CD.2 Flashcards
Evidence for the delocalized model of benzene structure
What does the Kekule’s model suggest about the structure of benzene?
It is a six-membered ring consisting alternating double and single bonds.
What is the enthalpy of hydrogenation for cyclohexene, so saturating one double bond?
-120kJmol-1
What is the enthalpy of hydrogenation of benzene that would be predicted from Kekule’s model?
Around -360kJmol-1, so three times the enthalpy for saturating cyclohexene, because it’s three double bonds in benzene.
What would be predicted about benzene’s shape from Kekule’s model?
An irregular hexagonal planar, because double bonds and single bonds would have different bond lengths.
What would the bond angle of benzene be as predicted by Kekule’s model?
120
What would be predicted about benzene’s bond lengths from Kekule’s model?
Double bonds and single bonds have different lengths resulting in an irregular hexagonal shape.
What are three problems of the Kekule’s model for the structure of benzene that doesn’t match real life observations?
- The observed value for the enthalpy of hydrogenation of benzene is much lower than the prediction from the model.
- Benzene doesn’t react with bromine water like other unsaturated hydrocarbons.
- The observed shape of benzene have equal bond lengths.
What model is now being accepted as the correct structure of benzene?
The delocalised model.
Describe the bonding in the delocalised model of benzene?
Each carbon atom has four outer electrons, which can be used to form bonds. Three of these electrons are used to form single σ-bonds with carbons and hydrogen, leaving one electron on each carbon to spare. Instead of forming three separated double bonds, the six electrons delocalise and spread out evenly around the ring. These electrons form a delocalised charge cloud above and below the plane of the flat ring.
What would be predicted about benzene’s shape from the delocalised model?
A regular hexagonal planar, because all the bonds are single so they are all the same length?
Does the observation of benzene’s shape in reality match what would be predicted from the delocalised model?
Yes, it is true that all the bonds in benzene are the same length, which is around 0.140nm, shorter than C-C but longer than C=C.
What would the bond angle of benzene be as predicted by the delocalised model?
120
What would enthalpy of hydrogenation be as predicted by the delocalised model?
Much less negative than -360, benzene is quite stable due to the delocalisation.
Can the delocalised model explain benzene’s unreactivity with bromine water which Kekule’s model couldn’t explain?
Yes, because the structure doesn’t really contain double bonds so it doesn’t behave in the same way as alkenes. The delocalisation also mean the structure is very stable and therefore benzene doesn’t undergo addition reactions which would disrupt the delocalisation.
What is the general priniciple linking delocalisation in a molecule and its stability?
The more delocalised a structure is, the more stable the molecule becomes.
