Carbonyl compounds

Question 1

Preparation of Aldehydes: Oxidation of 1’ alcohols

Answer 1

Reagents & Conditions: K2Cr2O7 (aq), dilute H2SO4, heat with immediate distillation
Observation : Orange K2Cr2O7 solution turns green
Note:
KMnO4 is NOT used for the preparation of aldehydes because it is too strong an oxidising agent. Carboxylic acids will be formed instead.

Question 2

Preparation of Ketones: oxidation of 2’ alcohols.

Answer 2

Reagents & Conditions: K2Cr2O7 (aq), dilute H2SO4 OR KMnO4 (aq), dilute H2SO4, heat

Observation: Orange K2Cr2O7 solution turns green or Purple KMnO4 solution is decolourised

Question 3

Intro to carbonyl compounds: Why do Carbonyl compounds undergo Nucleophilic addition

Answer 3

The highly electronegative O atom draws electron density towards itself. This polarises the C=O bond, making the carbonyl C atom electron deficient

The electron deficient carbonyl C atom is susceptible to attack by nucleophiles. The carbonyl C atom is unsaturated (it contains a C=O double bond) and
hence it undergoes addition reactions.

Question 4

Reactions of Carbonyl Compounds: NA of HCN

Answer 4

Reagents & Conditions: HCN with trace amounts of NaCN (or NaOH) as catalyst cold (10C to 20C)

Application: Increase the no. of C atoms in a chain (1C for each addition of HCN)—known as “step–up” reaction. Resulting hydroxynitrile product can be hydrolysed to form carboxylic acid or reduced to form amines.

Question 5

The Nucleophilic Addition Mechanism

Answer 5

The nucleophile, CN– attacks the electron–deficient carbonyl carbon atom to form an alkoxide intermediate.
Protonation of intermediate to form the stable addition product, with regeneration of CN- catalyst.

Question 6

Common questions to the NA mechanism

Answer 6

1. What is the role of HCN?
   - HCN acts as an acid in step 2 to donate proton in order to generate the hydroxynitrile / cyanohydrin product.
2. What is the role of the trace amount of base (NaOH) or NaCN added?
   - The slow (rate–determining) step involves the CN– nucleophile. However HCN being a weak acid is a poor source of the nucleophile CN– i.e. dissociates partially to give very small amounts of CN–
3. Suggest why the temperature of the reaction is controlled at 10C to 20C?
   - The temperature cannot be too high so as to prevent the poisonous HCN gas from escaping to the environment as the boiling point of HCN is
   26C.
   - The temperature cannot be too low so as to ensure rate of reaction is reasonably high.
4. Would a racemic mixture be obtained for reactions involving nucleophilic addition? Why?
   - There is equal probability of the CN– nucleophile attacking the trigonal planar electron–deficient carbonyl C atom from top and bottom of the
   plane.
5. Why are ketones less reactive than aldehydes?
   -Steric reason:
   In ketones, the presence of 2 alkyl (R) groups results in greater steric hindrance compared to aldehydes where there is only 1 R group. It is more difficult for the nucleophile to attack the electron–deficient
   carbonyl C atom in the nucleophilic addition reaction.
   -Electronic Factor Reason:
   Ketones have 2 alkyl/ R groups which are are electron–donating, thus makes the carbonyl C less electron deficient and hence less susceptible to
   nucleophilic attack. (compared to aldehyde with only 1 R group)

Question 7

Important reactions of Hydroxynitrile: Reduction

Answer 7

Reagents & Conditions:
(1) LiAlH4 in dry ether, r.t.p. or
(2) H2 with Pd or Pt catalyst, r.t.p or
(3) H2 with Ni catalyst, heat

Question 8

Important reactions of Hydroxynitrile: Acidic Hydrolysis

Answer 8

Reagents & Conditions: HCl (aq) or H2SO4 (aq), heat under reflux

Forms RCOOH, and NH4+

Question 9

Important reactions of Hydroxynitrile: Basic Hydrolysis

Answer 9

Reagents & Conditions: NaOH (aq) or KOH (aq), heat under reflux

Forms: RCOO- and NH3

Question 10

Reaction of Carbonyl Compounds: Condensation with 2,4-DNPH

Answer 10

Application: Chemical test for BOTH aldehydes and ketones (To test for presence of carbonyl (C=O) compounds)

Test: Add 2,4–dinitrophenylhydrazine or 2,4–DNPH

Observation: Orange ppt of 2,4-dinitrophenylhydrazone are formed.

Question 11

Reaction of Carbonyl Compounds: Mild Oxidation of Tollens Reagent

Answer 11

Chemical test for aromatic and aliphatic aldehydes
(To distinguish aldehydes from ketones)

Test: Add Tollens’ reagent (aqueous NH3 with AgNO3, also known as ammonical AgNO3) to compound and warm

Observation: A silver mirror or black ppt is formed.

Question 12

Reaction of Carbonyl Compounds: Mild Oxidation of Fehlings Reagent

Answer 12

Application: Chemical test for aliphatic aldehyde only
(To distinguish aliphatic from aromatic aldehydes)

Test: Add Fehling’s reagent to compound and heat

Observation: Brick red precipitate of Cu2O is formed.

Question 13

Reaction of Carbonyl Compounds: Iodoform test

Answer 13

Application: This test specifically identifies aldehydes and ketones with the structure -CO(CH3)

Test: Add aqueous iodine in sodium hydroxide to a solution of the compound with the above structure. Heat the solution.

Observation: Pale yellow precipitate of tri–iodomethane (CHI3) will be observed.