topic 18A Flashcards
aliphatic chemicals
straight or branched chain organic substances
aromatic/arene chemicals
includes one or more rings of 6 carbon atoms with delocalised bonding
structure of benzene
- C6H6
- six C atoms in a hexagonal ring with one H atom bonded to each C
- bonded by single covalent sigma bonds
- one unused electron on each C atom in a p orbital
Kekule’s representation
- he suggested that the structure of benzene consisted of alternate single and double covalent bonds between the carbon atom
- this structure is incorrect
bonding in benzene
- 6 electrons in the pi bonds are delocalised
- they spread out over the whole ring
- benzene is planar molecule
- all the C-C bonds are the same
- bond energy between a C-C single and C=C double bond
bond angle in benzene
120 degrees
how can the stability of benzene be measured
- by comparing the enthalpy changes of hydrogenation
evidence for the delocalised model
- hydrogenation of cyclohexene = -120 kJ/mol
- you would assume because there are 3 double bonds in benzene, the ΔH = -360 kJ/mol
- the actual ΔH is less; -208 kJ/mol
- more energy is actually required to break the bonds in benzene
- this suggests benzene is more stable than cyclohexene
- this stability is due to the delocalised electrons
why is benzene resistant to bromination
- benzene is stable so doesn’t undergo electrophilic addition reactions
- benzene has high electron density hence undergoes electrophilic substitution
- delocalised ring of electrons is attracted to electrophiles
why does benzene have high electron density
- due to the delocalised ring of electrons
why does benzene prefer electrophilic substitution rather than addition
- the addition of an electrophile would disrupt the stability of the ring of electrons
reaction of benzene with oxygen (combustion)
- C6H6 + 7.5 O2 -> 6 CO2 + 3 H2O
- produces a smoky flame (soot)
halogenation of benzene
- change in functional group
- reagents
- conditions
- mechanism
- benzene to bromobenzene
- bromine
- iron bromide catalyst
- electrophilic substitution
equation for the formation of electrophiles
- AlCl3 + Cl2 -> AlCl4- + Cl+
- FeBr3 + Br2 -> FeBr4- + Br+
nitration of benzene
- change in functional group
- reagents
- mechanism
- electrophile
- benzene to nitrobenzene
- conc nitric acid in the presence of conc sulfuric acid
- electrophilic substitution
- NO2+
equation for the formation of electrophile
- HNO3 + 2H2SO4 -> NO2+ + 2HSO4- + H3O+
importance of nitration
- synthesise useful compounds
- done at 60 degrees
hydrogenation of benzene
- change in functional group
- reagents
- conditions
- mechanism
- benzene to cyclohexane
- hydrogen
- nickel catalyst at 200 C and 30 atm
- addition and reduction
friedel crafts alkylation
- change in functional group
- reagents
- conditions
- mechanism
- benzene to alkylbenzene
- chloroalkane in the presence of anhydrous aluminium chloride catalyst
- heat under reflux
- electrophilic substitution
formation of the electrophile
- AlCl3 + CH3CH2Cl -> CH3CH2+ + AlCl4-
Friedel crafts acylation
- change in functional group
- reagents
- conditions
- mechanism
- benzene to phenyl ketone
- acyl chloride in the presence of anydrous aluminium chloride catalyst
- heat under reflux (50C)
- electrophilic substitution
equation for the formation of electrophile
AlCl3 + CH3COCl -> CH3CO+ + AlCl4-`
phenols
OH group attached directly to benzene ring
are phenols weakly or strongly acidic
- very weakly acidic
phenol’s and carboxylic acids
- weaker acids than carboxylic acids
- both react with sodium metal and sodium hydroxide
- only carboxylic acids react with sodium carbonates
why can a phenol not react with sodium carbonate
it is not strong enough an acid to react
reaction of phenol with bromine
- reagent
- conditions
- bromine water
- room temperature
what is more reactive - phenol or benzene?
phenol is more reactive
why is phenol more reactive
- due to the electron density in the ring being higher
- electrons in the p-orbital of the oxygen overlap with the delocalised ring structure
- electron density increases within the ring structure hence it is more susceptible to attack from electrophiles
- Br2 is more polarised
bromination of phenol compared to benzene
- phenol does not need an FeBr3 catalyst like benzene
- undergoes multiple substitution whereas benzene will only add one Br
what are phenols used in the production of?
- plastics
- antiseptics
- disinfectants
- resins for paints
bromination of phenol reaction
phenol + 3Br2 –> 2,4,6-tribromophenol + 3HBr
phenol reaction with alkalis
phenol + NaOH –> sodium phenoxide + water
phenol + alkali -> salt + water
observation of bromination of phenol
brown to colourless
