6.1.3: Electrophilic substitution Flashcards
Electrophilic addition can be considered a series of reaction steps:
Step 1:
Electrons above and below the plane of the ring attract an electrophile.
Electrophilic addition can be considered a series of reaction steps:
Step 1: Electrons above and below the plane of the ring attract an electrophile.
Step 2:
The electrophile accepts a pair of pi-electrons from the delocalised ring and makes a covalent bond.
This is the slowest step and is known as the rate determining step.
Electrophilic addition can be considered a series of reaction steps:
Step 2: The electrophile accepts a pair of pi-electrons from the delocalised ring and makes a covalent bond.
Step 3:
A reactive intermediate is formed where the delocalised electrons have been disrupted.
What is a nitration reaction?
An electrophilic substitution reaction where a hydrogen is exchanged for a nitro group (-NO2)
What are the reagents for the nitration of benzene?
Concentrated nitric acid, with concentrated sulfuric acid as a catalyst.
What is the balanced symbol equation summarising the nitration of benzene?
C6H6 + HNO3 —> C6H5NO2 + H2O
Describe the steps and conditions of the nitration of benzene.
- The concentrated nitric acid and concentrated sulfuric acid are mixed together in a flask and held in an ice bath.
- Then benzene is added and a reflux condenser is set up, keeping the mixture at 50 degrees to prevent further substitution reactions from occurring.
Why is sulfuric acid needed in the nitration of benzene?
To generate the NO2+ electrophile from the nitric acid.
The sulfuric acid is regenerated at the end and is therefore a catalyst.
What is the symbol equation for the generation of NO2+ using H2SO4?
HNO3 + H2SO4 —> NO2+ + HSO4- + H2O
What is the symbol equation for the regeneration of H2SO4 after the nitration of benzene?
H+ + HSO4- —> H2SO4
Why does benzene not directly react with a halogen?
The aromatic ring is too stable.
What is used to allow halogens to react with benzene?
- A halogen carrier such as iron (which forms an iron halide in-situ).
- Iron halides.
- Aluminium halides.
what does bromine form when it reacts with iron(III) bromide?
A positive bromine ion that can act as an electrophile.
What is the balanced symbol equation for bromine reacting with iron(III) bromide?
Br2 + FeBr3 —> Br+ + FeBr4-
(the Br+ is generated in situ. It an then attack the benzene ring and an electrophilic substitution occurs.
How do you know that the halogen carrier is a catalyst?
It is regenerated at the end of the halohenation.
