17: Carbonyl compounds ✅

Question 1

the reaction (reagents and conditions) by which aldehydes can be produced

Answer 1

the oxidation of primary alcohols and immediate distillation

(distillation to avoid carboxylic acid forming from further oxidation)

Oxidising agents: acidified potassium dichromate, K₂Cr₂O₇, or acidified potassium permanganate, KMnO₄

Question 2

colour change when primary/secondary alcohols are oxidised by acidified potassium dichromate, K₂Cr₂O₇

Answer 2

🟠 → 🟢

Cr₂O₇ 2- ions reduced to Cr3+ ions

Question 3

colour change when primary/secondary alcohols are oxidised by acidified potassium permanganate, KMnO₄

Answer 3

🟣 → colourless

MnO4- ions are reduced to Mn2+ ions

Question 4

the reaction (reagents and conditions) by which ketones can be produced

Answer 4

the oxidation of secondary alcohols and eventual distillation

Oxidising agents: acidified potassium dichromate, K₂Cr₂O₇, or acidified potassium permanganate, KMnO₄

Question 5

steps in the reduction of aldehydes and ketones

Answer 5

C=O is broken and primary/secondary alcohols are produced

Reducing agents: sodium bromohydride, NaBH₄, or lithium aluminium hydride, LiAlH₄

Question 6

type of reaction that 2,4-DNPH with carbonyl compounds is

Answer 6

a condensation reaction

Question 7

positive result for testing for carbonyl group by addition of 2,4-DNPH

Answer 7

formation of deep 🟠 precipitate that is purified by recrystallisation

Question 8

identification of which specific aldehyde/ketone has reacted with 2,4-DNPH

Answer 8

melting point of the formed precipitate is measured and compared to literature values for specific aldehydes and ketones

Question 9

reaction used to detect the presence of a carbonyl group

Answer 9

reaction with 2,4-DNPH

Question 10

reactions used to deduce the nature of an unknown carbonyl compound

aldehyde or ketone

Answer 10

1. Warming with Fehling’s
2. Warming with Tollens’ reagents

Question 11

composition of Fehling’s reagent

Answer 11

Alkaline solution containing Cu2+ ions (oxidising agent)

Question 12

composition of Tollens’ reagent

AKA ammoniacal silver nitrate solution

Answer 12

an aqueous alkaline solution of silver nitrate in excess ammonia solution

Question 13

products formed when aldehyde is warmed with Fehling’s solution

Answer 13

• the aldehyde is oxidised to a carboxylic acid
• Cu+ ions form from reduction of Cu2+ ions

Question 14

positive result for aldehydes

in testing with Fehling’s reagent

Answer 14

clear 🔵 → opaque 🔴 precipitate

Question 15

positive result for ketones

from testing with Fehling’s reagent

Answer 15

no change in clear 🔵 solution

Question 16

products formed when aldehyde is warmed with Tollens’ reagent

Answer 16

• the aldehyde is oxidised to a carboxylic acid
• the Ag+ ions are reduced to Ag atoms

Question 17

positive result for aldehyde

from testing with Tollens’ reagent

Answer 17

a silver ‘mirror’ forms on the inside of the tube

Question 18

positive result for ketone

from testing with Tollens’ reagent

Answer 18

no silver ‘mirror’

Question 19

reaction used to deduce the presence of a methyl ketone or ethanal

Answer 19

heating with alkaline I₂ (aq)

Question 20

product of heating methyl ketones or ethanal with alkaline I₂ (aq)

Answer 20

tri-iodomethane / “iodoform”

yellow precipitate with an antiseptic smell