Chapter 25 - Aromatic Compounds

Question 1

What is an aromatic compound?

Answer 1

A compound that contains a benzene ring

Question 2

What is benzene and describe the structure:

Answer 2

Benzene, C6H6, is the simplest arene (aromatic hydrocarbon)

Question 3

What are some derivatives of benzene?

Answer 3

Benzaldehyde
Thymol

Question 4

What are the three pieces of evidence which disprove Kekule’s model?

Answer 4

The lack of reactivity of benzene
The lengths of the carbon-carbon bonds in benzene
Hydrogenation enthalpies

Question 5

How does the lack of reactivity of benzene disprove Kekule’s model?

Answer 5

If benzene contained C=C bonds it should decolourise brome in an electrophilic addition reaction
Benzene does not decolourise bromine under normal conditions
Benzene does not undergo electrophilic addition reactions

Question 6

How do the lengths of the carbon-carbon bonds in benzene disprove Kekule’s model?

Answer 6

Using X-ray diffraction enables to measure the C-C bond lengths in benzene
X-ray diffraction produces an electron density map, which reveals the distribution of the electrons
All C-C bond lengths in benzene are equal
These are inbetween the length of a C-C bond and a C=C bond

Question 7

How does hydrogenation enthalpies disprove Kekule’s model?

Answer 7

It would expected to have an enthalpy change of hydrogenation that is 3 times that of cyclohexene
Benzene has a less exothermic enthalpy change of hydrogenation than expected
Benzen has a more stable structure than theoretical Kekule model

Question 8

What is the expected enthalpy change for Kekule’s benzene?

Answer 8

360kJmol^-1
It is actually -208kJmol^-1
Less exothermic than expected

Question 9

What is benzene structurally?

Answer 9

Benzene is a planar, cyclic, hexagonal hydrocarbon containing 6 carbon atoms and 6 hydrogen atoms

Question 10

In the delocalised model of benzene, how many electrons does each carbon atom use?

Answer 10

3 of its available 4 electrons in bonding to 2 other carbon atoms and to 1 hydrogen atom

Question 11

What happens to the 1 electron on each a carbon atom of the delocalised model of benzene which isn’t used in bonding?

Answer 11

Is in a p-orbital which sticks out above and below the planar ring

Question 12

What do the lone electrons in the p-orbitals combine to form?

Answer 12

A delocalised ring of electrons
Adjacent p-orbitals overlap sideways in both directions
This overlapping of the p-orbitals creates a system of π-bonds which spread over all 6 of the carbons

Question 13

Name this molecule:

Answer 13

Methylbenzene

Question 14

Name this molecule:

Answer 14

Nitrobenzene

Question 15

Name this molecule:

Answer 15

Chlorobenzene

Question 16

How does the naming of benzene change when it is attached to an alkyl chain with 7 or more carbon atoms or an alkyl chain with a functional group?

Answer 16

The benzene ring becomes a substituent, and phenyl is used as a prefix

Question 17

Name this molecule:

Answer 17

2-phenyloctane

Question 18

Name this molecule:

Answer 18

Phenylethanone

Question 19

Name this molecule:

Answer 19

1,3-dinitrobenzene

Question 20

Name this molecule:

Answer 20

1,2,4-trimethylbenzene

Question 21

Name this molecule:

Answer 21

2-chloromethylbenzene

Question 22

Name this molecule:

Answer 22

3-chloromethylbenzene

Question 23

Name this molecule:

Answer 23

4-chloromethylbenzene

Question 24

Name this mono-substituted benzene derivate:

Answer 24

Benzoic acid

Question 25

Name this mono-substituted benzene derivate:

Answer 25

Benzaldehyde

Question 26

Name this mono-substituted benzene derivate:

Answer 26

Phenylamine

Question 27

Name this mono-substituted benzene derivate:

Answer 27

Phenol

Question 28

What happens when multiple benzene rings join together?

Answer 28

Fused ring systems form Delocalisation extends over all of the rings so they can't be represented using circles So Kekule model is used

Question 29

Why is benzene not reactive? Use comparisons

Answer 29

Benzene and other aromatic compounds undergo SUBSTITUTION reactions rather than addition reactions to preserve the delocalised electron syste, A catalyst is also need because the π-electrons are delocalised above and below plane The electron density around any 2 carbon atoms is much lower than that in a C=C double bond in an alkene Insufficient electron density to polarised a non-polar molecule Prevents a reaction taking place without catalyst

Question 30

What reactions do benzenes usually undergo and what happens?

Answer 30

Electrophilic substitution reactions A hydrogen atom on the benzene ring is replaced by another atom or group of atoms

Question 31

What must be present for halogens to react with benzene?

Answer 31

A suitable catalyst called a halogen carrier e.g. AlCl3, FeCl3, AlBr3, FeBr3

Question 32

Describe the bromination of benzene:

Answer 32

At RTP and pressure and in the presence of a halogen carrier, benzene reacts with bromine in an electrophilic substitution reaction

Question 33

Describe the electrophilic substitution mechanism for the bromination of benzene

Answer 33

Br+ accepts a pair of electrons from the benzene ring Froms unstable intermediate Bromobenzene and a hydrogen ion are formed Hydrogen bromide is also formed Hydrogen ion reacts with FeBr4 to regenerate the FeBr3 catalyst

Question 34

Describe the electrophilic substitution mechanism for the chlorination of benzene

Answer 34

Cl+ accepts a pair of electrons from the benzene ring Unstable intermediate Chlorobenzene and a hydrogen ion are formed Hydrogen chloride is also formed The hydrogen ion reacts with AlCl4- to regenerate the AlCl3 catalyst

Question 35

What are the conditions for the nitration of benzene to take place?

Answer 35

Catalysed by conc. sulfuric acid The reaction is maintained at a temperature of 50*C by using a water bath

Question 36

Describe the electrophilic substitution mechanism for the nitration of benzene

Answer 36

NO2+ accepts a pair of electrons from the benzene ring Unstable intermediate forms Nitrobenzene and a hydrogen ion are formed The hydrogen ion reacts with HSO4- to regenerate the H2SO4 catalyst

Question 37

What is a Friedel-Crafts (alkylation) reaction?

Answer 37

These reactions involve the substitution of a hydrogen atom in the benzene ring by an alkyl group Benzene is reacted with a haloalkane in the presence of a suitable halogen carrier catalyst Alkylation increases the number of carbon atoms in a compound

Question 38

Describe the electrophilic substitution mechanism for the alkylation of benzene

Answer 38

CH3+ accepts a pair of electrons from the benzene ring Unstable intermediate forms Methylbenzene and a hydrogen ion are formed HCl is also formed Hydrogen ion reacts with AlCl4- to regenerate the AlCl3 catalyst

Question 39

What is an acyl group?

Answer 39

An alkyl group attached to a carbon-oxygen double bond

Question 40

What is a Friedel-Crafts (acylation) reaction?

Answer 40

Substituting an acyl group into a compound (in this case a benzene ring)

Question 41

What happens when benzene reacts with an acyl chloride?

Answer 41

An aromatic ketone is formed In the presence of a halogen carrier such as AlCl3

Question 42

Describe the electrophilic substitution mechanism for the acylation of benzene

Answer 42

CH3C+O accepts a pair of electrons from the benzene ring Unstable intermediate forms Phenylethanone and a hydrogen ion are formed The hydrogen ion reacts with AlCl4- to regenerate the AlCl3 catalyst

Question 43

Compare the solubility of phenols with alcohols

Answer 43

Less soluble in water than alcohols due to the presence of the non-polar benzene ring When dissolved in water, phenol partially dissociates to form a phenoxide ion and a proton

Question 43

What is a phenol?

Answer 43

Are a type of organic compound in which an -OH group is attached directly to a benzene ring HOWEVER if the OH groups is bound to a carbon side chain, rather than the aromatic ring, it is an alcohol, not a phenol