Benzene

Question 1

Evidence against the Kekule structure

Answer 1

1. benzene doesn’t decolourise Br2 at room temp= benzene doesn’t contain C=C bonds, less reactive than expected
2. Bond lengths from X-ray diffraction analysis; All C to C Bond lengths are the same, the C to C bond length is between that of the C-C single bond and C=C double bond= benzene does not contain isolated, alternating C-C or C=C bonds.
3. The enthalpy change for hydrogenation is less exothermic than the enthalpy change for hydrogenation predicted for the triene = benzene does not contain 3 C=C bonds, benzene is more stable than expected.

Question 2

The delocalised structure of benzene

Answer 2

A cyclic molecule- closed ring of 6 C atoms
The shape around each C atom is trigonal planar with a H-C-C bond angle of 120 degrees
There are sigma bonds between the C atoms and between the C-H atoms, which are formed by single overlap of orbitals directly between the C atoms
There is one pi system above and below the plane of the C ring, which is formed by the side ways double overlap of p orbitals on each C atom of the C ring
The pi system contains 6 delocalised electrons, which are mobilised across all 6 C atoms of the benzene ring.

Question 3

Similarities between cyclohexene and benzene

2

Answer 3

The pi bond is formed by the the double sideways overlap of a p orbital on each C atom involved in the Pi bond

The pi bond is above and below the plane of the C atoms involved in the pi bonding

Question 4

differences between benzene and cyclohexene

Answer 4

In benzene, the pi system is made up of 6 electrons whereas in cyclohexene the pi bond is only made up of 2 electrons

In benzene the pi system electrons are delocalised across all 6 C atoms, wheras in cyclohexene, the pi bond electrons are localised across 2C atoms

Question 5

Explain why cyclohexene is more reactive towards electrophiles, such as bromine, than benzene?

Answer 5

The pi bond in an Alkene is more electron dense than the pi systems in benzene
Because the 2pi electrons in an Alkene are localised between 2 C atoms, whereas the 6pi electrons are delocalised across all 6 C atoms in benzene
The pi bond in the Alkene is sufficiently electron dense to induce a dipole in the Br2. Molecule and so attracts electrophiles more strongly

Question 6

Electrophilic substitution reactions

Answer 6

A H atom on the benzene ring is replaced by an electrophile. The electrophile is attracted to the electron rich pi system, where it accepts a pair o electrons from the pi system to form a new covalent bond with a c atom of the ring.

Question 7

The intermediate and organic product have different structures
Deduce how many electrons are involved in the pi bonding in each structure and describe how their arrangements are different

Answer 7

Carbocations intermediate has 4 electrons in pi system
Delocalised about 5 C atoms of the ring
The product contains 6 electrons in the pi system
Delocalised about all 6 C atoms of the benzene ring

Question 8

Nitration of benzene

Answer 8

Reagents: conc nitric acid]
Conditions: conc sulfuric acid, heat under reflux at 50 degrees