CD 3 - Organic Reactions

Question 1

How does benzene undergo nitration?

Answer 1

Benzene reacts with concentrated nitric acid at a temperature of 50-60 degrees celsius with a concentrated sulfuric acid catalyst to form nitrobenzene and water:
C6H6 + HNO3 –> C6H5NO2 + H2O

Question 2

How is sulfuric acid acting as a catalyst?

Answer 2

• Generation of electrophile:
  H2SO4 + HNO3 –> NO2+ + H2SO4- + H2O
• Sulfuric acid is regenerated by the hydrogensulfate ion reacting with the proton expelled from the intermediate
  HSO4- + H+ –> H2SO4

Question 3

What is the role of the halogen carrier?

Answer 3

• To generate the electrophile:
  Br2 + FeBr3 –> FeBr4- + Br+
• FeBr4- then reacts with the proton expelled from the intermediate to regenerate the halogen carrier:
  FeBr4- + H+ –> FeBr3 + HBr

Question 4

How does benzene undergo halogenation?

Answer 4

E.g. Bromination:
- Benzene only reacts with bromine if a halogen carrier i.e. FeBR3 (or AlCl3 if chlorine is being used) is present
- This forms bromobenzene and hydrogen bromide:
C6H6 + Br2 –> C6H5Br + HBr

Question 5

How does benzene undergo an alkylation reaction?

Answer 5

React benzene with a haloalkane in the presence of a halogen carrier (i.e. AlCl3):
C6H6 + C2H5Cl –> C6H5(C2H5) + HCl

Question 6

How does the halogen carrier generate the electrophile? (alkylation)

Answer 6

• Halogen carrier reacts with haloalkane to generate electrophile:
  AlCl3 + C2H5Cl –> AlCl4- + C2H5+
• AlCl4- reacts with the proton expelled from the intermediate to regenerate the halogen carrier:
  AlCl4- + H+ –> AlCl3 + HCl

Question 7

How does the halogen carrier generate the electrophile? (acetylation)

Answer 7

• Halogen carrier reacts with acyl chloride to generate electrophile:
  AlCl3 + CH3COCl –> AlCl4- + CH3CO+
• AlCl4- reacts with the proton expelled from the intermediate to regenerate the halogen carrier:
  AlCl4- + H+ –> AlCl3 + HCl

Question 7

How does benzene undergo an acetylation reaction?

Answer 7

Reflux benzene with an acyl chloride in the presence of a halogen carrier (i.e. AlCl3):
C6H6 + CH3COCl –> C6H5COCH3 + HCl

Question 8

How does benzene undergo sulfonation?

Answer 8

Heat benzene under reflux with (fuming) concentrated H2SO4:
C6H6 + SO3 –> C6H5SO3H

Question 9

How is diazonium compound formed?

Answer 9

• Hydrochloric acid reacts with sodium nitrate to form an unstable nitrous acid:
  NaNO2 + HCl –> HNO2 + NaCl
• Nitrous acid then reacts with aminobenzene:
  {}-NH2 + HNO2 –> {}-+NNCl- + 2H2O {} represents benzene and NN are triple bonded

Question 10

How do diazonium compounds form azo dyes?

Answer 10

• In a coupling reaction: The diazonium salt reacts with the coupling agent (which is another arene)
• The diazonium salt reacts with the benzene ring of the arene (coupling agent) as it is acting as an electrophile

Question 11

What can diazonium salts react with to form different dyes?

Answer 11

• When reacted with a phenol, a yellow/orange azo compound is formed
• A red azo compound is formed when the salt reacts with an alkaline solutions of 2-Naphthol
• A yellow dye is often formed when a diazonium salt is reacted with phenylamines

Question 12

How can an aldehyde form a carboxylic acid?

Answer 12

• By heating under reflux with acidified potassium dichromate(VI)
• The aldehyde is oxidised into a carboxylic acid:
  CH3CHO + [O] –> CH3COOH
• There will be a colour change from orange to green

Question 13

What are the two tests for an aldehyde?

Answer 13

1. Fehling’s solution
2. Tollen’s reagent

Question 14

How do you test for an aldehyde with Tollen’s reagent?

Answer 14

Warm the sample with Tollen’s reagent. If an aldehyde is present, a silver mirror will form

Question 15

How do you test for an aldehyde using Fehling’s solution?

Answer 15

Warm the sample with Fehling’s solution. If an aldehyde is present, a red precipitate will form

Question 16

How do aldehydes react with cyanide ions to form a cyanohydrin?

Answer 16

They are reduced by nucleophilic addition:
CH3CHO + HCN –> CH3CH(OH)CN

Question 17

How do ketones react with cyanide ions to form a cyanohydrin?

Answer 17

They are reduced by nucleophilic addition:
CH3COCH3 + HCN –> CH3CN(OH)CH3