Benzene

Question 1

What is the structure of benzene?

Answer 1

C6 H6Cyclic structure- with its six carbon atoms joined together in a ring.

Question 2

What are the two ways of representing it?

Answer 2

The Kekulé model and the delocalised model.

Question 3

Kekulé suggested that benzene was…

Answer 3

PlanarCyclicand had alternating double and single bonds

Question 4

Kekulé ‘s structure of benzene is incorrect. Why?

Answer 4

Bromine water reaction did not occur with benzene suggesting it to not be a normal alkene.
Benzene is thermodynamically more stable than would be expected from kekules structure.
All the C-C bond lengths in benzene are the same

Question 5

How are the bond length observations explained with the delocalised model?

Answer 5

1) The delocalised model says that the p-orbitals of all six carbon atoms overlap to create pi bonds.
2) This creates two ring shaped clouds of electrons- one above and one below the plane of six carbon atoms.
3) All the bonds in the ring are the same length because all the bonds are the same.
4) The electrons in the rings are said to be delocalised- because they don’t belong to a specific carbon atom.

Question 6

Whats the difference between benzene and other alkenes that means benzene is very unwilling to undergo addition reactions?

Answer 6

The delocalised electron rings above and below the plane of carbon atoms.They make the benzene ring very stable, and spread out the negative charge.

Question 7

In what two ways do arenes undergo electrophillic substitution reactions?

Answer 7

1) With halogens using a halogen carrier.2) With nitric acid and a catalyst.

Question 8

How do arenes undergo electrophillic substitution reactions with halogens using a halogen carrier?

Answer 8

1) Benzene will react with bromine, Br-Br in the presence of aluminium chloride.
2) Br-Br is the electrophile.
3) Aluminium chloride acts as a halogen carrier which makes the electrophile stronger. Without it the electrophile doesn’t have a strong enough positive charge to attack the stable benzene ring.
4) A Br atom is substituted in place of an H atom.

Question 9

How do arenes undergo electrophillic substitution reactions with nitric acid and a catalyst?

Answer 9

When you warm benze with concentrated nitric and sulfuric acid, you get nitrobenzene.
Sulfuric acid works as a catalyst- it helps make the nitronium ion which is the electrophile but is regenerated at the end of the reaction mechanism.

Question 10

How did kekule describe benzene?

Answer 10

Kekulé suggested that the structure of benzene was a regular hexagon of carbon atoms with alternating single and double bonds, each carbon being bonded to a hydrogen atom

Question 11

Equation for production of NO2+ in nitration.

Answer 11

HNO3 + H2SO4 → NO2+ + HSO4- + H2O

Question 12

Equation for production of electrophile in chlorination

Answer 12

Cl2 + FeCl3 → Cl+ + FeCl4-

Question 13

Equation for production of electrophile in bromination

Answer 13

Br2 + FeBr3 → Br+ + FeBr4-

Question 14

A chemist found that when benzene reacts with bromine, a halogen carrier is required as a catalyst.Write an equation for this reaction.

Answer 14

C6H6 + Br2 → C6H5Br + HBr

Question 15

A chemist observed that bromine decolourises when it reacts with phenol.What other observation would she have made?

Answer 15

White precipitate

Question 16

Cyclohexene also decolourises bromine.Name the organic product formed.

Answer 16

1,2-Dibromocyclohexane

Question 17

Explain the relative resistance to bromination of benzene compared to phenol and compared to cyclohexene.

Answer 17

Benzene electrons or π-bonds are delocalised
Phenol a lone or non-bonded pair of electrons on the oxygen
Cyclohexene electrons are localised Benzene has a lower electron density
Benzene cannot polarise or induce a dipole in Br2

Question 18

In contrast to benzene, the reaction of an alkene with bromine does not need a halogen carrier.Compare the different reactivities of benzene and alkenes towards chlorine.

Answer 18

In benzene, π electrons are delocalised/spread out In alkenes, π electrons are concentrated between 2 carbons Electrophiles attracted more to greater electron density in alkenes

Question 19

Describe with the aid of suitable diagrams the bonding and structure of a benzene molecule.

Answer 19

P-orbitals overlapAbove and below the ring (to form) π-bonds / orbitals(π-bonds / electrons) are delocalised Ring is planar /C-C bonds are equal length σ-bonds are between C-C and/or C-HBond angles are 120°

Question 20

Name an important chemicals that are made from benzene.

Answer 20

Acetone

Question 21

What type of reaction takes place between alkenes and bromine?

Answer 21

Electrophillic addition reaction.

Question 22

Define delocalised electrons

Answer 22

Electrons that are not associated with a single atom or a single covalent bond.

Question 23

Define addition reaction

Answer 23

A reaction in which a reactant is added to an unsaturated molecule to make a saturated molecule.

Question 24

Define substitution reaction

Answer 24

A reaction in which an atom or group of atoms is replaced by a different atom or group of atoms.

Question 25

Define electrophile

Answer 25

An atom that is attracted to an electron rich centre or atom, where it accepts a pair of electrons to form a new covalent bond.

Question 26

The shape around each carbon is…

Answer 26

tetrahedral.

Question 27

How is benzene different from cyclohexene?

Answer 27

Benzene is an aromatic ring with alternating single and double bonds, while cyclohexene is all single bonds.