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
If you’re asked to show the role of the catalyst in electrophilic substitution of benzene, you need to write…?
- An equation showing the generation of the electrophile
AlBr3 + Br2 -> AlBr4- + Br+ - An equation representing the regeneration of the catalyst
AlBr4- + H+ -> AlBr3 + HBr
What type of reaction is Friedel-Crafts acylation?
Electrophilic substitution reaction
An acyl group is substituted onto benzene.
What is the halogen carrier for Freidel-Crafts acylation/alkylation?
AlCl3
What is the equation for the generation of the electrophile in Friedel-Crafts acylation (using CH3COCl)?
CH3COCl + AlCl3 -> CH3CO+ + AlCl4-
Overall equation for reaction between ethanoyl chloride and benzene
C6H6 + CH3COCl -> C6H5COCH3 + HCl
What is Freidel-Crafts alkylation?
Electrophilic substitution. Alkyl group substituted onto benzene ring.
Haloalkane used.
What is the equation for the generation of the electrophile in Friedel-Crafts alkylation (using CH3CH2Cl)?
CH3CH2Cl + AlCl3 -> CH3CH2+ + AlCl4-
Name the reagents required for the nitration of benzene
Conc HNO3 & conc H2SO4
Equation for the nitration of benzene
C6H6 + HNO3 -> C6H5NO2 + H2O
What is the catalyst for the nitration of benzene reaction?
Conc H2SO4
What is the electrophile in the nitration of benzene?
NO2+
Equation for the reaction between nitric acid and sulfuric acid
HNO3 + H2SO4 -> NO2+ + HSO4- + H2O
Why does water need to be kept out of electrophilic substitution reactions of benzene?
The electrophiles are very reactive, so will react with water rather than benzene if it is present.
What happens to phenol in water?
Phenol partially dissociates as it is a weak acid.
How do you test for the phenol functional group?
- Measure pH - should be less than 7 as phenol is a weak acid.
- Add sodium carbonate - as phenol is a weak acid, it shouldn’t react with a weak base, so no reaction should occur.
Why does phenol react with NaOH?
It is a weak acid and NaOH is a strong base.
What does phenol react with to form an ester?
Acyl chlorides, acid anhydrides. Not carboxylic acids because they aren’t reactive enough.
Why does bromine react more readily with phenol than with benzene?
What is produced in the reaction between phenol and bromine?
2,4,6-tribromophenol
Halogen carrier/acid catalyst not needed.
What do we need for the nitration of phenol?
Dilute HNO3.
What products are formed when phenol is reacted with dilute HNO3?
2-nitrophenol & 4-nitrophenol
Explain why electrophiles react more readily with phenol than with benzene
- A lone pair of electrons on oxygen is delocalised into phenol’s pi system.
- This increases the electron density of the pi system.
- This results in increased attraction between phenol and the electrophile.
What are electron donating groups?
Substituents that donate electrons into a pi system e.g. OH, NH2
What are 2,4-directing groups?
OH and NH2.
Direct substitutents e.g. Br to 2 or 4 carbon
Why does nitrobenzene react less readily with electrophiles than benzene?
NO2 withdraws electrons from the pi system.
This reduces the elctron density of the pi system and reduces attraction between the electrophile and nitrobenzene.
What are electron withdrawing groups?
Substituents that withdraw electrons from a pi system e.g. NO2
What are 3-directing groups?
NO2.
Directs substituents e.g. Br to 3 carbon.
Why when reacting phenol with nitric acid do we produce twice as much 2-nitrophenol than 4-nitrophenol?
There are two carbon 2s (carbon 6 and carbon 2 are equivalent), whereas there is only 1 carbon 4.
