6.1.1: Benzene and its structure Flashcards
What is the molecular formula of benzene?
C6H6
What state is benzene at room temperature and what is it primarily used for?
- It is liquid at room temperature
- It is a key ingredient added to gasoline as it increases the efficiency of car engines.
What was Kelkule’s model of benzene?
He suggested that benzene was a six-membered ring with alternating single and double bonds between carbon atoms.
cyclo-1,3,5-hexatriene
What experimental evidence did he use to support his theory?
- He discovered that when one group was added to benzene, only one isomer was ever made.
- When two groups were added, there were always three structural isomers produced.
What are the three pieces of experimental evidence that do not support the kelkule model?
- Unlike alkenes, benzene is resistant to addition reactions.
- Enthalpy of hydrogenation of benzene shows that benzene is much more stable than was predicted.
- All six carbon bonds in benzene are the same length.
How did Kelkule try to justify benzene being resistant to addition reactions?
He said that the double and single bonds changed position in a very fast equilibrium so an approaching bromine molecule could not be attracted to the double bond before it changed.
What is hydrogenation?
The addition of hydrogen to an unsaturated chemical.
What would the expected enthalpy change for hydrogenation of benzene be using kelkule’s model?
What is the enthalpy change for hydrogenation of benzene?
- 360 KJ mol-1
- 208 KJ mol-1
(-152 KJ mol-1 more energetically stable than predicted)
What do x-ray diffraction techniques show about the length of the six carbon bonds in benzene?
That they are 0.140 nm, which is between a single and a double bond. Kelkule’s model suggests that there should be three shorter double bonds and three single bonds.
how did x-ray diffraction techniques disprove kelkule’s model of benzene?
It showed that all six bonds are the same length.
Why is the delocalised structure of benzene better?
Because it explains all three pieces of experimental evidence that do not support kelkule’s structure.
Describe the delocalised structure.
Each of the six carbon atoms donates one electron from its p-orbital. These electrons combine to form a ring of delocalised electrons above and below the plane of the molecule.
Why are the electrons in the ring said to be delocalised?
they are able to move freely within the ring and do not belong to a single atom. Therefore, unlike kelkule’s structure, all bonds in this ring are identical, so they are the same length.
Why does the delocalisation of electrons lead to benzene being about 152 KJ mol-1 more stable than expected when using kelkule’s model?
Because so much energy is needed to disrupt this delocalisation, benzene is very stable and resistant to addition reaction.
