17B: Carbonyl compounds

Question 1

Identify the functional group.

Answer 1

Aldehyde

Question 2

Identify the functional group.

Answer 2

Ketone

Question 3

Describe the intermolecular forces in aldehydes and ketones

Answer 3

London forces,
Permanent dipole - dipole forces

Question 4

Explain the solubility of aldehydes and ketones.

Answer 4

Soluble, because they can form hydrogen bonds with water.

Question 5

Reaction of carbonyls with Fehling’s or Benedict’s solution:
Observation from aldehydes,
Observation from ketones,
Product.

Answer 5

Aldehyde: red precipitate forms,
Ketone: solution stays blue,

Aldehyde -> Carboxylate ion.

Question 6

Give the equation for the reaction of carbonyls with Fehling’s or Benedicts solution.

Answer 6

Aldehyde + [O] + OH^- -> carboxylate ion + H₂O

Question 7

Reaction of carbonyls with Tollen’s reagent:
Observation from aldehydes,
Observation from ketones,
Product.

Answer 7

Aldehyde: silver mirror forms,
Ketone: no silver mirror,

Aldehyde -> Carboxylate ion.

Question 8

Give the equation for the reaction of carbonyls with Tollen’s reagent.

Answer 8

Aldehyde + [O] + OH^- -> carboxylate ion + H₂O

Question 9

Describe the reactions
Aldehyde -> Carboxylate ion

Answer 9

Warm with Fehling’s or Benedict’s solution,
red precipitate forms.

or

Warm with Tollen’s reagent,
silver mirror forms.

Question 10

Reaction of carbonyls with acidified dichromate(VI) ions:
Observation from aldehydes,
Observation from ketones,
Product.

Answer 10

Aldehydes: orange -> green,
Ketones: stays orange,

Aldehyde -> carboxylic acid.

Question 11

Describe the reaction
Aldehyde -> Carboxylic acid

Answer 11

Warm with potassium dichromate in sulfuric acid,
Colour change orange -> green

Question 12

Reaction of carbonyls with lithium tetrahydridoaluminate:
Condition,
Product.

Answer 12

In dry ether,
Alcohol.

Question 13

Give the equation for the reaction of carbonyls with lithium tetrahydridoaluminate.

Answer 13

Carbonyl + 2[H] -> Alcohol

Question 14

Describe the reaction:
Carbonyl -> Alcohol

Answer 14

React with LiAlH₄ in dry ether.

Question 15

Reaction of carbonyls with HCN:
Condition,
Type of reaction,
Purpose,
Observation,
Product.

Answer 15

In the presence of KCN,
Nucleophilic addition,
Increases chain length,
No optical activity (racemic mix),
Hydroxynitrile.

Question 16

Give the equation for the reaction of carbonyls with HCN in the presence of KCN.

Answer 16

Carbonyl + HCN -> hydroxynitrile

Question 17

Describe the reaction
Carbonyl -> Hydroxynitrile

Answer 17

React with HCN in the presence of KCN,
Nucleophilic addition reaction,
Increases chain length.

Question 18

Describe the test for carbonyl groups.

Answer 18

React with 2,4-DNPH,
- Orange precipitate forms

Question 19

Describe the test to identify a carbonyl compound given data
for the melting temperatures of derivatives.

Answer 19

React with 2,4-DNPH,
Purify orange precipitate by filtration and recrystallisation,
Measure the melting point and compare with database.

Question 20

Reaction of carbonyls with iodine:
Condition,
Test for,
Observation,
Product.

Answer 20

In alkali.
Methyl ketones, ethanal or methyl alcohol,
Pale yellow precipitate,
Iodoform.

Question 21

Give the equation for the reaction of carbonyls with iodine in the presence of alkali.

Answer 21

RCOCH₃ + 3I₂ + 4NaOH -> CHI₃ + 3NaI + RCOONa + 3H₂O

Question 22

Describe the reaction
Methyl ketone, ethanal or methyl alcohol -> iodoform

Answer 22

Warm with iodone in alkali,
Pale yellow precipitate forms.