How was Kekules model of Benzene denounced
- X ray crystallography
- Benzene appeared perfectly hexagonal in its angles and intensitys
- this meant that there could not be 3 long and 3 short sides
Why is Benzene so attractive to potential electrophiles
- delocalised electrons form pi systsems above and below
- highly electron dense
Why does substituion ooccur rathern than addition in the reactions of Benzene
- energetically unfavourable to break the pi system permanetly in an addition reaction
- instead ring often undergoes substitution.
Why does the adition of halogenoalkanes to alkenes happen more readily than to Benzene
- The pi bond in alkene is localised opposed to delocalised
- the localised pi bond has a greater electron density and hence polarises the Br-Br bond more effectivley
What is the reaction and conditions for the substituion reactions of halogens
Room temperature, anhydrous, Alcl3 for chlorine and Febr3 for bromine
why is the substotution of Halides in Anhydrous conditions
to prevent the reaction of ALCl3 with water, therefore Cl2 and Br2 should not be delivered in aqueous solution
What is the mechanism for halogenation of Benzene
What are the conditions for Nitration
- Conc H2SO4 Conc HNO3, temperature below 50
- conc sulphuric acid acts as a catalyst
What is the first step of nitration
- generation of electrophile NO2+
What is the mechanism for nitration
What are the conditions for chlorine and bromine reacting with Methyl Benzene
the same
what is the directing nature of the methyl group in methyl benzene
- pushes groups to substitute at positions 2 and 4
What is Friedel crafts acylation
where acyl compounds are specifically substiuted onto aromatic ring
what are the conditions for Freidel crafts acylation
AlCL3, catalyst, reflux, anhydrous
Why in freidel crafts reactions with methyl benzene will acyl group only join at 4
steric hindrance at position 2 means there is not enough space on carbon 2 for substituion to occur
what is the effect of alkyl groups on activating the ring
- alkyl groups activate the ring
- alkyl groups donate electron density increasing stabillity of carbocation
What groups activate the Benzene ring
- OH, NH2, NHR, OCOR, alkyl
what is the effect of groups that deactivate the ring
- more likley to join at position 3
- make the ring less electron dense and less attractive to electrophiles
What is the exception to the effects of deactivating the ring
- although halides are withdrawing they still direct to positions 2 and 4
What groups deactivate the ring
halides, carboxylic acids, Nitro, Aldehyde
