Benzene

Question 1

Which model of benzene is this?

Answer 1

Kekulé’s model

Question 1

Which model of benzene is this?

Answer 1

Delocalised model

Question 2

Describe the structure and bonding in Kekulé’s model of benzene.

Answer 2

P-orbitals overlap to form pi-bonds.

Pi-electrons are localised between carbon atoms.

Alternating pi-bonds.

Question 3

What is the similarity between Kekulé’s benzene model and the delocalised benzene model?

Answer 3

Both have overlap of p-orbitals.

Question 4

Describe the delocalised model of benzene. key words

Answer 4

P-orbitals overlap to form pi-bonds,

pi electron are delocalised

more stable

Question 5

What is the evidence to suggest that Kekulé’s model of benzene isn’t correct?

Answer 5

Carbon-carbon bond lengths are all the same size in benzene, but not in Kekulé’s model.

Enthalpy of hydrogenation of benzene is less exothermic than Kekulé’s model.

Benzene is less reactive than alkenes - bromination requires a catalyst for benzene but not for alkenes.

Question 6

Name this molecule.

Answer 6

Phenylamine

Question 7

Name this molecule.

Answer 7

Benzonic acid

Question 8

Catalyst for benzene electrophilic substitution - nitration.

Answer 8

Concentrated Sulfuric Acid

Question 9

Conditions for benzene electrophilic substitution - nitration, and why.

Answer 9

Less than 50°C to prevent further substitutions occurring.

Question 10

Inorganic product for benzene electrophilic substitution - nitration.

Answer 10

Water

Question 11

Electrophile generator step for benzene electrophilic substitution - nitration.

Answer 11

Question 12

Mechanism step for benzene electrophilic substitution - nitration.

Answer 12

Question 13

Catalyst regeneration step for benzene electrophilic substitution - nitration.

Answer 13

Question 14

Catalyst for benzene electrophilic substitution - halogenation.

Answer 14

Halogen carrier - AlCl3 or FeCl3

at room temp

Question 15

Inorganic product for benzene electrophilic substitution - halogenation.

Answer 15

Hydrogen halide

e.g. HBr

Question 16

Catalyst generation for benzene electrophilic substitution - halogenation.

Answer 16

Question 17

Mechanism for benzene electrophilic substitution - halogenation.

Answer 17

Question 18

Catalyst regeneration for benzene electrophilic substitution - halogenation.

Answer 18

Question 19

Reactant for Friedel-Crafts alkylation.

Answer 19

Haloalkane

Question 20

Catalyst and conditions for Friedel-Crafts alkylation.

Answer 20

Halogen carrier - AlCl3 or FeCl3
and reflux

Question 21

Inorganic product of Friedel-Crafts alkylation.

Answer 21

Hydrogen halide

e.g. HCl

Question 22

Reactant of Friedel-Crafts acetylation.

Answer 22

Acyl chloride

e.g.CH3COcl

Question 23

Catalyst for Friedel-Crafts acetylation.

Answer 23

Halogen carrier - AlCl3 or FeCl3

and reflux

Question 24

Inorganic product of Friedel-Crafts acetylation.

Answer 24

Hydrogen halide

Question 25

Structure of acyl chloride.

Answer 25

Question 26

Describe and explain why benzene is more resistant to bromination than alkenes.

Answer 26

In alkenes, electrons in the pi-bond are localised between carbon atoms. In benzene, electrons in the pi-bond delocalise into the delocalised ring pi-system. Alkenes therefore have a higher electron density, so can polarise electrophiles more, making them more susceptible to electrophilic attack.

Question 27

Benzene is... (properties)

Answer 27

* A colourless, sweet smelling, highly flammable liquid * Found naturally in crude oil, is a component of petrol and cigarette smoke * Classified as a carcinogen All bond angles are 120o

Question 28

benzene is a....

Answer 28

simple aromatic hydrocarbon

Question 29

A B Describe with aid of diagram showing overlapping orbitals, the difference in bonding between structure A and B.

Answer 29

1- p orbitals overalp to form pi bonds 2- B - Pi bonds are delocalised 3- A - Pi bonds are localised between 2 carbons 4- diagram

Question 30

predictions from both models

Answer 30

Question 31

name this molecule

Answer 31

nitrobenzene

Question 32

name this molecule

Answer 32

benzaldehyde

Question 33

name this molecule

Answer 33

phenyl ethanone

Question 34

name this molecule

Answer 34

2- phenyl octane

Question 35

name this

Answer 35

3- chloromethylbenzene

Question 36

name this

Answer 36

1,3 -dimethylbenzene

Question 37

name this

Answer 37

4- chloromethyl benzene

Question 38

name this

Answer 38

2- hydroxy benzoic acid

Question 39

describe in terms of orbital overlap the similarities and differences between the bonding in the kekule model and delocalised model of Benzene (3 marks)

Answer 39

Similarities: 1- Overlap of P orbitals 2- Pi bonds above and below bonding c atoms Difference: 3- K - alternating Pi bond 4- D- Pi ring, all P orbitals overalp 5- Pi electron spread around ring 6- diagram

Question 40

why does bromine react more readily with cyclohexene than benzene

Answer 40

- in benzene the pi electrons are delocalised and in cyclohexene they are localised - benzene has a lower electron density - so benzene polarises bromine less

Question 41

molecular formula of benzene

Answer 41

C6H6

Question 42

Compare the combustion of ethane and ethene

Answer 42

ethene - sooty flame - less H (benzene same)

Question 43

define electrophilic substitution

Answer 43

replacing an atom/ group of atoms with an electrophile

Question 44

steps for reaction of benzenes

Answer 44

1- formation of electrophile 2- substation with benzene ring 3- regeneration of catalyst

Question 45

whole equation for nitration of benzene

Answer 45

Question 46

why do we need a halogen carrier

Answer 46

electrons in the ring is not high enough to polarise the Br2

Question 47

what is a alkylation reaction

Answer 47

Alkylation reactions substitute a hydrogen atom on a benzene ring for an alkyl group

Question 48

why is benzene unreactive

Answer 48

pi electrons delocalsied across the ring electron density is spread out (delocalised) across the ring less reactive towards electrophiles cant polarise molecule

Question 49

The student is told by a friend that the fecl3 catalyst is not needed in quinol (2 lots of OH in benzene) as it is more reactive than benzene, explain why this is correct

Answer 49

1- lone pairs of electron on the oxygen atom 2- is donated to/ partially delocalised into the Pi system 3- making quinol more susceptible to electrophile attack 4- can polarise the molecules more

Question 50

Answer 50

D - only 1

Question 51

explain why chlorine reacts much more readily with c6H5N(ch3)2 than with benzene

Answer 51

1- lone pair of electron on N is partially delocalised into the ring 2- electron density increases 3- cl2/ electrophile is more polarised 4- cl2 is more attacked

Question 52

Answer 52

Pi bond in benzene are delocalised

Question 53

A - kek B- deloc model

Answer 53

1- structure B is more stable 2- structure B better as enthalpy change of hydration of benzene is less exothermic than -357kj mol-1

Question 54

what is the number of sigma bonds in a benzene molecule

Answer 54

12

Question 55

delocalised structure pi bonds are...

Answer 55

delocalised

Question 56

Kekules structure (alkenes) Pi bonds are

Answer 56

localised

Question 57

why is benzoic acid least reactive (over benzene and phenol)

Answer 57

-COOH group is electron withdrawing group - electron density lower - ring less susceptible to attack

Question 58

using 13C NMR

Answer 58

state environment e.g. 6 environments therfore 6 peaks if same environment cant distinguish