Benzene - AD THE HESS CYCLE W KUKULELE AND NORM

Question 1

Molecular formula of benzene

Answer 1

C6H6

Question 2

Describe the nature of the carbon-carbon bonds in benzene

Answer 2

They are all the same lengths
The bond length in benzene is an intermediate between C=C and C-C

Question 3

Shape of the benzene molecule

Answer 3

Planar and hexagonal

Question 4

What structure of benzene did Kukule suggest

Answer 4

A cyclic arrangement of carbon atoms joined by alternate single and double bonds

Question 5

What was the evidence that led scientists to doubt and reject Kukule’s model

Answer 5

The carbon-carbon bond length is intermediate between C=C and C-C

Benzene only reacts with bromine in presence of a halogen carrier so is less reactive than alkenes

Enthalpy change of hydrogenation for benzene is less exothermic/negative than that of cycloheaxa-1,3,5-triene

Benzene does not readily react by addition but reacts by substitution

Question 6

Why does benzene react by substitution and not by addition

Answer 6

Benzene does not have three C=C double bonds but has a delocalised pi electron system.
The 6 p orbitals overlap sideways, above and below the ring to form a system of pi bonds.
The pi electrons are delocalised over all the 6 carbon atoms in the ring which gives the molecule greater stability.

When benzene reacts by substitution the delocalised pi electron system is retained, whereas in addition reactions the delocalised pi electron system is disrupted. Therefore substitution is energetically more favourable than addition.

Question 7

Bond angles in benzene

Answer 7

120 degrees

Question 8

How are the carbon atoms bonded in benzene

Answer 8

Each carbon atom is bonded to two other carbon atoms and to one hydrogen atom using three of its available 4 electrons.

Question 9

Why is benzene stable

Answer 9

Each carbon atom has one electron in a p orbital. The six p orbitals overlap sideways, above and below the ring to form a system of pi bonds. The pi electrons are delocalised over all the six carbon atoms in the ring which gives the molecule a greater stability.

Question 10

Explain in terms of structure and bonding why benzene and alkenes react differently with electrophiles like bromine

Answer 10

Benzene does no have 3 C=C bonds but has a delocalised pi electron system.
The six p orbitals overlap above and below the ring to form a system of pi bonds.
The pi electrons are delocalised over all six carbon atoms in the ring which gives the molecule greater stability.
Benzene reacts by substitution rather than addition.
When benzene reacts by substitution the delocalised pi electron system is retained, whereas in addition reactions the delocalised pi electron system is disrupted.

Question 11

Differences between the pi bonding in Kekule’s model and the accepted model of benzene

Answer 11

Kekule’s model has three pi bonds. The two electrons in each pi bond are localised between the two carbon atoms in the double bond.

Benzene has a system of pi bonds where the pi electrons are delocalised over all six carbon atoms in the ring.

The p orbitals in Kekule’s model overlap in one direction, whereas the p orbitals in benzene overlap in both directions.

Question 12

Although the enthalpy of hydrogenation of benzene might be expected to be -360 kJ/mol it was found to be -208 kJ/mol. Give a reason why -360 was not the actual value.

Answer 12

The six p orbitals overlap sideways to form a system of pi bonds.
The pi electrons are delocalised over all six carbon atoms in the ring which gives the molecule greater stability. More energy is needed to break the bonds in benzene, hence the enthalpy change of hydrogenation of benzene is less exothermic.

Question 13

Observation for the combustion of benzene

Answer 13

Burns with a yellow smoky flame and leaves a black residue

Question 14

What type of reaction is the nitration of benzene

Answer 14

Electrophilic substitution

Question 15

Reagents needed for the nitration of benzene

Answer 15

Conc nitric acid and conc sulphuric acid

Question 16

Conditions for the nitration of benzene

Answer 16

Heat under reflux at 55 degrees

Question 17

What is formed in the nitration of benzene

Answer 17

Nitrobenzene + water

Question 18

Why must the temperature of the reaction mixture in the nitration of benzene be kept under 55 degrees

Answer 18

To prevent further substitution and to stop di or trinitrobenzene forming

Question 19

Why is the nitration of benzene called an electrophilic substitution reaction

Answer 19

The electrophile NO2 + accepts a pair of electrons.
It is a substitution because NO2 replaces one hydrogen atom in the benzene ring

Question 20

Reaction for the formation of the electrophile in the nitration of benzene

Answer 20

HNO3 + 2H2SO4 ——-> NO2+ + 2HSO4- + H3O+

Question 21

Function of sulphuric acid in the nitration of benzene

Answer 21

Catalyst

Question 22

Give two safety precautions for the nitration of benzene

Answer 22

Wear gloves because concentrated sulfuric acid is corrosive
Carry out the experiment in a fume cupboard as benzene is carcinogenic

Question 23

Reagents for the reaction of benzene with bromine

Answer 23

Bromine, FeBr3 as catalyst

Question 24

Conditions for reaction of benzene and bromine

Answer 24

Room temperature

Question 25

Type of reaction between benzene and bromine

Answer 25

Electrophilic substitution

Question 26

Equation showing how the electrophile Br+ is formed

Answer 26

Br2 + FeBr3 ——–> Br+ + FeBr4 -

Question 27

State the main use of phenylethene

Answer 27

To make polymers which is used for packaging and insulation

Question 28

Explain why benzene does not react with bromine or chlorine at room temperature but cycloalkenes do

Answer 28

In benzene, the pi electrons are delocalised all over the 6 carbon atoms in the ring. Therefore, benzene has a lower electron density than cyclohexene. The electron density around any two carbon atoms in the benzene ring is insufficient to polarise bromine, so a halogen carrier is needed to generate the stronger electrophile.

In cycloalkenes the pi electrons are localised between the two carbon atoms in the double bond. This produces an area of high electron density so they can polarise bromine.

Question 29

Why is the nitration of benzene an important

Answer 29

It is an important step in the synthesis of useful compounds including explosives and formation of amines.

Question 30

What are the reagents, conditions, product formed and the type of reaction in the reaction of benzene with a halogenoalkane

Answer 30

Reagent : chloroalkane, anhydrous AlCl3 as catalyst
Conditions : Heat under reflux
Product : Alkylbenzene
Reaction : Electrophilic substitution

Question 31

What are the reagents, conditions and the type of reaction in the acylation of benzene

Answer 31

Reagent: Acyl chloride using AlCl3 as catalyst
Condition: Heat under reflux
Product: phenylketone
Reaction: Electrophilic Substitution

Question 32

Equation for the formation of the electrophile in the acylation of benzene

Answer 32

AlCl3 + CH3COCl ——-> [CH3CO]+ + [AlCl4]-

Question 33

Important of Friedel-Crafts acylation iin organic synthesis

Answer 33

They introduce a reactive functional group on the benzene ring

Question 34

What is nitrobenzene reduced to

Answer 34

phenylamine

Question 35

Reaction for the reduction of nitrobenzene

Answer 35

nitrobenzene + 6[H] —–> phenylamine + 2H2O

Question 36

Reagent, condition and mechanism for the reduction of nitrobenzene

Answer 36

Reagent: Fe and HCl
Condition: Heating
Mechanism: Reduction

Question 37

Explain the bonding in and the shape of a benzene molecule. Compare the stability of benzene with that of the hypothetical cyclohexa-1,3,5-triene molecule. Use the data in your answer.

Answer 37

Bonding:

Each C has three (covalent) bonds
Spare electrons (in a p orbital) overlap (to form a π cloud)
delocalisation

Shape:

Planar
Hexagon/6 carbon ring/120° bond angle
C–C bonds equal in length / C–C bond lengths between single and double bond

Stability:

Expected ∆H hydrogenation of cyclohexatriene = –360 kJ mol–1
∆H hydrogenation benzene (is less exothermic) by 152 kJ mol–1
Benzene lower in energy than cyclohexatriene / Benzene is more stable

Question 38

State, in terms of its bonding, why benzene is more stable than cyclohexa−1,3,5−triene.

Answer 38

Electrons delocalised

Question 39

When methylbenzene reacts with ethanoyl chloride and aluminium chloride, a similar substitution reaction occurs but the reaction is faster than the reaction of benzene.
Suggest why the reaction of methylbenzene is faster.

Answer 39

M1 about electrons

methyl group has (positive) inductive effect OR increases electron density on
benzene ring OR pushes electrons OR is electron releasing

M2 about attraction

electrophile attracted more

or benzene ring better nucleophile

Question 40

The enthalpy of hydrogenation of cyclohexa-1,3-diene is not exactly double that of cyclohexene.

Suggest a value for the enthalpy of hydrogenation of cyclohexa-1,3-diene and justify your value.

Answer 40

Value within range –239 to –121

Double bonds separated by one single bond / alternating

Allows some delocalisation/overlap of p orbitals