Test For Carbonyls

Question 1

Outline Fehling’s solution test

Answer 1

-bridge blue and when two parts added and warmed with aldehyde a red precipitate of copper (I) oxide produced, ketone does not react
-aldehyde oxidised to carboxylic acid
-Cu2+ ions in solution reduced to Cu+

Question 2

What is Benedicts solution

Answer 2

Similar to fehlings but more stable and less corrosive

Question 3

What do tests for aldehydes involve

Answer 3

Mild oxidising agents oxidising the aldehyde to carboxylic acid (RCOO-) which is present as a carboxylate ion in the alkaline conditions

Question 4

Explain the Tollen’s reagent test

Answer 4

Tollen’s agent is amoniacal silver nitrate high when heated with aldehyde produces a silver mirror
-silver ions reduced to silver atoms and aldehyde oxidised to carboxylic acid

Question 5

Test for all carbonyl compounds and how to find specific ketone or aldehyde

Answer 5

Treat with 2,4-dinitrophenylhydrazine (2,4-DNPH) which produces an orange precipitate
-this derivative formed has a specific melting point that can be tested and checked in data book after product filtered, recrystallised and dried then melting point cetermined

Question 6

Explain the iodoform test and what it tests for

Answer 6

Reagent is iodine in potassium hydroxide which when warmed with compound containing CH3CO- group (methyl ketone, methyl group next to carbonyl group) produces yellow precipitate of CHI3 (triiodomethane)

Question 7

Overall triiodomethane reaction equation

Answer 7

CH3COR + 3I2 + 4NaOH —-> RCOO-Na+ + CHI3 (yellow precip) + NaI + 3H2O

Question 8

What are the two steps in the triiodomethane reaction

Answer 8

Substitution of three iodine atoms on three hydrogen atoms of methyl group then hydrolysis when carbon carbon bond broken

Question 9

Use of triiodomethane reaction

Answer 9

Can be used to shorten carbon chain by removing methyl group

Question 10

Which aldehyde gives positive result for iodoform test

Answer 10

Ethanal