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
What are the 6 key point about the delocalised model of Benzene?
- Benzene is
-a planar, cyclic, hexagonal hydrocarbon containing 6 x C atoms and 6 x Hydrogen atoms
-Each C atom uses 3 e- to bond to 2 other carbons and a hydrogen atom out of the 4 e- it has
-Each C atom has 1 e- in a p orbital at right angles to the plane of the bonded C and H atoms
-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 overlapping 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 e- occupying this system of pi bonds are =delocalised
When naming an aromatic compound how do we count around the benzene ring
Count around the shortest part, e.g. the shortest part until you get to the next group
NO₂ groups are called what
Nitro-
e.g. 1,3-dinitrobenzene
The benzene ring, when attached to other compounds is called a what group
phenyl- group
If there is an ethene joined onto a benzene ring what is this called
Phenylethene
When a carboxylic acid is bonded to a benzene, what is this called?
benzoic acid
NOT
phenylmethanoic acid
What is it called when the -OH group is attached to a benzene?
phenol
What is it called when the NH₂ group is attached to a benzene ring?
Phenylamine
Rules of naming aromatic compounds (1)
If alkyl groups with:
-less than 7 carbons
-Nitro groups
-halogens
are attached directly to a benzene ring, they are the prefixes to benzene (put in alphabetical order, use di, tri and number)
