module 6.1.1 - aromatic compounds

Question 1

briefly describe Kekule’s Benzene

Answer 1

six memebered cyclic structure with alternating single and double bonds (cyclohexa - 1,3,5, - triene)

Question 2

give the 3 pieces of evidence as to why scientists disagreed with Kekule’s model of benzene:

Answer 2

1. reactivity with bromine
2. bond lengths
3. enthalpy of hydrogenation

Question 3

explain why benzenes reactivity with bromine disproved Kekule’s model of benzene

Answer 3

benzene does not undergo electrophilic addition with bromine like alkenes typically do, and there is no colour change when bromine is added to benzene. Benzene can react with bromine in the presence of a halogen carrier catalyst in electrophilic substitution.

Question 4

explain why bond lengths in benzene disproved Kekule’s model of benzene

Answer 4

benzene has a regular structure which is planar with equal carbon carbon bond lengths. C C double bonds are shorter than C-C bonds so Kekule’s model didn’t predict a regular hexagon for benzene.

Question 5

explain why the enthalpy of hydrogenation of benzene disproves Kekule’s model of Benzene

Answer 5

the actual enthalpy of hydrogenation of benzene is less exothermic than Kekule’s suggested cyclohex - 1,3,5 - triene

Question 6

describe the shape of benzene

Answer 6

benzene is a planar cyclic molecule with bon angles of 120 degrees between carbon atoms.

Question 7

describe the electron structure/bonding of benzene

Answer 7

each carbon forms 3 sigma bonds. the 4th electron of each carbon is contained in a p-orbital. the p-orbitals overlap sideways, above and below the plane of the ring to form a pi bond. 6 pi electrons are delocalised over the 6 carbon atoms.

Question 8

in terms of a question about the delocalised model of benzene, describe an alkene

Answer 8

1. sideways overlap of p-orbitals
2. pi bonds above and below carbon ring
3. 3 localised pi bonds
4. 2 electrons in each pi bond
5. overlap of orbitals in one direction.

Question 9

in terms of a question about the delocalised model of benzene, describe the structure of benzene

Answer 9

1. sideways overlap of p - orbitals
2. pi bond (ring) above and below carbon ring
3. one delocalised pi ring
4. 6 electrons in one pi bond (ring)
5. overlap in both directions.

Question 10

compare the reactivity of an alkene and benzene in terms of reactions with Br2

Answer 10

in alkenes, the pi electrons are localised and there is a high electron density, which is able to polarise and attract the Br2 molecule for and electrophilic addition reaction.
in benzene, the pi electrons are delocalised, giving a lower electron density and benzene cannot polarise and attract a Br2 molecule, so it cant undergo electrophilic addition.
benzene can react with Br2 in electrophilic substitution reactions in the presence of a halogen carrier catalyst.

Question 11

define arene

Answer 11

hydrocarbon containing at lease one benzene ring

Question 12

define aromatic

Answer 12

compounds containing benzene rings

Question 13

when is the word phenyl used?

Answer 13

when a benzene ring is already a part of an already well known and named molecule. eg. phenol or phenylethene.

Question 14

explain why benzene cant undergo electrophilic addition.

Answer 14

in alkenes (which can undergo electrophilic addition) the localised 2 pi electrons cause a high electron density, so the alkene can polarise and attract an electrophile. benzene has 6 delocalised pi electrons, so has a lower electron density, and cant polarise or attract an electrophile, and cant undergo electrophilic addition

Question 15

what is the name of the mechanism that allows benzene to react with halides? what are the reagents?

Answer 15

electrophilic substitution. a halogen carrier catalyst is required.

Question 16

in electrophilic substitution: what produces an electrophile for benzene to react with?

Answer 16

reaction between catalyst and the other reactant. produces a positive ion (heterolytic fission) that is attracted to the electrons in the delocalised pi ring.

Question 17

explain what happens to the delocalised pi ring during electrophilic substitution.

Answer 17

there are 6 pi electrons in benzene spread over the 6 ring carbon atoms.
during the reaction, 2 of these electrons go into making a bond between the electrophile and a carbon.
the intermediate now has 4 pi electrons spread over 5 carbon atoms.
a H atom is lost in the last step, which gives 2 electrons back to the benzene ring, which re-established the delocalised pi system.

Question 18

what is the name of the reaction when a Br+ electrophile reacts with an aromatic compound?

Answer 18

halogenation

Question 19

what is the name of the reaction when an “acyl” electrophile reacts with an aromatic compound?

Answer 19

Friedel-Crafts Acylation

Question 20

what is the name of the reaction when a NO2+ electrophile reacts with an aromatic compound?

Answer 20

nitration

Question 21

what is the name of the reaction when an alkyl electrophile reacts with an aromatic compound?

Answer 21

Friedel-Crafts Alkylation

Question 22

give and example of the uses of Bromobenzene

Answer 22

preparation of pharmaceuticals

Question 23

give 2 examples of what chlorobenzene is used for

Answer 23

as a solvent and in the preparation of pesticides

Question 24

what are the conditions for the nitration of an aromatic compound?

Answer 24

Concentrated HNO3, contrentrated H2SO4 and 50-60 degrees celsius

Question 25

what happens when temperatures higher than 50-60 degrees celsius are used in the nitration of benzene?

Answer 25

multi-substitution

Question 26

what is the nitrobenzene used for

Answer 26

starting material in the preparation of dyes and pharmacuticals

Question 27

what can the nitration of methylbenzene produce?

Answer 27

TNT: 2,4,6 - trinitromethylbenzene aka trinitrotoluene

Question 28

what is Friedel-Crafts alkylation used for

Answer 28

forming C-C bonds

Question 29

what is a phenol?

Answer 29

phenols have an -oh group attached to the benzene ring.

Question 30

why is phenol partially soluble in water?

Answer 30

hydrogen bonds can be formed between the -OH and the water molecules, but the benzene ring is non-polar and not soluble in water

Question 31

what is the equation for the dissociation of phenol to form a weak acid?

Answer 31

C6H5OH ⇌ C6H5O– + H+

Question 32

give 2 uses of phenol.

Answer 32

an antiseptic in disinfectants, and in the synthesis of dyes and pharmaceuticals.

Question 33

what is the name of the salt formed when phenol reacts with a metal or metal hydroxide

Answer 33

phenoxide salt

Question 34

what type of metal compounds does phenol not react with?

Answer 34

doesn’t react with metal carbonates, as it is not acidic enough.

Question 35

explain why phenol can undergo bromination without the presence of a halogen carrier catalyst.

Answer 35

1. phenol reacts with bromine at room temperature in an electrophilic substitution reaction without a halogen carrier catalyst
2. there is a lone pair of electrons on the O atom, which is in a p-orbital, which are delocalised into the benzene ring.
3. this increases the electron density of the molecule, activating it.
4. the greater electron density allows the ring to polarise and attract a Br2 molecule.
5. produces 2,4,6 - tribromophenol

Question 36

what would be observed in the bromination of phenol?

Answer 36

colour change from orange to colourless
white precipitate formed (2,4,6-tribromophenol)

Question 37

why does multi-substitution occur in the bromination of phenol

Answer 37

phenol has a high electron density.

Question 38

what are the reagents for nitration of phenol?

Answer 38

dilute HNO3. no H2SO4 catalyst is required.

Question 39

what are the 2 possible products of nitration of phenol

Answer 39

2-nitrophenol and 4-nitrophenol

Question 40

what is a directing group?

Answer 40

a group attached to a benzene ring will direct incoming electrophiles to a certain carbon atom.

Question 41

what is an activating group?

Answer 41

have a lone pair that is delocalised into the ring, which activates it, this causes higher electron density at carbons 2 and 4. e.g.. -OH or -NH3

Question 42

what is a deactivating group?

Answer 42

withdraw electron density from the ring, deactivating it. there is a high electron density remaining over carbon-3. e.g. NO2

Question 43

what is the directing effect of OH?

Answer 43

activating and directs to carbons 2 and 4

Question 44

what is the directing effect of

Answer 44

activating and directs to carbons 2 and 4

Question 45

what is the directing effect of

Answer 45

deactivating and directs to carbon 3