Electrophilic substitution

Electrophilic substitution is the main type of reaction of benzene and other aromatic compounds. An electrophile, a particle that accepts a pair of electrons, is attracted to the ring of delocalised electrons and replaces one hydrogen atom on the ring. The ring itself survives, which is why benzene undergoes substitution rather than the addition reactions alkenes do. At A-level you need two examples: nitration and Friedel-Crafts acylation.

What is electrophilic substitution?

Break the name into its two parts:

• Electrophilic: the reagent that attacks is an electrophile, an electron pair acceptor. It is usually a positive ion, such as the nitronium ion, NO₂⁺.
• Substitution: one atom or group is swapped for another. In benzene, a hydrogen on the ring is swapped for the electrophile, so C₆H₆ becomes C₆H₅X.

The general equation is C₆H₆ + X⁺ → C₆H₅X + H⁺. Every electrophilic substitution you meet follows the same two-step pattern, so once you know one mechanism you know them all; only the way the electrophile is made changes.

Why benzene does substitution, not addition

Benzene, C₆H₆, is a flat (planar) ring of six carbon atoms, each bonded to one hydrogen. Every carbon has one electron in a p orbital, and these p orbitals overlap above and below the ring to form a ring of six delocalised electrons. That is why benzene is drawn as a hexagon with a circle inside, not with three separate double bonds.

Two pieces of evidence show the electrons really are spread round the ring:

• Bond lengths. All six carbon to carbon bonds are the same length, 139 pm, between a typical C-C single bond (154 pm) and a C=C double bond (134 pm). A Kekulé ring with three separate C=C bonds would have three short bonds and three long ones.
• Enthalpy of hydrogenation. Benzene releases much less energy when it is hydrogenated than a ring with three separate double bonds would, so it is more stable.

The enthalpy of hydrogenation evidence

Adding hydrogen to cyclohexene, which has one C=C bond, gives out 118 kJ mol⁻¹. The imaginary molecule cyclohexa-1,3,5-triene, with three C=C bonds, would be expected to give out about 3 × 118 = 354 kJ mol⁻¹. Benzene actually gives out only 206 kJ mol⁻¹ when it is hydrogenated to cyclohexane.

The difference, 354 − 206 = 148 kJ mol⁻¹, is the extra stability that delocalisation gives benzene. Many textbooks round the figures to 120, 360 and 208 kJ mol⁻¹, which gives 152 kJ mol⁻¹; the idea is the same.

This explains benzene's reactions. Addition would use up the delocalised electrons and lose that stability. Substitution puts the delocalised ring back at the end, so it is the favoured route. The spread-out electrons are also less concentrated than the electrons in an alkene's C=C bond, so benzene needs a strong electrophile; it does not decolourise bromine water the way an alkene does. Compare electrophilic addition to alkenes.

The electrophilic substitution mechanism

All of these reactions share one mechanism once the electrophile E⁺ exists:

1. Make the electrophile. A strong electrophile is generated from the reagents, usually with a catalyst.
2. Attack. Two electrons from the delocalised ring form a bond to E⁺. Draw a curly arrow from the circle to the electrophile.
3. The intermediate. One carbon now holds both H and E, so it has left the delocalised system. Draw a horseshoe over the other five carbons, open towards that carbon, with a + inside. This positive intermediate is less stable than benzene.
4. Lose H⁺. The C-H bond on that carbon breaks and its two electrons return to the ring. Draw a curly arrow from the C-H bond into the horseshoe. The full delocalised ring is restored.
5. Regenerate the catalyst. The H⁺ that leaves reacts with the ion formed in step 1, so the catalyst is made again.

Build benzene and nitrobenzene in the Molecule Builder to see that the ring is flat and that only one hydrogen has changed.

Nitration of benzene

Benzene reacts with a mixture of concentrated nitric acid and concentrated sulfuric acid, kept at no more than about 50 °C. Hotter mixtures make it more likely that a second nitro group goes onto the ring.

Making the nitronium ion

Sulfuric acid protonates nitric acid, which then loses water. The electrophile is the nitronium ion, NO₂⁺:

HNO₃ + 2H₂SO₄ → NO₂⁺ + 2HSO₄⁻ + H₃O⁺

After the ring has lost its H⁺, that H⁺ reacts with HSO₄⁻ to give back H₂SO₄, so sulfuric acid is a catalyst. The overall equation is:

C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O

The product is nitrobenzene. AQA singles nitration out as an important step in synthesis, including the manufacture of explosives and the formation of amines: reducing the nitro group gives phenylamine, C₆H₅NH₂, and this nitration then reduction route is used to make many dyes and medicines.

Check your understanding of the mechanism.

Nitration of benzene in the lab: methyl 3-nitrobenzoate and phenylamine

AQA suggests a practical alongside the theory: preparing methyl 3-nitrobenzoate by nitrating methyl benzoate, purifying it by recrystallisation and finding its melting point. It is the same electrophilic substitution as the nitration of benzene, on a ring that already carries an ester group. The Royal Society of Chemistry's version of the method goes like this:

1. Dissolve methyl benzoate in concentrated sulfuric acid, in a flask cooled in an ice-water bath.
2. Make the nitrating mixture fresh by adding concentrated sulfuric acid to concentrated nitric acid, cooled in ice.
3. Add the nitrating mixture slowly, over about 15 minutes with stirring, keeping the temperature below 6 °C. Then leave the mixture at room temperature for 15 minutes.
4. Pour it onto crushed ice. The solid product forms, because it is insoluble in water.
5. Filter under suction and wash with a little ice-cold water.
6. Recrystallise from a hot mixture of water and ethanol, filter again under reduced pressure, and dry.
7. Measure the melting point.

The RSC notes that the ester group directs the nitro group mainly to the 3-position, which is why the main product is methyl 3-nitrobenzoate (see directing groups below). For safety, the concentrated acids are highly corrosive and nitric acid is also oxidising, so goggles and gloves are needed, and the ethanol is heated on an electric hot plate because it is highly flammable.

The melting point is a test of purity. AQA's practical handbook explains that a pure, dry solid melts at a precise temperature, while an impure one melts over a range of temperatures below the pure compound's melting point. Compare your value with the data book value.

From nitrobenzene to phenylamine

Nitration is often the first step towards an amine. OCR A's specification gives the reagents: aromatic amines are made by reducing nitroarenes using tin and concentrated hydrochloric acid. With [H] for the reducing agent, the equation for nitrobenzene is:

C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O

AQA notes that aromatic amines made this way are used in the manufacture of dyes. How phenylamine behaves as a base is covered in amines and amides.

Friedel-Crafts acylation

The other substitution AQA asks for adds an acyl group, RCO, to the ring, using an acyl chloride with aluminium chloride as the catalyst. AlCl₃ makes the electrophile, the acylium ion RCO⁺, and the ring attacks it in exactly the same way as it attacks NO₂⁺, giving an aromatic ketone. The conditions, the full mechanism, naming the product and worked examples are in the Friedel-Crafts acylation guide.

Halogenation of benzene (OCR)

OCR A also asks for halogenation. Benzene reacts with bromine or chlorine only in the presence of a halogen carrier, such as iron, an iron halide or an aluminium halide. The halogen carrier polarises the halogen enough to act as the electrophile, which OCR lets you treat as Br⁺ or Cl⁺:

Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻, then C₆H₆ + Br₂ → C₆H₅Br + HBr

The product is bromobenzene, and H⁺ + FeBr₄⁻ → FeBr₃ + HBr regenerates the halogen carrier.

Directing groups: where a second group goes (OCR)

When the ring already carries a group, that group decides where the next one goes. OCR A expects you to know three:

• -OH and -NH₂ direct to positions 2 and 4. They also donate electron density into the ring, which makes it react much more easily than benzene. For example, bromine water at room temperature puts three bromine atoms onto phenol, at carbons 2, 4 and 6, with no halogen carrier needed. Dilute nitric acid gives a mixture of 2-nitrophenol and 4-nitrophenol.
• -NO₂ directs to position 3. So nitrating nitrobenzene further gives 1,3-dinitrobenzene.

Positions are numbered round the ring from the carbon that holds the first group, which is carbon 1. AQA only asks about monosubstitution, so this section is for OCR students.

Worked examples

Example 1: the overall equation and the electrophile

Methylbenzene, C₆H₅CH₃, is nitrated to give one nitro group on the ring. Write the equation that makes the electrophile, and the overall equation.

• The electrophile is the same as for benzene: HNO₃ + 2H₂SO₄ → NO₂⁺ + 2HSO₄⁻ + H₃O⁺.
• One H on the ring is swapped for NO₂: C₆H₅CH₃ + HNO₃ → CH₃C₆H₄NO₂ + H₂O.
• Check it balances: 7 C, 9 H, 1 N and 3 O on each side.

Example 2: the stability of benzene from data

Cyclohexene has an enthalpy of hydrogenation of −120 kJ mol⁻¹ and benzene −208 kJ mol⁻¹. Use these values to find how much more stable benzene is than cyclohexa-1,3,5-triene.

1. Expected value for three C=C bonds: 3 × (−120) = −360 kJ mol⁻¹.
2. Difference: −208 − (−360) = +152 kJ mol⁻¹.
3. Benzene is 152 kJ mol⁻¹ more stable, because its electrons are delocalised.

Example 3: atom economy of nitration

Find the atom economy for making nitrobenzene from benzene. Use C = 12, H = 1, N = 14, O = 16.

1. Mr of C₆H₅NO₂ = 6 × 12 + 5 × 1 + 14 + 2 × 16 = 123.
2. Mr of the reactants, C₆H₆ + HNO₃ = 78 + 63 = 141.
3. Atom economy = 123 ÷ 141 × 100 = 87.2%. The only waste product is water. See the atom economy guide for more.

Example 4: predicting a product

Benzene is warmed with propanoyl chloride, CH₃CH₂COCl, and AlCl₃. Name the organic product.

• The electrophile is CH₃CH₂CO⁺, which replaces one H on the ring.
• The product is C₆H₅COCH₂CH₃, a ketone with three carbons in its chain and the ring on carbon 1: 1-phenylpropan-1-one. The other product is HCl.

Common mistakes

• Calling it electrophilic addition. Benzene keeps its ring and swaps an H; alkenes add across the C=C.
• Drawing the curly arrow from the electrophile to the ring. Arrows show electrons moving, so they start at the ring and point to E⁺.
• Closing the horseshoe. The horseshoe covers five carbons and leaves out the one bonded to both H and E, with the + inside it.
• Forgetting to regenerate the catalyst. Show H⁺ + HSO₄⁻ → H₂SO₄, or H⁺ + AlCl₄⁻ → AlCl₃ + HCl.
• Writing NO₂⁻ or NO₃⁺. The nitronium ion is NO₂⁺.
• Saying benzene has three double bonds. The Kekulé structure is a model that the bond length and enthalpy evidence rules out.

Practise with a short timed paper

Aromatic chemistry is in the second year of A-level Chemistry for AQA and OCR. The daily A-level Chemistry: Year 2 paper is a short timed paper of 8 questions, with a new one every day. For the reaction of halogenoalkanes, see nucleophilic substitution.

Read next

Chemistry · Organic chemistryElectrophilic addition: mechanism and major productsWhat electrophilic addition is, the mechanism for alkenes with HBr, bromine and sulfuric acid, carbocation stability, and how to predict the major product.