Benzene and Phenol Flashcards
(20 cards)
Properties of Benzene? (10)
- Colourless
- Sweet smelling
- Highly flammable
- Liquid at room temp.
- Non-polar (requires non-polar solvent)
- Similar boiling point to hexane
- Higher melting point than hexane (can closely pack together)
- Flame is orange when burnt (carbon proportion)
- Found in crude oil
- Can cause cancer
How is benzene formed from a straight chain? (5)
- Uses platinum/aluminium oxide mix catalyst
- High temp. (500ºC)
- Low pressure (20atm)
- Cyclisation (make into cyclic) (releases H2)
- Aromatisation (releases 3 H2)
What was the Kekulé model?
Six carbon length ring with alternating double and single bonds
Explain the experimental evidence that lead to the development of the delocalised model from the Kekulé structure. (3)
- Bond lengths are all the same in benzene and are not alternating between double and single carbon bonds
- Doesn’t undergo addition reactions or decolorisation bromine water
- Enthalpy change of hydrogenation is LESS EXOTHERMIC than expected
How did bond length disprove the Kekulé model? (4)
- X-ray diffraction used to find bond length
- Should be unequal and have three C=C length and three C-C length
- Was equal all the way round
- Was an intermediate between the two lengths
How does lack of reactivity disprove the Kekulé model? (3)
- Doesn’t undergo electrophilic substitution reactions
- Does not decolourise bromine under standard conditions
- So cannot contain C=C bond
How does hydrogenation enthalpies disprove the Kekulé model? (4)
- When cyclohexene is reduced, releases 120kJ
- So theoretically Kekulé’s model would release 360kJ (3x120)
- Actually releases 208kJ
- 152kJ more stable (delocalisation energy)
What are the features of the delocalised model? (6)
- Each carbon uses 3 electrons to form sigma bonds with two other carbons and a hydrogen
- It donated an electron from its p-orbital to form pi bonds with the neighbouring carbons
- Bond lengths are the same
- One free electron is spread across the the whole molecule
- Two areas of high electron density (one above and one below the plane of the molecule)
- Delocalised electron density
How are mono-substituted benzenes named? (2)
Alkylbenzene
- Alkyl substituted benzenes with six or less carbons in the alkyl chain
n-phenylalkene
- Alkyl substituted with more than six carbons in the alkyl chain
How are di-substituted benzenes named?
Alphabetically
What are the three steps in the electrophilic substitutions of benzene?
- Mechanism
- Electrophilic substitution
- Regeneration of catalyst
Why do you need to use a catalyst during the electrophilic substitution of benzene?
The delocalised electrons aren’t concentrated enough to induce a dipole - not enough electron density
What catalysts are used for:
- Nitration
- Halogenation
- Alkylation
- Acylation
- conc. H2SO4 (sulfuric acid)
- Halogen carrier (e.g. FeX or AlX)
- AlCl3
- AlCl3
What is phenol?
Benzene with a hydroxyl (OH) group directly attached to it
Why is phenol not very soluble?
Due to a higher area of non-polar region
Why is phenol classified as a weak acid? (3)
When dissolved in water, phenol partially dissociates to release a H+
- More acidic than alcohols
- Less acidic than carboxylic acids
Reaction of phenol with sodium hydroxide. (3)
Forms C6H5ONa
- O- and Na+ charges on the atoms
- Neutralisation reaction
Bromination of phenol? (3)
- Decolourises bromine water
- Forms a white precipitate (2,4,6-tribromophenol)
- Halogen carrier not required
Nitration of phenol? (2)
- Dilute nitric acid
- Mixture of 2-nitrophenol and 4-nitrophenol
Why does phenol not need a halogen carrier like benzene?
- Lone pair of electrons from OH group is donated to the pi bond
- Higher electron density attracts electrophiles more
- More susceptible to attack than benzene
