Arenes Flashcards
Benzenes physical properties
Description of state
Boiling temp
Solubility
Where is it found
- colourless liquid
- boiling temp = 80°C
- insoluble in water
Found in crude oil
Benzene molecular formula
C6H6
Is benzene saturated or unsaturated
Unsaturated
How was the empirical formula of CH of benzene found
Through combustion analysis
Uses of benzene
- organic synthesis
- manufacture of detergents, explosives
- was used for decaffeinated coffee - dissolves caffeine
Describe and draw Kekules structure
Six member ring, alternate single and double bonds
Check oneNote for drawing
List the problems with Kekules structure
- Bromine test and reactivity
- Isomers
- Bond length
- Enthalpy changes
Describe the issues with reactivity and the bromine test with kekules structure
If benzene had 3x C=C it should decolourise bromine, it does not suggesting no double bonds
Benzene is much less reactive than equivalent alkenes - doesn’t take part in electrophilic addition.
Describe the issues with isomers with kekules structure
If Kekules model is correct there should be 4 isomers however there are only 3 found to exist.
Suggests bonds in benzene are the same and not different
Describe the issues with bond length with kekules structure
What was used to discover this
C-C bond length = 0.153nm
C=C bond length = 0.134nm
In Benzene all bond lengths = 0.139nm
They are all the same (intermediate bonding) and this was determined using x-ray crystallography.
Describe the issues with enthalpy changes with kekules structure
Use numbers
The model predicts an enthalpy change of hydrogenation to the equivalent for 2x C=C: -360 KJ mol-1
Actual enthalpy change = -208 kj mol-1
Meaning the change is lower by 152kj mol-1
Describe the new benzene model
Draw it
After all sigma bonds have formed there is one electron in each p orbital.
One large pi bond forms from all 6 electrons, three above and three below the atom.
6 electrons form a delocalised pi bond.
Check drawing on oneNote
How does the new model agree with the bromine test and reactivity
There are no individual C=C bonds so less reactivity or addition reaction with bromine
How does the new model agree with isomers
3 not 4 isomers as there is now no difference in the arrangement of electrons between these atoms
How does the new model agree with bond length
The carbon-carbon bonds are now all the same length as they are not individual c-c or c=c
How does the new model agree with enthalpy
As charge is spread around the species there is an increased stability explaining why benzene has a greater stability.
2 electron clouds, delocalised electrons cause charge to spread around the species.
Why does a reaction with Br2 only let one Br bond to benzene
As it preserves the stability of the delocalised electrons in the pi bonds.
What does aromatic mean
And does it apply
Draw the new structure
A cyclical structure in chemistry
Yes
Check on OneNote
List the general reactions of benzene
Hydrogenation
Combustion
Equation for the hydrogenation of benzene
What is the product called
Draw the new structure
C6H6 + 3H2 —> C6H12
Cyclohexane
Check drawing on oneNote
Conditions for the hydrogenation of benzene
Benzene and hydrogen react under reflux with a nickel catalyst
Equation for the combustion of benzene
C6H6 + 7 1/2 O2 —> 6CO2 + 3H2O
Conditions and observations - why
Burning in oxygen
Burns with a smoky flame due to high C to H ratio
List the substitution reactions of benzene
Bromination
Nitration
Sulfonation
