6.1.1 Aromatic compounds

What is the formula of benzene?

C6H6

What is the empirical formula of benzene?

CH

What is the state of benzene at room temperature?

Liquid

What is the bond angle of benzene?

120°

What are the 3 features of benzene that don't support Kekule's model?

Benzene is resistant to addition reactions; enthalpy change of hydrogenation is more stable than predicted; all carbon-carbon bonds are the same length.

What technique was used to find the bond lengths of benzene?

X-ray diffraction

What happens to the 4th electron in the p-orbital of each carbon atom in benzene?

It delocalises to form rings of electron density above and below the hexagon.

How do the rings of electron density affect the stability of benzene?

They make benzene very stable even though it is unsaturated, giving it aromatic stability.

What is the skeletal structure of cyclohexa-1,3,5-triene?

A six-membered carbon ring with alternating C=C bonds.

Why does benzene have a relatively high melting point?

Because flat hexagonal benzene molecules can pack closely together when solid.

Is benzene soluble in water? Why?

No, because benzene is non-polar.

Why is benzene not used in schools?

It is a carcinogen.

How do you name compounds containing a benzene ring?

Use -benzene or phenyl-; numbers can designate the positions of substituents if there is more than one.

Why is benzene attacked by electrophiles?

It has a high electron density above and below the ring due to delocalised electrons.

What type of reaction is the general reaction of benzene with an electrophile?

Electrophilic substitution.

What type of reaction is nitration of benzene?

Electrophilic substitution.

Which ion is used to nitrate benzene?

NO2+

What is the catalyst in the nitration of benzene?

Sulfuric acid, H2SO4.

How is the NO2+ ion generated?

Concentrated H2SO4 and concentrated HNO3 react to generate NO2+.

What is the overall equation for generating NO2+?

H2SO4 + HNO3 → HSO4− + NO2+ + H2O

How is the H2SO4 catalyst regenerated during nitration of benzene?

HSO4− + H+ → H2SO4

What is the overall equation for nitration of benzene?

C6H6 + HNO3 → C6H5NO2 + H2O

What type of catalyst is used for a Friedel-Crafts reaction?

A halogen carrier, e.g. AlCl3.

Why does benzene not react directly with halogens?

The aromatic ring is too stable.

What happens when AlCl4− is formed in terms of electrons?

The lone pair of electrons on chlorine forms a coordinate bond to aluminium.

How is the AlCl3 catalyst reformed?

AlCl4− + H+ → HCl + AlCl3

How can a methyl group be added to a benzene ring using a Friedel-Crafts mechanism?

Use a halogenoalkane and AlCl3 to create an electrophile that can attack benzene.

What is the electrophile used in the acylation of benzene?

RCO+

What is the structure of phenol?

A benzene ring bonded to an -OH group.

How can you demonstrate the weak acidity of phenol?

Phenol reacts with NaOH in a neutralisation reaction but does not react with carbonates.

What happens when phenol reacts with bromine?

It forms 2,4,6-tribromophenol.

How does phenol react with dilute nitric acid?

It undergoes electrophilic substitution to form nitrophenols.

Why is electrophilic substitution easier with phenol than with benzene?

The lone pair on oxygen in the -OH group is partially delocalised into the ring, increasing its electron density and attracting electrophiles.

What is the directing effect of the electron-donating groups OH and NH2?

They direct substitution to the 2 and 4 positions of the aromatic ring.

What is the directing effect of the electron-withdrawing group NO2?

NO2 directs substitution to the 3 position of the aromatic ring.