Kekule Model
Six membered carbon ring, alternate single and double bonds
Delocalised Model
Planar six membered carbon ring, each carbon atom uses
... [Show More] 3 electrons for bonding to other atoms, each carbon has one electron in a p orbital at right angles to the plane, p orbitals can overlap above and below the plane to create a system of pi bonds, electrons making up the network are delocalised
Evidence to disprove the Kekule model
Bond length measured in x-ray crystallography found to be between that of single and double bonds, benzene doesn't do electrophilic addition reactions or decolourise bromine water, enthalpy change of hydrogenation should be thrice that of cyclohexene but it isn't
Bond length in benzene
0.139nm
Bond lengths that should be present in benzene
0.153nm for single bonds, 0.134nm for double bonds
Expected enthalpy change of hydration of benzene
-360 kJ mol^-1
Actual enthalpy change of hydration of benzene
-208 kJ mol^-1
How to nitrate a benzene molecule
React benzene with concentrated nitric acid in the presence of concentrated sulfuric acid at a high temperature
What happens if you react benzene with concentrated nitric acid in the presence of concentrated sulfuric acid?
Nitration
What happens if the temperature gets too high when you're nitrating benzene?
Multiple substitutions
Mechanism for the nitration of benzene
NO2+ generated, electrophile accepts pair of electrons from benzene ring to form a dative covalent bond, intermediate breaks down, forms organic product and ion, catalyst reforms
Equation for generation of electrophile in nitration
HNO3 + H2SO4 -> NO2+ + HSO4- +H2O
How to halogenate a benzene molecule
React benzene with a halogen in the presence of a halogen carrier at room temperature
Kind of reaction nitration and halogenation of a benzene molecule is
Electrophilic substitution
Equation for generation of the electrophile in halogenation of benzene
X2 + FeBr3 -> FeBr3X^- + X^+
How to alkylate a benzene ring
React benzene with a haloalkane or acyl chloride in the presence of a halogen carrier
Name for alkylation of a benzene ring
Friedel-Crafts Reaction
Why is alkylation important?
Increases the number of carbon atoms present by forming carbon-carbon bonds to an aromatic ring
Are alkenes or benzene more reactive?
Alkenes
Explanation for difference in reactivity between alkenes and benzene
The pi electrons in benzene are delocalised above and below the plane of the ring structure, localised in alkenes, electron density around 2 carbon atoms is less than that in the C=C bond, the electron density is insufficient to polarise a molecule, less able to attract electrophiles
What can phenol be classed as?
Weak acid
Why can phenol be classed as a weak acid?
Can partially dissociate to form a phenoxide ion and a proton
What will phenol react with as a weak acid?
Sodium hydroxide
Relative acidity of phenol compared with other molecules
Stronger than most acids, weaker than carboxylic acids
How to distinguish between phenols and carboxylic acids
React them with sodium carbonate
Will phenol react with sodium carbonate?
No
Reaction which shows that phenol is a weak acid
Sodium hydroxide
Reactions phenol can do
Bromination, nitration
Type of reaction the bromination and nitration of phenol is
Electrophilic substitution
What does bromination of phenol make?
2,4,6-tribromophenol, hydrogen bromide
Conditions for bromination of phenol
No halogen carrier, room temperature
Observations of bromination of phenol
White precipitate, decolourises bromine water
Products of nitration of phenol
2-nitrophenol, 4-nitrophenol, water
Conditions for nitration of phenol
Dilute nitric acid, room temperature
Which is more reactive: Phenol or benzene?
Phenol
Why is phenol more reactive than benzene?
Phenol activates the ring, lone pair of electrons in oxygen p-orbital is donated into the pi system, increases electron density of phenol, attracts electrophiles more strongly, ring is more susceptible to attack, benzene is more stable
2,4 directing groups
OH, NH2
3-directing groups
NO2
What type of groups are 2,4-directing groups?
Electron donating, activating
What type of groups are 3-directing groups?
Electron withdrawing, deactivating
Use of phenols
Antiseptics
Uses of nitrobenzene
Explosives, dyes, pharmaceuticals [Show Less]