Chapter 25 - Aromatic Chemistry

Question 1

What are the two models for benzene?

Answer 1

1) kekulè model
2) delocalised model

Question 2

What is the kekulè model?

Answer 2

In 1865, Kekulè proposed that benzene was made up of a planar ring of carbon atoms with alternating single and double bonds between them. Each carbon atom was bonded to one hydrogen atom.

Question 3

What is benzene’s molecular formula

Answer 3

C6H6

Question 4

Why was Kekulè’s model disproved ?

Answer 4

1) the C=C bond should decolourise bromine water during an electrophillic addition reaction however benzene does not decolourise bromine water under normal conditions so it does not undergo electrophillic addition reactions, suggesting benzene does not have double bonds.

2) Bond lengths can be measured using x-ray diffraction. The bond length of a single bond is 0.153nm and for a double bond is 0.134nm however in benzene all bonds were the same length (0.139nm).

3) cyclohexene has a hydrogenation enthalpy of -120 kj/mol therefore you would expect benzene to be 3x this so -360 kj/mol however it was actually -208 kj/mol so it was less exothermic than expected. This suggests benzene was more stable than the kekulè model.

Question 5

What is the delocalised model of benzene?

Answer 5

1) The delocalised model says that the p-orbitals of all six carbon atoms overlap to create a pi system.

2) the Pi system is made up of two ring-shaped clouds of electrons, one above and below the plane of the six carbon atoms.

3) all the bonds in the ring are the same length because all the bonds are the same.

4) the electrons in the rings are said to be delocalised because they don’t belong to a specific carbon atom. They are represented as a circle inside the ring of carbons rather than double or single bonds.

Question 6

When is benzene the parent chain?

Answer 6

1) halogens attached eg chlorobenzene
2) alkyl group (hydrocarbon less than 7Cs)
3) Nitro groups - NO2

Question 7

When is benzene considered the substituting chain?

Answer 7

1) an alkyl chain with a functional group - 3- phenylpropanoic acid
2) an alkyl group with seven or more carbon atoms

In this case, the prefix phenyl- is used eg phenylamine

Question 8

What are the exceptions when naming aromatic compunds?

Answer 8

1) phenylamine
2) benzoic acid
3) benzaldehyde

Question 9

What type of reaction does benzene undergo?

Answer 9

Electrophillic substitution

Question 10

What is Electrophillic substitution?

Answer 10

This is when a hydrogen atom is being substituted by an electrophile.

Question 11

How does Electrophillic substitution work?

Answer 11

1) arrow from delocalised ring to electrophile

2) incomplete ring covering 4 carbons with + in the middle and electrophile and hydrogen attached

3) arrow from bond on hydrogen to incomplete ring.

4) final product without hydrogen attached.

Question 12

What are the conditions for the nitration of benzene?

Answer 12

1) Sulfuric acid catalyst
2) heat to 50 degrees Celsius (water bath)
3) concentrated nitric acid

Question 13

What is the mechanism for nitration of benzene?

Answer 13

1) HNO3 + H2SO4 -> NO2+, + HSO4 - + H20

2) NO2 + is electrophile and reacts with benzene in regular Electrophillic substitution reaction.

3) H+ + HSO4- -> H2SO4 - catalyst reformed

Question 14

What are the conditions for the halogenation of Benzene?

Answer 14

1) halogen carrier catalyst eg AlCl3, FeCl3, AlBr3 etc
2) room temperature and pressure

Question 15

Mechanism for the bromination of benzene?

Answer 15

1) Br2 + FeBr3 -> FeBr4 - + Br+

2) the electrophile Br+ undergoes regular Electrophillic substitution with benzene.

3) H+ + FeBr4- -> FeBr3 + HBr

Question 16

What is alkylation of benzene?

Answer 16

H atom replaced with an alkyl group, which increases the number of c-c atoms in a compound.

It is sometimes called friedel-crafts alkylation

Question 17

What are the conditions for alkylation of benzene?

Answer 17

1) reacts with haloalkane
2) AlCl3 acts as halogen carrier catalyst

Question 18

Mechanism for alkylation of benzene to form methylbenzene?

Answer 18

1) AlCl3 + CH3Cl -> CH3+ + AlCl4-

2) CH3+ undergoes regular Electrophillic substitution reaction with benzene.

3) H+ + AlCl4- -> HCl + AlCl3

Question 19

What are the conditions for Acylation of Benzene?

Answer 19

Reacts benzene with an acyl chloride in the presence of a halogen carrier catalysts (AlCl3) to form an aromatic ketone.

Acyl groups contain a C=O bond

Question 20

Mechanism for acylation of benzene to form phenylethanone?

Answer 20

1) CH3COCl + AlCl3 -> CH3CO+ + AlCl4-

2) CH3CO+ undergoes regular Electrophillic substitution with benzene .

3) H+ + AlCl4- -> HCL + AlCl3

Question 21

What is phenol?

Answer 21

An -OH bonded directly to benzene.
Molecular formula is C6H5OH

Question 22

Why is phenol less soluble in water than alcohols?

Answer 22

Due to the presence of the non-polar benzene ring

Question 23

What type of acid is phenol?

Answer 23

A weak acid

Question 24

What does phenol partially dissociate in water to form?

Answer 24

A phenoxide ion (O- bonded to benzene) and H+

Question 25

Phenol reacts with NaOH to form?

Answer 25

A salt (sodium phenoxide - O-Na+ bonded to benzene) and water

Question 26

What is the conditions for bromination of phenol?

Answer 26

Room temperature
No halogen carrier catalyst

Question 27

Equation for the bromination of phenol?

Answer 27

Phenol + 3Bromine →2,4,6-tribromophenol + 3hydrogen bromide

Question 28

What are the observations for the reaction of bromine water with phenol?

Answer 28

1) decolourise bromine water
2) white precipitate forms from tribromophenol

Question 29

Conditions for the nitration of phenol?

Answer 29

1) dilute nitric acid
2) room temperature
3) no catalyst required

Question 30

Why does phenol react more readily than benzene?

Answer 30

1) the lone pair of electrons from the oxygen p-orbital of the -OH group is donated into the delocalised Pi-system of phenol.

2) the electron density of benzene ring in phenol is increased

3) therefore the ring is more likely to attract and polarise elctrophiles than benzene.

Question 31

What is an electron donating functional group?

Answer 31

1) -OH or -NH2 functional group attached

2) substitution occurs more readily than benzene.

Question 32

Why do electron donating functional groups allow the molecule to react more readily?

Answer 32

1) functional group will donate electrons into the delocalised ring in benzene

2) this increases the electron density in the ring, allowing the molecule to react more readily with electrophile

Question 33

Where do electron donating groups direct substitution?

Answer 33

-2 and -4 positions

Question 34

What is an electron withdrawing functional group?

Answer 34

1) -NO2 group attached - nitrobenzene

2) substitution occurs less easily than benzene. It requires extreme conditions eg halogen carrier and high temperatures

Question 35

Why do electron withdrawing functional groups result in the molecule reacting less readily?

Answer 35

1) the functional group will withdraw electrons from the delocalised ring in benzene

2) this decreases the electron density in the ring resulting the molecule reacting less readily with electrophiles

Question 36

Where do electron withdrawing groups direct substitution?

Answer 36

At the 3- position

Question 37

What are an exception to the electron withdrawing/donating groups?

Answer 37

Halogens are Not electron donating groups however still direct substitution to the 2- and 4- position