Benzene & Phenol Flashcards
Why does carbon form 4 bonds?
It promotes an electron from a 2s orbital to a 2p orbital.
Leaves it with 4 unpaired electrons, which can all form covalent bonds.
Problem 1 with Kekule’s model
Bromine does not turn brown -> colourless
Problem 2 with Kekule’s model
Carbon-carbon bond lengths are identical
Problem 3 with Kekule’s model
Enthalpy of hydrogenation is not -360kJ mol-1 as it should have been - it is -208kJ mol-1.
How many sigma bonds and pi bonds would carbon form in Kekule’s benzene model?
Each carbon would form 3 sigma bonds and 1 pi bond.
Why does benzene not react with bromine?
Benzene has delocalised electrons, so has a lower electron density than alkenes.
It cannot induce a strong enough dipole in bromine.
Why does benzene have a less exothermic enthalpy of hydrogenation than expected?
The enthalpy of hydrogenation value for benzene is less exothermic than expected because benzene’s delocalised electrons increase its stability .
Why does benzene have identical carbon-carbon bond lengths?
Benzene has identical carbon-carbon bond lengths because each p orbital overlaps both of its neighbouring p orbitals by the same amount above and below the ring.
Explain the bonding in benzene
Each carbon atom forms three covalent bonds.
The remaining p orbitals overlap above and below the ring to form a pi system.
The electrons in these p orbitals are delocalised.
Explain the shape of benzene
Benzene is planar.
The bond angles in benzene are 120°.
The C-C bonds are equal in length.
Why does benzene have a pi system?
Instead of a p orbital on each carbon overlapping with just one other neighbouring p orbital, forming a double bond, a p orbital from each carbon overlaps with two neighbouring p-orbitals. The electrons are then delocalised across the ring.
When benzene is the highest priority functional group, what do we call it?
-benzene
When benzene is not the highest priority functional group, what do we call it?
phenyl-
How do you create a Br+ ion for electrophilic substitution?
Use a halogen carrier e.g. AlBr3.
AlBr3 is electron deficient, and so reacts with Br2 to form AlBr4- and Br+. Br+ is a strong electrophile.
Br2 + AlBr3 -> Br+ + AlBr4-
How does Br+ replace a hydrogen atom on the benzene ring?
- 2 electrons from the pi system are donated to bromine.
- This means a C-Br bond is formed, and the pi system breaks, so we form an unstable intermediate.
- C-H bond breaks as H donates both electrons to pi system, causing H+ to form and pi system to be reformed.
Why is the formation of bromobenzene an electrophilic substitution reaction?
H is substituted for an electrophile.
Why is AlBr3 a catalyst in the formation of bromobenzene?
AlBr3 is used to generate the electrophile and is reformed at the end of the reaction.
Write an equation to show how the halogen carrier catalyst is regenerated e.g. AlBr3.
AlBr4- + H+ -> AlBr3 + HBr
