The benzene problem
Near the start of this century, there was much debate about the
structure of a useful and ubiquitous compound called benzene. These
pages are designed to lead you to the structure using the information
available
1. The percentage composition by mass was determined
experimentally. Benzene was found to contain 92.31% carbon and
7.69% hydrogen. Calculate its empirical formula
2. Mass spectrometry showed the peak with the highest m/z ratio
(the parent peak) to be at 78. What is the formula of benzene?
3. Try and draw as many possible structures of benzene as you can.
Remember to count to 4 for carbon!
4. X-ray diffraction, where X-rays are scattered off the atoms,
showing where they are in space, gave the following information
about benzene
a. It is a planar molecule
b. It has a hexagonal shape
This led a chemist called Kekulé to give it a structure with double
bonds like below:
5. There was a problem with this structure, though. Complete the
table below to show the formula and how many moles of Br2 each
compound can react with. This is a measure of how unsaturated
they are. Think about the space for benzene but don’t write
anything yet
Compound name
Formula
Hexane
Moles of Br2 that can
be added due to
unsaturation
None – it’s saturated
Hexene
Hexyne
C6H10
Benzene
If you shake benzene with bromine water, it does not decolourise –
benzene does not easily undergo addition reactions. This meant that
it can not have a structure like the Kekulé structure.
6. Further X-ray diffraction showed the carbon to carbon bond
lengths to all be equal. This further ruled out Kekulé’s structure –
it would have required two different bond lengths.
The carbon to carbon bond length in benzene is 0.139nm. Use the
graph below to work out the ‘bond order’ (single, double, triple etc.)
in benzene. Plot the points, draw a smooth curve through them and
use it to find out what order 0.139nm corresponds to
Bond length (nm)
Bond order
0.154
0.134
0.120
0.139
1 (ethane)
2 (ethene)
3 (ethyne
?? (benzene)
Bond order in benzene  _____
7. Benzene was thus found to be a very stable, hexagonal molecule,
formula C6H6. A carbon atom in it forms three single covalent
bonds (two with its neighbouring carbons and one with a hydrogen)
and the spare p-electron is shared in delocalised clouds above and
below the ring. Draw this below showing the molecule (a) from
above, (b) from the side and (c) how it is represented in Chemistry
(a)
(b)
(c)