26.2 - Identifying Aldehydes & Ketones

Question 1

How do you identify carbonyl compounds?

Answer 1

USE 2,4-DNP (Brady’s Reagent)

• In the presence of carbonyl, a yellow/orange precip. forms
• Solid 2,4-DNP is dangerous, friction causes explosion
  
  • typically dissolved in methanol and sulfuric acid

1. Add 5cm³ 2,4,-DNP in excess to clean test tube
2. Add 3 drops of the unknown compound
3. If no crystals form, add sulfuric acid
4. In the presence of C=O, a yellow/orangle precip. forms

Question 2

How do you identify carbonyls by melting point?

Answer 2

• The 2,4 - dinitrophenylhydrazone precipitate from the 2,4- DNP test is filtered from solution and recrystallised giving a pure sample of crystals
• The melting point of the pure 2,4- dinitrophenylhydrazone is measured, recorded and compared to a set of knwon melting point values to identify the original carbonyl compound

Question 3

How do you distinguish between aldehydes and ketones?

Answer 3

• After using 2,4-DNP to identify the carbonyl
  
  • TOLLEN’S REAGENT (AgNO₃ in NH₃) is used to distinguish between aldehydes and ketones
  • Forms a silver mirror in the presence of aldehydes
  • Tollen’s has a short shelf-life, it must be used as soon as prepared

Question 4

How do you make Tollen’s Reagent?

Answer 4

1. Add 3cm depth AgNO₃, then NaOH_(aq) until the brown precip. of Ag₂O forms
2. Add dilute NH₃ unti lthe brown precip forms and disappears
   
   • That is Tollen’s

Question 5

Describe the test using Tollen’s Reagent.

Answer 5

1. 2cm depth of unknown solution in a test tube
2. Add equal volume of Tollen’s
3. Leave the testu tube to stand in 50^oC water bath (10-15mins), look for silver mirror (aldehyde)

Tollen’s has Ag⁺ ions (oxidising agent)

Ag⁺ + e^- → Ag(s)