Chapter 25 - Aromatic Compounds

Question 1

What is the molecular, empirical and skeletal formula of benzene?

Answer 1

C6H6
CH
Hexagon with three double bonds = Kekulé model (disproved)
Hexagon with a circle in the middle = delocalised model

Question 2

What was the evidence to disprove Kekulé’s model?

Answer 2

1. Lack of reactivity
2. Carbon-carbon bond lengths
3. Hydrogenation enthalpies

Question 3

Explain how the lack of reactivity of benzene disproves the Kekulé model?

Answer 3

• if benzene contain C=C bonds, it should react in a similar way to alkenes and discolouring bromine water
• benzene does not undergo electrophilic addition reactions expected of C=C bonds and does not de colourise bromine under normal conditions

Question 4

Explain how the carbon-carbon bond lengths disprove the Kekulé model?

Answer 4

X-ray diffraction studies showed that the carbon-carbon bond lengths in benzene are all the same length which are between the C-C and C=C lengths.

Question 5

Explain how the hydrogenation enthalpies disproves the Kekulé model?

Answer 5

• the hydrogenation enthalpy of benzene is less exothermic than expected for the Kekulé model
• this means that Benzene is more stable (un reactive) than expected for the Kekulé model

Question 6

What is the subsistent group NO2 named?

Answer 6

Nitro (nitrogen is bonded to the ring)

Question 7

What is the rule for naming compounds with benzene bonded to an alkyl group and halogens?

Answer 7

The alkyl group is priority so it is assumed to be at position 1 and the other substituent groups are named relative to the alkyl group

Question 8

What is -C6H5?

Answer 8

A way of representing a phenyl group

Question 9

When is benzene considered a substituent?

Answer 9

Ring is attached to:
- an alkyl chain with a functional group
- alkyl chain with 7 or more carbon atoms

Question 10

What prefix is used for benzene?

Answer 10

Phenyl

Question 11

What exceptions are there for naming benzene?

Answer 11

1. Connected to a carboxylic acid = benzoic acid
2. Connected to an amine group = phenyl amine
3. Connected to an aldehyde group = benzaldehyde
4. Connected to an alcohol group = phenol

Question 12

Why is benzene so stable?

Answer 12

Its delocalisation

Question 13

Why does benzene not take part in addition reactions?

Answer 13

As the pi system would break if it did making it less stable which is energetically unfavourable

Question 14

What are the conditions for nitration of benzene?

Answer 14

• presence of concentrated sulfuric acid catalyst
• reaction with concentrated nitric acid

Question 15

What is the overall reaction for nitration of benzene?

Answer 15

Benzene + nitric acid -> (sulfuric acid catalyst) nitro benzene + water

Question 16

What are the 3 general steps for substitution reactions with benzene?

Answer 16

Step 1: generate very reactive electrophile (catalyst + reactant)
Step 2: substitution (curly arrows)
Step 3: regenerating the catalyst

Question 17

What are the conditions for halogenation of benzene?

Answer 17

• halogen carrier must be present as halogens do not react with benzene otherwise (AlCl3, FeBr3)

Question 18

What are halogen carriers?

Answer 18

Generated in a reaction between the metal and halogen

Question 19

What is the overall reaction for halogenation (bromine) of benzene?

Answer 19

Benzene + Br2 -> bromobenzene + HBr

Question 20

What are the conditions for the alkylation of benzene (Friedel-Crafts)?

Answer 20

• uses a haloalkane and a halogen carrier catalyst to replace one of the hydrogen atoms on benzene with an alkyl group

Question 21

What is the overall reaction of alkylation of benzene to form the organic product ethylbenzene?

Answer 21

Benzene + chloroethane -> ethyl benzene + hydrogen chloride

Question 22

What are the conditions for the acylation of benzene?

Answer 22

• benzene reacts with an acyl chloride
• presence of an AlCl3 catalyst
• aromatic aldehyde/ketone is formed

Question 23

What is the overall reaction of ethanoyl chloride with benzene?

Answer 23

Benzene + ethanoyl chloride -> phenylethanone + HCl

Question 24

Describe the bonding and structure of benzene?

Answer 24

P orbitals overlap above and below the ring to form pi-bonds. The electrons are delocalised to form a pi-system

Question 25

What are phenols?

Answer 25

A type of organic compound containing a hydroxyl (-OH group) **directly bonded** to an aromatic ring

Question 26

How are phenols different to alcohols?

Answer 26

2-hydroxybenzoic acid is an example of a phenol as the -OH group is directly on the ring however 2-phenylethanol is an example of an alcohol functional group as the -OH group is on the alkyl chain

Question 27

Are phenols acidic, if so why?

Answer 27

They are weakly acidic - when dissolved in water, phenol partially dissociates forming a phenoxide ion (benzene ring with a O-) and proton

Question 28

Compare the solubility of phenols with alcohols?

Answer 28

Phenols are less soluble in water due to the presence of the non-polar benzene ring

Question 29

Draw the reaction of Phenol + NaOH ->?

Answer 29

ACID + BASE -> SALT + WATER Phenol + NaOH -> sodium phenoxide + water

Question 30

Draw the reaction of Phenol + Mg ->?

Answer 30

ACID + METAL -> SALT + HYDROGEN Phenol + Mg -> magnesium phenoxide + hydrogen

Question 31

Compare the reactivity of phenols and carboxylic acids with carbonates? How could we tests to differentiate them?

Answer 31

Phenols are not acidic enough to react with carbonates but carboxylic acids are so effervescence would be observed This can be tested to see if it CO2 (bubble through limewater, should turn from colourless to cloudy)

Question 32

What are the different observations of alkene, benzene, phenol on addition of bromine water?

Answer 32

Alkene - decolourises bromine (orange -> colourless) Benzene - no reaction Phenol - decolourises bromine and forms a white precipitate

Question 33

What is the overall equation for the bromination of phenol?

Answer 33

Phenol + 3Br2 -> (no catalyst) 2,4,6 - tribromophenol + 3HBr

Question 34

Describe why phenol is more reactive than benzene and so can trisubstitute?

Answer 34

- lone pair of electrons on O from -OH group becomes delocalised into the pi system - this makes the aromatic ring in phenol more susceptible from attacks from electrophiles as the electron density in the phenol ring is sufficient to polarise molecules so no catalysts are required

Question 35

What are electron donating groups? Examples?

Answer 35

- like the -OH group in phenols,other substituents can also donate electrons into the aromatic ring to increase the electron density in the pi system and become more susceptible to attack by electrophiles - these groups have a 2,4 - directing effect - -OH group, -NH2 group

Question 36

What are electron withdrawing groups?

Answer 36

- substituents that withdraw electrons from an aromatic ring - this decreases the electron density of the pi system and makes it less susceptible to attack by electrophiles - usually have 3-directing effect - -NO2 group

Question 37

Draw the mechanism of bromination of phenol:

Answer 37

Notes section

Question 38

What is the overall equation for nitration of phenol?

Answer 38

Phenol + HNO3 (**dilute**/concentrated) -> water + either 2-nitrophenol/4-nitrophenol **NO CATALYST**

Question 39

Why does phenylamine react similarly to phenol? What are the similarities of the bromination of phenylamine and phenol?

Answer 39

It also has 2 extra lone pair of electrons that delocalise into the pi system Similarities: - no halogen carrier catalyst (more reactive than benzene) - -NH2 group activates the aromatic ring - 2,4-directing - trisubstitued product

Question 40

What is the overall equation for the bromination of nitrobenzene?

Answer 40

Nitrobenzene + bromine -> (AlBr3) 3-bromonitrobenzene + HBr - halogen carrier catalyst and high temperature needed - 3-directing - mono-substitutes - same mechanism for bromination of benzene (3 steps)