18:Organic Chemistry III
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What is benzene?
A cyclic, planar molecule with the formula C6H6
What are the physical properties of benzene?
1) Colourless liquid 2) Insoluble in water 3) Toxic and carcinogenic
What is Kekulé’s proposed benzene structure?
C6H6 with double bonds
What were the four problems with the Kekulé structure of benzene?
1) Benzene doesn’t decolorise bromine liquid/solution 2) There are only 3 isomers for dibromobenzene when there should be 4 (1,6 - isomer is not distinguishable from 1,2-isomer 3) X-ray studies revealed that benzene has a perfect hexagon structure (they all bonding is identical) 4) The enthalpy change of hydration is lower expected
Why is it a problem for the Kekulé structure that, benzene doesn’t decolorise bromine liquid/solution?
This disproves the presence of double bonds
Why is it a problem for the Kekulé structure that, there are only 3 isomers for dibromobenzene when there should be 4?
Suggests bonds between the carbon atoms in the ring are the same
Why is it a problem for the Kekulé structure that, x-ray studies revealed that benzene has a perfect hexagon structure?
X-ray studies showed that all internal bond angles were 120° and that all bonds were equal length of 0.139nm (which is inbetween the length of and single and double bond)
Why is it a problem for the Kekulé structure that, the enthalpy change of hydration is lower than expected?
It showed that benzene is more stable than predicted
What is the new model for benzene?
Cyclic hydrocarbon, perfect planar hexagonal molecule with 120° internal bond angle. - drawn as a hexagonal ring with a circle inside it
What is carbon bonding in the new model for benzene?
• 3 binds are sigma bonds • 4th electron in the 2p oribtal, orbits above and blow the plane (8 shape for p-orbitals) -> neighbouring p orbitals overlap, resulting in a greater electron density -> these rings spread over whole structure and are delocalised
How does the new model for benzene answer: benzene doesn’t decolorise bromine liquid/solution?
No individual C=C bonds edicts in benzene so there no addition of bromine
How does the new model for benzene answer: that there are only 3 isomers for dibromobenzene when there should be 4?
Bromine atoms are on adjacent carbon atoms, there is no different in the arrangement of electrons between atoms
How does the new model for benzene answer: x-ray studies revealed that benzene has a perfect hexagon structure?
All carbon bonds are the same length/strength -> no individual C-C single bonds or C=C double bonds can be found
How does the new model for benzene answer: that the enthalpy change of hydration is lower than expected?
Charge is spread around the benzene structure -> so extra stability is found
What forms when benzene undergoes a substitution reaction with Br2?
C6H5Br, not C6H6Br - one bromine atom replaces hydrogen
Why are substitution reactions more common with benzene than addition reactions?
• Substitution is the only reaction that preserves the stability of delocalised electrons in the pi bond, -> addition reaction will form a product with 2 C=C which removes the extra stability formed by delocalised electrons
What are aromatic compounds?
Compounds that contain a ring of continuously overlapping p-orbital with delocalised electrons (benzene)
What are the conditions required for hydrogenation of benzene?
Heating under pressure with a nickel catalyst
For the reactions with benzene, what is hydrogenation?
Mixing benzene with hydrogen (H2)
What is the equation for hydrogenation of benzene?
C6H6 + 3H2 -> C6H12 - basically goes from benzene to cyclohexane cause delocalised system is removed
What is observed in the combustion of benzene?
A smoke flame is produced, due to a high C:H ratio
For the reactions with benzene, what is combustion?
Burning of benzene in the air
What is the equation for combustion of benzene?
C6H6 + 71/2 O2 —> 6CO2 + 3H2O
Why do benzene (and their derivatives) not readily take part in typical reactions of alkenes?
Because of the stability of the delocalised ring structure