Electrophilic Substitution Flashcards
What is electrophilic aromatic substitution?
This is where a hydrogen on the aromatic ring is exchanged with an electrophile
Why does benzene not react in addition reactions?
The aromatic ring would be destroyed making the products much less stable than benzene
This makes the reaction unfavourable
Outline the basic mechanism of electrophilic substitution
1) pi electrons donate to electrophile
2) forms a positively charged intermediate (wheland intermediate)
3) proton is lost to reform the aromatic ring (different to alkenes)
Describe how the wheland intermediate is stabilised even though it is no longer aromatic
It is stabilised by the delocalisation of positive charge over 3 carbon atoms
What causes the difference in reactivity between alkenes and aromatics?
The difference is due to the extra stabilisation that aromatic compounds have
(So they want to stay aromatic)
Outline the energy diagram of electrophilic substitution
There are two transition states (one where E is joining and one where H is leaving)
The 1st transition state is higher in energy as it has lost its aromaticity, 2nd is lower as it is regained
Wheland intermediate is high in energy despite delocalisation as it is not aromatic
Why is the first step of the reaction slower?
2st step= adding E
This is because the Gibbs energy of formation of wheland intermediate is greater than gibbs energy of activation for deprotonation of the wheland intermediate (greater Ea for 1st)
This means the first step is the rate determining step
What is the overall Gibbs energy change of electrophilic substitution
The overall change is negative as the energy of the products is less than that of the reactants
This means products will be favoured in equilibrium
How is nitrobenzene formed?
Benzene reacts with a mixture of concentrated nitric acid and sulfuric acids
H2SO4 and HNO3 react together first= NO2+, H2O + HSO4-
H2SO4 donates H and water is a leaving group
(O=N=O reacts with benzene)
Why does addition of a nitrogroup make benzene less electron rich?
The nitrogroup is strongly electron withdrawing due to the positive charge on the nitrogen atom
This positive charge can be delocalised around the ring hence making it less electron rich
How do you ensure only monosubstitution of NO2?
Keep the reaction below 100 degrees
How can you form di or tri NO2 substituted benzenes?
At higher temperatures, further substitutions can occur
This is because the nitro group is electron withdrawing which makes the ring less reactive
This means higher temperatures are required for it to react
Describe the bromination/chlorination of benzene
You need catalyst such as FeBr3 or AlBr3 (Lewis acid)
Electrosubtitution of halogen onto the ring
The Lewis acid polarises the halogen so the other halogen has a partial positive charge and can react with benzene
The FeBr4- removes theH from benzene
The FeBr3 is reformed and HBr is a byproduct
Why does only monosubstitution of halogens occur?
Chlorine and bromine are inductively electron withdrawing groups do the product is less reactive than the starting material
Their -I is stronger than their +M
What is the problem with fluorination of benzene?
Fluorine reacts extremely rapidly and so it is difficult to control the number of fluorines that are attached to the ring
This means this is not a useful reaction and is causes is number of products
