Carbonyl Compounds

Quick Notes

• Aldehydes and ketones contain carbonyl (C=O) functional groups.

• Physical Properties:
  • Cannot form hydrogen bonds with each other (have relatively low boiling points compared to alcohols)
  • Can hydrogen bond with water meaning small carbon chain aldehydes and ketones are water-soluble
• Reactions of Aldehydes and Ketones:
  • Reduction to Alcohols:
    Use NaBH₄ or LiAlH₄.
    Aldehydes to primary alcohols
    Ketones to secondary alcohols
  • Reaction with HCN/KCN:
    Forms hydroxynitriles (adds –OH and –CN across the C=O group).
    Occurs via nucleophilic addition.
• Tests for Carbonyl Compounds:
  • 2,4-DNPH Test:
    Reacts with both aldehydes and ketones forms an orange precipitate.
  • Fehling’s Solution / Tollens’ Reagent:
    Distinguish between aldehydes and ketones.
    Aldehydes are easily oxidised to give a positive result.
    (Fehling’s gives a red ppt, Tollens’ a silver mirror).
    Ketones give no reaction.
  • Iodoform Test (I₂/OH⁻):
    Detects a CH₃CO– group (typically methyl ketones or ethanal).
    Gives a yellow precipitate of CHI₃ (iodoform) with antiseptic smell.

Full Notes

Both aldehydes and ketones contain the carbonyl group (C=O)

• Aldehydes have a –CHO group at the end of the chain
• Ketones have a C=O group in the middle of the chain

Aldehydes and ketones Physical Properties

Aldehydes and ketones can’t form hydrogen bonding between molecules as they have no O–H groups, giving relatively low boiling points.

They can however form hydrogen bonds with water as the lone pair on the oxygen in the C=O bond can attract the δ⁺ hydrogen atoms in H₂O.

This makes short-chain aldehydes and ketones soluble in water.

Solubility decreases with chain length.

Oxidation Reactions

Aldehydes can be oxidised to carboxylic acids using oxidising agents.

Acidified potassium dichromate, Fehling’s solution and Tollens’ reagents can all be used and give a colour change

• The oxidising agent is shown as [O].

For Example Ethanal can be oxidised to ethanoic acid
CH₃CHO + [O] → CH₃COOH

Acidified Potassium Dichromate(VI) (K₂Cr₂O₇/H⁺)

Reagents: K₂Cr₂O₇/H⁺

Conditions: Acidic

Aldehydes are oxidised to carboxylic acids (RCOOH)
Ketones: No reaction

Observation: Orange solution → Green solution (Cr³⁺ formed)

Example: CH₃CHO + [O] → CH₃COOH

Solution turns from orange to green due to reduction of dichromate ions Cr₂O₇²⁻ to Cr³⁺ ions. (Cr₂O₇²⁻ (orange) + 14H⁺ + 6e⁻ → 2Cr³⁺ (green) + 7H₂O)

Fehling’s or Benedict’s Solution

Reagents: Fehling’s Solution (also called Benedict’s solution)

Conditions: Alkaline

Aldehydes are oxidised to carboxylate ions (RCOO⁻)
Ketones: No reaction

Observation: Blue solution → Brick-red precipitate (Cu₂O)

Example: CH₃CHO + [O] → CH₃COO⁻ + H₂O

Brick red precipitate forms due to reduction of Cu²⁺ ions (2Cu²⁺ + H₂O + 2e⁻ → Cu₂O (s) + 2OH⁻)

Tollens’ Reagent (ammoniacal silver nitrate)

Reagents: Tollens’ Reagent

Conditions: Alkaline

Aldehydes are oxidised to carboxylate ions (RCOO⁻)

Observation: Colourless → Silver mirror (Ag metal)

Example: CH₃CHO + [O] → CH₃COO⁻ + H₂O

Silver mirror forms due to reduction of Ag⁺ ions (Ag⁺ + e⁻ → Ag(s))

Because ketones do not oxidise easily under standard conditions, we can use the above reactions to distinguish between aldehydes and ketones.

Reduction Reactions

[H] in reactions represents the addition of hydrogen, indicating a reduction reaction. It is often used as shorthand for a reducing agent, such as LiAlH₄ when reducing carbonyls.

Both aldehydes and ketones can be reduced back to alcohols using lithium tetrahydridoaluminate (LiAlH₄) in dry ether.

LiAlH₄ is a strong reducing agent and must be used under anhydrous conditions as it reacts with water.

For example

Ethanal → Ethanol
CH₃CHO + 2[H] → CH₃CH₂OH

Propanone → Propan-2-ol
CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃

Matt’s Exam Tip

Remember in organic chemistry that oxidation is the gaining of a bond to a more electronegative element (such as oxygen or nitrogen) and, or the loss of a bond to a less electronegative element (hydrogen). Reduction is the opposite of this - the losing of a bond to a more electronegative element (such as oxygen or nitrogen) and the gaining of a bond to hydrogen.

Nucleophilic Addition with HCN

Aldehydes and ketones react with KCN followed by dilute acid to form hydroxynitriles.

• Reagent: KCN followed by H⁺.
• Mechanism: Nucleophilic addition of CN⁻ to the carbonyl carbon.

The cyanide (CN⁻) ion comes from KCN which dissociates to release K⁺ and CN⁻.

Key Point: The product contains a new chiral centre, so if starting with an carbonyl bonded to two different groups, a racemic mixture of enantiomers forms.

Reason: The planar carbonyl group can be attacked from either side, forming two mirror-image products.

A racemic mixture forms when equal attack occurs from both sides.

2,4-DNPH Test

Carbonyl containing compounds (aldehydes and ketones) form an orange precipitate when added to a solution of 2,4-DNPH (Brady’s reagent)

• Reagent: 2,4-dinitrophenylhydrazine
• Observation: Bright orange or yellow precipitate
• Explanation: Forms a hydrazone derivative with C=O compounds (both aldehydes and ketones).

The precipitate’s melting point can be used to identify the original carbonyl compound.

Iodoform Reaction

The iodoform test detects compounds with the CH₃CO– group (methyl ketones) or ethanal.

• Reagent: I₂ in alkali (e.g. NaOH)
• Positive test: Yellow precipitate of CHI₃ (iodoform)
Example: CH₃COCH₃ + 3I₂ + 4OH⁻ → CHI₃ + CH₃COO⁻ + 3I⁻ + 3H₂O

Summary

• Aldehydes and ketones contain the C=O group and have lower boiling points than comparable alcohols.
• Short-chain carbonyls are water-soluble due to hydrogen bonding with water.
• Aldehydes oxidise easily giving positive Fehling’s and Tollens’ tests while ketones do not.
• LiAlH₄ reduces aldehydes to primary alcohols and ketones to secondary alcohols.
• HCN/KCN adds across C=O to give hydroxynitriles often forming racemic mixtures.
• 2,4-DNPH gives an orange precipitate with carbonyls and iodoform detects CH₃CO– groups.