C4.2 Aromaticity

Question 1

Benzene molecular formula

Answer 1

C6H6

Question 2

Aliphatic

Answer 2

straight chains

Question 3

Aromatic (and one property)

Answer 3

ring structure
sweet smelling

Question 4

naming if benzene is the main group

Answer 4

-benzene

Question 5

If benzene is a side chain

Answer 5

-phenyl

Question 6

Describe chlorobenzene’s structure

Answer 6

benzene with one chlorine functional group

Question 7

Describe nitrobenzene’s structure

Answer 7

benzene with one NO2 functional group

Question 8

Describe methylbenzene’s structure

Answer 8

Benzene with one CH3 functional group

Question 9

Describe Kekule’s structure of benzene

Answer 9

6 carbons in a ring with alternating C-C and C=C bonds with one hydrogen bonded to each carbon

Question 10

List 3 three pieces of evidence against Kekule’s model

Answer 10

• The molecule’s carbon to carbon bonds all having the same bond length.
• Lack of electrophilic addition reactions.
• Benzene’s lower than expected enthalpy of hydrogenation.

Question 11

Describe why The molecule’s carbon to carbon bonds all having the same bond length disproves Kekule’s structure.

Answer 11

Using x-rays we can study the location of atoms in a molecule. A double bond is much shorter than a single bond, but all the bonds are the same length in a benzene molecule. Each bond within the structure is the same length and between the expected length of a single and double bond. If Kekule’s structure was correct, half the bond would be single bond length and the other half at double bond length.

Question 12

Describe why benzene not undergoing electrophilic addition disproves Kekule’s structure.

Answer 12

the double bonds within alkenes are reactive functional groups and they undergo electrophilic addition reaction to get rid of the double bond.
With benzene, electrophilic substitution occurs. The benzene circle remains unchanged at the end of the reaction, suggesting that the benzene circle is more stable than double bonds in alkenes.

Question 13

Describe why benzene’s lower than expected enthalpy of hydrogenation disproves Kekule’s structure.

Answer 13

Hydrogenation enthalpy is the energy change as hydrogen is added to an unsaturated molecule (C=C) to change it into a standard molecule (C-C). It’s possible to estimate the hydrogenation enthalpy of benzene from information about the hydrogenation enthalpy of one double bond.
Benzene is +152kJ/mol more stable than Kekule’s structure.

Question 14

What is meant by delocalisation enthalpy or resonance enthalpy?

Answer 14

The difference between any enthalpy value for Kekule’s structure and any value measured for benzene.

Question 15

What are the bon angles between the sp2 orbitals?

Answer 15

120

Question 16

What happens to C third p orbital in benzene?

Answer 16

Each p orbital can overlap side by side with the two p orbitals by its side to create a circle of 6 overlapping orbitals.

Question 17

The p orbitals combine to form a cloud of…

Answer 17

electron density above and below the carbon circle.

Question 18

The circle in benzene represents the cloud of electron density called…

Answer 18

π system

Question 19

Electrophilic substitution reactions maintain the ________ of the benzene ring.

Answer 19

stability

Question 20

Nitration of benzene reagents, conditions and reaction classification.

Answer 20

Reagents: concentrated sulphuric acid and concentrated nitric acid.
Conditions: 50*C
Classification: Electrophilic substitution reaction (NO2 is an electrophile)

Question 21

During the nitration of benzene, a hydrogen is substituted for a…

Answer 21

NO2 group

Question 22

Halogenation of benzene requires a catalyst, usually:

Answer 22

Aluminium chloride or Iron(111) chloride (replaced with the halogen)

Question 23

Halogenation of benzene conditions and reaction classification:

Answer 23

Conditions: room temperature in darkness
Classification: Electrophilic substitution

Question 24

During the second step of nitration and halogenation of benzene, a ___________ is produced

Answer 24

Wheland intermediate

Question 25

Product of benzene nitration

Answer 25

Benzene with NO2 group attached and H+ ion

Question 26

Product of the halogenation of benzene

Answer 26

Benzene with a halogen group and HX.
X representing the halogen

Question 27

What occur during the Friedel Crafts Reaction

Answer 27

Forming a C-C bond.
Substitution reaction between halogenoalkane and benzene ring.

Question 28

What catalyst must be present for Friedel crafts Reaction?

Answer 28

Aluminium chloride

Question 29

If chlorobenzene and chloroalkanes are heated to 100*C with sodium hydroxide(aq), what would happen and explain.

Answer 29

Chloroalkane substitutes Cl for OH group. No reaction with chlorobenzene.
much more energy is require to break the C-Cl bond in chlorobenzene, which leads to a far higher activation energy.

Question 30

Explain the acidity of phenol.

Answer 30

Donates protons.
More acidic than an aliphatic alcohol.
One of the lone pairs on the oxygen atom overlaps with the delocalised electrons on the benzene ring.
interact with pi system to strengthen the bond.

Question 31

Explain the basicity of phenylamine

Answer 31

Base accepts protons
Lone pair of electrons from the NH3 interact with pi system.
The compound is less likely to accept H= ions.

Question 32

Classify the reaction mechanism for benzene nitration.

Answer 32

Substitution reaction.

Question 33

Why do you need a catalyst in the reaction between chloromethane and benzene?

Answer 33

cause polarity

Question 34

Describe the structure of, and bonding in benzene and explain why benzene is less ready to undergo addition reactions than alkenes.

Answer 34

• C6H6 ring hexagonally shaped
• each C is covalently bonded to 2 other C ad one H
• planar molecule
• 120* bond angles
• each C has an unbonded p-orbital and they all overlap to form a cloud of electron density above and below the carbon ring
• π system
• alkenes have C=C, attracts electrophiles more readily, causing an addition reaction.
• benzene’s π system makes it more stable that C=C so takes a lot more delocalisation energy to break the bond, so benzene undergoes substitution.