Arenes - Benzene

What are arenes (aromatic compounds)

Compounds with a delocalised π system

In terms of bond lengths, comment on the accuracy of the Kekulé structure of benzene.

Not accurate - all carbon-carbon bonds in benzene are same length, there are no longer discrete C—C and shorter C=C present.

How many π electrons are there in a benzene molecule

6 (one per carbon)

What is needed for the hydrogenation (reduction) of benzene

Nickel catalyst

What is the extra thermochemical stability associated with the delocalisation of the π electrons in benzene

152kJ/mol (360-208)

What happens when cyclohexene is added to bromine water

Decolourises

Name and outline the mechanism for the reaction between cyclohexane and bromine

Electrophilic addition

What happens when bromine water is added to benzene and how does this provide evidence for the delocalised model of benzene

• no reaction

• Delocalisation provides extra stability

• Electrophilic addition reaction would remove stable arrangement as it would involve the loss of a double bond

Why does benzene undergo electrophilic substitution, not addition

• it would involve the loss of delocalisation

• Retains stability

• Requires stronger electrophiles and more forcing conditions

What are the conditions for the nitration of benzene

• mixed with conc nitric acid and conc sulfuric acid

• Below 50˚C

Give the equation between conc nitric acid and conc sulphuric acid.

2H2SO4 + HNO3 → NO2+ + 2H2SO4- + H3O+

• NO2 + acts as electrophile

Draw the mechanism for the reaction between NO2 + and benzene

What does H2SO4 act as in the nitration of benzene and why

Catalyst, the H+ lost from the benzene reacts with the HSO4 - to regenerate H2SO4

Why does the temperature of the nitration of benzene need to be kept below 50˚C

Prevents the likelihood of multiple substitution of the benzene ring, potentially producing a hazardous, explosive product (TNT)

What is needed for the formation of aromatic amines from nitrobenzene and what type of reaction is it

• Tin and concentrated hydrochloric acid

• Reduction

What is a common use of aromatic amines

Used to make as azo-dyes, used to treat textiles, leather and some foods

What is the name of the reaction between acyl chlorides and benzene

Freidel-Crafts Acylation of Benzene

What is needed for freidel-crafts acylation and alkylation of benzene

anhydrous AlCl3 catalyst

Give the chemical equation for the reaction between an acyl chloride (RCOCl) and AlCl3

RCOCl + AlCl3 → AlCL4- + RCO+

Draw the mechanism between benzene and RCO+ (electrophile)

• H+ produced reacts with the AlCl4 - forming HCl and reforming AlCl3

What is the name for the reaction between chloroalkanes and benzene

Freidel-crafts alkylation of benzene

Give the equation for the reaction between a chloroalkane (RCH2Cl) and AlCl3

AlCl3 + RCH2Cl → AlCl4- + RCH2 + + AlCl4-

Draw the mechanism for the reaction between benzene and RCH2+

What is needed for the bromination of benzene (benzene + bromine)

AlBr3 (or iron) catalyst

Give the mechanism for the reaction between benzene and Br+

Give the chemical equation for the reaction between bromine and AlBr3

AlBr3 + Br2 → AlBr4- + Br+

• Br+ is used as an electrophile for the bromination as the delocalisation within the ring is too stable for just bromine water.

Name this molecule

Toluene

State and explain the reactivity of toluene compared to benzene

• more reactive

• Alkyl groups have positive inductive effect pushing electron density to ring

• Ring more electron dense, more attractive to electrophiles

State and explain the reactivity of phenylamine compared to benzene

• more reactive

• Lone pair on nitrogen delocalised into ring

• More electron dense, more attractive to electrophile

State and explain the reactivity of chlorobenzene compared to benzene

• less reactive (ignore lone pairs)

• Very electronegative, negative inductive effects, electron pulling

• Benzene ring less electron dense, less attractive to electrophiles

State and explain the reactivity of phenol compared to benzene

• more reactive

• Lone pair delocalises into ring

• Increased electron density, more attractive to electrophiles

How does phenol react with bromine (in the absence of a catalyst

Complete the mechanism