Evidence for Structure of Arenes
Chemistry: Cambridge International A Level, topic 3.2.11
Benzene Experimental Evidence (row)
The delocalised picture of benzene’s bonding is supported by a variety of experimental evidence.
X-ray diffraction
- X-ray diffraction allows you to measure the distances between atoms.
- This experiment confirms that the bond length between every carbon is the same, rather than the alternating long and short bonds expected from Kekulé’s structure.
Hydrogenation enthalpies
- The delocalised model of benzene implies that the six overlapping p-orbitals provides more stability than three separate pi bonds.
- We can test this by hydrogenating benzene.
- When cyclohexene is hydrogenated, the enthalpy change is -120kJmol-1
- This corresponds to the enthalpy change of hydrogenating one pi bond.
- Cyclohexane is more stable than cyclohexene because it is saturated.
Hydrogenating benzene
- The enthalpy change for hydrogenating benzene is -207kJmol-1
- This is smaller by 153kJmol-1 than the value you’d expect for three isolated pi bonds, -360kJmol-1
- This implies that there is a significant extra stability to benzene.
- This is due to the delocalisation.
Evidence for the Structure of Benzene
Many pieces of evidence have been used to prove the structure of benzene.
Structure of benzene
- For years, chemists argued over what the best structure was to represent benzene.
Bond lengths
- All carbon-carbon bonds have been shown to be the same length.
- This length (0.139 nm) is somewhere between the C-C and C=C bond lengths.
Bond angles
- The structure has been proved to have a bond angle of 120o at each carbon.
- This means the structure must be planar.
Enthalpy of hydrogenation
- The enthalpy of hydrogenation for benzene is more positive than for cyclohexa-1,3,5-triene.
- Hydrogenation is a form of addition reaction.
- So this reaction breaks the aromaticity in the benzene ring, which is why it is unfavourable.
- This reflects how benzene is more stable due to aromaticity.
Test yourself on Evidence for Structure of Arenes
What does x-ray diffraction allow you to measure?
What is the enthalpy change when benzene is hydrogenated?
Which experiments support the delocalised picture of benzene's bonding?
What are the three pieces of evidence for the structure of benzene?