Organic Chemistry and Analysis

Question 1

Formula for benzene

Answer 1

C6H6

Question 2

what model of benzene is most frequently used and draw it

Answer 2

the delocalised model - has a circle in the middle of a hexagon

Question 3

what evidence is there for the delocalised model of benzene

Answer 3

1. Lack of reactivity
   - doesn’t decolourise bromine water
   - doesn’t undergo electrophilic addition
   - undergoes electrophilic substitution
2. length of carbon bonds - the lengths are all the same in benzene despite expectation that double bonds would be shorter
3. Hydrogenation enthalpies
   - enthalpy change of 1 double bond hydrogenation is ΔH=-120kJmol-1
   - would expect it to be -120x3= - 360 BUT it is less exothermic as it is actually 208kJmol-1

Question 4

what are the orbitals like in the delocalised model of benzene

Answer 4

• sideways overlap, delocalised electrons are above and below the plana

Question 5

compare the electron density of benzene to cyclohexene

Answer 5

electron density is lower

Question 6

when naming benzene compounds, what is the parent name when there is a monosubstituted ring

Answer 6

benezene
eg, methylbenzene

Question 7

what is the parent name for benzene compounds that have functional groups that have higher priority than the benzene

Answer 7

phenyl
NH2 - phenylamine
alkanes - phenylxane
ketones - phenylxone
alcohols - hydroxy -/ phenol

Question 8

what is benzene considered as when there is an alkyl group with 7+ C atoms/ functional group

Answer 8

benzene is the substituent

Question 9

what does the kekule model look like

Answer 9

a hexagon with 3 double bonds

Question 10

describe the lack of reactivity of benzene

Answer 10

doesn’t undergo electrophilic addition
doesn’t decolourise bromine under normal conditions