Aromatic compounds Flashcards
(27 cards)
What is benzene?
A colourless, sweet smelling, highly flammable liquid.
What is the molecular formula of benzene?
C6H6
What is the kekule model of benzene?
A cyclic 6 carbon ring with alternating double and single bonds
What is the delocalised model of benzene?
- Benzene is a planar, cyclic, hexagonal hydrocarbon containing 6 carbons and 6 hydrogens
- Each carbon atoms uses 3 of its available 4 electrons in bonding to 2 other carbons and 1 hydrogen
- Each carbon has 1 electron in a p-orbital at right angles to the plane of the bonded carbon and hydrogen bonds
- Adjacent p-orbital electrons overlap sideways, in both directions above and below the plane of the carbon atoms to form a ring of electron density
- This over-lapping of the p-orbitals creates a system of pi bonds which spread over all 6 of the carbon atoms in the ring structure
- The 6 electrons occupying this system of pi bonds are said to be delocalised
What are the 3 pieces of evidence that disprove the kekule model of benzene?
1) The lack of reactivity of benzne
2) The lengths of the carbon-carbon bonds in benzene
3) The hydrogenation enthalpies
How does the lack of reactivity of benzene disprove the kekule model?
If it contained C=C bonds it should decolorise bromine in an electrophillic addition reaction but:
- benzene does not undergo electrophilic reactions
- benzene does not decolourise bromine under normal conditions
How does the lack of reactivity of benzene disprove the kekule model?
If it contained C=C bonds it should decolorise bromine in an electrophillic addition reaction but:
- benzene does not undergo electrophilic reactions
- benzene does not decolourise bromine under normal conditions
How do the lengths of carbon-carbon bonds in benzene disprove the kekule model?
Using x-ray diffraction it was found that all the bonds in benzene were 0.139nm. This bond length is between the length of a single bond (0.153nm) and the length of a double bond (0.134nm)
How do hydrogenation enthalpies disprove the kekule model?
If benzene did have the kekule structure it would be expected to have an enthalpy change of hydrogenation that is 3 times that of cyclohexene. The enthalpy change of hydrogenation for cyclohexene is -120 kJ mol^-1 ; this means the expected enthalpy change of hydrogenation for cyclohexene is -360 kJ mol^-1. However the actual value for benzene was found to be -208 kJ mol^-1. This means the actual structure of benzene is more stable than the kekule structure
Nitrobenzene
Benzoic acid
Phenyl amine
Benzaldehyde
Phenol
What is the prefix when benzene is considered to be the substituent?
Phenyl-
If there is an alkyl group and a halogen group on the benzene, what acts as the substituent?
The halogen
When is benzene considered to be the substituent?
- Attached to an alkyl chain with a function group
- Attached to an alkyl chain with 7 or more carbon atoms
H2SO4
50 degrees C
What happens if the temperature during the nitration of benzene rises?
Further substitution may occur - leading to dinitrobenzene
H2SO4
70 degrees C
What is the mechanism for the nitration of benzene at 50 degrees C?
What type of reaction is the nitration of benzene?
Electrophillic substitution
What must be present for halogens to react with benzene?
A catalyst called a halogen carrier
What are some common halogen carriers?
AlCl3, FeCl3, AlBr3 and FeBr3
