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
name: a benzene connected to COOH
benzoic acid
name a benzene connected to CHO
benzaldehyde
how do you name arenes with more than one substituent groups
The ring is now numbered starting with one of the substituent groups.
The substituent groups are listed in alphabetical order using the smallest numbers possible
what reaction does benzene undergo
electrophilic substitution
what is formed in the nitration of benzene when the temperature ≤ 50°
one of the hydrogen atoms on the benzene will be replaced by a nitro -NO₂ group.
what is formed in the nitration of benzene when the temperature is > 50°
at a temperature greater than 50° further substitution may occure leading to the production of dinitrobenzene
what is the electrophile in the nitration of benzene
- NO₂⁺
- this ion is formed from:
HNO₃ + H₂SO₄ → NO₂⁺ + H₂SO₄⁻ + H₂O
The H₂SO₄ catalyst is reformed at the end from the H⁺ off of benzene
What is required for the halogenation of benzene
halogens do not react with benzene unless a halogen carrier is present
a halogen carrier is a type of catalyst
list some halogen carriers
- FeCl₃
- AlCl₃
- AlBr₃
- FeBr₃
Why does a benzene require a halogen carrier to react with halogens
benzene is too stable to react with a non-polar bromine/chlorine molecule.
What is the first step of a halogen reacting with benzene
e.g. Bromine
formation of the electrophile
Br₂ + FeBr₃ → FeBr₄⁻ + Br⁺
compare the reactivity of alkanes with arenes
- alkenes react in addition reactions, benzene reacts in substitution reactions - though both are elecrophilic
- π-electrons in alkenes are localised to the C=C bond. So there is enough electron density to polarise the Br₂ and induce a dipole. HOWEVER the π-electrons in benzene are delocalised around the ring resulting in a lower electron density so a halogen carrier is requires to form Br⁺
