C6.1.1 Benzene and it's structure

What are the bond lengths for a 1) C-C bond 2) C=C bond

1) 0.147 nm 2) 0.135mn

What is benzenes : 1) molecular formula 2) empirical formula

1) C6H6 2) CH

Give three pieces of evidence that prove the structure of benzene that kekulé suggested was wrong.

1. Unlike alkenes , benzene is resistant to addition reactions 2. The Enthalpy of hydrogénation of benzene shows that benzene is much more stable than was predicted. 3. All six carbon bonds in benzene are the same length

What is an Arene?

A compound that contains a benzene ring

What is benzene?

A naturally occuring aromatic compound

What is a model?

A simplified version that allows us to make predictions and understand observations more easily

What are the bond lengths in benzene?

All the same All 0.140nm

How did the evidence of bond lengths disprove kekulé's theory?

As all bonds are the same length and his diagram suggested there were 3 shorter bonds and 3 longer bonds

How can we investigate the Enthalpy data for hydrogenation of benzene?

Calculate the Enthalpy change for hydrogenation of cyclohexane and cyclo-1,3,5 hexatrine which is the Kekulé model. And compare the Enthalpy change with cyclo-1,3,5hexatrine with 3x the Enthalpy Change of cyclohexane

Describe the delocalised structure of benzene

Each of the six carbon atoms donate one electron from its p orbital These electrons combine to form a ring of delocalised electrons above and below the plane of the molecule

What is benzene used for in industry?

Key ingredient for gasoline as it increases the efficiency of the car engine.

Does the délocalisation of electrons make benzene more or less stable?

More stable because so much energy is needed to disrupt delocalisation so benzene is stable and resistant to addition reaction.

Describe the structure of benzene simply

Stable planar ring structure with delocalised electrons

What evidence can explain all the evidence that disproves kekulé's theory?

That benzene has a delocalised electron structure

What did Kelulé discover about isomers of benzene?

That when one group was added to benzene only one isomer was ever made but when two groups were added there were always three structural isomers produced.

What is hydrogenation?

The addition of hydrogen to a unsaturated chemical

How did kekulé explain the resistance to reaction of benzene?

The double and single bonds change position in a very fast equilibrium

Why are electrons said to be delocalised in benzene, and how does this disprove kekulé's model?

The electrons are delocalised because they can move freely and do not belong to a single atom. Therefore unlike kekulé's structure all bonds in the ring are identical so are the same length.

How would we expect benzene to react and how does it actually react?

We would expect it to undergo similar reactions to alkenes like addition reactions however it will normally undergo substitution of a hydrogen atom

kekulé (1865)

proposed the commonly accepted structure for benzene carbon ring with six carbons and alternating single and double bonds between carbon atoms