Benzene - Organic Chem III

Question 1

What is aromatic chemistry?

Answer 1

When a molecule contains a benzene ring

Question 2

What is the empirical and molecular formula of benzene?

Answer 2

CH
C6H6

Question 3

Why does benzene burn with a very smoky flame?

Answer 3

Unsaturated

Question 4

What are some of the properites of benzene compared to the Kekulé structure?

Answer 4

1. Bond lengths: two different bond lengths in Kekule but all carbon-carbon bond lengths are of an equal length in benzene
2. Tendency to undergo reactions: Kekule structure would have a high tendency to undergo addition reaction due to having 3 double bonds. Benzene does not readily undergo addition reaction
3. Enthalpy of hydrogenation for reaction: Kekule = 3 x -120 = -360 kJ mol-1 but the enthalpy change for the hydrogenation of benzene is -208kJ mol-1

Question 5

What is the enthalpy of hydrogenation of benzene?

Answer 5

-208 kJ mol-1 (the enthalpy change when one mole of C=C double bonds is reduced to single bonds by reacting with gaseous hydrogen)

Question 6

What would the theoretical value of the enthalpy of hydrogenation of benzene be?

Answer 6

-360 kJ mol-1

Question 7

Why is there a difference in the value of enthalpy of hydrogenation of benzene?

Answer 7

The difference is due to electron delocalisation. (152 kJ mol-1)
Where the molecule becomes more stable due to the fact that all of the p-orbitals on the carbon atoms are aligned parallel to each other, and they can all overlap in a ring above and below the plane of the sigma-bond network, rather than being discrete π bonds

Question 8

What does the delocalised model for the structure of benzene look like?

Answer 8

In benzene, each C forms 3 sigma-bonds
- Two to neighbouring carbon atoms and one to a H atom. The result of this is the sigma framework of benzene.
- Forming a ring electron density above and below the plane of the benzene molecule

Question 9

What does the combustion of benzene look like?

Answer 9

C6H6 (l) + 15/2 O2 (g) -> 6CO2 (g) + 3H2O (l)
- Benzene is unlikely to completely combust due to the high proportion of carbon in the molecule

Question 10

What does the bromination of benzene look like? What catalyst and condition is needed?

Answer 10

Benzene + Br2 –> bromobenzene + HBr
Catalyst: anhydrous aluminium bromide AlBr3/FeBr3
Warm

Question 11

What does the nitration of benzene look like? What catalyst and condition is needed?

Answer 11

Benzene + cHNO3 –> nitrobenzene + H2O
Catalyst: cH2SO4, warm to 55 C

Question 12

What does the Freidel-Crafts alkylation of benzene look like? What catalyst and condition is needed?

Answer 12

Benzene + CH3Cl –> methylbenzene + HCl
Catalyst: anhydrous AlCl3 or FeCl3, heat under reflux

Question 13

What does the Freidel-crafts acylation of benzene look like? What catalyst and condition is needed?

Answer 13

benzene + CH3COCl –> phenyl ethanoate + HCl
Catalyst: anhydrous AlCl3/FeCl3, heat under reflux

Question 14

Why is benzene susceptible to electrophilic attack?

Answer 14

Rings of electron density above and below the plane of the sigma framework (electron rich molecule)

Question 15

What is the mechanism associated with all of the reactions of benzene?

Answer 15

Electrophilic substitution

Question 16

Why does benzene not undergo addition reactions?

Answer 16

It will cause the benzene molecule to lose its delocalised π system. Substitution allows benzene to retain its delocalised π system and therefore retain its stability

Question 17

What does the general mechanism for electrophilic substitution look like?

Answer 17

E+ = electrophile

Question 18

Show the equation and mechanism (and the regeneration of the catalyst) of the nitration of benzene?

Answer 18

Generation of the electrophile NO2+: HNO3 + H2SO4 –> H2NO3+ + HSO4-
H2NO3+ –> NO2+ + H2O

Re-generation of the catalyst: HSO4- + H+ –> H2SO4

Question 19

Why is a catalyst needed for the bromination of benzene?

Answer 19

Because the delocalised π electron system is more stable than the simple π bond in alkenes.

Question 20

What are the typical Lewis acids used?

Answer 20

Anhydrous AlBr3/FeBr3 (species that are electron deficient)

Question 21

Why can iron also be described as the catalyst?

Answer 21

The iron can react with Br2 to form FeBr3 in situ (within the reaction mixture)

Question 22

Why are anhydrous conditions necessary?

Answer 22

To prevent hydrolysis of the Lewis acid

Question 23

What group attached in alkylation?

Answer 23

Attaches an alkyl groyp (R eg/ CH3, CH2CH3 etc)

Question 24

What group attached in acylation?

Answer 24

Attaches an acyl group (COR eg/ COCH3, COCH2CH3 etc)

Question 25

What does it mean for alkyl groups to be electron donating?

Answer 25

Makes the ring more reactive and therefore more susceptible to further electrophilic attack. There is a good chance that further alkyl groups will be substituted onto the ring, giving a mixture of products

Question 26

Why is the product of the Freidel-Crafts acylation less reactive than the original benzene?

Answer 26

The acyl group withdraws electron density and so deactivates the ring, making it less susceptible to further electrophilic attack. As a result, the initial product is unlikely to react with a second acyl group.

Question 27

What is the molecular formula of phenols?

Answer 27

C6H5OH
- OH group is attached directly to a benzene ring

Question 28

What does the structure of phenol look like?

Answer 28

C6H5OH

Question 29

Explain the difference in the flames when phenol and ethanol burn

Answer 29

Phenol - very smoky
Ethanol - clean

Phenol - higher proportion of carbon therefore not all the carbon in phenol will oxidise fully

Question 30

Give the equation to show what happens to some molecules of phenol in water:

Answer 30

C6H5OH ⇌ C6H5O- (phenoxide) + H+

Question 31

Why is the loss of proton possible in phenoxide?

Answer 31

Loss of a proton is possible because the negative charge on the oxygen in the phenoxide ion overlaps with the delocalised π electron system in the benzene ring. Therefore, the phenoxide ion is stabilised.

Question 32

What happens when phenol reacts with a strong base?

Answer 32

C6H5OH + NaOH –> C6H5O-Na+ + H2O

Question 33

Reaction of phenol with bromine water and its observations:

Answer 33

The reaction occurs at room temperature. The milder conditions necessary for phenol to react means that it must be more reactive than benzene

• Forms 2,4,6 - tribromophenol (white ppt) + 3HBr
• Orange –> colourless

Question 34

Why is phenol more reactive than benzene?

Answer 34

The oxygen in phenol has two lone pairs of electrons. One of the lone pairs is at the correct orientation to overlap with the delocalised π system. This increases the electron density in the ring, making phenol more susceptible to electrophilic attack and therefore more reactive than benzene

Question 35

Compare the properties of phenol and ethanol:

Question 35

What happens if the -OH group is not directly attached to the benzene ring?

Answer 35

The compound is an alcohol and not a phenol
Eg/ phenylmethanol

Question 36

Describe the process of recrystallisation:

Answer 36

1. Dissolve the solid in the minimum volume of hot solvent (to obtain a hot saturated solution) - if the solvent is flammable, it must be pre-heated in a water bath
2. Filter the solution while hot (to remove any insoluble impurities)
3. Cool the filtrate (so that, as the solubility drops on cooling, the solid will crystallise out)
4. Filter at the pump, and wash with a little cold solvent
5. Blot the crystals dry between filter papers - or place in an oven at low heat (to dry the crystals)