[Year 2] CD Flashcards
What shape is benzene?
Cyclic planar.
Describe the arrangement of electrons in benzene.
- Each carbon has 4 valent electrons.
- 3 of these are used to bong with 2 carbons and a hydrogen.
- The lone electron exists in the p-orbital above and below the planar ring.
- This creates a delocalised ring of electrons.
What is Kekule’s structure of benzene?
- Hexagonal ring.
- Alternating double bonds.
How do we know benzene is more stable than cyclohexa-1,3,5-triene (Kekule’s structure)?
- By measuring and comparing their enthalpy changer of hydrogenation.
- Benzene has a larger enthalpy of hydrogenation.
- Therefore it is more stable.
How do we know Kekule’s structure is incorrect?
- X-ray diffraction shows benzene has the same bond lengths, in Kekule’s structure, they would be different.
- Enthalpy change of hydrogenation, is not what it is theoretically for Kekule’s structure.
- Benzene doesn’t react with bromine water (electrophilic addition of bromine), as no colour change is observed.
Why doesn’t benzene react with bromine water?
- Benzene is stable due to its stable ring of delocalised electrons.
- So if an addition reaction were to take place it would make benzene less stable.
Why is benzene able to undergo electrophilic substitution?
- Benzene has a high electron density due to its delocalised ring.
- This is attractive to an electrophile.
- So a substitution can take place kicking off hydrogen for the attracted electrophile.
How do you name arene compounds?
- ‘-benzene’ (e.g. bromobenzene)
- ‘phenyl-‘ (e.g. phenylamine)
Describe the general process of electrophilic substitution of benzne
- Delocalised electrons on benzene are attracted to the
positive charge on the electrophile. - This breaks the ring and forming an unstable intermediate with the electrophile bonded with carbon on benzene.
- The area of positive charge (where the initial electrons left) attracts electrons from the C-H of the same carbon.
- This causes H⁺ to act as a leaving group (which is substituted for) and restores the delocalised ring.
(Diagram: https://bit.ly/3mNH9Zb )
What is required for the electrophilic substitution of benzene?
A very strong electrophile as benzene is stable.
- This can be done by a halogen carrier catalyst like Aluminium chloride or, Iron and iron chloride
Describe Friedel-Crafts acylation?
- An acylchloride is not reactive enough by its self to react with benzene.
- So it is reacted with AlCl₃ first to give…
R-C⁺=O and AlCl₄⁻
… as the halogen carrier pulls electrons from the chlorine on the acylchloride. - Under reflux and using a dry ether solvent electrophilic substitution takes place with the carbonation.
- The hydrogen on the intermediate is attacked by electrons from a Cl-Al bond reforming the catalyst and forming HCl.
( Diagram: https://bit.ly/3dmnigq )
Describe Friedel-Crafts alkylation?
- A halogenoalkane is not reactive enough by its self to react with benzene.
- So it is reacted with AlCl₃ first to give…
R⁺ and AlCl₄⁻
… as the electrons in the R-Cl bond is attracted to the delta negative chlorine. Which then breaks the bond and is attracted to the delta positive aluminium halogen carrier. - Under reflux and using a dry ether solvent electrophilic substitution takes place with the carbocation.
- The hydrogen on the intermediate is attacked by electrons from a Cl-Al bond reforming the catalyst and forming HCl.
(Diagram https://bit.ly/2RGGbT7 )
Describe the mechanism for the halogenation of benzene?
- A halogen is not reactive enough by its self to react with benzene.
- AlCl₃ polarises Cl₂
- Delacalised electron in benzene attack the delta positive end and the electrons from the Cl-Cl bond is attack the delta positive AlCl₃
- (is an electrophilic substitution.)
- The hydrogen on the intermediate is attacked by electrons from a Cl-Al bond reforming the catalyst and forming HCl.
([1] Diagram: https://bit.ly/3mQgBq1 )
([2] Diagram: https://bit.ly/2Q8Pbj8 )
Describe the mechanism for the nitration of benzene?
- Nitric acid is not reactive enough by its self to react with benzene.
- So it is reacted with a sulphuric acid cataylst…
HNO₃ + H₂SO₄ → H₂NO₃⁺ + HSO₄⁻
(nitric acid acts as a base) - H₂NO₃⁺ decomposes to form a nitronium ion…
H₂NO₃⁺ → NO₂⁺ + H₂O - electrophilic substitution of NO₂⁺ to form nitrobenzene.
- The H⁺ leaving group reacts with HSO₄⁻ to reform H₂SO₄.
(Diagram: https://bit.ly/3ge6VEK )
What are the conditions for the nitration of benzene?
- below 55ᵒC
- Sulphuric acid catalyst.
Describe the mechanism for the sulfonation of benzene?
- Sulphuric acid is not reactive enough by its self to react with benzene.
- So it is turned into sulfur trioxide first…
H₂SO₄ → H₂O + SO₃ - SO₃ can then act as the electrophile in electrophilic substitution.
- in the electrophilic substitution a lone pair of lectron from the R-SO₃⁻ attacks the H on the benzene forming R-SO₂OH as the substituted group
([1] Diagram: https://bit.ly/32dXZqC )
([2] Diagram: https://bit.ly/3gaElDV )
