Aromatic Compounds- Benzene

3 pieces of evidence to disprove kekules model

lack of reactivity, bond lengths, hydrogenation enthalpies

Lack of reactivity theory

If benzene had c=c bonds it would decolourise bromine in an Electrophilic addition reaction but actually benzene doesn’t tunderogo EA and doesn’t decolourise bromine water under normal conditions

Bond length theory

Used a x ray diffraction to measure bond lengths- found that all were 0.139nm in length= between the lengths of a single and double bond (0.153nm and 0.134 nm)

Hydrogenation enthalpy es theory

Kekules structure is expected to have enthalpy change of hydrogenation that is 3x that of cyclohexene= -360kj mol but actual enthalpy is -208 therefore is 153 less than expected meaning the actually structure is more stable

Replacement for kekules structure

Delocalised model of benzene

Delocalised model of benzene

Benzene is planar, cyclic, hexagonal hydrocarbon with 6 carbon and 6 hydrogen atoms

Bonding in delocalised model

Each carbon atom uses ¾ electrons in bndonign to 2 other carbon atoms and a hydrogen atom- last electron is in a p orbital above and below the plane of the carbon and hydrogen bond

How is the delocalised pi system formed

Adjacent p orbitals overlap sideways above and below the plane to create a ring of electron density, overallping creates a system of pi bonds which spread over the 6 carbons in the ring structure= delocalised pi system

Nitration of benzene word equation

Benzene + nitro acid → nitrobenzene

Conditions and reagents needed for nitration of benzene

Conc HNO3, conc H2SO4, 50deg c

What is nitrobenzene used for

Feedstock for dyes, pharmaceuticals + pesticides

What kind of reaction is nitration of benzene

Electrophilic substitution

Electrophilic in nitration of benzene

NO2+, nitronium ion

How is the electron pile produced

HNO3 + H2SO4 → NO2+ + HSO4- + H2O

How does nitration of benzene occur

NO2+ accepts pair of electrons from benzen ring to form dative covalent bond. Organic intermediate formed is unstable and breaks down to form nitrobenzene and a H+ ion, stable benzene ring is reformed

How is the H2SO4 catalyst regenerated

H+ ion reacts with HSO4- ion from step 1 to regenerate H2SO4

What is the catalyst needed for halogenation of benzene

Halogen carrier

Examples of halogen carriers

AlCl3, FeCl3, FeBr3

Reactants and conditions for bromination of benzene

Room temperature and pressure, halogen carrier

What kind of reaction is bromiantion of benzene

Electrophilic substitution

Equation for bromination of benzene

Benzene + Bromine → bromobenzene + HBr

What is the Electrophile used in bromination of benzene

Br+

How is Br+ generated

When the halogen carrier reacts with bromine in the first stage of the reaction

Step 1 bromiantion of benzene

Br2+ FeBr → FeBr4- + Br+

Step 2 bromiantion of benzene

bromonium ion accepts pair of electron form the benzen ring to form a dative covalent bond. Organic intermediate is unstable and breaks down to form bromobenze and a H+ ion

How is the FeBr3 catalyst regenerated

H+ + FeBr4- → FeBr3 + HBr

Chlorination of benzene

Sma e as bromine

Reactants and conditions for alkylation

Haloalkane, AlCl3

Reactants and conditions for acylation of benzene

AlCl3

What is formed during acylation reaciotyns

An aromatic ketone

Do alkenes decolourise bromine

Yes though Electrophilic addition reactions

Why doesn’t benzene react with bromine without a halogen carrier catalyst

The delocalised pi electrons spread above and below the plan of the bonds mean the electron density around ay 2 carbon atoms in the benzene ring is less than in a c=c bond in an alkene

Is benzene polar or non polar

Non polar

What is the significance of benzene being non polar

Has insufficient electron density to polarise a bromine molecule preventing a reaction from taking place