Electrophilic substitution

Which kinds of reactions does benzene undergo and why?

Electrophilic substitution

• Benzene has a region of high electron density above and below the plane of the carbon ring, which attracts nucleophiles

• The delocalisation makes the benzene very stable, and the reactions benzene undergoes aims to preserve this stability

  • Substitution means that they retain their delocalisation energy while addition reactions disrupt the delocalised system

When drawing mechanisms for benzene, what are the general steps?

• Overall equation

• Forming the electrophile

• Mechanism

• Reforming the catalyst

Give the general equation for all reactions of benzene

What is nitration?

When one of the hydrogen atoms on the benzene ring is replaced by a nitro, -NO₂, group

Give the conditions for nitration of benzene

• Concentrated nitric acid (nitrates ring)

• Concentrated sulfuric acid (catalyst)

• Around 50°C

  • If below 55°C - monosubstituion

  • Above 55°C - multisubstitution

Show the overall reaction of the nitration of benzene

Show the whole mechanism of the nitration of benzene

1. Electrophile (NO₂⁺) is produced by the reaction of concentrated nitric acid and concentrated sulfuric acid

2. Electrophile accepts a pair of electrons from the benzene ring to form a dative covalent bond

   • The organic intermediate formed is unstable and breaks down to form the organic product and a H⁺ ion - the benzene ring is now stable

3. H⁺ ion reacts with HSO₄^- ion to regenerate catalyst

What is required for the halogenation of benzene and why? Give some examples

A catalyst known as the halogen carrier

• Benzene will not react with a halogen alone as it is too stable

• E.g. AlBr₃ / FeBr₃ and AlCl₃/FeCl₃

Give the overall equation for the bromination of benzene

Give the overall mechanism of bromination of benzene

1. Electrophile (Br⁺) is generated when halogen carrier catalyst reacts with bromine

2. Bromonium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond

   • Organic intermediate is unstable and breaks down to form the organic product (bromobenzene) and a H⁺ ion

3. H⁺ reacts with FeBr₄^- ion to regenerate FeBr₃ (catalyst)

Give the overall equation for the chlorination of benzene

What is alkylation?

The substitution of a hydrogen atom in the benzene ring by an alkyl group (R) where benzene is reacted with a haloalkane

Give the reagents and conditions needed for alkylation

• Alkyl chloride (RCl)

• Halogen carrier catalyst (AlCl₃) which generates the electrophile

• Anhydrous conditions

How does alkylation increase the number of carbon atoms?

By forming carbon-carbon bonds

Give the equation for the alkylation of benzene with chloroethane

Give the mechanism for alkylation of benzene with CH₃Cl

What is acylation?

When benzene reacts with an acyl chloride to form an aromatic ketone

Give the conditions and reagents for acylation

• Acyl chlorde (RCOCl)

• Halogen carrier catalyst (AlCl₃)

• Anhydrous conditions

Give the overall equation for acylation of benzene

Give the mechanism for the acylation of benzene with C₂H₅COCl

What type of reaction is the reaction of alkenes with bromine? Give the reaction of cyclohexene and bromine

Electrophilic addition

Give the mechanism for the reaction of cyclohexene and bromine

1. π bond in alkene contains localised electrons above and below plane of the two carbon atoms to produce an area of high electron density

2. Dipole is induced in non-polar bromine molecule, to make Br₂ polar

3. Br₂ is now an electrophile

Explain why unlike alkenes, benzene does not react with bromine unless a halogen carrier is present

• Benzene has delocalised π electrons above and belwo the plane of carbon atoms in a ring structure while alkenes have π electrons localised above and below carbon atoms in a double bond

  • The electron denisty around any two carbon atoms in a benzene ring is less than that in a C=C double bond in alkenes

• Insufficient electron density around any two carbons to polarise Br₂, preventing any reaction from taking place

• Halogen carrier is needed to generate a more powerful electrophile (Br⁺)

Give the similaries between cyclohexene and benzene

• Six carbon ring

• Hydrocarbons

• Flammable

• Non-polar

Give the differences between cyclohexene and benzene