Aldehydes & Ketones: Nomenclature, Reactions & Uses

TAKEAWAYS:

• Aldehydes (R-CHO) and ketones (R-CO-R’) differ by their carbonyl carbon’s bonding and reactivity.
• Nucleophilic addition (e.g., with HCN, Grignard reagents) is their defining reaction, forming new carbon-carbon bonds.
• Aldehydes oxidize easily (to carboxylic acids), while ketones resist oxidation under mild conditions.
• Tollens’ and Fehling’s tests distinguish aldehydes from ketones via silver mirror and red precipitate formation.
• Aldehydes and ketones are vital in industry (e.g., formaldehyde in resins) and biology (e.g., glucose metabolism).

1. Introduction: What Are Aldehydes and Ketones?

Aldehydes and ketones are carbonyl compounds—organic molecules containing a C=O (carbonyl) group. They differ in where this group is located:

• Aldehydes: The carbonyl carbon is bonded to at least one hydrogen (R-CHO). Example: Ethanal (CH₃CHO), formaldehyde (HCHO).
• Ketones: The carbonyl carbon is bonded to two carbon atoms (R-CO-R’). Example: Propanone (acetone, CH₃COCH₃).

Why are they important?

• Found in food flavors (vanilla, cinnamon), pharmaceuticals (aspirin), and plastics (formaldehyde in Bakelite).
• Key intermediates in organic synthesis (e.g., making alcohols, acids).

classDiagram
    class Aldehyde {
        +Carbonyl carbon bonded to H
        +General formula: R-CHO
        +Oxidizes to carboxylic acid
        +Example: CH₃CHO (ethanal)
    }
    class Ketone {
        +Carbonyl carbon bonded to 2 carbons
        +General formula: R-CO-R'
        +Resists oxidation (mild conditions)
        +Example: CH₃COCH₃ (propanone)
    }
    Aldehyde --> Ketone : "Both have C=O but differ in bonding"

2. Nomenclature: Naming Aldehydes and Ketones

Use IUPAC rules to name them systematically.

A. Aldehydes

1. Find the longest carbon chain containing the -CHO group.
2. Replace the -e ending of the alkane with -al.
   • Example: CH₃CH₂CHO → Propanal (3 carbons + "-al").

B. Ketones

1. Find the longest chain with the C=O group.
2. Number the chain to give the carbonyl the lowest possible number.
3. Replace -e with -one.
   • Example: CH₃COCH₂CH₃ → Butan-2-one (4 carbons, carbonyl on C2).

Common Names (Still Used!)

• Formaldehyde: HCHO (methanal).
• Acetone: CH₃COCH₃ (propanone).
• Benzaldehyde: C₆H₅CHO (from bitter almonds).

Structural difference between aldehydes and ketones (Image: Bin im Garten, CC BY-SA 3.0, via Wikimedia Commons)

3. Preparation Methods

A. Oxidation of Alcohols

• Primary alcohols → Aldehydes (then further to acids if oxidized strongly). Reagent: K₂Cr₂O₇/H₂SO₄ (acidified dichromate) or PCC (pyridinium chlorochromate). Example:

  CH₃CH₂OH (ethanol) --[PCC]--> CH₃CHO (ethanal)
  

• Secondary alcohols → Ketones. Example:

  CH₃CH(OH)CH₃ (propan-2-ol) --[K₂Cr₂O₇]--> CH₃COCH₃ (propanone)
  

B. Ozonolysis of Alkenes

• Cleaves C=C bonds to form aldehydes/ketones. Example:

  CH₃-CH=CH-CH₃ (but-2-ene) --[O₃, Zn/H₂O]--> 2 × CH₃CHO (ethanal)
  

C. From Alkynes

• Hydration of alkynes (with HgSO₄/H₂SO₄) gives methyl ketones. Example:

  CH≡CH (ethyne) --[H₂O, Hg²⁺]--> CH₃CHO (ethanal)
  

flowchart TD
    A["Primary Alcohol"] -->|"PCC"| B["Aldehyde"]
    A -->|"K₂Cr₂O₇ (strong)"| C["Carboxylic Acid"]
    D["Secondary Alcohol"] -->|"K₂Cr₂O₇"| E["Ketone"]
    F["Alkene"] -->|"O₃, Zn/H₂O"| G["Aldehyde/Ketone"]
    H["Alkyne"] -->|"H₂O, Hg²⁺"| I["Methyl Ketone"]

4. Physical Properties

Why the difference?

• Aldehydes can H-bond with water (due to -CHO hydrogen), while ketones cannot.

5. Chemical Reactions

Aldehydes and ketones undergo nucleophilic addition (not substitution!) because the C=O bond is polar (δ⁺ on C, δ⁻ on O).

A. Nucleophilic Addition Reactions

1. Addition of Hydrogen Cyanide (HCN)
   • Forms hydroxynitriles (cyanohydrins). Example:

   CH₃CHO + HCN → CH₃CH(OH)CN (2-hydroxypropanenitrile)
   

   Use: Preparing α-hydroxy acids (e.g., lactic acid).

2. Addition of Grignard Reagents (RMgX)

   • Forms alcohols after hydrolysis. Example:

   CH₃CHO + CH₃MgBr → CH₃CH(OH)CH₃ (propan-2-ol)
   

3. Addition of Water (Hydration)

   • Catalyzed by acid/base, forms gem-diols (unstable, revert to carbonyl). Example:

   CH₃CHO + H₂O ⇌ CH₃CH(OH)₂
   

flowchart LR
    A["CH₃CHO"] -->|"HCN"| B["CH₃CH(OH)CN"]
    A -->|"CH₃MgBr"| C["CH₃CH(OH)CH₃"]
    A -->|"H₂O"| D["CH₃CH(OH)₂"]

B. Oxidation Reactions

• Aldehydes oxidize easily to carboxylic acids. Reagents: K₂Cr₂O₇/H₂SO₄, Tollens’ reagent (Ag(NH₃)₂⁺), Fehling’s solution (Cu²⁺). Example:

  CH₃CHO --[Tollens']--> CH₃COOH + Ag (silver mirror)
  

• Ketones resist oxidation (except strong oxidizers like KMnO₄, which cleave the C-C bond).

Test for Aldehydes vs. Ketones:

C. Reduction Reactions

• Lithium Aluminium Hydride (LiAlH₄) reduces both to alcohols. Example:

  CH₃CHO --[LiAlH₄]--> CH₃CH₂OH (ethanol)
  CH₃COCH₃ --[LiAlH₄]--> CH₃CH(OH)CH₃ (propan-2-ol)
  

• Catalytic Hydrogenation (H₂/Ni) also works.

D. Cannizzaro Reaction (Aldehydes Only!)

• No α-hydrogen aldehydes (e.g., formaldehyde) disproportionate in base to give:
  • Alcohol (reduced product).
  • Carboxylate salt (oxidized product). Example:

  2 HCHO --[OH⁻]--> CH₃OH + HCOO⁻
  

flowchart TD
    A["CH₃CHO"] -->|"LiAlH₄"| B["CH₃CH₂OH"]
    C["CH₃COCH₃"] -->|"LiAlH₄"| D["CH₃CH(OH)CH₃"]
    E["HCHO"] -->|"OH⁻"| F["CH₃OH + HCOO⁻"]

**6. Comparison Table: Key Differences

7. Applications in Daily Life

8. Solved Examples

Example 1: Nomenclature

Name the following:

1. CH₃CH₂CH(CH₃)CHO
2. CH₃COCH₂CH₂CH₃

Solution:

1. 4-Methylpentanal (5 carbons, -CHO on C1, methyl on C3).
2. Pentan-2-one (5 carbons, C=O on C2).

Example 2: Reaction Prediction

What happens when butan-2-one reacts with:

1. HCN
2. LiAlH₄

Solution:

1. 2-Hydroxy-2-methylbutanenitrile (nucleophilic addition).
2. 2-Methylpropan-2-ol (reduction to alcohol).

9. NEB Board-Style Questions

Short Answer (5 marks)

1. Explain why aldehydes are more reactive than ketones toward nucleophilic addition. Give one example of each.
2. Write the IUPAC names of:
   • CH₃(CH₂)₃CHO
   • (CH₃)₂CHCOCH₃

Long Answer (10 marks)

1. Describe the preparation of ethanal from ethanol. How would you distinguish ethanal from propanone using:
   • Tollens’ reagent
   • Fehling’s solution
   • 2,4-DNP reagent
2. What is the Cannizzaro reaction? Write the reaction for formaldehyde undergoing this reaction in the presence of NaOH.

Exam Tip

1. Memorize Tests:

   • Tollens’/Fehling’s = Aldehydes only!
   • 2,4-DNP = Both aldehydes and ketones (orange precipitate).

2. Reaction Mechanisms:

   • Always show the nucleophile attacking the carbonyl carbon in addition reactions.

3. Naming Tricks:

   • Aldehydes: -al suffix, carbonyl carbon is C1.
   • Ketones: -one suffix, give carbonyl the lowest number.

4. Common Mistakes:

   • Don’t confuse oxidation (aldehydes → acids) with reduction (both → alcohols).
   • Ketones do not give Tollens’/Fehling’s tests—write "no reaction" if asked!

5. Industrial Applications:

   • Link aldehydes/ketones to plastics (formaldehyde), perfumes (citral), and medicine (aspirin synthesis).

Final Note: Practice drawing mechanisms for nucleophilic additions—they fetch high marks in NEB exams! Use curly arrows to show electron movement.