Carbonyl Chemistry 2: Chemical Tests

Question 1

How do you distinguish between aldehydes and ketones?

Answer 1

The Fehling’s test
Reagent is Cu2+ (Fehling’s A) in sodium tartrate and NaOH solution (Fehling’s B)
Cu2+ reduced to Cu+ (in copper oxide)
Aldehyde is oxidised to carboxylic acid
No change for ketones
Forms a red precipitate

Question 2

How do you test for methyl ketones?

Answer 2

Iodoform test
Qualitative
Reagent is iodine in NaOH solution
Methyl ketones will react to form a carboxylic acid and iodoform (CHI3)
Forms a yellow solid precipitate of iodoform

Question 3

How do you test for alkyl halides?

Answer 3

Silver nitrate test
Reagent is AgNO3 solution
Ag+ ions react with halide to form AgX
Forms a white precipitate with R-Cl
Forms a cream precipitate with R-Br
Forms a yellow precipitate with R-I

Question 4

What is in Fehling’s A?

Answer 4

Copper 2+ ion solution

Question 5

What is in Fehling’s B?

Answer 5

Sodium tartrate and sodium hydroxide solution

Question 6

Why can’t ketones be oxidised?

Answer 6

They don’t have a hydrogen atom on the carbonyl carbon - this is necessary for oxidation