Module 6: Benzene and Phenol

Question 1

The Kekule Model:

Answer 1

Ring of 6 carbons joined by alternate double and single bonds.

Question 2

Evidence disproving the Kekule model:

Answer 2

Lack of Reactivity with bromine
Length of carbon-carbon bonds
Hydrogenation enthalpies.

Question 3

How did Benzene’s lack of reactivity disprove the Kekule model?

Answer 3

Benzene doesn’t undergo electrophilic addition reactions, doesn’t decolourise bromine under standard conditions, therefore there are no C=C bonds.

Question 4

How did Benzene’s carbon-carbon bond lengths disprove the Kekule model?

Answer 4

Using X-ray diffraction, it was found that all bond lengths were 1.39nm, when there should have been 1.53nm and 1.34nm.

Question 5

What method can be used to measure bond lengths?

Answer 5

X-ray diffraction.

Question 6

Hydrogenation Enthalpies:

Answer 6

Benzene under the the Kekule Model is cylcohex-1,3,5-triene and-so it’s expected hydrogenation enthalpy would be -360kJmol-1. The actual hydrogenation enthalpy is -208kJmol-1. Therefore the actual structure is much more stable than the Kekule model.

Question 7

Delocalised Model of Benzene:

Answer 7

Benzene is a planar, cyclic, hexagonal hydrocarbon containing six carbon atoms and six hydrogen atom.
Each carbon atom uses three of its 4 available electrons in bonding two other carbon atoms and one hydrogen atom. Each carbon has one electron in a p-orbital perpendicular to the plane of the carbon atoms. The overlapping p-orbital creates a ring of pi-bonds. The six electrons occupying the ring are delocalised.

Question 8

Naming aromatic compounds:

Answer 8

Short alkyl chains, halogens and nitro groups are shown as prefixes to benzene.
Long alkyl chains or short alkyl chains with functional groups - benzene is considered as a substituent, and-so the phenyl prefix will be used.

Question 9

What reaction does Benzene typically undergo?

Answer 9

electrophilic substitution.

Question 10

Nitration of Benzene: Conditions and catalysts

Answer 10

50’C with a sulfuric acid catalyst. Benzene + concentrated nitric acid.

Question 11

Alkylation reaction with phenol:

Answer 11

AlCl3 catalyst acts as a halogen carrier, generating the electrophile.

Haloalkane + phenol -> Hydrogen halide + alkyl phenol

Question 12

What reactions involving benzene use AlCl3 as a catalyst?

Answer 12

Acylation, alkylation, Halogenation.

Question 13

Why does cyclohexene react with bromine but benzene doesn’t?

Answer 13

The electron density of benzene is insufficient to induce the appropriate charge on the bromine.

Question 14

What is the difference between phenol and an aromatic alcohol?

Answer 14

In phenols the -OH group is attached to the carbon ring however in alcohols the OH group will be attached to a carbon chain branching off the benzene ring.

Question 15

Phenol as a weak acid:

Answer 15

When in water, phenol dissolves into a phenoxide ion and a proton ion

Question 16

Bromination of Phenol:

Answer 16

Phenol reacts with aqueous bromine solution, because the oxygen’s lone pair is donated to the delocalised ring, increasing the electron density. The solution is decolourised from orange to colourless. Forms 2,4,6 Tribromophenol.

Question 17

Nitration of Phenol:

Answer 17

Phenol + HNO3 will form 2-nitrophenol or 4-nitrophenol and H2O

Question 18

Why is phenol more reactive than benzene?

Answer 18

The lone pair of electrons in the oxygen p-orbital of the OH group is donated into the pi-system. This increases the electron density of the benzene ring in phenol, which increases the attraction towards electrophiles.

Question 19

Nitration of benzene:

Answer 19

Benzene reacts slowly with nitric acid to form nitrobenzene. The reaction is catalysed by H2SO4 and heated to 50’C.

Question 20

What group activates benzene?

Answer 20

NH2 - amine

Question 21

What group deactivates benzene?

Answer 21

NO2 - nitrate

Question 22

Where does a NH2 group direct substituents?

Answer 22

2nd, 4th and 6th position

Question 23

Where does a NO2 group direct substituents?

Answer 23

3rd position and 5th position

Question 24

What is reacted with benzene in a nitration reaction?

Answer 24

Nitric Acid HNO3

Question 25

Nitration of Benzene is an example of what?

Answer 25

Electrophilic substitution

Question 26

What catalyst is needed for the halogenation of benzene?

Answer 26

A halogen carrier e.g AlCl3, AlBr3, FeBr3

Question 27

What type of mechanism is the halogenation of benzene?

Answer 27

Electrophilic Substitution

Question 28

Alkylation of Benzene: | Catalyst and Reactants:

Answer 28

Halogen Carrier Catalyst Haloalkane + benzene -> extended carbon chain + hydrogen halide

Question 29

Acylation of Benzene: | Catalyst and reaction:

Answer 29

Halogen Carrier Catalyst: | Acylchloride + benzene -> phenyl(-------)one + hydrogen halide

Question 30

Why does benzene need a halogen carrier to undergo halogenation however alkenes don't?

Answer 30

C=C in an alkene has a higher pi-electron density, because the electron density around any two carbons in a benzene will be lower as it's pi-electrons are distributed across all carbons. Therefore the benzene has an insufficient pi- electron density to polarise a halogen.

Question 31

Reaction of phenol with NaOH:

Answer 31

Reacts with sodium hydroxide to form sodium phenoxide and water (neutralisation)

Question 32

How do you recrystallise impure crystals to obtain a pure sample

Answer 32

Recrystallize. Dissolve impure solid in minimum volume of hot water/solvent.. Cool solution and filter solid. Wash with cold water/solvent and dry.

Question 33

preparation of an organic solid: filtration under reduced pressure

Answer 33

1. connect one end of the pressure tubing to the vacuum outlet or to the filter pump whilst attaching the other end of the rubber tubing to the buchner flask. 2. Fit the buchner funnel to the buchner flask ensuring that there is a good tight fit. This is usually obtained using a buchner ring or a bung. 3. switch on the vacuum pump, or the tap to which your filter pump is attatched. 4. Check for good suction by placing your hand across the top of the funnel. 5. Place a piece of filter paper inside the buchner funnel and wet this with the same solvent used in preparing your solid. 6. To filter your sample, slowly pour the mixture from a beaker into the centre of the filter paper. 7. rinse out the beaker with the solvent so that all of the solid crystals collect in the buchner funnel. 8. Rinse the crystals in the buchner funnel with more solvent and leave them under suction for a few minutes so that the crystals start to dry.

Question 34

Preparation of an organic solid: recrystallisation

Answer 34

1. Pour a quantity of the chosen solvent into a conical flask. If the solvent is flammable warm the solvent over a water bath. If the solvent is water, place the conical flask on a tripod and gauze over a bunsen and warm the solvent. 2. Tip the impure sample into a second conical flask or beaker. 3. Slowly add the solvent to the impure sample until it dissolves in the solvent. You should add the minimum volume of solvent needed to dissolve the solid. 4. Once the solid has dissolved, allow the solution to cool. Crystals of the desire product should form in the conical flask or beaker. When no more crystals form, filter the crystals under reduced pressure to obtain the dry crystalline solid.

Question 35

Preparation of an organic solid: melting point determination

Answer 35

1. Before taking the melting point of a solid you should ensure that the sample is completely dry and free flowing. 2. Take a glass capillary tube or melting point tube. Hold one end of the capillary tube in the hot flame from the bunsen burner. Rotate the tube in the flame until the end of the tube is sealed.. 3. The capillary tube is allowed to cool, an is then filled with crystals to about a 3mm depth. This is usually carried out by pushing the open end of the capillary tube into the solid sample to force some of the solid into the tube. 4. Once you have prepared your sample you will need to take its melting point.

Question 36

Using an electrically heated melting point apparatus to determine melting point

Answer 36

1. Place the capillary tube containing the sample into the sample hole and a 0-300 degree thermometer in the thermometer hole of the melting point apparatus. 2. Using the rapid heating setting, start to heat up the sample whilst observing the sample through the magnifying window. 3. Once the solid is seemed to melt, record the melting point. Allow the melting point apparatus to cool. 4. prepare a second sample in a new capillary tube an place in the melting point apparatus an again heat up the sample. 5. As the melting point is approached, set to low and raise the temperature slowly whilst observing the sample.

Question 37

Using an oil bath or thiele tube method to determine melting point

Answer 37

1. Set up the thiele tube or oil bath. 2. Insert the thermometer through a hole in the cork if using a thiele tube or clamp the thermometer if using an oil bath. The end of the thermometer and the end of the capillary tube should dip into the oil. 3. Using a micro-burner, slowly heat the side arm of the thiele tube or the oil bath whilst observing the solid. When the solid starts to melt, remove the heat and record the temperature at which all of the solid has melted. It is important to heat the oil slowly when approaching the melting point and repeat the melting point determination to obtain an accurate value.

Question 38

Explain how the number of isomers of C6H4Cl2 provides evidence for the delocalized structure of benzene

Answer 38

If benzene had a delocalized structure, only one 1,2 substituted isomer is possible. If benzene has the kekule structure, there would be 2 different 1,2 substituted isomer. Only one 1,2 substituted isomer has been found.