Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 15.8 Reactions of carbonyls-Study Notes - New Syllabus

15.8 Reactions of Carbonyl Compounds

(i) Reactions with Fehling’s/Benedict’s Solution, Tollens’ Reagent and Acidified Dichromate(VI)

Aldehydes are readily oxidised because the carbonyl carbon is bonded to a hydrogen atom. Ketones are generally resistant to oxidation under mild conditions.

Aldehydes

• Oxidised to carboxylic acids.
• Positive results with oxidation tests.

Ketones

• Not oxidised by mild oxidising agents.
• Negative results with these tests.

1. Fehling’s Solution / Benedict’s Solution

Reagent

• Alkaline copper(II) solution.

Observation with Aldehydes

• Blue solution forms a:

Brick-red precipitate

due to formation of \( \mathrm{Cu_2O} \).

Observation with Ketones

• No change.

General Oxidation Equation

\( \mathrm{RCHO + [O] \rightarrow RCOOH} \)

2. Tollens’ Reagent

Reagent

• Ammoniacal silver nitrate solution.

Observation with Aldehydes

• Formation of a:

Silver mirror

due to deposition of silver metal.

Observation with Ketones

• No reaction.

Importance

• Distinguishes aldehydes from ketones.

3. Acidified Dichromate(VI) Ions

Reagent

• \( \mathrm{K_2Cr_2O_7/H^+} \)

Observation with Aldehydes

• Orange solution changes to:

Green

Observation with Ketones

• No change.

Oxidation Equation

\( \mathrm{RCHO + [O] \rightarrow RCOOH} \)

Why Ketones Resist Oxidation

• No hydrogen attached directly to carbonyl carbon.
• Oxidation would require breaking C–C bonds.

(ii) Reduction with Lithium Tetrahydridoaluminate(III)

Reagent

\( \mathrm{LiAlH_4} \) in dry ether (ethoxyethane)

\( \mathrm{LiAlH_4} \) is a strong reducing agent supplying hydride ions (\( \mathrm{H^-} \)).

In equations, reduction may be represented by:

\( \mathrm{[H]} \)

Reduction of Aldehydes

• Aldehydes are reduced to:

Primary alcohols

General Equation

\( \mathrm{RCHO + 2[H] \rightarrow RCH_2OH} \)

Example

\( \mathrm{CH_3CHO + 2[H] \rightarrow CH_3CH_2OH} \)

Reduction of Ketones

• Ketones are reduced to:

Secondary alcohols

General Equation

\( \mathrm{RCOR’ + 2[H] \rightarrow RCH(OH)R’} \)

Example

\( \mathrm{CH_3COCH_3 + 2[H] \rightarrow CH_3CH(OH)CH_3} \)

Conditions

• Dry ether solvent required because \( \mathrm{LiAlH_4} \) reacts violently with water.

Summary Table

Key Features

• Aldehydes are easily oxidised.
• Ketones resist mild oxidation.
• \( \mathrm{LiAlH_4} \) reduces carbonyl compounds to alcohols.
• Oxidation tests distinguish aldehydes from ketones.

(iii) Reaction with HCN in the Presence of KCN

Aldehydes and ketones undergo nucleophilic addition reactions with hydrogen cyanide, \( \mathrm{HCN} \), to form hydroxynitriles.

Role of Reagents

• \( \mathrm{KCN} \) provides the nucleophile:

\( \mathrm{CN^-} \)

• \( \mathrm{HCN} \) provides \( \mathrm{H^+} \).

Why Carbonyl Compounds React

• Carbonyl bond is polar.
• Oxygen is more electronegative than carbon.

\( \mathrm{C^{\delta +}=O^{\delta -}} \)

Therefore, the carbonyl carbon is electrophilic.

Mechanism (Nucleophilic Addition)

Step 1: Nucleophilic Attack

• Lone pair on \( \mathrm{CN^-} \) attacks carbonyl carbon.
• \( \mathrm{\pi} \)-bond electrons move to oxygen.

Step 2: Protonation

• Alkoxide ion gains \( \mathrm{H^+} \) from HCN.
• Hydroxynitrile forms.

General Equation

\( \mathrm{RCHO + HCN \rightarrow RCH(OH)CN} \)

Example

\( \mathrm{CH_3CHO + HCN \rightarrow CH_3CH(OH)CN} \)

Optical Activity Evidence

• Carbonyl carbon is planar.
• \( \mathrm{CN^-} \) can attack from either side equally.
• If a chiral centre forms, both enantiomers form.
• Product becomes a racemic mixture.

(iv) Reaction with 2,4-Dinitrophenylhydrazine (2,4-DNPH)

2,4-DNPH is used as a qualitative test for aldehydes and ketones.

Purpose of the Test

• Detect presence of carbonyl group.
• Identify unknown carbonyl compounds using melting point data.

Observation

• Formation of an:

Orange precipitate

due to formation of a 2,4-dinitrophenylhydrazone derivative.

Importance of Melting Point

• Different derivatives have different melting points.
• Measured melting point compared with data table to identify compound.

Limitations

• Cannot distinguish aldehydes from ketones.
• Only confirms carbonyl group presence.

(v) Iodoform Test

The iodoform test identifies compounds containing the:

\( \mathrm{CH_3CO-} \)

group.

Reagents

• Iodine
• Aqueous alkali (e.g. NaOH)

Positive Result

• Formation of a:

Yellow precipitate

of iodoform:

\( \mathrm{CHI_3} \)

Compounds Giving Positive Test

• Ethanal
• Methyl ketones
• Secondary alcohols containing:

\( \mathrm{CH_3CH(OH)-} \)

Reason

• These compounds can form the \( \mathrm{CH_3CO-} \) group under reaction conditions.

Example

\( \mathrm{CH_3COCH_3} \) gives positive iodoform test.

Summary Table

Key Features

• \( \mathrm{HCN} \) reactions proceed by nucleophilic addition.
• Planar carbonyl groups can produce racemic mixtures.
• 2,4-DNPH detects aldehydes and ketones.
• Iodoform test identifies methyl ketones and ethanal.