in the gas pipes of London and called it bicarburet of hydrogen. In 1834 Eilhardt Mitscherlich of the University of Berlin made the same substance by heating benzoic acid with lime (calcium oxide) and found that its empirical formula is CH.

Decarboxylation: lime removes the $-\mathrm{COOH}$ group as carbonate and leaves the benzene ring.

Benzoic acid was obtained from gum benzoin, a resin of a balsam tree of the island of Java (Indonesia), so the product was called benzin, later benzene. All compounds related to benzene and toluene (methylbenzene) are called aromatic hydrocarbons.

The word aromatic comes from aroma, the Latin word for fragrance, because the first members came from pleasant-smelling plant extracts. Later it was found that many aromatic compounds are not pleasant-smelling and some have harmful vapours. What they share is a benzene ring and the unusual stability it gives, so today aromatic means a compound containing a benzene ring, not a compound with a pleasant smell.

Physical properties of benzene

• Colourless liquid with a characteristic smell; boiling point $80^{\circ }\mathrm{C}$, melting point $5.5^{\circ }\mathrm{C}$.
• Density $0.88\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$, so it floats on water.
• Non-polar molecule: insoluble in water but a good solvent for fats, oils and other non-polar substances.
• Only weak London dispersion forces act between its molecules (no dipoles, no hydrogen bonding).
• Burns in air with a luminous, very smoky (sooty) flame, because its carbon content is high (C : H = 1 : 1).

Complete combustion of benzene in excess oxygen.

Grade 11 recapRecap: alkanes and free-radical substitution

The book opens this unit by recalling the aliphatic hydrocarbons studied in Grade 11. You need them here because benzene is compared with alkanes and alkenes throughout the unit, and because the old-syllabus 2026 paper asked about them. This section and the next one are a short revision.

Structure and preparation

Alkanes (${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2n+2}$) are saturated hydrocarbons. Every carbon is $s{p}^3$ hybridised and forms four σ bonds at $109.5^{\circ }$. Methane, ethane, propane and butane are gases; natural gas and petroleum are the main sources.

Why alkanes are unreactive

Alkanes were once called paraffins (from Latin parum affinis, little affinity) because very few reagents attack them. Acids, alkalis, oxidising agents such as $\mathrm{KMnO}{}_4$ and bromine water in the dark have no effect. The reasons are:

1. Strong σ bonds only. All bonds are strong σ bonds (C–C about $348\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$, C–H about $413\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$). There are no π electrons and no lone pairs, so nothing is loosely held for a reagent to attack, and much energy is needed to break a bond.
2. Non-polar bonds. Carbon (electronegativity 2.5) and hydrogen (2.1) differ very little, so C–H bonds are almost non-polar. The molecule has no electron-rich or electron-poor site to attract an electrophile or a nucleophile.
3. Saturation. Each carbon already has four σ bonds, so an alkane cannot add a reagent. It can react only by substitution, and only after a strong bond has been broken by light or heat.

Free-radical substitution: chlorination of methane

In ultraviolet (UV) light or diffused sunlight, or at about $400^{\circ }\mathrm{C}$, methane reacts with chlorine. One H atom at a time is replaced by a Cl atom.

With more chlorine the substitution continues, $\mathrm{CH}{}_3\mathrm{Cl}\to \mathrm{CH}{}_2\mathrm{Cl}{}_2\to \mathrm{CHCl}{}_3\to \mathrm{CCl}{}_4$, so a mixture of products forms. The reaction is a chain reaction with three stages.

Alkanes burn in excess air to give carbon dioxide and water with a large release of heat, which is why they are used as fuels:

Grade 11 recapRecap: alkenes, alkynes and electrophilic addition

Alkenes

Alkenes (${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2n}$) contain a C=C double bond made of one σ bond and one π bond. The two carbons are $s{p}^2$ hybridised, and the π bond comes from the sideways overlap of two 2p orbitals, above and below the plane of the molecule. The π electrons are loosely held and exposed, so they attract electrophiles (electron-seeking species). This is why alkenes are far more reactive than alkanes, and why their typical reaction is electrophilic addition.

Alkynes

Alkynes (${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2n-2}$) have a C≡C triple bond: one σ bond and two π bonds. The carbons are $sp$ hybridised, so the C–C≡C–C unit is linear ($180^{\circ }$). Ethyne is made from calcium carbide and water, or by removing two molecules of HBr from 1,2-dibromoethane:

An alkyne can add two molecules of a reagent, one for each π bond:

Hydration of ethyne gives ethanal.

The H atom on a triple-bond carbon is weakly acidic, because an $sp$ carbon holds its electrons tightly. Terminal alkynes therefore give a white precipitate with ammoniacal silver nitrate and a red precipitate with ammoniacal copper(I) chloride; alkenes do not.

Conjugated dienes are extra stable

In 1,3-butadiene ($\mathrm{CH}{}_2=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}{}_2$) the two double bonds are separated by one single bond; they are conjugated. All four carbons are $s{p}^2$, so their four 2p orbitals lie side by side and the π electrons spread (delocalise) over all four carbons. The central C–C bond gains some double-bond character: it is 1.47 Å long instead of 1.54 Å.

Heats of hydrogenation show the extra stability. 1-Butene releases $127\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$ for its one C=C bond, so two separate C=C bonds should release about $2\times 127=254\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$. 1,3-Butadiene releases only $239\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$ (about $119.5\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$ per C=C). The difference, about $15\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$, is the delocalisation energy of the conjugated system. Benzene shows the same effect on a much larger scale ($152\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$, section 8.2.1).

8.1Nomenclature of benzene derivatives

The simplest aromatic compound is benzene, \ce{C6H6}. Many aromatic compounds carry one or more groups (substituents) in place of H atoms. The IUPAC system names them, but still keeps several common names.

Monosubstituted benzenes

One H of benzene is replaced by a group. All six carbons of benzene are identical, so there is only one compound of each kind and no number is needed. These names are used by IUPAC:

More monosubstituted benzenes with retained common names:

Disubstituted benzenes

Two groups can be placed on the ring in three ways, giving three position isomers. Their positions are shown by numbers or by the prefixes ortho, meta and para:

When one group gives the compound a special name (as in toluene, phenol or aniline), the compound is named as a derivative of that parent, and the special group is at position 1. IUPAC keeps the name xylene for the three dimethylbenzenes:

When neither group gives a special name, the groups are written in alphabetical order before the word benzene, and the group named first gets the lower number:

Polysubstituted benzenes

1. Use numbers (not o, m, p) when there are three or more groups.
2. If one group gives a special name, name the compound as a derivative of that parent; that group is at position 1.
3. Number the ring in the direction that gives the lowest set of numbers.
4. Write the other groups in alphabetical order (bromo, chloro, ethyl, nitro) before the parent name or before 'benzene'.

The first compound is named as a derivative of toluene and the second as a derivative of phenol. The third has no group with a special name, so its carbons are numbered to give the lowest set (1,2,4), and the three groups are listed alphabetically before 'benzene'.

8.2The structure of benzene: from open chains to Kekulé

After its discovery, the structure of benzene puzzled chemists for about forty years. Analysis showed the molecular formula \ce{C6H6} and empirical formula CH. A saturated hydrocarbon with six carbons would be \ce{C6H14}; benzene has 8 hydrogens fewer, so it should contain the equivalent of four double bonds or rings. Chemists concluded that benzene is highly unsaturated.

The first suggestions were open chains with double and triple bonds, for example:

These structures do not fit benzene's behaviour. Such molecules would quickly decolourise bromine water and cold \ce{KMnO4}, like alkenes and alkynes; benzene does neither. They would also give several different products when one H atom is replaced, but benzene gives only one monosubstituted product (for example only one bromobenzene), which means all six H atoms are equivalent. An open chain cannot explain this.

Kekulé's structure

In 1865 the German chemist August Kekulé proposed a ring (cyclic) structure: six carbon atoms at the corners of a regular hexagon, each bonded to one H atom, joined by alternate single and double bonds. He described getting the idea from a dream of chains of atoms turning into a snake biting its own tail. To explain why only one 1,2-disubstituted product is found, he said that the double bonds switch between two positions so fast that the two forms cannot be told apart.

Objections to Kekulé's structure

Kekulé's ring explained why all six H atoms are equivalent, but it raised three questions that it could not answer:

1. If benzene has three C=C bonds like alkenes, why does it not react like alkenes (for example, why does it not decolourise bromine water)?
2. If benzene has three double bonds, why can it not easily add three molecules of bromine?
3. Why does benzene prefer substitution reactions to addition reactions?

Later measurements added two more problems: X-ray diffraction shows that all six C–C bonds are the same length (1.39 Å), whereas Kekulé's formula has three short C=C and three long C–C bonds; and benzene's enthalpy of hydrogenation is 152\,\mathrm{kJ\,mol^{-1}} smaller than expected for three C=C bonds, so benzene is much more stable than Kekulé's structure (section 8.2.1).

8.2Atomic (molecular) orbital model of benzeneOften examined

In the 1930s Linus Pauling used two new ideas, hybridisation and resonance, to give the first satisfactory picture of benzene. The orbital model builds the molecule in two layers: a flat framework of σ bonds, and a cloud of π electrons above and below it.

The σ framework

The delocalised π electron cloud

After hybridisation each carbon still has one unhybridised 2p orbital containing one electron. These six 2p orbitals stand perpendicular to the plane of the ring and parallel to one another. Each one overlaps sideways, equally well, with the 2p orbitals on both its neighbours, not with just one. So the overlap runs all the way round the ring and forms one continuous π system that holds all six π electrons.

The π electron density lies in two doughnut-shaped rings (tori): one above the plane of the ring and one below it. The six π electrons are delocalised: they belong to the whole ring, not to any particular pair of carbon atoms.

• All six C–C bonds are identical (1.39 Å), because the π electrons are shared equally by all six carbons.
• The molecule is flat and unusually stable: the delocalised electrons are attracted by six nuclei instead of two, which lowers the energy.
• The π cloud is an electron-rich region above and below the ring, so benzene is attacked by electrophiles.
• Because the delocalised system is so stable, benzene prefers substitution (which keeps it) to addition (which destroys it).

8.2The resonance model of benzeneOften examined

Resonance

The delocalisation of electron pairs (π pairs or lone pairs) over several atoms, so that a single Lewis structure cannot represent the molecule. The atoms stay in the same positions; only the electrons are spread out.

Resonance (canonical or contributing) structures

The different possible Lewis structures of such a molecule. They are imaginary: none of them exists on its own.

Resonance hybrid

The real structure: a weighted average of all the canonical structures. It is lower in energy (more stable) than any single canonical structure.

The real structure of benzene is a resonance hybrid of the two Kekulé structures. Each C–C bond is single in one Kekulé structure and double in the other, so in the hybrid every C–C bond has a bond order of 1.5, exactly as the equal bond lengths (1.39 Å) show. The hybrid is drawn as a hexagon with a circle inside; the circle stands for the six delocalised π electrons in the six 2p orbitals of the ring.

Why a single Kekulé structure is still used

Although benzene exists as the hybrid, chemists often draw just one Kekulé structure, for two reasons:

1. it shows the tetravalency of carbon and makes it easy to count the electrons (three double bonds = six π electrons);
2. it makes it easy to show the movement of electron pairs with curly arrows in reaction mechanisms.

From Thiele to Pauling

In 1899 Johannes Thiele proposed the idea of partial valence to explain the unexpectedly high stability of benzene: each carbon uses only part of its fourth valency in the ring, shown as a dotted circle inside the hexagon (the Thiele structure). Werner Heisenberg later described resonance using quantum mechanics, and Linus Pauling (1931 to 1933) developed the theory of resonance, applied it to organic molecules and showed that it explains the high stability of benzene.

8.2.1Resonance energy of benzeneOften examined

Resonance energy (delocalisation energy)

The difference in energy between the real benzene molecule and the hypothetical cyclohexa-1,3,5-triene, which has three localised double bonds. It measures the extra stability that delocalisation of the π electrons gives: 152\,\mathrm{kJ\,mol^{-1}} for benzene.

Enthalpy of hydrogenation

The heat change when one mole of an unsaturated compound reacts completely with hydrogen to give a saturated compound.

We cannot measure the energy of a molecule directly, but we can measure the heat released when it is hydrogenated. Cyclohexene, the hypothetical cyclohexa-1,3,5-triene and benzene all give the same product, cyclohexane. So the heat released compares their energies: the less heat a molecule releases, the lower (more stable) its starting energy.

The book's diagram marks the gap as −152 kJ/mol. The resonance energy itself is quoted as a positive amount of stabilisation, 152\,\mathrm{kJ\,mol^{-1}}.

8.3Reactivity of benzene: the three types of reaction

The delocalised π ring of benzene is so stable that it stays intact in most reactions. Benzene shows three types of reaction:

• Electrophilic substitution (the typical reaction): an electrophile replaces an H atom and the aromatic ring is kept. Examples: halogenation, nitration, sulphonation, Friedel–Crafts alkylation and acylation.
• Addition: only under harsh conditions, because it destroys the aromatic ring. Example: hydrogenation to cyclohexane.
• Oxidation: the ring resists ordinary oxidising agents, but an alkyl side chain is oxidised to \ce{-COOH}. Benzene also burns in air.

Why substitution rather than addition? After substitution the product is still aromatic, so the 152\,\mathrm{kJ\,mol^{-1}} of resonance energy is kept. After addition the product is no longer aromatic, so this stability would be lost. Substitution is therefore the lower-energy path.

8.3.1Electrophilic aromatic substitution: the general mechanismOften examined

Electrophile

An electron-deficient species (a positive ion, or the positive end of a polar molecule) that accepts an electron pair to form a new bond. Examples: \ce{Br+}, \ce{NO2+}, \ce{CH3+}, \ce{CH3CO+}.

Electrophilic aromatic substitution (EAS)

A reaction in which an electrophile attacks the π electrons of an aromatic ring and replaces one hydrogen atom, the aromatic ring being restored at the end.

Arenium ion (σ complex)

The carbocation intermediate formed when an electrophile bonds to one ring carbon. That carbon becomes sp^3 and carries both H and E; four π electrons and a positive charge are delocalised over the other five carbons.

The π cloud above and below the benzene ring is a rich source of electrons, so benzene attacks electrophiles. But its delocalised π electrons are far more stable than the localised π pair of an alkene. Molecules such as \ce{Br2} or \ce{CH3Cl} are not strong enough electrophiles to react with benzene on their own. A catalyst, usually a Lewis acid such as \ce{FeBr3}, \ce{FeCl3} or \ce{AlCl3}, is needed to produce a strong electrophile.

The arenium ion is stabilised by resonance: the positive charge is shared by the three carbons that are ortho and para to the sp^3 carbon.

8.3.1Halogenation of benzene and methylbenzeneOften examined

Halogenation is the reaction in which a halogen molecule reacts with benzene and one H atom of the ring is replaced by a halogen atom. It needs a catalyst, usually an iron(III) halide (a halogen carrier).

X = Cl or Br; the product is a halobenzene.

Bromination

Benzene reacts with liquid bromine at room temperature in the presence of iron(III) bromide, \ce{FeBr3}, to form bromobenzene. Iron filings can be used instead, because iron reacts with bromine to make the catalyst: \ce{2Fe + 3Br2 -> 2FeBr3}.

Observation: the red-brown colour of bromine fades, and steamy fumes of hydrogen bromide are given off.

Chlorination

A similar reaction happens when chlorine gas is bubbled through benzene at room temperature in the presence of a catalyst such as \ce{FeCl3} or aluminium chloride, \ce{AlCl3}. The products are chlorobenzene and hydrogen chloride. The mechanism follows the same three steps as bromination.

Ring halogenation of methylbenzene

With a halogen carrier (\ce{AlCl3} or \ce{FeCl3}), at room temperature and in the dark, chlorine attacks the ring of methylbenzene. The methyl group releases electrons into the ring, so methylbenzene reacts faster than benzene, and the electrophile attacks positions 2 and 4 (ortho and para to \ce{CH3}). Hydroxyl and amino groups activate the ring in the same way, by pushing electrons into it through p orbitals (section 8.4). Two products form:

a mixture of 2-chloromethylbenzene and 4-chloromethylbenzene

With excess chlorine, more Cl atoms enter the ring at the remaining activated positions (2, 4 and 6), giving 2,4-dichlorotoluene, 2,6-dichlorotoluene and finally 2,4,6-trichlorotoluene.

Side-chain halogenation

When chlorine is passed into boiling methylbenzene in UV light (or strong sunlight) with no catalyst, the ring is not attacked. Instead the H atoms of the methyl side chain are replaced one at a time by free-radical substitution, just as in methane.

In excess chlorine all three side-chain H atoms are replaced: overall \ce{C6H5CH3 + 3Cl2 -> C6H5CCl3 + 3HCl}.

8.3.1Nitration of benzeneOften examined

Nitration is the introduction of a nitro group (\ce{-NO2}) into the benzene ring. Benzene is warmed with a nitrating mixture of concentrated nitric acid and concentrated sulphuric acid. The electrophile is the nitronium ion (nitryl cation), \ce{NO2+}.

The book says the mixture is refluxed with benzene at between 25^\circ\mathrm{C} and 60^\circ\mathrm{C}. Keep the temperature below about 55^\circ\mathrm{C}: at higher temperatures a second nitro group enters and 1,3-dinitrobenzene is formed.

extraSulphonation of benzene

Sulphonation introduces the sulphonic acid group (\ce{-SO3H}) into the ring. Benzene is warmed at about 40^\circ\mathrm{C} with fuming sulphuric acid (oleum: sulphur trioxide dissolved in concentrated sulphuric acid), or heated under reflux with concentrated \ce{H2SO4} for several hours. The product is benzenesulphonic acid.

Sulphonation is reversible. Heating benzenesulphonic acid with dilute acid or superheated steam (about 150^\circ\mathrm{C}) removes the \ce{-SO3H} group and gives back benzene. Sulphonic acids are used to make detergents, dyes and sulpha drugs.

8.3.1Friedel–Crafts alkylationOften examined

In 1877 the French chemist Charles Friedel and his co-worker, the American chemist James Crafts, found that benzene heated with an alkyl halide in the presence of aluminium chloride gives an alkylbenzene, and with an acyl halide gives an aryl ketone. These two Friedel–Crafts reactions are the main ways of joining a new carbon group to a benzene ring.

Friedel–Crafts alkylation is the introduction of an alkyl group (R) into the benzene ring by heating benzene with a haloalkane in the presence of a Lewis acid, anhydrous aluminium chloride. \ce{AlCl3} acts as a catalyst.

General reaction: the product is an alkylbenzene.

Example: chloromethane gives methylbenzene.

Role of the Lewis acid (\ce{AlCl3})

• Electron-pair acceptor: Al in \ce{AlCl3} has an incomplete octet (6 valence electrons) and an empty 3p orbital, so it accepts a lone pair from the halogen atom of the haloalkane.
• Makes the electrophile: by pulling the halogen away as \ce{AlCl4-} it turns the weakly electrophilic haloalkane into a strong electrophile, the carbocation \ce{R+} (or a strongly polarised complex \ce{R^{\delta+}-Cl-AlCl3^{\delta-}}).
• Catalyst: it is given back in step 3 (\ce{AlCl4- + H+ -> AlCl3 + HCl}), so it is not used up. Because it carries the halogen from the haloalkane to the HCl, the book also calls it a halogen carrier.

Limitations of Friedel–Crafts alkylation

• Polyalkylation: an alkyl group activates the ring, so the product (for example methylbenzene) reacts faster than benzene and more alkyl groups enter. A large excess of benzene is used to reduce this.
• Rearrangement: a primary carbocation can rearrange into a more stable secondary one, so 1-chloropropane gives mainly (propan-2-yl)benzene (cumene), not propylbenzene.
• Deactivated rings do not react: nitrobenzene and rings with other strongly electron-withdrawing groups do not undergo Friedel–Crafts reactions. This decides the order of steps in a synthesis (Long question ix).
• Aryl halides such as chlorobenzene cannot be used as the halide, because their C–Cl bond is too strong to give a carbocation.

8.3.1Friedel–Crafts acylationOften examined

Friedel–Crafts acylation is the reaction in which an acyl group (\ce{RCO-}) is introduced into the benzene ring by warming benzene with an acyl halide in the presence of the catalyst aluminium trihalide (\ce{AlX3}). The product is an aromatic ketone (a phenyl ketone).

General reaction with an ethanoyl halide.

Example: ethanoyl chloride gives phenylethanone.

8.3.2Addition reactions of benzeneOften examined

Benzene has a π electron cloud, so we might expect addition reactions like those of alkenes. Under normal conditions it does not add bromine or hydrogen. Its π electrons are part of a highly stable conjugated system, the aromatic ring: they circulate over all six carbons and are less available for reaction than the localised π pair of an alkene. Addition would destroy this system and lose the resonance energy, so it needs harsh conditions: high temperature, high pressure, a catalyst or UV light.

Hydrogenation

Hydrogenation is the addition of hydrogen molecules across the carbon atoms of benzene, converting it into cyclohexane. It takes place in the presence of a finely divided metal catalyst, nickel, platinum or palladium, at 150–200^\circ\mathrm{C} and 20–30 atm pressure. The reaction is exothermic.

Methylbenzene is hydrogenated to methylcyclohexane under the same conditions:

Addition of chlorine

In bright sunlight or UV light, with no catalyst and without heating, chlorine adds to benzene by a free-radical mechanism, giving 1,2,3,4,5,6-hexachlorocyclohexane (benzene hexachloride, BHC). One isomer of it (gammexane or lindane) was used as an insecticide.

Addition versus electrophilic substitution

In both reactions an electrophile first attacks π electrons and a positive intermediate forms. The key difference is what happens to this intermediate. In the halogenation of an alkene (Grade 11) a bridged halonium ion forms first, and then a halide ion attacks the positive intermediate, giving a saturated product. In benzene the arenium ion is stabilised by losing a hydrogen ion, not by the attack of a nucleophile. So an electrophile takes the place of a hydrogen atom and a substituted product forms.

8.3.3Oxidation reactions: side-chain oxidationOften examined

Benzene, like the alkanes, resists common oxidising agents such as potassium manganate(VII), whereas alkenes and alkynes are oxidised easily. The benzene ring can be oxidised only under very aggressive conditions (for example by oxygen over a \ce{V2O5} catalyst at about 450^\circ\mathrm{C}), and it burns in air with a smoky flame.

An alkyl side chain on a benzene ring behaves differently. Alkanes are not oxidised by \ce{KMnO4}, but the methyl group of methylbenzene is oxidised to a \ce{-COOH} group by hot alkaline (or hot acidic) potassium manganate(VII) on prolonged heating. The ring makes the side chain more reactive: the C–H bonds on the carbon next to the ring (the benzylic carbon) are weakened, because a radical or cation formed there is stabilised by delocalisation into the ring.

[O] stands for oxygen supplied by the oxidising agent.

In alkaline solution the product is the salt, potassium benzoate; acidifying the mixture then gives benzoic acid as a white solid. The purple \ce{KMnO4} is reduced to a brown precipitate of \ce{MnO2} in alkaline solution (in acidic solution the purple colour fades as \ce{Mn^2+} forms).

Rules for side-chain oxidation

• The carbon atom attached to the ring is always oxidised to \ce{-COOH}, whatever the length of the chain.
• In side chains longer than methyl, the carbon atoms beyond the first one are lost as carbon dioxide, so ethylbenzene and propylbenzene also give benzoic acid.
• Every alkyl group on the ring is oxidised, so a ring with two alkyl groups gives a dicarboxylic acid.
• The carbon attached to the ring must carry at least one H atom; tert-butylbenzene, \ce{C6H5C(CH3)3}, is not oxidised.

Ethylbenzene also gives benzoic acid; the second carbon leaves as \ce{CO2}.

Mechanism of side-chain oxidation (as given in the book)

methylbenzene → phenylmethanol → benzaldehyde → benzoic acid: the first carbon of the side chain is oxidised step by step.

8.3Reactivity of benzene compared with alkanes and alkenesOften examined

One learning outcome of this unit asks you to compare the reactivity of benzene with alkanes and alkenes. The comparison depends on what electrons each molecule offers to a reagent.

So the order of reactivity is alkene > benzene > alkane. Alkenes are the most reactive because their π electrons are localised between two carbons, exposed and easily polarised. Benzene also has π electrons, but they are delocalised and stabilised by 152\,\mathrm{kJ\,mol^{-1}}, so benzene reacts only with strong electrophiles made with the help of a catalyst, and it prefers substitution to addition. Alkanes have no π electrons, only strong, non-polar σ bonds, so they react only with free radicals under UV light or at high temperature.

Benzene therefore behaves partly like an alkane (it undergoes substitution and resists oxidation) and partly like an alkene (it has π electrons and can add hydrogen or chlorine under harsh conditions).

8.4Electrophilic substitution in substituted arenes: directing effectsOften examined

Benzene has six identical carbon atoms, so an electrophile can attack any of them and only one monosubstituted product forms. When a second electrophile attacks a monosubstituted benzene (\ce{C6H5G}), three disubstituted isomers are possible, because the second group can go ortho (positions 2 and 6), meta (3 and 5) or para (4) to the first group G.

If substitution were random, the two ortho positions, two meta positions and one para position would give ortho : meta : para = 2 : 2 : 1, that is 40 : 40 : 20. Experiments give very different results:

From these data we conclude:

1. The orientation of the incoming group is decided by the nature of the substituent already bonded to the ring, not by the incoming electrophile.
2. Some substituents direct the electrophile to positions 2 and 4 (ortho and para), while others direct it to position 3 (meta).

The group already present also changes the rate. Substituents are therefore put into two classes (with halogens as a special case).

(a) Ring-activating, ortho/para-directing substituents

These substituents donate electrons to the ring. They increase the electron density over the whole ring, and especially at carbons 2 and 4. The ring becomes more reactive than benzene (activated), and the next electrophile attacks the ortho and para positions. Examples: alkyl groups (\ce{-R}, such as \ce{-CH3}), hydroxyl (\ce{-OH}), amino (\ce{-NH2}) and alkoxy (\ce{-OR}, such as \ce{-OCH3}).

Groups like \ce{-OH}, \ce{-NH2} and \ce{-OR} have a lone pair on the atom joined to the ring; they push it into the ring through p-orbital overlap (the resonance or mesomeric effect). Alkyl groups have no lone pair; they release electrons through the σ bond (the inductive effect, helped by hyperconjugation).

Why ortho and para? Look at the arenium ion. Draw the three canonical structures of the arenium ion formed by attack at each position of methylbenzene. In ortho attack and in para attack, one canonical structure puts the positive charge on the ring carbon that carries the \ce{CH3} group. That structure is a tertiary carbocation, and the electron-releasing \ce{CH3} group next to the charge stabilises it strongly. In meta attack, none of the three structures puts the charge on that carbon. So the ortho and para arenium ions are more stable, they form faster, and the ortho and para products dominate.

(b) Ring-deactivating, meta-directing substituents

These substituents withdraw electrons from the ring by the inductive effect and by resonance. The whole ring becomes electron-poor and less reactive than benzene (deactivated), and the incoming group goes mainly to carbon 3 (meta). In each of them the atom joined to the ring carries a full or partial positive charge. Examples: \ce{-NO2}, \ce{-CHO}, \ce{-COOH}, \ce{-COR}, \ce{-CN}, \ce{-SO3H}.

Why meta? Look at the arenium ion again. For nitrobenzene, ortho attack and para attack each give one canonical structure in which the positive charge sits on the ring carbon bonded to the nitrogen of the nitro group, and this nitrogen already carries a positive charge. Two positive charges on neighbouring atoms repel strongly, so these arenium ions are very unstable. Meta attack never puts the charge on that carbon, so the meta arenium ion is the least destabilised. It forms fastest, and the meta product dominates (93.5%).

In short, electron-withdrawing (meta) substituents destabilise the arenium ions of ortho and para attack more than the arenium ion of meta attack. Therefore they send electrophiles to the meta position and are called meta-directing substituents.

Halogens: deactivating but ortho/para-directing

Halogen atoms (\ce{-Cl}, \ce{-Br}) are a special case. They are strongly electronegative, so their inductive effect withdraws electrons and makes the ring less reactive than benzene (deactivating). But they also have lone pairs that can be donated into the ring by resonance. When an electrophile attacks ortho or para to the halogen, a lone pair of the halogen can stabilise the positive charge (an extra canonical structure with \ce{C=Cl+}); this is not possible in meta attack. The inductive effect controls the rate, and the resonance effect controls the orientation. So chlorobenzene reacts more slowly than benzene but gives mainly the 2- and 4-products.

a mixture of 1-chloro-2-nitrobenzene and 1-chloro-4-nitrobenzene (the para isomer is major)

Using directing effects in synthesis

When a target molecule has two groups on the ring, the order of the steps matters: the group put in first decides where the second one goes.

Formula sheet

Key terms

Hydrocarbon

A compound that contains only carbon and hydrogen.

Aliphatic hydrocarbon

An open-chain or non-aromatic ring hydrocarbon: alkanes, alkenes and alkynes.

Aromatic hydrocarbon (arene)

A hydrocarbon that contains one or more benzene rings, for example benzene, methylbenzene and naphthalene.

Aromaticity

The special stability of a planar, cyclic, fully conjugated ring with a delocalised π electron cloud, as in benzene (6 π electrons).

Phenyl group

The group \ce{C6H5-} left when one H atom is removed from benzene.

Ortho, meta, para

Positions on a benzene ring relative to a substituent at carbon 1: ortho = 2 (and 6), meta = 3 (and 5), para = 4.

Kekulé structure

A six-membered carbon ring with alternate single and double bonds, proposed by Kekulé in 1865 for benzene.

Delocalisation

Spreading of π electrons over more than two atoms; in benzene the six π electrons are shared by all six carbon atoms.

Resonance

The description of a molecule by two or more Lewis structures (canonical forms) that differ only in the position of electrons; the real molecule is a hybrid of them.

Resonance hybrid

The real structure of a molecule, intermediate between its canonical forms and lower in energy than any of them.

Resonance energy

The difference in energy between the real molecule and its most stable canonical structure; for benzene 152\,\mathrm{kJ\,mol^{-1}}.

Enthalpy of hydrogenation

The enthalpy change when one mole of an unsaturated compound reacts completely with hydrogen to form a saturated compound.

Free radical

An atom or group with an unpaired electron, formed by homolytic fission of a covalent bond.

Free-radical substitution

Replacement of an H atom of an alkane (or of a side chain) by a halogen atom through a chain reaction of free radicals, started by UV light or heat.

Electrophile

An electron-deficient species (positive ion or molecule with a partial positive centre) that accepts an electron pair, for example \ce{Br+}, \ce{NO2+}, \ce{R+}.

Electrophilic addition

The typical reaction of alkenes and alkynes in which an electrophile attacks the π bond and the two parts of the reagent add across the multiple bond.

Electrophilic aromatic substitution

A reaction in which an electrophile replaces an H atom of an aromatic ring while the aromatic ring is kept.

Arenium ion (σ complex)

The resonance-stabilised carbocation formed when an electrophile bonds to one ring carbon of an arene; it has lost aromaticity and loses \ce{H+} to become aromatic again.

Lewis acid

An electron-pair acceptor, for example \ce{AlCl3}, \ce{FeBr3}.

Halogen carrier

A Lewis acid catalyst (\ce{FeBr3}, \ce{AlCl3}) that takes a halogen from the reagent to make the electrophile and gives it back as HX when it is regenerated.

Nitrating mixture

A mixture of concentrated nitric acid and concentrated sulphuric acid that produces the nitronium ion \ce{NO2+}.

Nitronium ion

The linear cation \ce{O=N+=O}, the electrophile in nitration.

Sulphonation

Introduction of the \ce{-SO3H} group into an aromatic ring with fuming or concentrated sulphuric acid; it is reversible.

Friedel–Crafts alkylation

Introduction of an alkyl group into an aromatic ring by a haloalkane and anhydrous \ce{AlCl3}.

Friedel–Crafts acylation

Introduction of an acyl group (\ce{RCO-}) into an aromatic ring by an acyl chloride and anhydrous \ce{AlCl3}, giving an aryl ketone.

Acylium ion

The cation \ce{RC+=O} (for example \ce{CH3CO+}), the electrophile in Friedel–Crafts acylation.

Polyalkylation

Entry of more than one alkyl group in Friedel–Crafts alkylation, because the first alkyl group activates the ring.

Side-chain oxidation

Oxidation of an alkyl group attached to a benzene ring to a \ce{-COOH} group by hot alkaline \ce{KMnO4} followed by acid.

Benzylic position

The carbon atom of a side chain that is directly attached to the benzene ring.

Activating group

A substituent that donates electrons to the ring and makes it react faster than benzene, for example \ce{-OH}, \ce{-NH2}, \ce{-OCH3}, \ce{-CH3}.

Deactivating group

A substituent that withdraws electrons from the ring and makes it react more slowly than benzene, for example \ce{-NO2}, \ce{-CHO}, \ce{-COOH}, halogens.

Ortho/para-directing group

A substituent that sends the incoming electrophile mainly to the 2- and 4-positions, for example \ce{-OH}, \ce{-NH2}, \ce{-CH3}, \ce{-Cl}.

Meta-directing group

A substituent that sends the incoming electrophile mainly to the 3-position, for example \ce{-NO2}, \ce{-CHO}, \ce{-COOH}, \ce{-CN}, \ce{-SO3H}.

Inductive effect

Shift of σ bond electrons towards the more electronegative atom, which weakens with distance.

Resonance (mesomeric) effect

Donation or withdrawal of electrons by a substituent through overlap of its lone pair or π bond with the π system of the ring.

FBISE 2026 questions

Questions from the FBISE HSSC-II 2026 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

Exam focus

1. Explain the structure of benzene by the atomic orbital treatment (sp^2 hybridisation, σ frame, doughnut-shaped π cloud). (long)
   An SLO; short questions viii and ix and long question iii test it.
   Go to answer
2. Compare the enthalpy of hydrogenation of benzene with that of 1,3,5-cyclohexatriene and explain the significance (resonance energy 152\,\mathrm{kJ\,mol^{-1}}). (short)
   Asked in 2026 Set B and the old-syllabus paper (2+1); long question iv asks the same with the energy diagram.
   Go to answer
3. Write the mechanism of nitration of benzene, including the formation of the nitronium ion. (short)
   Asked in 2026 Set D Q.2 (vi) OR and with sulphonation in the old-syllabus Q.3 OR.
   Go to answer
4. Explain the mechanism of Friedel–Crafts alkylation and the role of the Lewis acid \ce{AlCl3}. (long)
   2026 Set B Q.5 (2+4); long question ii and short question iv.
   Go to answer
5. Bromination of benzene with \ce{FeBr3}: equation, catalyst, electrophile, mechanism and observation. (long)
   Long question i and MCQ ix; the halogenation SLO.
   Go to answer
6. Halogenation of methylbenzene: \ce{Cl2} with \ce{AlCl3} (ring) against \ce{Cl2} with UV light (side chain). (short)
   Short question v; 2026 Set D Q.2 (vii) OR asked the ring product of methylbenzene with \ce{2Cl2}.
   Go to answer
7. Explain ortho/para and meta direction from the arenium-ion structures; classify activating and deactivating groups. (long)
   Long questions v to viii; the directing-effects SLO decides every 'predict the product' question.
   Go to answer
8. Plan two-step syntheses from benzene in the correct order (4-nitrotoluene, m-bromonitrobenzene, 3-chlorobenzoic acid). (long)
   Long questions ix and x and short question ii.
   Go to answer
9. Side-chain oxidation of alkylbenzenes to benzoic acid. (short)
   2026 Set B MCQ 10 and short question ii.
   Go to answer
10. Compare the reactivity of alkanes, alkenes and benzene; why alkanes are the least reactive. (short)
    2026 Set D MCQ 8 and old-syllabus Q.2 (iv) OR; short questions i and vii.
    Go to answer
11. Why cyclohexene decolourises bromine water but benzene does not. (short)
    Short question i; tests the stability of the delocalised π system.
    Go to answer