Chemistry 6.1

Aromatic compounds, carbonyls and acids

What is an arene?

An aromatic compound that contains a benzene ring

What is benzene?

An arene consisting of a ring of 6 carbon atoms each bonded to one hydrogen atom, and has a ring of delocalised electrons

What is the molecular formula of benzene?

C6H6

How does benzene have a ring of delocalised electrons?

The outer electron from the p-orbital of each carbon atom is delocalised into the centre. This results in pi bonds forming

Is benzene stable and why?

Yes due to the delocalised ring structure

What is the kekulé structure?

• originally thought that benzene had this structure

• Cyclic hydrocarbon with 3 double bonds

What evidence was found against the kekulé structure?

• X-ray structure determination showed all c-c bond lengths were identical, where as a kekule structure would have shorter bond lengths for c=c

• Benzene didn't react with halogens but should've according to kekulé structure

• Enthalpy of hydrogenation of benzene should've been 3x that of cyclohexene but was found to be much less

What happens in benzene electrophilic substitution reactions?

• delocalised electron ring is an area of high electron density

• This means its susceptible to attack from electrophiles

• Electrophiles attack the electron ring, partially destroying it, before it is restored to form the aromatic product

What happens in benzene halogeration reactions?

• type of electrophilic substitution

• Catalyst ( e.g. Iron 3 bromide) required to generate the electrophile

• Iron 3 bromide acts as a halogen carrier

What happens in benzene nitration reactions?

• type of electrophilic substitution

• Electrophile is an NO2+ ion

• This is a reactive intermediate produced by the reaction of c.H2SO4 and c.HNO3

What happens in friedel-crafts reactions?

• delocalised electron ring acts as a nucleophile

• Nucleophilic attack on acyl chlorides