Chapter 26

Question 1

What was the Kekule model?

Answer 1

Structure of benzene was based on a 6 member Ed ring of carbon atoms joined by alternate single and double bonds

Question 2

Evidence which deprived Kekule’s model

Answer 2

Lack of reactivity:
Benzene does not undergo electrophilic addition reactions
Benzene does not decolourise bromine under normal conditions
This means it can’t contain any carbon to carbon double bonds
Length of bonds:
Benzene bonds’ length is different to single and double bonds
Hydrogenation enthalpies:
The hydrogenation of benzene should produce 360 kJ mol^-1 due to there being 3 double bonds according to Kekule’s model
It actually produces 208 kJ mol^-1

Question 3

Delocalised model of benzene

Answer 3

Benzene is a planar, cyclic, hexagonal hydrocarbon
Each carbon atom uses 3 of it’s available electrons in bonding to 2 other carbon atoms and one hydrogen atom
Each carbon atom has one electron in a p orbital at right angles to the plane of the bonded carbon and hydrogen atoms
Adjacent p orbitals overlap sideways in both direction above and below the plane of carbon a toms to form a ring of electron density
The overlapping of p orbitals creates a system of pi bonds which spread all over the 6 carbon atoms
The 6 electrons occupying this system of pi bonds are delocalised

Question 4

What type of substitution does benzene react by?

Answer 4

Electrophysiology substitution