C25 - Aromatic Compounds

Question 1

What’s benzene?

Answer 1

Benzene, C6H6 is a sweet smelling, highly flammable liquid found in crude oil and classified as a carcinogen.

Question 2

What 3 pieces of evidence disprove Kekulé’s model of benzene?

Answer 2

1) Benzene’s lack of reactivity. If benzene contains C=C bonds, it would undergo electrophilic addition and decolourise bromine water, however it does not.
2) All the bonds in benzene are the same length. It’s a perfect hexagon. However, Kekulé’s model would suggest that some bonds (C=C) are shorter than others (C-C) due to bind strength.
3) A single hydrogenation reaction would release -120kJ/mol energy therefore, for benzene, it would be expected that -360kJ/mol energy is released (3 double bonds). However it only releases -208kJ/mol (it is 152kJ/mol more stable).

Question 3

What are the features of the delocalised model of benzene?

Answer 3

Benzene is a planar, cyclic, hexagonal hydrocarbon containing 6 C and H atoms.

Each C uses 3 out of its 4 electrons in bonding to two other C atoms and to one H atom.
Each C atom has one electron in a p orbital at right angles to the plane of the bonded carbon and hydrogen atoms.

Adjacent p orbitals overlap sideways in both directions, above and below the plane of the carbon atoms to form a ring of electron density.

The overlapping of the p orbitals creates a system of pi bonds spread over all 6 carbon atoms in the ring.

The 6 electrons occupying this system of pi bonds are delocalised.

Question 4

What’s the benzene delocalised model?

Answer 4

Benzene has an extended pi bind region above and below the carbon planar structure due to the overlap of 1 electron from each of the 6 C atoms.

These electrons are delocalised across the entire ring.

Question 5

How are aromatic compounds named?

Answer 5

The benzene ring is considered to be the parent chain and any alkyl groups are considered to be prefixes to benzene. E.g. ethylbenzene or nitrobenzene.

However, when attached to an alkyl chain with a functional group or or an alkyl chain with 7+ carbon atoms, benzene is considered the substituent and the prefix, ‘phenyl’ is used.
E.g. phenylethanone or 2-phenyloctane.

Question 6

What occurs during the nitration of benzene?

Answer 6

Electrophilic substitution.
It reacts slowly with nitric acid, at 50°C catalysed by H2SO4.

HNO3 and H2SO4 react to form NO2+ as the electrophile.

The electrophile accepts an electron pair from the benzene ring (curly arrow from ring to + charge on electrophile).

This forms a dative covalent bond. (H and electrophile extending from benzene. Horse shoe electron density. Curly arrow from H bond to + charge at centre of benzene)

This is unstable and breaks down to form nitrobenzene (with a stable benzene ring) and H+ (which is used to regenerate the catalyst).

Question 7

What is necessary for the halogenation of benzene?

Answer 7

A halogen carrier catalyst.

Common halogen carriers include AlCl3 and FeCl3 (for chlorination) and AlBr3 and FeBr3 (for bromination).

Question 8

How does H2SO4 act as a catalyst in the nitration of benzene?

Answer 8

H2SO4 + HNO3 -> NO2+ + HSO4- + H2O

(NO2+ acts as the electrophile. After substitution, nitrobenzene and H+ are produced).

H+ + HSO4- -> H2SO4

Sulphuric acid is regenerated.

Question 9

What occurs during the bromination of benzene?

Answer 9

Electrophilic substitution.
A halogen carrier catalyst e.g. AlBr3 or FeBr3 is needed.

AlBr3 + Br2 -> Br+ + AlBr4-
(Br+ is the electrophile)

The electrophile accepts an electron pair from the benzene ring (curly arrow from ring to + charge on electrophile).

This forms a dative covalent bond. (H and electrophile extending from benzene. Horse shoe electron density. Curly arrow from H bond to + charge at centre of benzene)

This is unstable and breaks down to form bromobenzene (with a stable benzene ring) and H+ (which is used to regenerate the catalyst).

Question 10

What occurs during the chlorination of benzene?

Answer 10

Electrophilic substitution.
A halogen carrier catalyst e.g. AlCl3 or FeCl3 is needed.

AlCl3 + Cl2 -> Cl+ + AlCl4-
(Cl+ is the electrophile)

The electrophile accepts an electron pair from the benzene ring (curly arrow from ring to + charge on electrophile).

This forms a dative covalent bond. (H and electrophile extending from benzene. Horse shoe electron density. Curly arrow from H bond to + charge at centre of benzene)

This is unstable and breaks down to form chlorobenzene (with a stable benzene ring) and H+ (which is used to regenerate the catalyst).

Question 11

What is alkylation?

Answer 11

When a H atom is substituted for an alkyl group by reacting benzene with a haloalkane in the presence of a halogen carrier catalyst.

Question 12

What occurs during the alkylation of benzene?

Answer 12

Electrophilic substitution.
A halogen carrier catalyst e.g. AlCl3 is needed.

AlCl3 + CH3Cl-> CH3+ + AlCl4-
(CH3+ is the electrophile)

The electrophile accepts an electron pair from the benzene ring (curly arrow from ring to + charge on electrophile).

This forms a dative covalent bond. (H and electrophile extending from benzene. Horse shoe electron density. Curly arrow from H bond to + charge at centre of benzene)

This is unstable and breaks down to form methylbenzene (with a stable benzene ring) and H+ (which is used to regenerate the catalyst).

Question 13

What is acylation?

Answer 13

When benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst, to form an aromatic ketone.

Question 14

What occurs during the acylation of benzene?

Answer 14

Electrophilic substitution.
The halogen carrier catalyst AlCl3 is needed.

AlCl3 + CH3COCl-> CH3CO+ + AlCl4-
(CH3CO+ is the electrophile)

The electrophile accepts an electron pair from the benzene ring (curly arrow from ring to + charge on electrophile).

This forms a dative covalent bond. (H and electrophile extending from benzene. Horse shoe electron density. Curly arrow from H bond to + charge at centre of benzene)

This is unstable and breaks down to form phenylethanone (with a stable benzene ring) and H+ (which is used to regenerate the catalyst).

Question 15

How does cyclohexene react with bromine water?

Answer 15

By electrophilic addition.
The pi bond in the alkane has localised electrons above and below the carbon plane, producing a region of electron density.

The localised e- induce a dipole in the bromine molecule, making one bromine atom slightly positive.

The slight positive charge enables it to act as an electrophile.

Question 16

What are phenols?

Answer 16

Organic compounds with a hydroxyl group bonded directly to an aromatic ring.

Question 17

How acidic are phenols?

Answer 17

They’re more acidic than alcohols but not as acidic as carboxylic acids.

Alcohols don’t react with strong (NaOH) or weak (Na2CO3) bases

Phenol reacts with strong but not weak bases.

Carboxylic acids would react with strong and weak bases.

Question 18

What can be used to distinguish between a phenol and carboxylic acid?

Answer 18

Sodium carbonate.

Carboxylic acid will react with weak base, producing CO2 gas.
Phenols won’t react with weak base.

Question 19

What happens when phenol reacts with NaOH?

Answer 19

It reacts to form the salt, ‘sodium phenoxide’ and water (neutralisation reaction).

The H on the -OH is lost and replaced with Na+.

Question 20

How do phenols undergo electrophilic substitution?

Answer 20

Under milder conditions than benzene / catalyst not needed.

This is because the electron pair from the -OH contributes to the delocalised ring and increases charge density.

Question 21

What happens in the bromination of phenol?

Answer 21

It reacts with 3 Br2 to form white precipitate, ‘2,4,6-tribromophenol’.

This decolourises bromine water.
(Halogen carrier and high temp not required).

Question 22

What’s the product of the bromination of phenol and how can it be identified?

Answer 22

2,4,6-tribromophenol (3HBr)

It’s a white precipitate.

Question 23

What happens in the nitration of phenol?

Answer 23

It readily reacts with dilute nitric acid at room temp.

It forms a mixture of 2-nitrophenol and 4-nitrophenol (and water).

Question 24

Why is phenol more reactive than benzene?

Answer 24

The electron pair from the oxygen p orbital of the -OH is donated to the delocalised pi ring and increases electron density.

This attracts electrophiles more strongly.

Question 25

What’s the directing effect?

Answer 25

When groups bonded to a benzene ring dictate and direct which carbon atom the next group will bond to.

Question 26

What do activating groups do (to benzene)?

Answer 26

Donate electrons to the central ring, allowing it to react more readily with electrophiles.

Examples include -OH, -NH2 etc.

They direct additional groups to carbon 2 and 4.

Question 27

What do deactivating groups do (to benzene)?

Answer 27

They withdraw electrons from the central ring, making it react less readily with electrophiles.

Examples include -COOR, CHO, -NO2 etc.

They direct additional groups to carbon 3.