Redox reactions of carbonyl compounds

Question 1

when an aldehyde is reduced, it forms a

Answer 1

primary alcohol

Question 2

when an aldehyde oxidised it forms a

Answer 2

carboxylic acid

Question 3

when a ketone is reduced, it forms a

Answer 3

secondary alcohol

Question 4

when a ketone oxidises, it forms

Answer 4

ketones are not easily oxidised

Question 5

the reagent used to reduced ketones and aldehydes back to alcohols is

Answer 5

lithium tetrahydridoaluminate, LiAlH4

Question 6

the reduction of both carbonyl compounds is carried out with the reagent……………which is dissolved in…………..

Answer 6

lithium tetrahydridoaluminate dissolved in dry ether

Question 7

the symbol used to represent the reducing agent is

Answer 7

[H]

Question 8

the equation for the reduction of propanal to propan-1-ol is

Answer 8

CH3CH2CHO + 2[H] → CH3CH2CH2OH

Question 9

the equation for the reduction of butanone to butan-1-ol is

Answer 9

CH3COCH2CH3 + 2[H] → CH3CH(OH)CH2CH3

Question 10

the four reagents which can oxidise aldehydes to carboxylic acid are:

Answer 10

→acidified potassium dichromate (VI)
→Fehling’s solution
→Benedict’s solution
→Tollen’s reagent

Question 11

explain the colour change when an aldehyde is oxidised using acidified potassium dichromate

Answer 11

orange solution → green solution

Due o the reduciton of chromium from +6 to +3

Question 12

explain the colour change when an aldehyde is oxidised using Fehling’s solution

Answer 12

deep blue solution to red precipitate

here a copper (II) complex to copper (I) oxide

Question 13

explain the colour change when an aldehyde is oxidised using Benedict’s solution

Answer 13

deep blue solution to red precipitate

here a copper (II) complex to copper (I) oxide

Question 14

explain the colour change when an aldehyde is oxidised using Tollen’s reagent

Answer 14

colourless solution to silver mirror
due to the conversion of a silver (I) complex to a metallic silver, which often sticks to the inside of the tube. the reagent is also described as ammoniacal silver nitrate

Question 15

the equation for the oxidation of propanal to pronanoic acid is

Answer 15

CH3CH2CHO + [O] → CH3CH2COOH

Question 16

to distinguish between a ketone and an aldehyde, the possible tests are:

Answer 16

Fehling’s solution, Benedict’s solution and Tollen’s reagent

Question 17

the reaction with triiodomethane produces a ……………………..precipitate when a carbonyl compound containing the ………………. group is added

Answer 17

• pale yellow precipitate
• when the CH3CO group is present (the only aldehyde with this is ethanal while all ketones with the CH3CO group will test positive. While pentan-2-one will test positive, pentan-3-one will not as its CO group is part of the CH3CH2CO group not CH3CO)

Question 18

the triiodomethane test is carried out in the conditions:

Answer 18

in alkaline solution and warmed, then allowed to cool down

Question 19

as the triiodomethane reaction is carried out in oxidising conditions, some alcohols can also give positive results because

Answer 19

their OH group is located so that a suitable ketone or ethanal is produced with the CH3CO group present