Electrophilic Substitution, Benzene Mechanism - Organic Chem

In this video we want to discuss the electrophilic substitution mechanism of benzene using 2 reactions as example.
Nitration of Benzene
The first reaction is nitration of benzene to nitrobenzene.
This reaction is done using concentrated nitric acid in concentrated sulphuric acid catalyst at 50 to 60 degree celsius.
The first step involves the generation of electrophile NO2+ which is formed from the reaction between conc HNO3 and conc H2SO4.
In the second step, which is the rate determining step or slow step, benzene will react with the electrophile NO2+.
For arrow pushing we have to draw the full arrow, which represents the movement of 2 electrons, from benzene ring (electron rich region) to nitrogen in NO2+ (electron poor region).
The intermediate formed is highly unstable as the benzene is positively charged and it loses the resonance stability due to the opening of delocalised ring.
In the third step, the intermediate will deprotonate or remove a H+ to form product nitrobenzene. The H+ will combine with HSO4- intermediate to regenerate H2SO4 catalyst.
Bromination of Benzene
The second reaction is bromination of benzene to bromobenzene.
This reaction is done via Br2 in FeBr3 catalyst.
The mechanism is effectively the same, namely:
Step 1 - generate electrophile Br+ from Br2 and FeBr3
Step 2 - carbon in benzene uses 2 electrons from delocalised pi system to form a bond with Br+, generating an unstable intermediate
Step 3 - deprotonation of intermediate to form bromobenzene and regeneration of FeBr3 catalyst.
For the detailed step-by-step discussion on how to draw the mechanism, check out this video!
Topic: Benzene (Hydrocarbon), Organic Chemistry, A Level Chemistry, Singapore
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[@nicelookhairsalon4089]

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[@user-hx8dx1rs2k]

Excellent!
Now, would u b able to show me the mechanism of addition reaction of acidified aqueous potassium manganate(VII) into ethene to produce ethane-1,2-diol

[@ChemistryGuru]

Hi the reaction of alkene to di-ol via KMnO4 is a mid oxidation. Mechanisms of redox reaction is not covered in A Level Chemistry so I will not be going through that. 😀

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[@ChemistryGuru]

Thank you!

[@chefhamzawyyummy5995]

electrophilic substitution all reactions have the same general mechanisms ?

[@ChemistryGuru]

this should only apply to benzene. other functional groups that undergo electrophilic substitution will have different steps

[@jaydenong153]

Hi, do you know where does the 2 electrons come from when the intermediate loses the H+ ion? Thank you

[@ChemistryGuru]

Hi Jayden if you are referring to step 3 for electrophilic substitution, when the intermediate loses the H+ to form product, the 2 electrons come from the C-H covalent bond. Both electrons from the C-H bond will be donated to the delocalised pi system so there will be 6 delocalised electrons (where resonance is the most stable). Then H+ is being kicked out and combine with other intermediates to form by-products.

[@jaydong9985]

Chemistry Guru - #1 JC, A Level, H2 Chemistry Tuition ohhh okay but why is it specifically 2 electrons?

[@ChemistryGuru]

@@jaydong9985 in step 2, 2 electrons from delocalised pi system in benzene is used to form a bond with carbon and electrophile. The pi system is then short of 2 electrons, so in step 3 it will take back 2 electrons to restore the delocalisation.