Aromatic Compounds Flashcards
What is benzene and what family does it belong to?
the simplest aromatic hydrocarbon or arene
Molecular formula for benzene
C₆H₆
Where does benzene come from and what are some properties?
-found naturally in crude oil, is a component of petrol, and also found in cigarette smoke
-a colourless, sweet smelling, highly flammable liquid
-classified as a carcinogen (can cause cancer)
A benzene ________ consists of __________ _____ of _ C atoms joined together, with a _________ attached to each C atom
A benzene molecule consists of hexagonal rings of 6 C atoms joined together, with a hydrogen attached to each C atom
What is the empirical formula of Benzene
CH
Describe the structure of Benzene
- A ring structure where the spare P- electrons are delocalised around all 6 carbon atoms
- remaining electron from each Carbon atom (6 p-orbitals in total-each containing one e- ) overlap
- Delocalised pi -bond above and below the plane of atoms, containing a cloud of 6 ‘‘delocalised’’ electrons
(rather than pairs of e- localised in double bonds e.g. as they are in alkenes)
Explain all the bonds of one carbon atom in the benzene ring
-each C has 4 outer e-
1 is used to bond to a hydrogen
2 are used to bond to adjacent carbons
The remaining electron from each carbon (6 in all) form a cloud of “delocalised” electrons above and below the plane of the ring
What is delocalisation energy
Delocalisation gives rise to extra stability called “delocalisation energy”
What is Kekules Model?
- structure of benzene was based on a 6 membered ring of carbon atoms joined by alternate single and double bonds
-he said he had thought of this following a dream involving ouroboros- a mythical serpent seizing its own tail
Why did Scientists disprove kekules model?
model of benzene structure didn’t explain all chemical+ physical properties of benzene
What were the 3 main criticisms of Kekules model?
- Enthalpy of Hydrogenation
- Reactivity
- C-C bond lengths
-did not explain the structure and hence its chemical and physical properties
Explain the following criticism of Kekules model- Reactivity
-lack of reactivity of benzene
-if benzene contained C=C bonds, it should decolourise bromine in an electrophilic addition reaction
However:
-benzene doesn’t undergo E.A. reactions
- doesn’t decolourise bromine under normal conditions
- will only react with halogens with if presence of a catalyst, even then it undergoes Electrophilic Substitution reaction
-has led scientists to suggest it cannot have C=C bonds in its structure
Explain the following criticism of Kekules model- C-C bond lengths
-Benzene has conjugate double + single bonds
-with 2 different bond lengths (and enthalpies) around the ring…
-BUT
-X-ray analysis has shown that in a benzene ring there is only one bond length
How does Pauling make sense of Kekules second criticism?
Described the structure of Benzene as:
a ring structure where the spare p-electrons are delocalised around all six carbon atoms
(Rather than pairs of electrons localised in double bonds, as they are in an alkene.)
Explain the following criticism of Kekules model- Enthalpy of Hydrogenation
Kekules structure predicted to contain 3 x C=C bonds, so expected enthalpy change for reacting 3 x C=C bonds with H2 would be much higher than actual enthalpy change of hydrogenation
-the actual structure of benzene therefore is more stable than the theoretical Kekule model of benzene
