chapter 25: aromatic chemistry

Property of each carbon atom in a benzene

Forms three sigma bonds

Contributed their electron to a pi system

what are the two benzene model?

The kekule model and the delocalised model

What is the kekule model?

Alternating single and double bonds

What’s the delocalised model?

Six electrons are delocalised across the ring and each bond has the same length

What where evidence for the delocalised model?

All six carbon bonds in benzene have the same length and the entalphy change of hydrogenation releases less energy than expected

What time of reaction do benzene undergo?

Electrophilic substitution

What is an electrophile?

An electron pair acceptor

Why does benzene system attract electrophiles?

Because the benzene delocalised system has high electron density

What’s nitration?

Introduced a nitro group into the ring. NO₂

Conditions for nitration of benzene?

Concentrated nitric acid. Concentrated sulphuric acid and warm temp

Equation for the nitration of benzene

C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O

What’s a benzene that’s been nitrated called?

A nitrobenzene

How is the electrophile generated equation?

HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O

How is the electrophile generated?

Sulfuric acid reacts with nitric acid to produce nitronium ions

When does benzene react with an halogen?

In the presence of an halogen carrier

What’s an halogen carrier example

AlCl₃ or FeBr₃

How is halogen ion produced?

Cl₂ + AlCl₃ → Cl⁺ + AlCl₄⁻

Why alkene can react to bromination but benzene cannot

An alkene has a localised -bond with concentrated electron density between two carbon atoms, which attracts electrophiles readily.

What’s the friedel crafts alkynation?

Adding a carbon containing group onto a benzene ring and forming a carbon carbon bond

What conditions are needed for friedel crafts alkynation and acylation?

A catalyst AlCl₃

What’s friedel acylation?

When a benzene reacts with an acyl chloride

What kind of reaction is the friedel crafts?

Electrophilic substitution

Why is phenol a weak acid?

It can partially dissociate in water

What does phenol not react with?

Carbonates

Phenol reaction with sodium hydroxide, equation

C₆H₅OH + NaOH → C₆H₅ONa + H₂O

Why do phenol react more readily than benzene

The oxygen atom in the OH group has a lone pair of electrons that can be donated into the benzene ring through overlap with a p-orbital.

This increases the electron density of the ring, making it more susceptible to electrophilic attack.

Bromination of phenol

It reacts readily with bromine without an halogen carrier. The bromine water is delocalised and a white precipitate form

Equation for the reaction of phenol with bromine water

C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr

Where are the bromine in phenol substituted onto?

Carbon 2, 4 and 6

nitration of phenol

They react with dilute nitric acid without requiring concentrated sulphuric acid

In what position does the nitro group substitute on in phenol?

Carbon 2 and carbon 4

which groups are electron donating groups

OH and NH₂

Which groups are electron withdrawing groups

NO₂

Where does substitution occur on electron donating groups

2 and 4

Here does substitution occur in electron withdrawing groups?

3rd carbon

Why do groups on benzene direct substitution differently?

EWG withdraw electrons making the 3rd position more susceptible to attack. EDG donate electrons making the 2nd and 4th position more susceptible to attack

Example of naming with a nitro group

nitrobenzene