6.1 aromatic compounds, carbonyls and acids Flashcards
(64 cards)
describe kekulé’s for benzene model using 3 points
- sigma bonded ring
- 3 alternating pi bonds
- highly unsaturated
name 3 problems with kekulé’s benzene model
- benzene is resistant to addition reaction and undergoes substitutions. suggests there aren’t double bonds
- enthalpy of hydrogenation - benzene has lower enthalpy than predicted, value for benzene is LESS EXOTHERMIC
- all 6 carbon bonds are the same length so all bonds must be uniform
how many pi electrons are there in the benzene ring?
6 pi electrons
describe the structure/bonding/delocalised system of benzene in 3 points
- each C is sigma bonded to a H, forming a planar hexagonal ring
- p orbitals overlap above and below the ring to make pi bonds
- 6 electrons in pi bonds are delocalised around the ring
which mechanism does benzene undergo?
electrophilic substitution
describe the 3 steps of electrophilic substitution of benzene
- electron from benzene RING is accepted by the electrophile
- electrophile is bonded to a carbon with a hydrogen and the C-H bond is attracted to the plus in the middle of the broken benzene ring
- H+ is lost and remains as a product along with the benzene compound
what is halogenation?
substituting a halogen for a hydrogen on a benzene ring
what condition are required for a halogenation reaction?
halogen carrier catalyst
- FeCl₃
- AlCl₃
give the overall equation for the halogenation between benzene and chlorine
C₆H₆ + Cl₂ –> C₆H₅Cl + HCl
halogenation: give two equations showing the formation of an electrophile and the regeneration of the catalyst
forming electrophile:
Cl₂ + FeCl₃ –> FeCl₄⁻ + Cl⁺
regenerating catalyst:
H⁺ + FeCl₄⁻ –> FeCl₃ + HCl
what is nitration?
substituting a hydrogen for a nitronium ion -NO₂⁺ on a benzene ring
which reagents and conditions are required for the nitration reaction?
reagent: HNO₃
catalyst: concentrated H₂SO₄
give the overall nitration reaction
C₆H₆ + HNO₃ –> C₆H₅NO₂ + H₂O
nitration: give two equations showing the formation of the electrophile and the regeneration of the catalyst
formation of electrophile:
HNO₃ + H₂SO₄ –> NO₂⁺ + H₂O + HSO₄⁻
catalyst regen:
HSO₄⁻ + H⁺ –> H₂SO₄
what is a phenol molecule?
a benzene ring with an -OH attached to a carbon
what are friedel-crafts reactions?
reactions in which hydrogens are substituted with R groups on benzene
what is alkylation?
substituting a hydrogen for an akyl group on a benzene
how does alkylation occur? give the overall equation of the substitution of a methyl group to benzene
occurs like a halogenation reaction with same catalysts but reagent is a haloalkane
C₆H₆ + CH₃Cl –> C₆H₅CH₃ + HCl
give the functional group of an acyl chloride
RC=OCl
how can an acyl chloride be used in friedel-crafts reactions?
as a halogen carrier - the chlorine on the acyl group and the hydrogen on the benzene are removed and form HCl
list phenol, benzene and cyclohexene in order of electron density, from most dense to least dense
most - alkene
phenol
least - benzene
explain how phenol has a greater electron density than benzene
the lone pairs and p-orbital on the oxygen in the -OH group add to the delocalised of benzene and make it more electron dense
how does increased electron density in benzene, phenol and alkenes increase reactivity?
more electron density means the electrophile is more readily polarised and so is more likely to react
which two electrophilic subsitution reactions with pheno are you supposed to learn?
with Br
with DILUTE nitric acid
