19 aromatics 1

compare kekulé and the delocalised model of benzene

kekulé:

• alternating C-C and C=C bonds

• three pi bonds

delocalised:

• p orbitals overlap sideways

• forms a delocalised pi system

• pi bonds are spread over all 6 carbon atoms

• all C-C bond lengths are the same

why is benzene more stable than in kekulé’s model predicts?

• benzene contains a delocalised pi system

• pi bonds spreads all over 6 carbon atoms

• delocalised electrons stabilise benzene

what are the three evidence that the delocalised model is correct than kekule?

1) substitution takes place NOT addition eg decolourises Br2

2) all C-C bond lengths are equal

3) enthalpy change of hydrogenation of benzene was less exothermic than predicted

how is substituted aromatic compounds names?

Use Benzene as the parent name

Examples:

• methylbenzene

• chlorobenzene

• nitrobenzene

Number positions if more than one substituent:

• 1,2-dimethylbenzene

• make sure the long chain is the main chain eg. 1,2 -dimethylethylbenzene

what are the reagents and conditions for nitration of benzene?

reagents:

• conc HNO3

• conc H2SO4

conditions:

• 50°c

product: nitrobenzene

reagents and conditions for halogenation of benzene?

reagents:

• Br2 / Cl2

catalyst:

• FeBr3, FeCl3, or AlCl3

product: halogenobenzene

reagents and conditions for alkylation?

reagents:

• haloalkane

catalyst

• AlCl3

product: akylbenzene

forms C-C bond

reagents and condition for acylation

reagents: acyl chloride

catalyst: AlCl3

product: aromatic ketone

what mechanism type does aromatics do?

electrophilic substitution

why does alkenes undergo addition reaction more easily than benzene

Alkenes

• π bond electrons are localised

• high electron density

• easily attacked by electrophiles

Benzene

• electron density delocalised across ring

• benzene is more stable

• less reactive towards electrophiles