Chapter 25 - Aromatic Compounds

Question 1

What does the Kekule model suggest?

Answer 1

It suggests that benzene was based on a six members ring of carbon atoms joined by alternate single and double bonds

Question 2

What evidence is there to disprove the kekule model?

Answer 2

• doesn’t undergo electrophilic addition with bromine
• more endothermic than expected
• intermediate bond lengths

Question 3

Describe the structure of benzene

Answer 3

6 carbon atoms and 6 hydrogen atoms - hexagonal.
Therefore each carbon only used three of four electrons in bonding.
Each carbon has one electron in a p orbital and adjacent p orbitals overlap sideways and form a ring of electrons density.
This creates a pi-system which spreads over all 6 carbons.
The 6 electrons in the system are said to be delocalised.

Question 4

How do you name aromatic compounds (with benzene ring)?

Answer 4

Halogens and nitro groups are prefixes

When a benzene ring is attached to an alkyl chain of 7 or more or an alkyl chain with functional group the prefix phenyl is used (benzene is seen as a substituent)

Ring is numbered using lowest combination of numbers

Question 5

What are the conditions needed for nitration of benzene?

Answer 5

50 Celsius and a H2SO4 catalyst

• If above 50 Celsius further substitutions may take place

Question 6

Reaction mechanism of the nitration of benzene

Answer 6

1) electrophile NO2+ is produced by reaction of conc nitric acid and sulfuric acid (hno3 +h2so4 = no2+ + hso4- + h20)
2) electrophile accepts pair of electrons from benzene ring.
3) dative covalent bond formed
4) Pair of electrons transferred from carbon hydrogen bond to benzene ring and thus intermediate breaks down
5) Breaks down into nitrobenzene and H+ ions. H+ ions react with HSO4- to regenerate catalyst.

Question 7

What are the two halogen carriers?

Answer 7

Al and Fe

Question 8

Describe the bromination of benzene

Answer 8

1) bromonium Ion formed when halogen carrier reacts with bromine
2) Bromonium ion accepts a pair of electrons from benzene ring. Dative covalent bond formed. (elctrophilic sub)
3) Pair of electrons transferred from carbon hydrogen bond to benzene ring and thus the organic intermediate is unsatble and breaks to form bromobenzene and a hydrogen ion)
4) H+ ion reacts with FeBr4- ion to regenerate FeBr3 catalyst

Question 9

Describe the alkylation reaction of benzene.

Answer 9

Alkyl group replaces hydrogen atoms in the presence of AlCl3 ( which acts as a halogen carrier catalyst, generating the electrophile)

Formation of electrophile with haloalkane and alcl3 = Eg c2h5+ + alcl4-

Question 10

Reactivity of cyclohexene

Answer 10

Pi bond in cyclohexene contains localised electrons (between the bonding C atoms) above and below the plane. This high electron density induces dipole in non-polar bromine and the slightly positive side makes it act like a good electrophile

Question 11

Reactivity of benzene

Answer 11

Has delocalised pi electrons above and below the plane of the carbon atoms. It’s electrons are delocalised so the electron density is less than a double bond and insufficient to polarise bromine. No reaction can take place unless a halogen carrier is present.

Question 12

Solubility of phenol vs alcohol

Answer 12

Phenol is less soluble in water than alcohols due to non-polar benzene ring but it does dissociate in water.

Question 13

Compare the reactivity of phenols, carboxylic acids and ethanol.

Answer 13

Phenol is more acidic than alcohols but less acidic than carboxylic acids.
Phenols and carboxylic acids react with solutions of strong bases (NaOH)
Ethanol doesn’t react with strong or weak bases
Carboxylic acids are strong enough to react with weak bases (NaCO3)

Question 14

What is phenol?

Answer 14

A hydroxyl -OH group attached to a benzene ring directly

Question 15

Bromination of phenol

Answer 15

Halogen carrier not required

Bromine decolourises and a white precipitate forms

Question 16

Why is phenol more reactive that benzene?

Answer 16

Because the lone pair of electrons from oxygen p-orbitals are delocalised into the ring. This means the electron density in the benzene ring of phenol is increased and is higher than in benzene. Phenol is more susceptible to attack from electrons than benzene

Question 17

What type of group is -NH2?

Answer 17

2 or 4 directing group

Question 18

What type of group is -OH?

Answer 18

2 or 4 directing group

Question 19

What type of group is -C6H5?

Answer 19

2 or 4 directing group

Question 20

What type of groups are -F, -Cl, -Br, -I?

Answer 20

2 or 4 directing groups

Question 21

What type of group is -NO2?

Answer 21

3 directing group

Question 22

What type of group is -COOH ?

Answer 22

It is a 3 directing group

Question 23

What type of group is -CHO?

Answer 23

It’s a 3 directing group

Question 24

What type of group is -CN?

Answer 24

It is a 3 directing group

Question 25

Describe the alkylation of benzene

Answer 25

Hydrogen atoms are substituted by an alkyl group in the presence of AlCl3. Carried out by reacting benzene with a haloalkane

Question 26

What are the conditions for the nitration of phenol? And what are the products?

Answer 26

- They are milder than those required for the nitration of benzene and only dilute nitric acid is required - Makes two products: 2-nitrophenol and 4-nitrophenol (and water)

Question 27

What do all 2, 4 directing groups do? (Apart from the halogens)

Answer 27

They activate the benzene ring making it more susceptible to attacks from electrophiles

Question 28

What do all 3 directing groups do?

Answer 28

They deactivate the benzene ring making it less susceptible to attacks from electrophiles

Question 29

Catalyst and reagent for nitration of benzene

Answer 29

``` Catalyst = H2SO4 Reagent = HNO3 ```

Question 30

Catalyst and reagent for bromination of benzene

Answer 30

``` Catalyst = FeBr3 Reagent = Br2 ```

Question 31

Catalyst and reagent for chlorination of benzene

Answer 31

``` Catalyst = AlCl3 Reagent = Cl2 ```

Question 32

Naming benzene with a methyl and other functional groups

Answer 32

- You do not have to state the position of the methyl - Name the compound relative to the methyl group - *........* methylbenzene

Question 33

Name: 1) Benzene attached to -COOH 2) Benzene attached to -NH2 3) Benzene attached to -CHO

Answer 33

1) Benzoic acid 2) Phenylamine 3) Benzaldehyde These do not obey the normal naming method for benzene

Question 34

What type of reactions take place when a hydrogen atom on benzene is substituted by another atom or group of atoms?

Answer 34

Electrophilic substitution

Question 35

What is nitrobenzene used for?

Answer 35

Starting material for dyes, pharmaceuticals and pesticides

Question 36

What are phenols used in?

Answer 36

Antiseptics | Cleaners

Question 37

Are 2,4 or 3 directing groups electron withdrawing?

Answer 37

3 directing groups

Question 38

- Formula of benzene - Empirical formula of benzene - state of benzene at room temp

Answer 38

- C6H6 - CH - liquid

Question 39

3 features of benzene disproving kekules model

Answer 39

- Benzene is resistant to addition reactions - Enthalpy change of hydrogenation of benzene is more stable than predicted - all carbon bonds are same length

Question 40

What technique used to find bond lengths of benzene

Answer 40

X-ray diffraction

Question 41

What happens to 4th electron in the p-orbital of each carbon atom in benzene

Answer 41

It delocalises to form rings of electron density above and below the hexagon forming rings of delocalised electron density above/ below the hexagon

Question 42

How do rings of electron density affect stability of benzene

Answer 42

Makes benzene very stable, even though it’s unsaturated (aromatic stability)

Question 43

Why does benzene have a relatively high mp

Answer 43

Close packing of flat hexagonal molecules when solids

Question 44

Is benzene soluble in water | - dangers of benzene

Answer 44

No bc it’s non polar | -carcinogen

Question 45

How do you name compounds containing a benzene ring?

Answer 45

-benzene or phenyl- ; can designate position on ring unsing numbers if there is more than one substituent

Question 46

Why is benzene attacked by electrophiles?

Answer 46

High electron density above and below ring due to delocalised electron (electrophile =electron pair acceptor)

Question 47

What reaction is nitration of benzene

Answer 47

Electrophilic substitution

Question 48

- Which ion is used to nitrate benzene? | - catalyst in nitration of benzene

Answer 48

- NO2+ (nitronium ion) - sulfuric acid - >50degrees Celsius

Question 49

How is NO2+ ion generated (conditions and equations)

Answer 49

-Concentrated H2SO4 and Concentrated HNO3 - H2SO4 + HNO3 = H2NO3+ + HSO4- - H2NO3+ = H20 + NO2+ Overall = H2SO4 + HNO3 = HNO4- + NO2+ + H2O

Question 50

Overall equation for nitration of benzene

Answer 50

C6H6 + HNO3 = C6H5NO2 + H2O

Question 51

- What type of catalyst is used for a Acylation reaction ( Friedel-Crafts reaction) - Why does benzene not react directly with halogens

Answer 51

- A halogen carrier (eg AlCl3) | - The aromatic ring is too stable

Question 52

Nucleophile

Answer 52

A species that donates a pair of electrons

Question 53

Acylation reaction formation of reactive intermediate

Answer 53

Benzene delocalised leads to it acting as a nucleophile leading to their attack on Acyl chlorides. In order to take place a reactive intermediate must be produced from a reaction between acyl chloride and an aluminium chloride catalyst An acyl chloride is a ketone attached to a chloride and a r group ( an alkane) reacts with AlCl3 to form the electrophile (R-CO +) and AlCl4- === reactive intermediate

Question 54

Acylation of benzene reaction

Answer 54

The reactive intermediate (R-CO+) is then attacked by the benzene ring (nucleophile) R-CO+ attacked by benzene ring At the end of the reaction, H+ ion removed from ring reacts with AlCl4- ion ( formed from reactive intermediate formation) to reform AlCl3 bc it’s a catalyst. It also releases HCL gas

Question 55

Halogenation of benzene

Answer 55

Halogens don’t react with benzene unless a catalyst called a halogen carrier is present. They can be generated in situ from the metal and the halogen

Question 56

How could you use a acylation mechanism to add a methyl group to a benzene ring

Answer 56

Use a halogenoalkane and AlCl3 to create an electrophile that can attack benzene

Question 57

Phenol properties

Answer 57

- Phenol less soluble in water than alcohols due to presence of non-polar benzene ring - When dissolved in water, phenol partially dissociates to form a phenoxide ion and a proton. This ability to dissociate classifies phenol as a weak acid. - More acidic than alcohols but less than carboxylic acids as seen by comparing acid dissociation constant Ka -Phenols react with solutions of strong bases such as aqueous NaOH but too weak to react with weak bases such as Sodium carbonate = thus a reaction with sodium carbonate can be used to distinguish between a phenol and a carboxylic acid as the carboxylic acid will react with sodium carbonate to produce carbon dioxide which is evolved as a gas

Question 58

When reacting phenol with a metal or base what is substituted? What about if not w metal or base

Answer 58

The H from the OH Anything else (eg cation) the h from the ring

Question 59

What is phenol colour and standard state

Answer 59

Colourless solid bc of the presence of hydrogen bonds making the melting point higher than those of hydrocarbons of comparable molecular mass

Question 60

Why are pheonols stronger acids than alcohols

Answer 60

The ethoxide ion readily accepts a proton to form ethanol because alkyl groups will release electrons along the sigma mind which increases electron density of the oxygen (positive inductive effect = equilibrium lies to left) Whereas a phenoxide ion a P- orbital of oxygen atoms overlaps with the pi system of the ring of carbon atoms So the O- of pheonoxide ion is less likely to accept a proton to form C6H5OH than ethoxide ion C2H50- is Therefore the phenoxide ion is less basic than ethoxide ion which means equilibrium lies slightly further to right for phenol thus more acidic than ethanol

Question 61

What would you see if you reacted phenol and a sodium

Answer 61

Effervescence ( not with metal hydroxide = no obsevable reaction)

Question 62

What are reactions of phenol

Answer 62

Either: 1) reactions of the phenol group 2) substitutions in the aromatic ring (electrophilic)

Question 63

Esterification of phenol

Answer 63

Occurs less readily then it does with alcohols. By converting phenol into the phenoxide ion, it’ll react more readily with carboxylic acids These phenyl esters are used to make: - polyesters - some types of liquid crystal displays

Question 64

How do alkenes decolorise bromine and why can’t benzene in the same way

Answer 64

Alkenes do so by electrophilic addition (eg cyclohexene) as in this reaction the bromine adds across the double bond in cyclohexene 1) the pi bond in alkene contains localised electrons above and below the plane of two carbon atoms producing high electron density 2) the localised electrons in the pi bond induce a dipole in non polar bromine etc... (so on w electrophile and electrophilic addition) Whereas benzene can’t react w bromine unless halogen carrier present as benzene has DElocalised electrons spread above and below the plane of carbon atoms - electron density between any two carbon atoms in benzene is less than then in a CC double bond Thus there is insuffiecnt pi electron density around any two carbon atoms to polarise bromine molecules

Question 65

How does bromine react with phenol

Answer 65

Multiple substitutions to produce 2,4,6- tribromophenol which forms a white precipitate. It decolourises bromine water

Question 66

Why is phenol more reactive than benzene

Answer 66

Increased reactivity is due to a lone pair of electrons on the oxygen atom which is partially delocalised into the benzene ring structure. Increases electron density making less stable and thus more susceptible to attack from electrophile

Question 67

What will electrophilic substitution of phenol with dilute nitric acid form

Answer 67

A mix of 2-nitrophenol and 4-nitrophenol. Nitration with phenol doesn’t require a catalyst due to increased reactivity of phenol in comparison to benzene. The NO2 group is electron withdrawing and thus makes benzene ring more stable (decreased electron density) happens at room temp

Question 68

2 and 4 directing groups (ortho- and para directing)

Answer 68

Eg OH, NH2, the halogens All direct on the benzene ring to the positions 2 (ortho) and 4 (para) and 6 (ortho)because it’s the same carbon environment as 2 just opposite so they’re the same These directing groups dontate their p orbital ( lone pair oxygen electrons ) into the benzene ring thus increases electron density. They are ring activating groups (except the halogens) OCCUR TO FABOUR MOST STABLE INTERMEDIATE

Question 69

3 - directing groups (meta)

Answer 69

Eg NO2, COOH, CHO Direct the substituting compound to 3 (and 5) on the benzene ring Decrease electron density Ring deactivating OCCUR TO FABOUR MOST STABLE INTERMEDIATE