Redox Chemistry of (Hetero)Aromatic Compounds

Question 1

Which molecule is most reactive to nucleophilic attack

Answer 1

Answer = B

Question 2

The reaction below shows the selective hydrogenation of alkenes in the presence of benzene rings
Why is this easily achieved?

Answer 2

Benzene rings are much less reactive than alkenes and hence more difficult to reduce by catalytic hydrogenation
(distrupting aromaticity)

Question 3

What are the conditions required for hydrogenation of benzene rings into cyclohexane?

Answer 3

Hydrogen
Rh catalyst
100°C
1000psi
(vigorous conditions)

Question 4

Describe the Birch reduction

Answer 4

It is the easiest way to reduce a benzene ring
using sodium in liquid ammonia (sodium will dump off its electrons to form sodium cation)

Question 5

How does the mechanism work for the Birch reaction

Answer 5

• The first single e- transfer allows the electron from one dbl bond to move entirely onto 1 carbon and a single e- to exist on the other carbon
• Then adding ethanol, the LP will remove the proton from ethanol
• A second e- transfer, will mean the radical on the other carbon is now a LP
• Then the LP can remove another proton from ethanol (forming two quaternary carbons)

Question 6

Electron-donating groups promote ……. Birch reductions

Answer 6

ortho + meta

Question 7

Electron-withdrawing groups promote ….. Birch reductions

Answer 7

Ipso + Para Birch reductions

Question 8

Identify the product formed in the reaction shown below

Answer 8

Answer = B

Question 9

True of False?
In general, pyridine rings are easier to reduce than benzene rings?

Answer 9

True
Lower resonance stabilisation energy
(this is why in the quinoline example at the bottom the pyridine gets reduced but the benzene doesn’t)

Question 10

Pyridinium salts can be reduced under what conditions?

Answer 10

NaBH₄ and EtOH
(same conditions for aldehydes and ketones)

Question 11

Why are 5-membered heteraromatics generally inert to hydride reducing agents (e.g. NaBH₄)

Answer 11

due to being electron rich
Hence cannot reduce it with a nucleophilic type reducing agent
(Also inert to Birch like reductions too)

Question 12

How would you reduce Pyrrole then?

Answer 12

Pyrrole is readily hydrogenated using hydrogen, a carboxylic acid and a platinum catalyst

Question 13

Why is thiophene less susceptible to catalystic hydrogenation?
What instead can be done?

Answer 13

Sulphur poisions the catalyst
Can be desulphurised with Raney nickel

Question 14

Identify the product formed in the reaction shown below

Answer 14

Answer = B

Question 15

Benzene rings are inert to most oxidation
What are the two conditions under which benzene rings can be oxidised

Answer 15

• Under biological conditions using en enzyme
• Using high-valance metals

Question 16

What are the two high-valance metal, you can use to oxidise benzene

Answer 16

• Ruthenium (Vlll)
• Cerium (lV)

Question 17

Phenols are easier to oxidise than benzene because ….
What is the final product of this reaction

Answer 17

They are more electron-rich than benzene
(the intermediate radicals shown can couple together in a process called phenolic oxidative coupling)

Question 18

What do pyridines readily oxidise too?

Answer 18

To the corresponding N-oxide
Further oxidations are difficult

Question 19

How does pyrrole react towards oxidants

Answer 19

Pyrrole is very unstable towards oxidants and usually just degrade

Question 20

Can Furan be oxidised?

Answer 20

Furan is unstable to oxidiation under acidic conditions
BUT ring can be oxidised to a carboxylic acid with Ru(VIII)
(we are not doing an oxidiation on the furan entirely because the whole thing is being converted to a carboxylic acid)

Question 21

Can Thiophenes be oxidised?

Answer 21

Thiophenes can be oxidised to S,S-dioxide
(aromaticity is lost however)
ring acts as a diene in Diels-Alder reaction