Benzene and its compounds

What is an arene?

An aromatic hydrocarbon so it contains a benzene ring

What is a halogenoarene?

An aromatic hydrocarbon whereby one or more hydrogen atoms bonded to the ring are replaced with by a halogen atom

What is a phenol?

A phenol group bonded to a hydroxyl group

C6H5OH

What is an acyl chloride?

A carboxylic acid derivative where the -OH group in -COOH has been replaced by a chlorine atom to form a -COCl

Describe the shape and bond angle of benzene

Planar

120 degrees

Describe the bonding in benzene

Each carbon uses two electrons to form sigma bonds with adjacent carbon atoms, one electron is used to form a sigma bond with a hydrogen atom

The remaining electron on each caron atom is delocalised

What kind of reaction does benzene undertake?

Electrophilic substitution

Define the term electrophilic substitution

The replacement of an atom by another atom or group of atoms after initial attack by an electron deficient species

What happens in the reaction of benzene with bromine?

Benzene will react with bromine in the presence of an anhydrous iron (III) bromide catalyst. The catalyst is made in the reaction vessel by adding iron filings to the benzene and bromine

What are the conditions and reagents needed in the reaction of bromine with benzene?

Conditions: Room temperature, presence of catalyst

Reagents: Bromine and benzene

What are the catalysts that can be used during the electrophilic substitution with bromine?

FeB3

AlBr3

Describe the how the electrophile ,FeBr4-, is created

• The electrophile is created when an iron (III) bromide molecule polarises bromine molecule.

• The Br2 molecule forms a dative bond with iron (III) Bromide by donating a lone pair of electrons from one bromine atom into an empty 3d orbital of the iron.

• This draws electrons from the other bromine atom in the Br2 molecule makin it partially positive, creating the electrophile

What are the products when chlorine gas is bubbled through benzene at room temperature in the presence of a catalyst?

The products are chlorobenzene and HCl

The catalyst is AlCl3

When we halogenate methylbenzene or other akylarenes, what are the positions for the substitutes in the benzene ring?

2 and 4

What happens when we react methylbenzene and chlorine gas using AlCl3 catalayst?

2 products are made, 2-chloromethylbenzene and 4-chloromethlybenzene

If excess chlorine is bubbled through, 1-methyl-2,4-dichlorobenzene, 1-methyl-2,6-dichlorobenzene and 1-methyl-2,4,6-trichlorobenzene can be formed

Why is the carbon-halogen bond in halogenoarenes stronger than the equivalent bond in a halogenoalkane?

One of the lone pairs overlaps with the pi bonding system of the benzene ring giving the carbon-halogen bond a partial double bond character. This makes it less reactive than the equivalent in halogenoalkanes

What happens during free radical substitution for methylbenzene?

Chloromethlybenzene is formed and the reaction has to take place in the presence of UV light and methylbenzene has to be boiling

Give the equation for the formation of the NO2+ ion

HNO3 + H2SO4 ———>HSO4- + NO2+ + H3O+

Give the equation for the nitrating mixture

Benzene + HNO3 ———> Nitrobenzene + H2O

What are the conditions for the nitration of benzene?

Heat under reflux and 25-60 degrees

With either conc HNO3 or conc H2SO4

What is nitration?

Nitration refers to the introduction of the No2+(nitronium) group to a molecule

Outline the process of nitration of benzene

• The electrophile NO2+, is attracted to the high electron density of the pi bonding in benzene

• A pair of electrons form the benzene ring is donated to the nitrogen in NO2+ and forms a new covalent bond

• There are now four pi bonding electrons and a positive charge on the five carbons

• The delocalised ring is restored in when C-H bond breaks heterolytically

• Both the C-H covalent bond go into nitrobenzene pi system and H+ leaves and the benzene is now stable

What does the further nitration of benzene produce?

1,3-dinitrobenzene and 1,3,5-trinitrobenzene and activation takes place in positions 3 and 5

Define the term Friedel-crafts reaction

The electrophilic substitution of an alkyl or acyl group into a benzene ring

They result in the introduction of a side-chain benzene ring

What do the mechanisms of Friedel-Crafts reactions involve?

They involve an attack on the benzene ring by an electrophile

The carbocation also attacks

How is the electrophile in acylation formed?

By adding an aluminium chloride catalyst to a halogenoalkane, to make alkylbenzene or an acyl group to make acylbenzene

This create the carbocation electrophile to attack the benzene in the first step of the mechanisms

State the steps in the alkylation of benzene and draw it

The electrophile is often formed by addition of an aluminium chloride catalyst to halogenoalkane

The carbocation electrophile then attacks the benzene ring

The aluminium chloride catalyst is regenerated

State the steps in the acylation of benzene and draw it

The electrophile is made and attacks the benzene ring

The aluminium chloride catalyst is regenerated

What are the conditions for the oxidation of the side-chains in arene and what’s the product?

Acidified KMnO4 in alkaline solution or NaOH and heated under reflux at 55 degrees

The product is benzanoic acid

What are the conditions for the hydrogenation of benzene and what’s the product?

Conditions: Nickel or platinum catalyst at 140 degrees

The product is cyclohexane

How can we make phenol?

By reacting phenylamine, C6H5NH2, with nitric (III) acid, HNO2, using ice to keep the temperature below 10 degrees. This gives an unstable diazonium salt which is warmed in aqueous solution to produce phenol

Write the equation for producing nitric acid from sodium nitrate and dilute hydrochloric acid?

NaNO2 + HCl ——> HNO2 + NaCl

Give the equation when reacting nitrous acid with phenylamine and draw it

Phenylamine + HNO2 + HCl ——> Benezenediazonium chloride + 2H2O

Why is ethanol a weaker acid than water?

Because it’s alkyl group that is electron donating, this increases the electron cloud density around oxygen so it pulls hydrogen more so the bond between the O and H is not easy to break making ethanol the weaker acid

What happens when reacting alkaline with phenol?

Draw it

Phenol is a weak acid so it will react to give the salt, phenoxide and water

What happens when reacting sodium with phenol?

Draw it

Phenol reacts vigorously with sodium metal giving off hydrogen gas and forming sodium phenoxide

What occurs when bromine water reacts with phenol?

Phenol decolourises the orange bromine solution and forms a white precipitate of 2,4,6-tribromophenol

What are the requirements for the nitration of benzene?

Reagents: Conc H2SO4 or HNO3

Conditions: Heat under reflux

If conc HNO3 is used we get 2,4,6-trinitrophenol and if it’s dilute HNO3 2-nitrophenol