Benzene

Question 1

What did Kekulé suggest about benzene’s structure?

Answer 1

• C=C bonds 135pm & C-C bonds 147pm long

* alternating double bonds: flipping between isomers

Question 2

How did x-ray diffractions prove Kukulé wrong?

Answer 2

They showed that carbon to carbon bond lengths were 140 pm, between the length of C=C (135 pm) bonds and C-C (147 pm).

Question 3

How did enthalpy changes give more evidence for delocalisation?

Answer 3

Cyclohexene has one double bond. When dehydrogenated, EC is -120kjmol-1 so benzene would be expected to have -360kjmol-1 but it actually has an EC of -208kjmol-1.

More energy was needed to break bonds than expected.

Question 4

What is the delocalised model of benzene?

Answer 4

• p-orbitals of all 6 carbon atoms overlap to create a π bonds
• creates 2 ringed shaped clouds of electrons (1 up and 1 below
the carbon plane)
• bonds are the same length
• electrons don’t belong to specific carbon so is said to be
delocalised

Question 5

Why doesn’t benzene undergo addition reactions?

Answer 5

Delocalised rings make benzene stable and spread out the negative charge. Benzene is unwilling to undergo and addition reaction as it would destroy the stable ring. Less polar that phenol and alkenes so needs a catalyst. More energy needed to π bond.

Question 6

Describe the structure of benzene.

Answer 6

• planar structure
• (draw delocalised model labelled)
• stable structure
• bonds same length (140 pm)

Question 7

Explain why alkenes undergo addition reactions.

Answer 7

The C=C bond is an area of high electron density which strongly attracts electrophiles.

Question 8

Conditions, Reagents, Equation (incl catalysts’) of halogenation of benzene.

Answer 8

Reagents: chlorine & halogen carrier, Conditions: reflux in presence of halogen carrier e.g. Fe, FeCl3, FeBr3, AlCl3
C6H6 + Cl2 -> C6H5Cl + HCl

Question 9

What does the halogen carrier do for the reagent chlorine during the halogen action of benzene?

Answer 9

It increases the polarisation of chlorine.

Cl2 + AlCl3 -> AlCl4- + Cl+

H+ + AlCl4- -> AlCl3 + HCl

Question 10

Draw the mechanism of halogenation.

Answer 10

• arrow pointing out of beneze ring towards Cl+ or Br+
• Cl and H attatched to one corner of benzene, arrow coming out of hydrogen bond to cut ring positive symbol
• bromo/chloro benzene and proton

Question 11

What type of reaction is the nitration and halogenation of benzene?

Answer 11

Electrophilic substitution.

Question 12

Draw the mechanism of Nitration of benzene.

Answer 12

• benzene ring arrow pointing to NO2+
• NO2 and H at corner with arrow coming towards pos. U centre from middle of H bond
• nitrobenzene and proton

Question 13

What are the reagents, conditions and equations for the nitration of benzene?

Answer 13

Reagents: Conc. Nitric and Sulphuric Acid , Conditions: Warm

Electrophile: NO2+
HNO3 + H2SO4 -> H2NO3 + HSO4-
H2NO3+ -> NO2+ + H20

Question 14

What is the formula of phenol?

Answer 14

C6H5OH

Question 15

What are the uses of phenol?

Answer 15

Plastics, antiseptics, disinfectants, resins for paints

Question 16

What are the conditions, reagents and equations for the reduction of nitrobenzene?

Answer 16

Reagents: tin and conc. HCl
Conditions: reflux
Equations: C6H5NO2 + 6[H] -> C6H5NH2 + 2H2O

Question 17

What are the conditions, reagents and equations for the diazotisation of phenylamine?

Answer 17

Reagents: Nitrous acid HNO2 (NaNO2 + HCl -> HNO2 + NaCl)
Conditions: Below 10*C to prevent decomposition into phenol
Equations: C6H5NH2 + HNO2 + HCl -> C6H5N2+Cl- + 2H2O

Question 18

What are the conditions, reagents and equations for the substitution of benzenediazonium chloride to form phenol?

Answer 18

Reagents: water
Conditions: warm above 10*C
Equations: C6H5N2+Cl- + H2O -> C6H5OH + N2 + HCl

Question 19

What are the steps to prepare phenol starting from benzene?

Answer 19

Benzene -> Nitrobenzene -> Phenylamine -> benzene- N2+Cl-

-> phenol

Question 20

Describe the reactions of phenol with water.

Answer 20

• phenol is a weak acid but it is a stronger acid than aliphatic acids
• the aromatic rings helps weaken the O-H bond and stabilises the
  resulting anion
• phenol dissolves slightly in water to form weak acidic solution
  C6H5OH (aq) C6H5O- (aq) + H+ (aq)

Question 21

Describe the reactions of phenol with NaOH.

Answer 21

• phenol reacts with NaOH to form a salt: sodium phenoxide
• it is ionic and water soluable

C6H5OH (aq) + NaOH (aq) -> C6H5O-Na+ (aq) + H20 (l)

Question 22

Describe the reactions of phenol with sodium.

Answer 22

• forms ionic salt: sodium phenoxide and hydrogen
• similar reaction to aliphatic alcohols e.g. ethanol

2C6H5OH (s) + 2Na (s) -> 2C6H5O-Na+ (s) + H2 (g)

Question 23

What is the reaction of phenol with bromine?

Answer 23

phenol + 3Br2 -> 2,4,6 - tribromophenol + 3HBr

Question 24

Describe what the OH group does in phenol.

Answer 24

The OH group is electron releasing & electron pair donation takes place from a p orbital on oxygen. It increases the ELECTRON DENSITY of the delocalised system which makes substitution much easier that bromine. Electron density is greatest at 2, 4, 6 POSITIONS and this is site of substitution. NO CATALYST.