AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Electrophilic attack on benzene rings results in substitution, limited to…
Full specification wording
Electrophilic substitution (A-level only): Electrophilic attack on benzene rings results in substitution, limited to monosubstitutions.
The delocalised ring is a region of high electron density above and below the plane, so it attracts species that are short of electrons.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The delocalised ring is a region of high electron density above and below the plane, so it attracts species that are short of electrons. The attack happens in two steps. A pair of electrons leaves the delocalised system and forms a bond to the electrophile; that leaves an unstable positively charged intermediate in which the delocalisation is broken and only part of the ring, drawn as a horseshoe, is still delocalised. In the second step the C-H bond at the attacked carbon breaks heterolytically and the pair of electrons returns to the ring, restoring the full delocalisation and releasing H⁺. The net change is that a hydrogen atom is replaced, so only one substituent is introduced at A-level.
What earns the marks
- A curly arrow from inside the hexagon to the electrophile.
- The intermediate drawn with a horseshoe and a positive charge inside the ring.
- A curly arrow from the C-H bond back into the hexagon.
- Naming the mechanism as electrophilic substitution and identifying H⁺ as the species lost.
Common mistakes and how to put them right
Drawing the first arrow from a localised C=C bond rather than from inside the ring.
Leaving the hydrogen off the attacked carbon of the intermediate, so the second arrow has no bond to start from.
Showing the electrophile adding across the ring to give a non-aromatic product.
Omitting the positive charge from the intermediate.
Exam tips
- Draw the intermediate large, with the attacked carbon at the top carrying both the H and the new group side by side.
- The horseshoe must not pass through the attacked carbon; that carbon is now saturated.
Try the lesson
The start of the Electrophilic substitution (A-level only): Electrophilic attack on benzene rings results in substitution, limited to monosubstitutions. lesson, made from checked teaching notes for this specification point.
Have a go first
Why do benzene rings typically undergo electrophilic substitution rather than electrophilic addition reactions?
Listen
The delocalised ring is a region of high electron density above and below the plane, so it attracts species that are short of electrons. The attack happens in two steps. A pair of electrons leaves the delocalised system and forms a bond to the electrophile; that leaves an unstable positively charged intermediate in which the delocalisation is broken and only part of the ring, drawn as a horseshoe, is still delocalised. In the second step the C-H bond at the attacked carbon breaks heterolytically and the pair of electrons returns to the ring, restoring the full delocalisation and releasing H⁺. The net change is that a hydrogen atom is replaced, so only one substituent is introduced at A-level. A strong answer earns marks for these points. a curly arrow from inside the hexagon to the electrophile. the intermediate drawn with a horseshoe and a positive charge inside the ring. a curly arrow from the C-H bond back into the hexagon. naming the mechanism as electrophilic substitution and identifying H⁺ as the species lost. Exam tip: Draw the intermediate large, with the attacked carbon at the top carrying both the H and the new group side by side.
Key words
Say what each word means before you turn the card.
electrophile
Try answering aloud or in your head before turning the card.
two-step mechanism
Try answering aloud or in your head before turning the card.
positively charged intermediate
Try answering aloud or in your head before turning the card.
restoring delocalisation
Try answering aloud or in your head before turning the card.
See every definition
- electrophile
- A species that is short of electrons and is attracted to the delocalised ring, which is a region of high electron density above and below the plane.
- two-step mechanism
- The attack in which a pair of electrons leaves the delocalised system to form a bond, then the C-H bond breaks heterolytically and the pair returns to the ring.
- positively charged intermediate
- An unstable species in which the delocalisation is broken and only part of the ring, drawn as a horseshoe, is still delocalised.
- restoring delocalisation
- The return of the pair of electrons from the C-H bond at the attacked carbon to the ring, releasing H⁺.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Electrophilic substitution (A-level only): Electrophilic attack on benzene rings results in substitution, limited to monosubstitutions.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge