Aldehydes and Ketones: Structure, Preparation, and Reactions

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Topic 6 — Key Facts for Kenyatta University (Kenya) Core concept: Aldehydes have the –CHO group on a terminal carbon (R–CHO); ketones have the –CO– group internal (R–CO–R’); both contain the carbonyl (C=O) functional group, which is polar and electrophilic at the carbon atom High-yield point: Both aldehydes and ketones undergo nucleophilic addition reactions; NaBH₄ reduces them to alcohols; Tollens’ reagent (AgNO₃/NH₃) oxidises aldehydes to carboxylic acids (gives silver mirror) but does NOT oxidise ketones; Fehling’s solution (Cu²⁺) similarly distinguishes them ⚡ Exam tip: The nucleophilic addition mechanism involves: (1) nucleophile attacks carbonyl carbon, (2) carbonyl oxygen is protonated, (3) proton transfer; the carbonyl carbon must be sp² planar — this is frequently tested

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Aldehydes and Ketones: The Carbonyl Functional Group

Aldehydes and ketones are characterised by the presence of the carbonyl functional group (C=O). The carbonyl carbon is sp² hybridised with trigonal planar geometry. The C=O bond is polar — oxygen bears a partial negative charge (δ⁻) and carbon bears a partial positive charge (δ⁺).

Aldehydes: The carbonyl carbon is attached to at least one hydrogen atom and one alkyl/aryl group (terminal position). General formula: R–CHO. Suffix: -al.

Ketones: The carbonyl carbon is attached to two alkyl or aryl groups (internal position). General formula: R–CO–R’. Suffix: -one.

Nomenclature

Aldehyde Examples:

• Methanal (HCHO): Simplest aldehyde, also called formaldehyde
• Ethanal (CH₃CHO): Also called acetaldehyde
• Propanal (CH₃CH₂CHO): Three-carbon aldehyde
• Butanal (CH₃CH₂CH₂CHO): Four-carbon aldehyde

Ketone Examples:

• Propanone (CH₃COCH₃): Simplest ketone, also called acetone
• Butanone (CH₃COCH₂CH₃): Also called methyl ethyl ketone (MEK)
• Pentan-3-one (CH₃CH₂COCH₂CH₃)
• Cyclohexanone: Six-membered ring ketone

Physical Properties

Property	Aldehydes	Ketones
Polarity	Moderate dipole (1.2–1.8 D)	Moderate dipole (~2.7–3.0 D)
Boiling point	Lower than corresponding alcohol; higher than alkane	Higher than corresponding aldehyde of similar MW
Water solubility	Formaldehyde and acetaldehyde are miscible; larger aldehydes have limited solubility	Acetone and methyl ethyl ketone are miscible with water
Odour	Formaldehyde: pungent; others: fruity	Fruity to acetone-like

⚡ Exam Tip: Formalin is a 37% aqueous solution of formaldehyde. It is used as a preservative in biology. Propanone (acetone) is widely used as a solvent in nail polish remover and paint thinners.

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Preparation of Aldehydes and Ketones

1. Oxidation of Primary and Secondary Alcohols

• Primary alcohol → Aldehyde (requires mild oxidant, e.g., PCC, pyridinium chlorochromate)
• Secondary alcohol → Ketone (mild or strong oxidant, e.g., K₂Cr₂O₇/H₂SO₄, Jones reagent)

Example:

CH₃CH₂OH + [O] → CH₃CHO   (ethanol → acetaldehyde)
CH₃–CH(OH)–CH₃ + [O] → CH₃–CO–CH₃   (propan-2-ol → acetone)

⚡ Why Mild Oxidants for Aldehydes? Strong oxidants (like KMnO₄) further oxidise the aldehyde to a carboxylic acid. PCC (pyridinium chlorochromate) in CH₂Cl₂ stops at the aldehyde.

2. Ozonolysis of Alkenes

Ozonolysis cleaves C=C bonds:

• Each carbon of the double bond becomes a carbonyl carbon
• If each alkene carbon has one H: formaldehyde (HCHO)
• If each alkene carbon has one H and one alkyl: aldehyde
• If each alkene carbon has two alkyl groups: ketone

Example:

(CH₃)₂C=CHCH₃ + O₃ → (CH₃)₂C=O + CH₃CHO   (acetone + acetaldehyde)

3. From Acid Chlorides — Rosenmund Reduction

Acid chloride + H₂/Pd-BaSO₄ (poisoned catalyst) → aldehyde:

CH₃COCl + H₂ → CH₃CHO   (acetaldehyde from acetyl chloride)

This is a controlled reduction that stops at the aldehyde stage.

4. From Nitriles — partial hydrolysis

Nitrile + SnCl₂/HCl (Stephen reaction) → imine → hydrolysis → aldehyde:

CH₃CN + [H] → CH₃CHO   (acetaldehyde from acetonitrile)

Reactions of Aldehydes and Ketones

1. Nucleophilic Addition Reactions

The carbonyl carbon is electrophilic (δ⁺) due to oxygen’s high electronegativity. It is attacked by nucleophiles (Nu⁻). After attack, the carbonyl oxygen is protonated to give the addition product.

General Mechanism:

1. Nucleophile attacks the carbonyl carbon → tetrahedral alkoxide intermediate
2. Protonation of the oxygen → neutral addition product

Types of Nucleophiles:

• Hydride (H⁻) → alcohol
• Grignard reagents (R–MgBr) → alcohol
• Amines (RNH₂) → imine (via iminium ion)
• Alcohols (ROH) → hemiacetal → acetal
• Cyanide (CN⁻) → cyanohydrin

2. Reduction Reactions

Reduction to Alcohols:

• Aldehyde + NaBH₄ → primary alcohol
• Ketone + NaBH₄ → secondary alcohol

CH₃CHO + [H] → CH₃CH₂OH   (acetaldehyde → ethanol)
(CH₃)₂C=O + [H] → (CH₃)₂CHOH   (acetone → propan-2-ol)

Reduction to Alkanes (Clemmensen Reduction): Ketone + Zn(Hg)/HCl → alkane:

CH₃–CO–CH₃ → CH₃–CH₂–CH₃   (acetone → propane)

This is a method for converting a carbonyl group to a methylene (CH₂) group.

3. Oxidation Reactions — The Key Distinction

Aldehydes are oxidised; ketones are not (under mild conditions):

Tollens’ Test (AgNO₃ in NH₃ — Ag(NH₃)₂⁺): Aldehyde + [Ag(NH₃)₂⁺] → Carboxylic acid + Ag⁰ (silver mirror on test tube) Ketone: No reaction (no silver mirror)

Fehling’s Test (Cu²⁺ in alkaline tartrate solution): Aldehyde + Cu²⁺ → Carboxylic acid + Cu₂O↓ (brick-red precipitate) Ketone: No reaction

Benedict’s Test (Cu²⁺ in alkaline citrate solution): Similar to Fehling’s test; aldehydes give Cu₂O brick-red precipitate; ketones do not.

⚡ Exam Tip: These tests are routinely used to distinguish aldehydes from ketones. In the Tollens’ test, watch for a silver mirror on the inside of the test tube rather than a black precipitate (which indicates over-oxidation or decomposition).

4. Nucleophilic Addition of Cyanide — Cyanohydrin Formation

Aldehyde or ketone + HCN → cyanohydrin:

CH₃CHO + HCN → CH₃CH(OH)CN   (acetaldehyde cyanohydrin)
(CH₃)₂C=O + HCN → (CH₃)₂C(OH)CN   (acetone cyanohydrin)

Significance:

• The –OH and –CN groups end up on adjacent carbons
• The cyanohydrin can be hydrolysed to an α-hydroxy acid (–OH and –COOH on adjacent carbon)
• Example: Acetaldehyde cyanohydrin → lactic acid (CH₃CH(OH)COOH) upon hydrolysis

⚡ Exam Tip: HCN is a highly toxic gas (boiling point 26°C). In the laboratory, it is generated in situ by reacting NaCN with acid. Never store HCN — it decomposes over time and can polymerise explosively.

5. Addition of Amines — Imines and Enamines

Primary amine (R–NH₂): Aldehyde/Ketone + R–NH₂ → imine (Schiff base) + H₂O:

CH₃CHO + CH₃NH₂ → CH₃CH=N–CH₃ + H₂O   (ethanimine)

Secondary amine (R₂NH): Aldehyde/Ketone + R₂NH → enamine + H₂O:

(CH₃)₂C=O + (CH₃)₂NH → (CH₃)₂C=CH₂ + H₂O (after proton transfer) → enamine

6. Addition of Alcohols — Hemiacetals and Acetals

Step 1 — Hemiacetal formation: Aldehyde/Ketone + alcohol (ROH) + acid catalyst → hemiacetal:

CH₃CHO + CH₃OH → CH₃CH(OH)–OCH₃

Step 2 — Acetal formation (if excess alcohol and acid): Hemiacetal + ROH → acetal + H₂O:

CH₃CH(OH)–OCH₃ + CH₃OH → CH₃CH(–OCH₃)₂ + H₂O   (acetaldehyde dimethyl acetal)

Acetals as protecting groups: Since aldehydes and ketones are sensitive to acids and nucleophiles, they can be “protected” by converting them to acetals (stable to bases and nucleophiles but cleavable by acid).

7. The Haloform Reaction (Iodoform Test)

Methyl ketones (CH₃–CO–R) and acetaldehyde undergo the haloform reaction with I₂/NaOH:

CH₃–CO–R + 3I₂ + 4NaOH → CHI₃ (yellow precipitate) + R–COONa + 3NaI + 3H₂O

Iodoform Test:

• Compounds with CH₃–CO– group give yellow CHI₃ precipitate with I₂/NaOH
• Also positive for ethanol (oxidised to acetaldehyde first) and 2-propanol (oxidised to acetone)
• Acetone gives a positive iodoform test → CHI₃ yellow precipitate

⚡ Exam Tip: CHI₃ has a characteristic sweet odour and appears as yellow crystals. This is a key test for methyl ketones that is frequently tested in Kenyatta University organic chemistry examinations.

8. Condensation Reactions — aldol Condensation

In the presence of base (NaOH), aldehydes undergo aldol condensation:

Step 1 — Aldol addition: Two aldehyde molecules combine via C–C bond formation:

2CH₃CHO → CH₃–CH(OH)–CH₂–CHO   (3-hydroxybutanal, aldol)

Mechanism:

1. Base abstracts α-hydrogen from one aldehyde → enolate ion
2. Enolate attacks carbonyl carbon of second aldehyde
3. Protonation → β-hydroxy aldehyde

Step 2 — Aldol condensation: If heated, dehydration occurs to give an α,β-unsaturated aldehyde:

CH₃–CH(OH)–CH₂–CHO → CH₃–CH=CH–CHO + H₂O   (crotonaldehyde)

For ketones, the same reaction occurs but more slowly (acetone → diacetone alcohol → mesityl oxide).

⚡ Exam Tip: Aldol condensation requires an α-hydrogen (hydrogen on the carbon adjacent to the carbonyl). Formaldehyde (no α-H), benzaldehyde (no α-H on aryl carbon), and methyl ketones (the methyl group provides α-H) can participate if conditions allow.

Condensed Summary of Aldehyde vs Ketone Reactions

Reaction	Aldehyde	Ketone	Test Application
Tollens’ reagent	Oxidised to acid; Ag mirror forms	No reaction	Distinguish aldehydes from ketones
Fehling’s/Benedict’s	Cu₂O red precipitate	No reaction	Distinguish aldehydes from ketones
NaBH₄ reduction	Primary alcohol	Secondary alcohol	Identify functional group
HCN addition	Cyanohydrin	Cyanohydrin	Both react — chain extension
Iodoform test	Positive (if CH₃CHO)	Positive (if methyl ketone)	Identify methyl ketones
Aldol condensation	Yes	Yes (slower)	Both undergo aldol reactions

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