Carbonyl compounds Flashcards
Preparation of Aldehydes: Oxidation of 1’ alcohols
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.
Preparation of Ketones: oxidation of 2’ alcohols.
Reagents & Conditions: K2Cr2O7 (aq), dilute H2SO4 OR KMnO4 (aq), dilute H2SO4, heat
Observation: Orange K2Cr2O7 solution turns green or Purple KMnO4 solution is decolourised
Intro to carbonyl compounds: Why do Carbonyl compounds undergo Nucleophilic addition
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.
Reactions of Carbonyl Compounds: NA of HCN
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.
The Nucleophilic Addition Mechanism
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.
Common questions to the NA mechanism
- 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. - 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– - 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. - 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. - 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)
Important reactions of Hydroxynitrile: Reduction
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
Important reactions of Hydroxynitrile: Acidic Hydrolysis
Reagents & Conditions: HCl (aq) or H2SO4 (aq), heat under reflux
Forms RCOOH, and NH4+
Important reactions of Hydroxynitrile: Basic Hydrolysis
Reagents & Conditions: NaOH (aq) or KOH (aq), heat under reflux
Forms: RCOO- and NH3
Reaction of Carbonyl Compounds: Condensation with 2,4-DNPH
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.
Reaction of Carbonyl Compounds: Mild Oxidation of Tollens Reagent
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.
Reaction of Carbonyl Compounds: Mild Oxidation of Fehlings Reagent
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.
Reaction of Carbonyl Compounds: Iodoform test
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.
