4.4 - Aldehydes and Ketones

Question 1

How are aldehydes formed

Answer 1

Aldehyde (CHO)
- uses the oxidising agent Cr2O7 2-/H+
- from from the oxidation of primary alcohols (heat and distil)
CH3CH2OH + [O] > CH3C=OH +H2

Question 2

How are ketones formed

Answer 2

Ketones (COC)
- uses Cr2O7 2-/H+
- form from the oxidation of secondary alcohols (heat and reflux)
eg. CH3CHOHCH3 + [O] > CH3C=OCH3 + H2O

Question 3

What are the physical properties of aldehydes and ketones

Answer 3

Boiling point: have permanent dipole-dipole interactions from C=O bond but no hydrogen bonds for so weaker. boiling point is higher than alkanes but lower than alcohols, boiling point increases as carbon chain length increases as there are more intermolecular forces

Solubility: short chains are soluble due to the polar bond but as carbon chain length increases solubility decreases as the effect of the polar bond lessens

Aldehydes and ketones both undergo similar reactions from C=O bond, they undergo addition elimination reactions however they differ from each other as aldehydes can be oxidise whereas ketones are resistant to further oxidation

Question 4

What are the reduction reactions for aldehydes and ketones

Answer 4

• uses NaBH4 for reduction

NaBH4 is electron deficient so excepts lone pairs of electrons from the hydride ion forming a coordinate bond

Aldehydes > primary alcohol
eg. CH3C=OH + 2[H] > CH3CH2OH

Ketone > secondary alcohol
eg. CH3C=OCH3 + 2[H] > CH3CHOHCH3

Question 5

What is the reaction of aldehydes and ketones to hydroxynitriles

Answer 5

• carbonyls undergo nucleophilic addition due to the double bond and two reactants become one product
• HCN can be added to form hydroxynitriles (OH)CCN
• hydroxynitriles are regularly actively optic so have a chiral centre

CH3C=OCH3 + HCN > CH3CH3OHCN

MUST KNOW THE MECHANISM
- as HCN is weak and only partially dissociates KCN is added for cyanide and H2SO4 is also added slowly

Question 6

What is a positive Tollen’s test

Answer 6

Tollen’s (ammonical silver nitrate)
Positive: grey precipitate or silver mirror, shows an ALDEHYDE

Question 7

What is a positive Fehling’s test

Answer 7

Fehling’s (Cu2+ ions)
Positive: blue solution to a red precipitate, with an ALDEHYDE

Question 8

What is a positive acidified dichromate test

Answer 8

Acidified dichromate (Cr2O7 2-/H+)
Positive: orange to green solution when oxidised with an ALDEHYDE OR KETONE

Question 9

What is a positive iodoform test

Answer 9

Iodoform (I2 + NaOH)
Positive: a yellow precipitate of CHI3 (tests for methyl carbonyls)

Question 10

What is a positive 2,4-dinitrophenylhydrazine (2,4-DNPH) test

Answer 10

2,4-DNPH
Positive: a red-orange precipitate in ALDEHYDES AND KETONES

The derivative is then identified using melting point analysis by comparing with known values which can then identify the carbonyl