Identification Of Aldehydes And Ketones

Question 1

What is the structure of aldehydes (R-C(=O)-H)and ketones (R-C(=O)-R))?

Answer 1

Organic compounds that contains a carbonyl group (C = O)

Question 2

What are the uses of aldehydes and ketones?

Answer 2

1) formaldehyde (used in the plastic industry)

2) formaline (preserving biological specimen, disinfecting surgical instruments)

3) acetone (nail polish, solvent in many rxn’s)

Question 3

What are the different tests used to distinguish between aldehydes & ketones?

Answer 3

1) 2,4-Dinitrophenylhydrazine (2,4-DNPH) test: Used to distinguish between carbonyl compounds and non-carbonyl compounds (alcohol)

2) chromic test (K2CrO4/H2SO4): used to distinguish between aldehydes and ketones

3) Tollens test: used to distinguish between aldehydes and ketones

4) iodoform test: used to distinguish between methyl ketones and ketones

Question 4

How does 2,4-DNPH distinguishe between carbonyl and non-carbonyl compounds?

Answer 4

Aldehydes/ketones reacts with 2,4DNPH to give a orange-red ppt, while alcohols gives a negative test

Question 5

How does the chromic test distinguish between aldehydes and ketones?

Answer 5

Aldehydes give a positive green color as they can be oxidized to carboxylic acids (RCHO + H2CrO4 + 4H+ = RCO2H + Cr3+ +
3H2O), While ketones give no rxn

Question 6

How does the tollens (2Ag(NH3)2 OH) test distinguish between aldehydes and ketones?

Answer 6

Aldehydes give a positive silver mirror test, while ketones gives a negative test (aldehyde + 2Ag(NH3)2 OH = 2Ag + ammonium salt of carboxylic acid + H2O + 3NH3)

Question 7

How does the iodoform (3 I2) test distinguish between methyl ketone and ketone?

Answer 7

Methyl ketones gives a yellow precipitate, while other ketones gives a negative test