Aromatic compounds

Question 1

Write molecular formula for benzene

Answer 1

C6H6

Question 2

Describe the Kekule model for benzene

Answer 2

1. 6 membered ring of carbon atoms joined by alternative single and double bonds.

Question 3

Evidence against the Kekule model for benzene

Answer 3

1. Lack of reactivity of benzene- if it contains C=C bonds it should decolourise bromine in an electrophilic addition reaction. BUT it doesn’t undergo electorphilic addition reactions and does not decolourise bromine.
2. Lengths of carbon-carbon bonds- Using X-ray diffraction it was found that the bond lengths of benzene are all equal and between the bond length of a single and double C-C bond. Kekule’s structure would have 3 long single bonds and 3 shorter double bonds.
3. Benzene has a less exothermic enthalpy change of hydrogenation than expected- much more stable than Kekule’s model suggested.

Question 4

Descrie the delocalised model of benzene

Answer 4

1. Benzene is a planar, cyclic, regular, hexagonal hydrocarbon containing 6 carbon atoms and 6 hydrogen atoms- molecular formula of C6H6
2. Each carbon atoms uses 3 of its available four electrons in forming 2 sigma bonds with 2 other carbons and a sigma bond with 1 hydrogen atom.
3. Each carbon atom has one electron in a p-orbital perpendicular to the plane of the molecule.
4. Adjacent p-orbital electrons overlap sideways, in both directions above and below the plane of the carbon atoms to form a ring of electron density.
5. This overlapping of p-orbitals creates a system of pi-bonds which spread over all 6 carbon atoms in the ring structure.
6. The 6 electrons occupying this system of pi-bonds are said to be delocalised.

Question 5

Describe how to name simple benzene molecules

Methylbenzene, Chlorobenzene, Nitrobenzene, Benzoic acid

Answer 5

1. Methyl benzene- CH3 group is attached to benzene
2. Chlorobenzene- Cl is attached to benzene
3. Nitrobenzene- NO2 group is attached to benzene
4. Benzenecarboxylic (benzoic) acid- COOH group attached to benzene

Question 6

What would 2-hydroxybenzenecarboxylic acid look like

Answer 6

1. benzene ring with COOH attached

2. OH group on carbon after the carbon with the COOH attached

Question 7

What would 3,5-dinitrobenzenecarboxylic acid look like

Answer 7

1. Benzene ring with COOH group attached

2. NO2 on the 3rd carbon and 5th carbon counting from the COOH group.

Question 8

What is the name for a C6H5 group and when is it used.

Answer 8

1. Phenyl group
2. Used when the benzene ring is regarded as a substituent of a larger molecule.
3. e.g phenylethene- ethene with a benzene ring attached to the 1st carbon

Question 9

Draw the structures for phenyl amine, phenyl ethanoate and methyl benzenecarboxylate

Answer 9

1. Phenyl amine- NH2 group attached to benzene

2. Phenyl ethanoate- COOCH3 group attached to benzene

Question 10

What is phenol

Answer 10

1. Benzene ring with OH group attached

Question 11

Write the balanced equation, state any reaction conditions and name the type of reaction:
Benzene with chlorine

Answer 11

1. Halogenation of benzene
2. Benzene (hexagon with circle) + Cl2 → Chlorobenzene + HCl
3. Anhydrous AlCl3 needed

Question 12

When is benzene considered to be a subsituent

Answer 12

1. When a benzene ring is attached to an alkyl chain with a functional group or to an alkyl chain with seven or more carbon atoms.
2. Gets the prefix phenyl

Question 13

What is the general formula for the substitution reaction of benzene and name the type of substitution

Answer 13

1. Electrophilic substitution
2. Benzene + E+ → Benzene( attached to E) + H+
3. The benzene is reacting with an electrophile which replaces a hydrogen atom on the benzene ring.

Question 14

Describe the nitration of benzene including conditions and balanced equations

Answer 14

1. Benzene reacts with nitric acid to form nitrobenzene
2. Reaction is catalysed by concentration sulfuric acid and heated to 50 degrees to obtain a good rate of reaction.
3. Benzene + HNO3 → Nitrobenzene + H2O

Question 15

How can 1,3-dinitrobenzene be produced

Answer 15

1. React benzene with excess nitric acid

2. Temperature above 50 degrees so further substitution can happen.

Question 16

Draw the mechanism for the nitration of benzene and state the electrophile

Answer 16

1. Electrophile is nitronium ion NO2 +
2. Step 1- Electrophile is produced:
   HNO3 + H2SO4 →NO2+ + HSO4- + H2O
3. Step 2- Electrophile accepts a pair of electrons from the benzene ring to form a dative covalent bond. (Curly arrow from benzene circle to NO2+ )
4. The organic intermediate formed (Benzene with smiley face and + in the middle)i s unstable and breaks down to form nitrobenzene and the H+ ion (Curly arrow from bond connecting H and Benzene to + in middle of benzene)
5. Step 3- H+ + HSO4- → H2SO4

Question 17

Describe the halogenation of benzene

Answer 17

1. Halogens don’t react unless there is a catalyst called a halogen carrier present- AlCl3, FeCl3, AlBr3, FeBr3- generated in situ.
2. React at room temperature and pressure.
3. One of the hydrogen atoms in benzene is replaced by a halogen atom.
4. Benzene + X2 → Xbenzene + HX

Question 18

Draw the mechanism for the reaction of benzene with Bromine

Answer 18

1. Electrophile- Bromonium ion Br+ (generated when the halogen carrier catalyst reacts with Bromine)
2. Step 1- Formation of electrophile- Br2 + FeBr3 → FeBr4- + Br+
3. Step 2- Bromonium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond (Curly arrow from benzene ring to Br+)
4. The organic intermediate formed (Benzene with smiley face and + in middle attached to H and Br) is unstable and breaks down to form the organic product bromobenzene and H+ (Curly arrow from bond connecting H and Benzene to + in middle of benzene)
5. Step 3- Regeneration of catalyst- H+ + FeBr4- → FeBr3 + HBr

Question 19

Describe Alkylation reactions with benzene and give the equation to form ethylbenzene

Answer 19

1. Substitution of a hydrogen atom in the benzene ring with an alkyl group.
2. The reaction is carried out by reacting benzene with a haloalkane in the presence of AlCl3 which acts as a halogen carrier catalyst- generating the electrophile
3. Benzene + C2H5Cl → Ethylbenzene + HCl

Question 20

Describe the Acylation reaction with benzene and give the equation between benzene and ethanoyl chloride

Answer 20

1. When benzene reacts with an acyl chloride in the presence of AlCl3 catalyst an aromatic ketone is formed.
2. Benzene + ethanoyl chloride (CH3COCl) → Phenylethanone + HCl

Question 21

What is the difference between phenols and alcohols

Answer 21

1. If the OH group is bonded directly to the benzene then it is a phenol
2. If the OH group is bonded to a carbon side chain rather than the aromatic ring it is a alcohol
3. The alcohols and phenols have some common reactions
4. But many reactions are different as the proximity of the delocalised ring influences the OH group

Question 22

Describe what happens when phenol is dissolved in water.

Answer 22

1. Phenol is less soluble in water (described as slightly soluble) than alcohols due to the presence of the non-polar benzene ring.
2. When dissolved in water phenol dissociates forming the phenoxide ion and a proton
3. Because it can partially dissociate it is classified as a weak acid.
4. Phenol ↔ Phenoxide ion (Benzene attached to O-) + H+

Question 23

Compare the reactivity of phenol with alcohols and carboxylic acids

Answer 23

1. Phenol is more acidic than alcohols but less than carboxylic acids- can be seen by comparing Ka values
2. Ethanol does not react with sodium hydroxide (strong base) or sodium carbonate (weak base)
3. Phenols and Carboxylic acids react with Sodium hydroxides (strong bases) Phenol dissolves.
4. Only Carboxylic acids are strong enough acids to react with sodium carbonate (weak bases) Carboxylic acid produces CO2
5. Sodium Carbonate can be used to distinguish between the phenol and Carboxylic acid.`

Question 24

Describe the reaction and write the equation for phenol with NaOH

Answer 24

1. Phenol reacts with sodium hydroxide solution to give a colourless solution containing the salt sodium phenoxide and water.
2. Phenol + NaOH → sodium phenoxide (benzene attached to O-Na+) + H2O

Question 25

Why are phenols acidic even though alcohols have almost no acidic properties

Answer 25

1. In phenol electrons in the O-H bond are drawn into the aromatic ring weakening the bond and making it easier for the H+ to be released 2. Once the phenoxide ion is formed it is more stable as the electrons are dispersed across the pi cloud.

Question 26

State what is observed when bromine water is added to an aqueous solution of phenol at room temperature

Answer 26

1. Bromine water decolourises (orange to colourless) and a white precipitate

Question 27

Write the equation for the reaction of Phenol with Bromine and state the type of reaction

Answer 27

1. Electrophilic substitution 2. Phenol + 3Br2 → 2,4,6-tribromophenol + 3HBr 3. At room temperature and a halogen carrier catalyst is not required

Question 28

Write the equation for the reaction of Phenol with Nitric acid and state the type of reaction

Answer 28

1. Electrophilic substitution 2. Phenol + HNO3 → 2-nitrophenol or 4-nitrophenol + H2O 3. At room temperature and a halogen carrier catalyst is not required

Question 29

Compare the reactivity of phenol and benzene

Answer 29

1. Bromine and nitric acid react more readily with phenol than benzene 2. Phenol is nitrated with dilute nitric acid rather than needing concentrated nitric acid and sulfuric acids as with benzene. 3. The increased reactivity is caused by a lone pair of electrons form the oxygen p-orbital of the OH group being delocalised to the delocalised pi-system of electrons on the benzene ring. 4. This increases the electron density of the benzene ring in the phenol- attracts electrophiles more strongly than benzene 5. For bromine the electron density in the phenol ring structure is sufficient to polarise bromine molecules and so no halogen carrier catalyst is required.

Question 30

Describe what is meant by activating and deactivating groups

Answer 30

1. Activating groups- If the group activates the ring it means the aromatic ring reacts more readily with electrophiles. 2. Deactivating groups- If the group deactivates the ring it means the aromatic ring reacts less readily with the electrophiles

Question 31

List the 2,4 directing groups and state whether they are activating or not

Answer 31

All activating accept halogens 1. NH2 or NHR 2. OH 3. OR 4. R or C6H5 5. F, Cl, Br, I

Question 32

List the 3,5 directing groups and state whether they are activating or not

Answer 32

All deactivating 1. NO2- main 2. COOR 3. SO3H 4. CHO 5. COOH 6. CN 7. NR3+