25 Aromatic Compounds Flashcards
Whats the molecular formula of benzene?
C6H6
What is benzene classed as?
Aromatic hydrocarbon or arene
What was Kekule’s model/theory?
He suggested that the structure of benzene was based on a six membered ring of carbon atoms joined by alternate single and double bonds
What evidence was there to disapprove Kekule’s model?
1) Lack of reactivity of benzene
- If benzene contained C=C bonds it should decolourise bromine in an electrophilic addition reaction.
- But benzene doesn’t undergo electrophilic addition reactions or decolourise bromine
- This led scientists to suggest benzene doesnt have any C=C bonds
2) The length of carbon-carbon bonds in benzene
- When benzene was examined using a technique called X-ray diffraction the bond lengths were measured.
- It was found that all bonds in benzene were 0.139nm in length, which is between the length of a single bond and length of a double bond.
3) Hydrogenation enthalpies
- If benzene did have the Kekule structure, then it would be expected to have an enthalpy change of hydrogenation that is 3 times of cyclohexene.
- The enthalpy change of hydrogenation for cyclohexene is -120kJ
- So we would expect the Kekule structure to have an enthalpy change of -360kJ
- However, the actual enthalpy change is-208kJ
- The actual structure is benzene is more stable than Kekules model
What is the delocalised model of benzene?
- Benzene is a planar, cyclic, hexagonal hydrocarbon containing six carbon atoms and six hydrogen atoms.
- Each carbon atom uses three of its available four electrons in bonding to two other carbon atoms and to one hydrogen atom.
- Each carbon atom has one electron in a p-orbital at right angles to the plane of the bonded carbon and hydrogen atoms.
- Adjacent p-orbitals overlap sideways, in both directions, above and below the plane of the carbon atoms to form a ring of electron density.
- The six electrons occupying this system of pi-bonds are said to be delocalised.
What groups are considered to be the prefixes for benzene?
Alkyl groups, halogens, nitro groups
E.g ethylbenzene, chlorobenzene
When is benzene considered to be substituent?
When the benzene ring is attached to an alkyl chain with a functional group, or to an alkyl chain with seven or more carbon atoms
What prefix is used when benzene is substituent?
Phenyl
Noticeable exceptions to remember
Benzoic acid, phenylamine, benzaldehyde
What catalyses the reaction between benzene and nitric acid? (Nitrobenzene)
It is catalysed by sulfuric acid H2SO4 and heated to 50 degrees
Why is the nitration of benzene done at 50 degrees?
If the temperature of the reaction rises above 50 degrees, further substitution reactions may occur leading to the production of dinitrobenzene.
What is temperature of the reaction between benzene and excess nitric acid (dinitrobenzene) ?
70 degrees & H2SO4
Halogens do not react with benzene unless…
a halogen carrier is present
Some examples of halogen carriers
AlCl3, FeCl3, AlBr3, FeBr3
What is the halogen carrier for bromination of benzene?
FeBr3 or AlBr3
