3.3.10 - Aromatic chemistry

What are aromatic compounds?

Aromatic compounds are the unsaturated compounds having one or more planar rings of atoms with alternate single and bonds.

What is the structure of benzene called? Describe the structure.

Benzene has a kekule structure- the 6 carbons are arranged in a hexagonal ring with alternating double and single bonds.

What does benzenes structure suggest?

This suggests that benzene should react in the same way an unsaturated alkaline does, however this is not the case.

Describe the electron structure of benzene

Each carbon atom in the ring forms three sigma bonds using the sp oribitals. The remaining p orbitals overlap, this sideways overlap of p orbitals results in the electrons being delocalised, and able to freely spread over the entire ring. This causes two rings above and below the benzene molecule of electron density.

What is the shape of benzene and other aromatic compounds?

Benzene and other aromatic compounds are regular and planar compounds with bond angles of 120.

What are the bond lengths for benzene? Why?

The delocalisation of electrons means that all the C-C bonds in these compounds are identical, so the bonds all being the same length is evidence for the delocalised ring structure of benzene.

What is another evidence for the delocalised structure of benzene?

the enthalpy change of hydrogenation of cyclohexene is -120 kj/mol. Each molecule has 1 C=C bond. As benzene has 3 C=C bonds, the expected enthalpy change is -360kj/mol however, the enthalpy change is actually -208 kj/mol.

What are arenes?

Arenes - aromatic compounds that contain a benzene ring as part of their structure.

What reactions do arenes usually take part in due to the delocalization of electrons? Why?

The delocalisation of electrons in arenes is the reason why arenes mainly undergo substitution reactions over addition reactions. In substitution reactions, the aromaticity is restored however in addition reactions the aromaticity is not restored and is completely lost in some cases.

How to name benzene compounds?

When a functional group is present, benzene is a side chain. When naming, name the side chain first then benzene eg, propyl benzene. Alphabetical order. When OH is present it becomes phenol.

What happens when benzene undergoes electrophilic substitution?

Benzene undergoes electrophillic substitution. The hydrogen atom is substituted by the electrophile.

What mechanisms do you need to know for benzene and electrophilic substitution?

nitration, sulfonation and Friedel crafts

What are the three steps of electrophillic substitution?

Generation of an electrophile, electrophilic attack and regenerating aromaticity.

Why does the nitration of benzene take place?

This is used to make TNT and amines.

What happens in the nitration of benzene?

a H atom is replaced by a NO2 group.

How is the NO2+ electrophile generated?

by reacting concentrated nitric acid and concentrated sulfuric acid.

H2SO4+HNO3 → NO2+ +HSO4 - + H2O.

Why does the sulfonation of benzene exist?

Important for the manufacture of surfactant and sulfonamides

What happens in the sulphonification of benzene?

Sulfonyl group (-SO3H) replaces a hydrogen atom on the arene.

How is the so3+ electrophile generated?

The SO3 electrophile is generated by H2SO4 → SO3 +H2O.

What conditions does this reaction take place in?

The benzene is warmed with fuming sulfuric acid (sulfur trioxide in conc sulfuric acid).

Is SO3 polar?

Due to the electronegativity of the O atoms, SO3 is polarised.

What happens in friedel-crafts acylation and why does it take place.

Friedel-Crafts reactions → A hydrogen is substituted by an acycl group to make the benzene more reactive for synthesis of other organic compounds.

What catalyst is needed during friedel-crafts acylation?

AlCl3 catalyst.

How is the electrophile generated?

the chlorine atom becomes and ion and leaves the molecule.

What happens to chlorine and benzene under UV light?

Free radicle substitution

What is the formula for the homolytic fission of Cl2

(Cl-Cl → 2Cl*)

What is the product formed during the free radicle substitution of cl* and benzene?

1,2,3,4,5,6 -hexachlorocyclohexane.

Describe the substitution of Cl* and methylbenzene?

The methyl group in methyl benzene will undergo a substitution reaction. Each separate substitution reaction forms a molecule of HCl.

What two reactions can alkyl benzenes have halogen substitutions on?

The alkyl group - cl group undergoes nucleophillic substitution when hetaed with alkaline solution with OH- ions.

The ring - only substituted under very extreme conditions due to high electron density on the benzene ring.