Aromatic chemistry (benzene + phenol) Flashcards
What is benzene?
C6H6, is a colourless, sweet smelling, highly flammable liquid.
It is found naturally in crude oil, a component of petrol, and found in cigarette smoke.
It is a carcinogen.
What is the structure of benzene?
It is an aromatic hydrocarbon or arene.
It is a hexagonal ring of carbons.
The ring is planar with the hydrogens all in the same plane.
What is the Kekule model of benzene?
Suggested benzene was a ring of carbon atoms with alternating single and double bonds.
This was later adapted to say the benzene model is constantly flipping between two forms by switching over double and single bonds.
What is the evidence to disprove the Kekule model?
Reactivity, length of bonds, hydrogenation enthalpies.
How does the lack of reactivity of benzene disprove the Kekule model?
If benzene contained C=C bonds it should decolourise bromine in an electrophilic addition reaction.
But benzene does not undergo electrophilic addition reactions, and does not decolourise bromine under normal conditions, it requires a halogen carrier by electrophilic substitution.
How does the length of the carbon bonds in benzene disprove the Kekule model?
The X-ray diffraction data shows the bond length to all be 0.139nm, which is between the length of a single, 0.153nm, and a double bond, 0.134nm.
This is because the pi-bonds are delocalised.
How do hydrogenation enthalpies disprove the Kekule model?
Benzene would be expected to have an enthalpy change of hydrogenation 3x that of cyclohexene -120kjmol, as there would be x3 more double bonds.
But the actual enthalpy change is -208kjmol so the value is less exothermic than expected, and benzene is more stable.
What is the delocalised model of benzene?
Each carbon uses three of its available electrons in bonding to two other carbon atoms and to one hydrogen atom.
Each carbon has one electron in a p-orbital at right angles to the plane of the bonded carbon and hydrogens.
Adjacent p-orbitals overlap sideways in both directions above and below the plane of carbon atoms to form a ring of electron density.
This overlapping creates a system of pi-bonds spread over all 6 carbon atoms.
The electrons in the pi-bonds are delocalised.
What does the delocalised model of benzene look like?
Pg 434 textbook
Describe, in terms of orbital overlap, the similarities and differences between the bonding in the Kekulé model and the delocalised model of benzene.
Similarities: Sideways overlap of p orbitals.
π bond above and below bonding Carbon atoms.
Differences:
Kekule has: alternating π bonds and π electrons are localised and overlap in one direction.
Delocalised has: π ring and π electrons are spread around ring and overlap in both directions.
How are aromatic compounds named?
When the benzene is the main functional group the suffix is -benzene. (e.g. small alkyl groups, halogens and nitro groups). Substituent groups are listed alphabetically.
When it is not the functional group it is given the prefix phenyl-. (e.g. an alkyl group with more than 7 carbons).
What are the main exceptions to naming compounds?
Benzoic acid, phenylamine, benzaldehyde.
What is the nitration of benzene?
Benzene reacts slowly with concentrated nitric acid to form nitrobenzene by electrophilic substitution.
It is catalysed by concentrated sulfuric acid and heated to 50°C.
Water forms as a waste product.
If heated above 50°C dinitrobenzene may form.
What is nitrobenzene used for?
It is an important starting material in the preparation of dyes, pharmaceuticals and pesticides.
It can be used to make paracetamol.
What is the mechanism for nitration of benzene?
HNO3 + H2SO4 –> NO2+ + HSO4- + H2O
Arrow comes from the benzene ring to the NO2+
H+ +HSO4- –> H2SO4
