Arenes: Benzene and Phenol Flashcards
The Structure of Benzene
C6H6
Benzene has a ring structure with alternate double and single bonds
It has a ring of six carbon atoms joined by sigma bonds and a ring of six delocalised electrons above and below the plane of the carbon and hydrogen atoms
Evidence for the Structure of Benzene
Problems with Kekule’s theory
It has never been possible to isolate 2 isomers of any 1.2 - disubstituted benzene
X-ray diffraction measurements give all the same carbon-carbon bond lengths, shorter than single bonds and longer than double bonds - it is a perfectly regular hexagon
Benzene is much less reactive than alkenes and its characteristic reactions are substitution instead of addition
When benzene is hydrogenated, the measured enthalpy change is only -208 kJ mol-1, as opposed to the expected 360 kJ mol-1. This is because more energy is put in to break the bonds so the overall energy change is less negative.
Bonding in Benzene
Each carbon atom uses three of its electrons to form three σ bonds (covalent sigma bonds) with its three neighbours, leaving each carbon atom with one electron in a p orbital
The six spare p electrons are shared evenly between all six carbon atoms, resulting in clouds of negative charge above and below the ring of carbon atoms.
This is a delocalised π electron system. within which the electrons are free to move anywhere.
All bonds have the same length
Reactions of Benzene
Benzene undergoes Electrophilic addition as it has ring of delocalised electrons
An electrophile is substituted for the hydrogen
An electrophile is an electron deficient substance attracted to electron rich areas
When there are more electrons, it will be more reactive.
Resistance of Benzene to Bromination compared to Alkenes
Very easy for bromine to react with alkene as the π electrons induce a dipole in the bromine molecule and the ς+ bromine atom is a strong enough electrophile for the reaction to take place,
In Benzene, the greater stability of the benzene ring means that induced dipoles and the ς+ bromine atoms in Br2 are not electrophilic enough to react.
For bromine to react with benzene in an electrophilic substitution reaction, a catalyst must be used to produce a stronger electrophile.
Benzene and Bromine (in the presence of a catalyst)
Reagents: Bromine
Conditions: Iron (III) Bromide Catalyst
Electrophile: Brς+
Equation: C6H6 + Br2 –> C6H5Br + HBr
Iron reacts with Bromine to form iron (III) bromide
2Fe + 3Br2 –> 2FeBr3
Iron (III) Bromide polarises other bromine molecules to form a stronger electrophile
FeBr3 + Br2 –> FeBr4- + Br+
Nitration of Benzene
Reagents: Nitric acid + concentrated sulfuric acid
Conditions: heat to 50°C
Electrophile: NO2+
Equation: C6H6 + No2+ –> C6H5NO2 + H+
The nitryl cation electrophil is generated from a mixture of nitric acid and concentrated sulfuric acid
HNO3 + H2SO4 –> NO2+ + 2HSO4- + H3O+
Used in the manufacture of explosives
Sulphonation of Benzene
Reagents: Fuming sulfuric acid (SO3 in concentrated H2SO4)
Conditions: reflux
Electrophile: SO3
Equation: C6H6 + SO3 –> C6H6SO3
Benzene is less reactive than methoxybenzene so fuming sulfuric acid is required for the sulphonation of Benzene.
The product is used to make sulphonamide drugs and detergents
Friedel Crafts Alkylation Reaction
Reagents: Chloroalkane
Conditions: heat and AlCl3-
Electrophile: CH3ς+
Equation: C6H6 + CH3+ –> C6H5CH3
Important method for substituting and alkyl group for a hydrogen atom on a benzene ring.
The alkyl group comes from the appropriate halogenoalkane, which is refluzed with benzene in the presence of an aluminium chloride catalyst
The catalyst AlCl3 is used to increase the positive nature of the electrophile so it is more likely to attack the benzene ring.
C6H6 + CH3+ –> C6H5CH3 + H+
Used to manufacture ethylbenzene
Friedel Crafts Acylation Reaction
Reagents: acyl chloride
Conditions: heat and AlCl3- catalyst
Electrophile: CH3ς+
Equation: C6H6 + CH3COCl –> C6H5COCH3 + H+
Used to make polymers
AlCl3 + CH3COCl –> AlCl4- + CH3CO+
Electrophilic Substitution with Benzene
Benzene is highly attractive to electrophiles due to the delocalised electrons exposed above and bleow the plane of the rest of the molecule
Addition Reaction of Benzene and Hydrogen
Hydrogen in the presence of a nickel catalyst reacts with Benzene to form cyclohexane
Conditions: 200°C and 50 atm required to keep the benzene liquid
Takes 6 electrons from the inner ring to form covalent bonds with 6 hydrogens
Addition Reaction of Benzene with Chlorine
When Chlorine is bubbled into boiling benzene in the presence of UV light, 1,2,3,4,5,6,-hexachlorocyclohhexane is formed
The Structure of Phenol
Phenols have a benzene ring bonded directly to a hydroxyl group (-OH) which dramatically changes the way it reacts
Solubility of Phenol
Phenol is slightly soluble in cold water; the solubility increases as the water is warmed
The solution formed is weakly acidic (pH 6)
Phenol does not dissociate very easily
Melting Point of Phenol
Higher melting point than other hydrocarbons of similar Mr, and is soluble in water
When phenol loses a proton, a lone pair of electrons on the oxygen becomes delocalised so the negative charge is spread so it is more stable
More energy is required to overcome the intermolecular forces
Phenol as an Acid
When sodium metal is added to phenol, fizzing is observed as Hydrogen is produced
Sodium Phenoxide is formed
C6H5OH + Na –> C6H5O-Na+ + 1/2H2
When Phenol is added to sodium hydroxide, it is neutralised. Phenol is more soluble in NaOH than in water.
C6H5OH + Na –> C6H5O-Na+ + H2O
If concentrated hydrochloric acid is added, the sodium phenoxide is converted back to phenol (white solid at room temp)
Combustion of the Benzene Ring
Phenols burn with a smoky flame due to the high carbon: hydrogen ratio
Phenol and Bromine
When Bromine water is added to phenol compounds, it decolorises and a white precipitate is formed
