6.1.1 Benzene Flashcards
what is the general formula of benzene
C₆H₆
describe Kekulé’s model
Kekule’s model suggests that benzene consists of alternating double and single bonds
Why is kekule’s model no longer accepted
- lack of reactivity of benzene
- lengths of the C-C bonds
- hydrogenation enthalpies
explain how kekule’s model doesnt account for the lack of reactivity of benzene
- If benzene contained C=C, as kekule suggests, then it should decolourise bromine water which it does not do.
- This suggests benzene doesnt have any C=C bonds
explain how length of bonds in benzene disproves kekule’s model
- using X-ray diffraction its possible to measure bond lengths in a molecule.
- Kekule’s model suggests that the length would alternate - longer-shorter-longer-shorter
- however all the bonds have been found to be the same length
Explain why the hydrogenation enthalpy of benzene disproves kekule’s model
- The enthalpy change of hydrogenation of benzene is less exothermic than the one predicted by kekule
- this suggests the actual structure of benzene is more stable than kekules structure.
describe the delocalised model of benzene
If you draw the picture
- Sideways overlap of p-orbitals gives a ring of delocalised π-electrons above and below the plane of carbon atoms.
- There are sigma bonds between all the C-C and C-H
- All the bond angles are 120°
- benzene is planar
- all the C-C are the same length
Describe the model of Benzene without the aid of a diagram
- Benzene is a planar, cyclic, hexagonal hydrocarbon containing six carbon atoms and 6 hydrogen atoms
- each carbon atom uses three of its available four electrons in bonding to two other carbon atoms and to one hydrogen.
- Each carbon atom has one electron in a p-orbital at a right angle to the plane of the bonded carbon and hydrogen atoms
- Adjacent p-orbital electrons overlaps sideways in both directions, above and below the the plane of the carbon atoms to form a ring of electron density.
- This overlapping of the p-orbitals creates a system of π-bonds which spread over all 6 carbon atoms in the ring structure.
- The six electrons occupying this systrm of π-bonds are delocalised.
when is benzene the parent and when is it the substituent in naming aromatic compounds with one substituent group
- in monosubstituted aromatic compounds the benzene is often considered the parent chain to alkyl groups, halogens, and nitrogroups.
- However when a benzene ring is connected to an alkyl chain with a functional group or that is made up of 7 or more carbon atoms it is considered the substituent. In this case the prefix ‘phenyl’ is used.
name: benzene connected to a nitro group
nitrobenzene
name: benzene connected to a halogen (e.g. Cl) group
chlorobenzene
name: benzene connected to an alkyl group (e.g. C₂H₅)
ethylbenzene
name: a benzene connected to COCH₃
phenylethanone
name: a benzene connected to CH(CH₃)(CH₂)₅CH₃
2-phenyloctane
name: a benzene connected to NH₂
phenylamine
