Aldehydes and Ketones (DONE)

Question 1

How can primary and secondary alcohols form aldehydes and ketones respectively
- Conditions
- Reagents
- Type of reaction

Answer 1

Aldehydes
- Distillation as aldehyde forms + Heat
- Excess primary alcohol with (Potassium) Cr₂O₇²⁻ / H⁺ (from dilute sulfuric acid)
- Oxidation
Ketones
- Reflux
- Oxidation
- Secondary alcohol with excess Cr₂O₇²⁻ / H⁺

Question 2

What is the observation when primary and secondary alcohols are oxidised using
- Potassium dichromate (VI)
- Potassium manganate (VII)

Answer 2

• Orange to green
• Purple to colourless

Question 3

How can the following be turned into alcohols
- Aldehydes and ketones

Answer 3

Aldehydes and ketones
- NaBH₄ or LiAlH₄

Question 4

Describe two different tests to distinguish between Aldehydes and Ketones
- Name of reagents/what is contained within them
- Observation
- Explanation in terms of oxidation/reduction

Answer 4

Tollens’ Reagent / Silver mirror test
- (AgNO₃/NaOH/NH₃) solution
- Produces a silver mirror if an aldehyde is present
- Aldehyde is oxidised by silver ions and therefore silver ions gain an electron forming silver solid/ppt
- No change with a ketone present

Fehling’s solution/ Benedicts’ Test
- If an aldehyde is present colour change of blue solution to brick red ppt
- Copper (II) ions gain an electron to form copper (I) ions

Question 5

How can aldehydes/ketones be identified (using the same reagent)
- Name(s) of the reagent used
- Observation
- Type of reaction
- Further identification of the aldehyde/ketone present

Answer 5

• 2,4-dinitrophenylhydrazine reagent/Brady’s Reagent or 2,4-DNPH)
• If aldehydes/ketone present yellow/orange crystalline precipitate
• Nucleophilic addition/Condensation/Elimination
• Solid is isolated and melting point is identified to compare to know values in order to identify the ketone/aldehyde present

Question 6

Show the condensation reaction of 2,4-DNPH with ethanal
- Use structural equation

Answer 6

Refer to book for structural equation

Question 7

Describe the Iodoform/triiodomethane test
- What does it identify
- What are the two potential reagents
- Observation if positive

Answer 7

• Identify CH₃CO (ethanal/methyl ketones) and alcohols containing CH₃CHOH- (ethanol and methyl secondary alcohols)
• Reagents required are I₂/NaOH (alkaline iodine) or KI/NaClO aqueous sodium chlorate (1)
• Positive result will produce a yellow precipitate of CH₃I. Smells like antiseptic

Question 8

Describe the reaction of carbonyl compounds with hydrogen cyanide
- Type of reaction
- How the hydrogen cyanide is made
- Name of the final product (generally)
- The reaction between propanone and hydrogen cyanide

Answer 8

• Nucleophilic addition
• KCN + HCl -> HCN + HCl (in situ)
• 2-hydroxy [] nitrile
• Final product is 2-hydroxy-2-methylpropanenitrile