chapter 25

Question 1

simplest arene

Answer 1

benzene

Question 2

molecular formular of benzene

Answer 2

C6H6

Question 3

benzene is not a carcinogen, true or false

Answer 3

false

Question 4

how many models are there to explain benzene’s structure, name them

Answer 4

2
the Kekule model
the delocalized model

Question 5

state the three pieces of evidence to disprove the Kekule model for benzene structure and explain them

Answer 5

1. benzene is unreactive: if benzene has 3 double bonds as suggested by the Kekule model, it would undergo electrophilic addition reactions with Bromine. However, when reacter with bromine water, it does not decolorize it, meaning it does not react with bromine water
2. the lengths of the C-C single bonds in benzene: X-ray diffraction measurements suggest that the C-C bond lengths in benzene are equal, between single and double bond length
3. Hydrogenation enthalpies: when the Kekule version of benzene reacts with hydrogen, the enthalpy change of hydrogenation is estimated to be -360kJmol^-1. However, the actual enthalpy change of hydrogenation for benzene is -208kJmol^-1.

Question 6

the delocalized model of benzene structure suggests that benzene is more stable than estimated in the Kekules structure. what makes benzene more stable in the delocalized model

Answer 6

the cyclic delocalization in the benzene ring in the delocalized model

Question 7

describe the structure of benzene according to the delocalised model

Answer 7

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

Question 8

what does the circle in a benzene structure represent

Answer 8

the delocalised electrons

Question 9

phenyl group

Answer 9

this is a benzene group with one hydrogen atom less

Question 10

what happens when benzene is attached to an alkyl group with 7 or more hydrogen atoms, or with a functional group

Answer 10

the benzene ring becomes a substituent and phenyl is used as a prefix

Question 11

benzene groups can be joined to form fused ring systems. what happens to the delocalization, and also what do we do when drawing the fused ring system

Answer 11

the delocalization extends over all the rings
when drawing the fused ring systems, we cannot represent the delocalized electrons with circles, so we simply use double bonds

Question 12

electrophile

Answer 12

electron pair acceptor

Question 13

why do benzene and other aromatic compounds undergo substitution reactions instead of addition reactions

Answer 13

to preserve the delocalised electron system

Question 14

why is a catalyst needed in the electrophilic addition reactions of benzenes with bromine

Answer 14

pie electrons are delocalized above and below the plane of the carbon atoms

so electron density is more spread out in the benzene ring than in the alkene double bond

the pie electron density around any two carbon atoms is insufficient to polarise a bromine molecule, so a catalyst is required

Question 15

what happens in the electrophilic substitution reactions ofbenzene

Answer 15

a hydrogen atom on the benzene ring is replaced another atom or a group of atoms

Question 16

halogens only react with benzene rings when a …………….. is present

Answer 16

catalyst

Question 17

in electrophilic substitution reactions of benzenes, the catalyst is called a

Answer 17

halogen carrier

Question 18

how are halogen carriers (catalysts) generated in the electrophilic substitution reactions of benzenes

Answer 18

in situ, by adding the metal and the hydrogen in a reaction vessel

Question 19

what catalyst is used in the bromination of benzenes

Answer 19

iron (III) bromide or aluminium bromide

Question 20

products made from the bromination of benzenes

Answer 20

bromobenzene and fumes of hydrogen bromide

Question 21

how is the electrophile generated during electrophilic substitution reactions of benzenes

Answer 21

by reacting the hydrogen carrier(catalyst) with the halogen

Question 22

products from the chlorination of benzenes

Answer 22

chlorobenzene and fumes of hydrogen chloride are produced

Question 23

conditions required for the nitration of benzenes to occur

Answer 23

reaction should be catalysed by concentrated sulfuric acid and heated to 50 degrees celsius

Question 24

what happens if the reaction is allowed to heat above 70 degrees celsius in the nitration of benzenes

Answer 24

further substitution may occur to produce dinitrobenzene

Question 25

the electrophile during the nitration of benzenes is formed by?

Answer 25

reacting the sulfuric acid with the nitric acid to produce the nitronium ion, water and HSO4-

Question 26

the electrophile in the alkylation of benzenes is formed by?

Answer 26

reacting a haloalkane with with AlCl3

Question 27

catalyst in the alkylation of benzenes

Answer 27

AlCl3

Question 28

product formed when benzene reacts with an acyl chloride in the presence of AlCl3

Answer 28

An aromatic ketone

Question 29

catalyst used in acylation of benzenes

Answer 29

AlCl3 or FeCl3

Question 30

phenol

Answer 30

an organic compound in which an OH group is directly attached to the benzene ring

Question 31

if there is one or more carbons between the OH group and the benzene ring, the the compound is classified as an?

Answer 31

Alcohol

Question 32

explain the relative resistance to bromination of benzenes compared to phenol and compared to cyclohexene (think deeply)

Answer 32

in benzene, the pie electrons are delocalized
so the pie electrons have a low electron density
this means benzene is unable to polarise bromine

while
in phenols, the lone pair of electrons on the oxygen of the OH group is delocalized into the pie electron cloud.
so it has a higher electron density in it’s ring, and can therefore polarise bromine

while
in cyclohexene, the pie electrons are localized.
so the pie electrons have a high electron density
meaning cyclohexene can polarise bromine