Chem Chapt 13: Arenes

Why does benzene not undergo addition reaction but rather substitution reactions instead

They have a delocalised pi electron system that makes it resonance stabilised and addition reactions will destroy this stability, which is energetically unfavourable. Hence, it will undergo substitution to preserve aromaticity

Outline why benzenes are susceptible to electrophilic substitution

The presence of 6 pi electrons in benzene makes it electron rich and hence susceptible to electrophilic attack, and also to preserve the aromaticity

Outline the role of anhydrous AlCl3 in Friedel-Crafts reactions

They act as a catalyst that speeds up rate of reaction where AlCl4- is formed as an intermediate and AlCl3 is eventually regenerated. It also acts as a Lewis acid to abstract Cl- to form strong electrophiles which are necessary to attack the resonance-stabilised benzene ring

Outline why the attack of -NO2+ on benzene to give an arenium ion is the rate determining step

This is because the aromaticity is lost in this step and the delocalisation stabilises the arenium complex, but it is still much less stable than aromaticity

Outline why arenium ions are less stable than benzene

This is because aromaticity is temporarily lose and delocalisation in arenium ions is limited to only 5 carbons, the ring also carries a localised positive charge whereas benzene is resonance stabilised

Outline why during nitration of methylbenzene, there is a higher chance of forming substituents ar the 2-/4-directing positions

The electron donating methyl group increases electron density at the 2-/4- positions, while that at the 3-/5- positions are the lowest

Outline 2-/4- directing activators and deactivators

2-/4- activators are alkyl, aryl, OH and NH2 groups whereas deactivators are halogen substituents

Outline the effect of activating substituents on electrophilic substitution

Activating groups increase the rate of electrophilic substitution as they increase electron density of the benzene ring, increasing the availability of the pi electron cloud and hence stabilising the arenium ion to a greater extent by dispersing the positive charge

Outline the effect of deactivating substituents on electrophilic substitution

Deactivating substituents decrease the rate of electrophilic substitution as they decrease the electron density of the benzene ring, and hence decreases the availability of the pi electron ring and destabilises the arenium ion by intensifying the positive charge

Outline why nitration of methylbenzene requires a lower temperature than that for benzene

The methyl group is electron donating and will increase the electron density of the benzene ring, making it more susceptible to electrophilic attack

Outline the 4-directing position is preferred over the 2-directing one

The attack at 2-position has greater steric hindrance and hence, the longer the substituent, the less likely it will form at the 2-position

Outline why side chain reaction always forms -COOH regardless of the length of the carbon chain

This is because the benzylic carbon that is directly attached to the ring is activated by resonance stabilisation of the cation intermediate, resulting in the alkyl chain forever bring fully oxidised to -COOH

Outline the test to distinguish between benzene and methylbenzene

KMnO4 in dilute H2SO4 and heat. Benzene will not decolourise purple KMnO4 whereas methylbenzene will result in a white ppt and decolorisation of KMno4

Outline the distinguishing test for benzene and cyclohexane

Add Br2 with anhydrous FeBr3, in the dark. For benzene, reddish brown Br2 is decolourised and white fumes of HBr is evolved whereas no colour change is observed for cyclohexane

Outline the distinguishing test for benzene and cyclohexene

Add Br2 in CCl4, orange-red Br2 will not be decolourised for benzene but cyclohexene will decolourise orange-red Br2