F324: Module 1: Benzene and Phenol

Question 1

I) What is the molecular and empirical formula for Benzene?

Answer 1

Molecular - C6H6

Empirical - CH

Question 2

I) How can you tell whether a compound is an arene based on that compounds molecular formula?

Answer 2

If the numbers of Carbon and Hydrogen atoms are almost equal or there are more Carbon atoms than Hydrogen atoms then the compound is an arene.

Question 3

I) What 3 pieces of evidence suggest that Benzene has a delocalised structure and how does it show that the kekule structure is incorrect?

Answer 3

1) X-ray diffraction studies show that the C-C bond lengths are all the same, being somewhere between a C - C and
C = C bond which supports the delocalised systems since it predicts that all the bonds should have the same length. In the kekule structure there are 2 types of bonds (C -C and
C =C) and therefore there would be 2 different bond lengths.

2) The experimental enthalpy change of hydrogenation of Benzene is -208 kJ/mol not -360 kJ/mol which is 3 times that of cyclohexene (this is what the kekule structure predicts). Benzene is therefore more stable than expected. The delocalisation of the pi-electrons into a pi-system makes the structure very stable as the electronegativity is spread throughout the whole structure. Therefore less energy is released when hydrogenated.
3) Bromine doesn’t decolourise bromine water and needs a catalyst for the reaction. According to the delocalised model, the delocalised pi-electron system has insufficient electronegativity to polarise the Br - Br bond and react. This disproves the kekule structure since 3 isolated double bonds would have sufficient electron density to polarise the Br - Br bond and react. No catalyst would be required.

Question 4

I) What is the bond angle in Benzene?

Answer 4

120 degrees

Question 5

I) Describe the delocalised structure of benzene.

Answer 5

The delocalised model states that the p-orbitals of all six carbon atoms overlap to create pi-bonds.

This creates two ring shaped clouds of delocalised electrons, one above and one below the plane of the six carbons.

This is also known as a pi system.

Question 6

I) What do you need in order to react benzene with bromine water?

Answer 6

A halogen carrier (catalyst) such as AlCl3 or FeBr3

Question 7

I) What type of isomerism arises due to the ring structure of benzene?

Answer 7

Positional isomerism - where functional groups can be bonded to different carbons in the ring.

Question 8

I) Why is benzene liable to attack from electrophiles?

Answer 8

Because the ring has a high electron density above and below the carbon plane.

Question 9

I) What are electrophiles?

Answer 9

Electrophiles are electron pair receptors.

Question 10

I) Give two examples of electrophilic substitution.

Answer 10

Nitration and Halogenation.

Question 11

I) In the nitration of benzene, what is the electrophile and how is it formed?

Answer 11

NO2+ or Nitronium ion

HNO3 + H2SO4 = H2NO3+ + HSO4-
H2NO3+ = NO2+ +H2O

Question 12

I) What key things do you have to remember when drawing the nitration mechanism?

Answer 12

The curly arrows represent the movement of a pair of electrons.

Draw the curly arrows going from the ring to the Nitrogen in the first step.

The Incomplete ring (looks like a horse shoe) should always cover 5 carbons.

In the intermediate step draw the curly arrow going from the H - C bond clearly and don’t for get the ‘+’ sign in the middle of the ring.

In the final step remember to draw the H+ ion since that reacts with the HSO4- ion to reform H2SO4 (a catalyst).

Question 13

I) Why is sulphuric acid considered a catalyst in the Nitration of Benzene?

Answer 13

H2SO4 is used to make the nitronium ion (NO2+) and in the process from a HSO4- ion. This ion reacts with the substituted H+ ion after the reaction has taken place to reform H2SO4. Thus it is not used up in the reaction.

Question 14

I) What is a halogen carrier?

Answer 14

A halogen carrier is an electron-deficient molecule that helps to produce an electron deficient halogen atom or alkyl group (both electrophiles) which can then react with the benzene ring.

Question 15

I) What type of fission occurs in the formation of the electrophile in halogenation and what happens to the two ions?

Answer 15

Heterolytic fission since the distribution of electrons in the fission is uneven.

The X2 molecule goes to X+ and X- ions.

The X+ is the electrophile.

The X- becomes datively covalently bonded to the halogen carrier.

Question 16

I) How does the halogen carrier act as a catalyst in the halogenation of benzene?

Answer 16

The halogen carrier ion reacts with the substituted Hydrogen.

H+ + AlCl4- = AlCl3 +HCl
The halogen carrier is therefore reformed.

Question 17

I) What is the molecular formula of Phenol?

Answer 17

C6H5OH

Question 18

I) Why does phenol act as a weak acid?

Answer 18

Phenol can lose a hydrogen ion from its OH group because the phenoxide ion formed is stabilised to some extent. The negative charge on the oxygen atom is delocalised around the ring. The more stable the ion is, the more likely it is to form.

Question 19

I) What salt does Phenol form when reacted with a base?

Answer 19

A Phenoxide salt.

Question 20

I) Describe how the OH group impacts the electron density of phenol and how it effects its reactivity?

Answer 20

The lone pair of electrons on the oxygen atom combines with the delocalised ring of electrons to form a ‘saucepan’ shaped electron system. This increases the electron density at positions 2,4 and 6 on the ring, making those areas more reactive.

Therefore phenol reacts more readily than benzene with electrophiles. Substitution reactions can occur at positions 2,4 and 6 because of their increase in electronegativity.

Question 21

I) Describe the reaction between Phenol and Bromine stating the catalyst (if there is one).

Answer 21

No Catalyst (halogen carrier) is needed

The reaction occurs with bromine water unlike the reaction with benzene which requires pure bromine.

Substitution occurs at 3 carbons (2, 4 and 6) to give
2,4,6-tribromophenol.

C6H5OH + 3Br2 = C6H2Br3OH + 3HBr

Question 22

I) What are the conditions for the mono nitration of benzene?

Answer 22

HNO3 Acid
H2SO4 Catalyst
Below 55 degrees for mono-nitration

Question 23

I) List some of the uses of phenol.

Answer 23

Resins in paints - Phenolic resins are used in making varnishes.

Antiseptics - Compounds containing phenol are often good at killing microbes e.g. Thymol and TCP.

Disinfectants - e.g. Dettol