aldehydes and ketone Y2

Question 1

Why are small carbonyls soluble in water?

Answer 1

They can form hydrogen bonds with water molecules.

Question 2

What intermolecular forces are found between pure carbonyls?

Answer 2

Permanent dipole–dipole forces (they cannot hydrogen bond with themselves).

Question 3

Why is the C=O bond reactive towards nucleophiles?

Answer 3

O is more electronegative than
carbon.
oxygen is δ– and carbon is δ+, so nucleophiles attack the positive carbon atom.

Question 4

Why don’t carbonyls undergo electrophilic addition like alkenes?

Answer 4

The C=O bond is stronger than a C=C bond.

Question 5

What happens when a primary alcohol is oxidised?

Answer 5

It forms an aldehyde, then further oxidises to a carboxylic acid.

Question 6

What happens when a secondary alcohol is oxidised?

Answer 6

It forms a ketone, which cannot be oxidised further.

Question 7

What happens when a tertiary alcohol is oxidised?

Answer 7

It does not oxidise

Question 8

What reagent is used to oxidise aldehydes to carboxylic acids?

Answer 8

acidified Potassium dichromate(VI) solution (K₂Cr₂O₇) with dilute H₂SO₄, heated under reflux.

Question 9

What is observed when aldehydes are oxidised with acidified dichromate?

Answer 9

Orange → green (Cr₂O₇²⁻ reduces to Cr³⁺).

Question 10

Write the oxidation of ethanal with dichromate ions.

Answer 10

3CH₃CHO + Cr₂O₇²⁻ + 8H⁺ → 3CH₃COOH + 4H₂O + 2Cr³⁺

Question 11

What two reagents can also be used to test for aldehydes?

Answer 11

Tollens’ reagent (silver mirror) and Fehling’s solution (blue → red precipitate).

Question 12

What reagents are used to reduce carbonyls to alcohols?

Answer 12

NaBH₄ (sodium borohydride) or LiAlH₄ (lithium aluminium hydride).

Question 13

What type of alcohol is formed when an aldehyde is reduced?

Answer 13

Primary alcohol.

Question 14

What type of alcohol is formed when a ketone is reduced?

Answer 14

Secondary alcohol.

Question 15

Reagents for reducing carbonyls into alcohol and it’s conditions

Answer 15

Reagents: NaBH4 In aqueous ethanol

Conditions: Room temperature and pressure

Question 16

What ion from NaBH₄ acts as the nucleophile?

Answer 16

Hydride ion (:H⁻)..

Question 17

What is the reaction mechanism for carbonyl reduction by NaBH₄?

Answer 17

Nucleophilic addition.

Question 18

Draw the mechanism for nucleophillic addition of Reduction of carbonyls to alcohols

Question 19

What supplies the proton (H⁺) after nucleophilic attack in the mechanism for Reduction of carbonyls

Answer 19

Water or a weak acid.

Question 20

What are the reagents and conditions for catalytic hydrogenation of carbonyls?

Answer 20

Reagent: H₂ with a nickel catalyst.
Conditions: High pressure.

Question 21

: Example: CH₃CHO + H₂ → ?

Answer 21

CH₃CH₂OH (ethanol).

Question 22

What is formed when HCN adds to a carbonyl compound?

Answer 22

A hydroxynitrile.

Question 23

What reagents and conditions are used for the reaction of carbonyls with HCN?

Answer 23

Reagent: potassium cyanide (KCN) and dilute sulfuric acid.

Conditions: Room temperature and pressure

Question 24

Why is KCN/NaCN preferred over HCN?

Answer 24

higher concentration of the CN ion as these
compounds will completely ionise. HCN is a weak acid an will only partially ionise

Question 25

What is the nucleophile in this reaction?

Answer 25

The cyanide ion, CN⁻.

Question 26

What provides the proton (H⁺) in the second step for Addition of hydrogen cyanide to carbonyls to form hydroxynitriles

Answer 26

Dilute sulfuric acid.

Question 27

Why is a racemate formed in this reaction of hydroxynitirile forming

Answer 27

The carbonyl C=O is planar; CN⁻ can attack from either side with equal probability → equal amounts of both enantiomers.