International A Level (IAL)ChemistryEdexcelRevision Notes5. Transition Metals & Organic Nitrogen ChemistryOrganic Chemistry: ArenesBenzene - Electrophilic Substitution

Benzene - Electrophilic Substitution (Edexcel International A Level (IAL) Chemistry): Revision Note

Exam code: YCH11

Benzene - Electrophilic Substitution

  • The main reactions which benzene will undergo include the replacement of one of the 6 hydrogen atoms from the benzene ring

    • This is different to the reactions of unsaturated alkenes, which involve the double bond breaking and the electrophile atoms 'adding on' to the carbon atoms

  • These reactions are called electrophilic substitution reactions

    • This is where at least one of the H atoms of benzene are substituted by the electrophile

  • You must be able to provide the mechanisms for specific examples of the electrophilic substitution of benzene

General Electrophilic Substitution Mechanism:

7-4-1-general-electrophilic-substitution-mechanism-1-1
General electrophilic substitution mechanism 2, downloadable AS & A Level Chemistry revision notes

  • The delocalised π system is extremely stable and is a region of high electron density

  • Electrophilic substitution reactions involve an electrophile, which is either a positive ion or the positive end of a polar molecule

  • There are numerous electrophiles which can react with benzene

    • However, they usually cannot simply be added to the reaction mixture to then react with benzene

    • The electrophile has to be produced in situ, by adding appropriate reagents to the reaction mixture

  • The electrophilic substitution reaction in arenes consists of steps:

    • Generation of an electrophile

    • Electrophilic attack

    • Regenerating aromaticity

Nitration of benzene mechanism

  • One hydrogen atom is substituted by a nitro group - NO2

  • The overall reaction is:

Overall nitration reaction: benzene plus nitric acid forms nitrobenzene (benzene ring with NO₂ group) and water, with an arrow showing the conversion.

C6H6 + HNO3 → C6H5NO2 + H2O

  • The reaction is conducted under reflux at 55 °C or 60 °C

Step 1: Electrophile generation

  • The electrophile NO2+ ion is generated by reacting concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4)

  • The equation for the generation of the electrophile is:

Diagram titled “Step 1: Generating the electrophile”, showing HNO₃ + 2H₂SO₄ → NO₂⁺ + 2HSO₄⁻ + H₃O⁺, with NO₂⁺ labelled as the electrophile

Examiner Tips and Tricks

There are 2 accepted equations for the formation of the eletrophile:

HNO3 ​+ H2​SO4 ​→ NO2+ ​+ HSO4− ​+ H2​O

OR

HNO3 + 2H2SO4 → NO2+ + 2HSO4- + H3O+

Step 2: Electrophilic attack

  • A pair of electrons from the benzene ring is donated to the electrophile to form a covalent bond

    • This disrupts the aromaticity in the ring as there are now only four π electrons and there is a positive charge spread over the five carbon atoms

Diagram of step 2 electrophilic attack: nitronium ion NO2+ attacks benzene ring to form complex with attached NO2 and positively charged ring.
A pair of electrons from the benzene ring is donated to the nitronium ion electrophile forming a covalent bond and a loss in aromaticity

Examiner Tips and Tricks

Examiners are looking for very specific points in an electrophilic substitution mechanism:

  1. The initial arrow:

    • The curly arrow must start from on or within the circle/hexagon of the benzene ring and point to the NO2+ electrophile.

    • Do not forget to draw the + charge on the electrophile.

  2. The horseshoe shape:

    • The intermediate must be drawn with a 'horseshoe' (an incomplete circle) that faces the tetrahedral carbon, which is the carbon atom attached to the H and NO2 group.

  3. Horseshoe size:

    • The horseshoe must be large enough to cover at least 3 carbon atoms.

    • Ideally it should cover the 5 carbon atoms that have not been substituted.

  4. The charge:

    • The positive charge (+) must be drawn inside the horseshoe

Step 3: Regenerating / restoring aromaticity

  • Aromaticity is restored by heterolytic cleavage of the C-H bond

    • This means that the bonding pair of electrons goes into the benzene π bonding system

Diagram of step 3, restoring aromaticity: benzene ring loses a proton to reform aromatic π system, giving nitrobenzene and H⁺ as products.

Examiner Tips and Tricks

The curly arrow must start from the C–H bond and point to anywhere within the ring to show the reforming of the delocalised structure

Step 4: Regenerating the catalyst

  • The H+ ion (released in Step 3) reacts to reform the original catalyst:

H+ + HSO4− → H2​SO4​

Halogenation of benzene mechanism 

  • One hydrogen atom is substituted by a halogen atom

  • For bromine, the overall reaction is:

Overall reaction showing benzene bromination with Br₂ and anhydrous AlBr₃ catalyst to form bromobenzene and hydrogen bromide

C6H6 + Br2 → C6H5Br + HBr

  • The reaction is conducted using a halogen carrier catalyst and requires heat / heating under reflux

Step 1: Electrophile generation

  • The electrophile X+ ion is generated by reacting the halogen with a halogen carrier

  • The common halogen carriers are:

    • AlBr3

    • AlCl3

    • FeCl3 (or iron and bromine, which react to form FeBr3​)

  • The halogen molecules form a dative bond with the halogen carrier by donating a lone pair of electrons from one of its halogen atoms into an empty 3p orbital of the halogen carrier

Mechanism step showing Br₂ reacting with AlBr₃ via a dative covalent bond to form Br⁺ electrophile and [AlBr₄]⁻ in electrophilic substitution

Examiner Tips and Tricks

The halogen carrier used must correspond to the halogen that is involved in the substitution reaction:

Br-Br + AlBr3 → Br+ + [AlBr4]-

Cl-Cl + FeCl3 → Cl+ + [FeCl4]-

Step 2: Electrophilic attack

  • A pair of electrons from the benzene ring is donated to the electrophile to form a covalent bond

    • This disrupts the aromaticity in the ring as there are now only four π electrons and there is a positive charge spread over the five carbon atoms

Diagram of step 2 electrophilic attack: bromine cation Br⁺ attacks benzene ring, forming a σ-complex with Br and H attached and a positive ring charge
A pair of electrons from the benzene ring is donated to the bromine ion electrophile forming a covalent bond and a loss in aromaticity

Examiner Tips and Tricks

Examiners are looking for similar specific points to the nitration mechanism:

  1. The initial arrow:

    • The curly arrow must start from on or within the circle/hexagon of the benzene ring and point to the Br+ or Cl+ electrophile.

    • Do not forget to draw the + charge on the electrophile.

  2. The horseshoe shape:

    • The intermediate must be drawn with a 'horseshoe' (an incomplete circle) that faces the tetrahedral carbon, which is the carbon atom attached to the H and halogen atom.

  3. Horseshoe size:

    • The horseshoe must be large enough to cover at least 3 carbon atoms.

    • Ideally it should cover the 5 carbon atoms that have not been substituted.

  4. The charge:

    • The positive charge (+) must be drawn inside the horseshoe

Step 3: Regenerating / restoring aromaticity

  • Aromaticity is restored by heterolytic cleavage of the C-H bond

    • This means that the bonding pair of electrons goes into the benzene π bonding system

Diagram of step 3 electrophilic substitution: [AlBr4]– removes H from bromobenzene sigma complex, restoring benzene aromaticity and forming HBr and AlBr3

Step 4: Regenerating the catalyst

  • The H+ ion (released in Step 3) reacts to reform the original catalyst:

H+ + [AlBr4]- → HBr + AlBr3

Friedel-Crafts acylation of benzene mechanism

  • One hydrogen atom on the benzene ring is substituted by an acyl group (e.g., an ethanoyl group, −COCH3​).

  • For the reaction with ethanoyl chloride, the overall reaction is:

C6H6 + CH3COCl → C6H5COCH3 + HCl

  • The reaction is conducted using an anhydrous aluminium chloride (AlCl3) catalyst and requires heating under reflux

Step 1: Electrophile generation

The electrophile is an acylium ion (e.g., CH3CO+).

It is generated by reacting the acyl chloride with the halogen carrier catalyst, which accepts a lone pair of electrons from the chlorine atom.

The equation for the generation of the electrophile is:

CH3COCl + AlCl3 → CH3CO+ + [AlCl4]−

  • In the Friedel-Crafts acylation reaction, an acyl group is substituted into the benzene ring

    • An acyl group is an alkyl group containing a carbonyl, C=O group

Hydrocarbons - Friedel-Crafts Acylation (1), downloadable AS & A Level Chemistry revision notes

Step 2: Electrophilic attack

  • A pair of electrons from the benzene ring is donated to the electrophile to form a covalent bond.

    • This disrupts the aromaticity in the ring as there are now only four π electrons and there is a positive charge spread over the five carbon atoms.

Examiner Tips and Tricks

Examiners are looking for similar specific points to the nitration mechanism:

  1. The initial arrow:

    • The curly arrow must start from on or within the circle/hexagon of the benzene ring and point to the positively charged carbon atom of the CH3CO+ electrophile.

    • Do not forget to draw the + charge on the electrophile.

  2. The horseshoe shape:

    • The intermediate must be drawn with a 'horseshoe' (an incomplete circle) that faces the tetrahedral carbon, which is the carbon atom attached to the H and acyl group.

  3. Horseshoe size:

    • The horseshoe must be large enough to cover at least 3 carbon atoms.

    • Ideally it should cover the 5 carbon atoms that have not been substituted.

  4. The charge:

    • The positive charge (+) must be drawn inside the horseshoe

Step 3: Regenerating / restoring aromaticity

  • Aromaticity is restored by heterolytic cleavage of the C–H bond.

    • This means that the bonding pair of electrons goes into the benzene π bonding system.

Step 3 of Friedel–Crafts acylation: proton loss restores aromaticity, forming methyl phenyl ethyl ketone, with HCl and regenerated AlCl₃ catalyst.

Step 4: Regenerating the catalyst

  • The H+ ion (released in Step 3) reacts with the complex ion to reform the original catalyst and produce hydrogen chloride gas:

H+ + [AlCl4]− → HCl + AlCl3​

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

I would just like to say a massive thank you for putting together such a brilliant, easy to use website.I really think using this site helped me secure my top gradesin science and maths. You really did save my exams! Thank you.

Beth
IGCSE Student

This website is soooo useful and I can’t ever thank you enough for organising questions by topic like this. Furthermore, the name of the website could not have been more appropriate as it literally did SAVE MY EXAMS!

Fathima
A Level Student

Incredible! SO worth my money, the revision notes have everything I need to know and are so easy to understand. I actually enjoy revising! It makes me feel a lot more confident for my GCSEs in a few months.

Kate
GCSE Student

Absolutely brilliant, both my girls used it for A levels and GCSE. It's saves on paper copies, also beneficial exam questions ranked from easy to hard. It's removed a lot of stress from the exams.

Sameera
Parent

Just to say that your resources are the best I have seen and I have been teaching chemistry at different levels for about 40 years

Mark
Chemistry Teacher

Excellent

Our rating represented in Trustpilot stars
Read more reviews
Test yourself
Previous:Benzene ReactionsNext:Phenol Bromination