Chapter 25 - Aromatic chemistry *PHYSICAL FLASH CARDS REQUIRE DOING*

Question 1

Describe the delocalised model of benzene.

Answer 1

• Benzene is a planar, cyclic, hexagonal hydrocarbon
• 3/4 outer electrons per carbon atom are used in σ-bonds
• • 2 σ-bonds to the C atoms either side in the ring
• • 1 σ-bond to a H atom outside the ring
• • p-orbitals above and below plane of benzene ring
• sideways overlap of orbitals create a delocalised ring of electron density above and below the plane of the benzene ring

Question 2

What is the key evidence for the delocalised model for benzene as an improvement on the Kekulé model?

Answer 2

• Bond lengths are all the same (all same type of carbon-carbon bonds, somewhere between single and double bonds)
• Enthalpy change of hydrogenation is less exothermic than expected.
• Resistance to reaction (unlike alkenes, benzene does not decolourise bromine under normal conditions and does not react by electrophilic addition)

Question 3

What is a substituent group?

Answer 3

An atom or group of atoms taking the place of another atom or group.

Question 4

What are arenes?

Answer 4

Arenes are aromatic hydrocarbons.

Question 5

What are the general steps of electrophilic substitution?

Answer 5

Arenes undergo substitution reactions in which a hydrogen atom on the benzene ring is replaced by an electrophile.

Step 1: Formation of electrophile
- Reactant that forms the substituent group reacts with a catalyst.

Step 2: Formation of organic product

• The electrophile accepts a pair of electrons from the benzene ring to form an unstable intermediate.
• The intermediate then loses H+ to form the organic product. The stable benzene ring is reformed.

Step 3: Regeneration of catalyst
- H+ ion reacts with ion formed in step 1

Question 6

What is the name used to refer to catalysts used in the halogenation of benzene?

Answer 6

Halogen carrier

e.g. AlCl3, FeCl3, AlBr3, FeBr3.

Question 7

What are the conditions required for the nitration of benzene?

Answer 7

Benzene + Nitric acid → Nitrobenzene + Water

Concentrated sulfuric acid catalyst
50°C

Question 8

What are the conditions required for the halogenation of benzene?

Answer 8

Benzene + Halogen → Halobenzene + Hydrogen halide

Room temperature
Halogen carrier (catalyst)

Question 9

Alkylation and acylation of benzene is also referred to as…

Answer 9

…Friedel-Crafts reaction.

Question 10

What are the conditions required for the alkylation of an benzene?

Answer 10

Benzene + Haloalkane → Aromatic compound (with allyl group) + Hydrogen halide

Halogen carrier (catalyst)

Question 11

What are the conditions required for the acylation of benzene?

Answer 11

Benzene + Acyl chloride (RCOCl) → Aromatic ketone + Hydrogen halide

Halogen carrier (catalyst)

Question 12

Why are arenes less reactive than alkenes?

Answer 12

Benzene has DELOCALISED electrons π-electrons spread above and below the plane of the C atoms in the ring structure.

The electron density around any two C atoms in the benzene ring is less than in the LOCALISED C=C double bond in an alkene.

e.g.
When a Br2 molecule approaches the benzene ring, there is insufficient π-electron density around any two C atoms to polarise the bromine molecule. This prevents any reaction taking place without formation of Br+ in the presence of a halogen carrier.

Question 13

What are phenols?

Answer 13

Organic compounds with a hydroxyl functional group bonded directly to an aromatic ring.

Question 14

Are phenols alkalis or acids?

Answer 14

Phenols are weak acids, they do not react with weak bases such as carbonates.

Phenols partially dissociate in water to form the phenoxide ion and a proton.

Phenols react with strong bases like NaOH.

Question 15

How can a carbonate be used to distinguish between a phenol and a carboxylic acid?

Answer 15

Carboxylic acids react with a carbonate to form gas bubbles of carbon dioxide.

Phenols do not react with carbonates so there are no gas bubbles.

(This is because carboxylic acids are more acidic than phenols).

Question 16

Phenol + Bromine (in bromine water) →

Answer 16

Phenol + Bromine (in bromine water) → 2,4,6-tribromophenol + Hydrogen bromide

Question 17

Describe the nitration of phenol.

Answer 17

Phenol + Nitric acid (dilute) → 2-nitrophenol + 4-nitrophenol + Water

Dilute nitric acid without catalyst used as phenols are more reactive.

Question 18

Phenols are more reactive than benzene as electrophilic substitution reactions…

Answer 18

…take place under milder conditions (room temperature, without catalysts)

Question 19

Why are phenols more reactive than benzene?

Answer 19

The increased reactivity is caused by the interaction of the hydroxyl group with the aromatic ring.

• A lone pair of electrons from an oxygen p-orbital of the -OH group is donated into the π-system of phenol.
• The electron density of the aromatic ring in phenol increases.
• The increased electron density makes phenols more susceptible to attack from electrophiles.
• Phenols can induce more polarisation in non-polar molecules.

Question 20

Electron-donating groups are ____ directing.

Answer 20

Electron-donating groups are 2- and 4- directing meaning that they direct the next substituent group to position 2 or 4. They form a mixture of 2 products, one with the substituent group on the 2nd position and one with it on the 4th position.
The initial (electron donating) group is at position 1.

Question 21

Electron-withdrawing groups (except for _____) are ____ directing.

Answer 21

Electron-withdrawing groups (except for halogens) are 3- directing.

Question 22

Give 2 examples of 2- and 4- directing groups.

Answer 22

• OH

- NH2

Question 23

Give an example of a 3- directing group.

Answer 23

-NO2

Question 24

What is the molecular formula of benzene?

Answer 24

C6H6

Question 25

What are monosubstituted compounds?

Answer 25

Compounds in which one hydrogen atom has been replaced by an electrophile.
(e.g. benzene during electrophilic substitution)

Question 26

What is a disubstituted compound?

Answer 26

A compound that has two substituent groups.

Question 27

The position of disubstitution depends on whether the first substituent group is…

Answer 27

…electron-donating (2,4-directing) or electron-withdrawing (3-directing).