30.1 arenes

What is the mechanism for Arenes reacting with halogens

Draw the process

Electrophilic substitution

Describe the structure of a benzene molecule

• Cyclic, planar molecule

• Carbon forms 4 bonds

• Each carbon is bonded to 2 other carbons and 1 H atom

• Final lone electron is in a p-orbital which stick out above and below the planar ring

• The lone electrons in p-orbitals combine to form a delocalised ring

What reactions do arenes undergo

Electrophilic substitution

Why can arenes not undergo electrophilic addition reactions?

Benzene is very stable and electrophilic addition would disrupt the stable ring of electrons

Why is the carbon halogen bond in an arene stronger than the carbon halogen bond in a halogenoalkane

One of the lone pairs on the halogen atom overlaps slightly with the pi bonding system giving a partial double bond character

What would happen if chlorine gas was passed into boiling methylbenzene in the presence of UV light?

Free radical substitution occurs and hydrogens on the methyl chain are replaced by chlorine

Nitration of benzene

Electrophilic Substitution

Electrophile is NO2+

25-60C

Write the overall equation for forming the electrophile during nutrition of benzene

HNO3 + 2H2SO4 → NO2+ + 2HSO4- + H3O+

Why do ethene and benzene differ in their reaction with bromine?

The delocalised ring of electrons in benzene is stable so is reformed in the mechanisms

The electrons in ethene pi bond are localised so more available for reaction with electrophiles

Why is the carbon-hydrogen bond in halogenoalkanes stronger than in halogenoalkanes?

One of the lone pairs from each carbon atom overlaps slightly with the pi bonding system in the benzene ring

This gives the ch bond a partial double bond character