Aldehydes and Ketones

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Aldehydes and Ketones — Key Facts for MDCAT

Structure:

• Aldehyde: R–CHO (at least one H attached to carbonyl carbon; terminal group)
  • General formula: C$n$H${2n}$O (for saturated aldehydes)
  • Examples: Methanal (HCHO), Ethanal (CH$_3$CHO), Propanal (C$_2$H$_5$CHO)
• Ketone: R–CO–R’ (two alkyl/araryl groups attached to carbonyl carbon; internal group)
  • General formula: C$n$H${2n}$O (same as aldehydes — isomers of each other)
  • Examples: Propanone/Acetone (CH$_3$COCH$_3$), Butanone (CH$_3$COC$_2$H$_5$)

Nomenclature:

• Aldehydes: suffix -al (e.g., ethanal, propanal)
• Ketones: suffix -one with position number if needed (e.g., butan-2-one)
• The carbonyl carbon is always position 1 in aldehydes

Physical Properties:

• Lower aldehydes (methanal, ethanal) are water-miscible due to H-bonding with water
• Ketones are polar molecules with dipole-dipole interactions → higher boiling points than alkanes of similar MW but lower than alcohols
• Aldehydes and ketones cannot hydrogen bond with themselves (no O–H bond)

Key Distinguishing Tests:

Test	Aldehyde	Ketone
Tollens’ reagent (AgNO$_3$ + NH$_3$)	Silver mirror ($\ Ag^0$)	No reaction
Fehling’s solution (CuSO$_4$ + Rochelle salt)	Red/brick-red Cu$_2$O precipitate	No reaction
2,4-DNP test	Yellow/orange precipitate (crystalline)	Same (2,4-DNP reacts with ALL carbonyls)
Brady’s test	Same as above	Same

⚡ Exam tip: Tollens’ test (silver mirror) distinguishes aldehydes from ketones. Aromatic aldehydes (benzaldehyde) give a positive Tollens’ test but negative Fehling’s test. Formaldehyde is the ONLY aldehyde that gives a positive Fehling’s test despite being an aldehyde. This is a common MCQ trap.

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🟡 Standard — Regular Study (2d–2mo)

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Aldehydes and Ketones — Complete Study Guide

Nucleophilic Addition Reactions: The carbonyl carbon is electrophilic due to the electronegative oxygen pulling electron density. Nucleophiles (Nu:$^-$) attack the carbonyl carbon. The π bond electrons shift to oxygen, which is then protonated.

Step 1: Nu$^-$ attacks C=O → alkoxide intermediate Step 2: Protonation of O$^-$ → alcohol

Key Addition Reactions:

• HCN: $R–CHO + HCN \rightarrow R–CH(OH)–CN$ (cyanohydrin). Rate faster for aldehydes than ketones. Used in organic synthesis.
• NaHSO$_3$: Only aldehydes and methyl ketones react (solid white bisulfite addition product). This is a test for aldehydes/methyl ketones.
• Grignard reagents (RMgX): $R–CHO + RMgX \rightarrow$ secondary alcohol (from aldehyde); $R–COR’ + RMgX \rightarrow$ tertiary alcohol
• Ammonia and derivatives: $NH_2OH$ (oxime), $NH_2NH_2$ (hydrazone), $C_6H_5NHNH_2$ (phenylhydrazone) — used in structure elucidation

Reduction Reactions:

• NaBH$_4$ (mild reducing agent): Reduces aldehydes to 1° alcohols, ketones to 2° alcohols. Does NOT reduce esters, carboxylic acids, or amides.
• LiAlH$_4$ (strong reducing agent): Reduces ALL carbonyl compounds including esters, amides, carboxylic acids.
• Clemmensen reduction: Zn(Hg)/HCl reduces C=O to CH$_2$ (for ketones). Used to determine structure of ketones.
• Wolf-Kishner reduction: Same as Clemmensen but uses hydrazine and base. Better for acid-sensitive compounds.

Oxidation:

• Aldehydes oxidise easily to carboxylic acids: $R–CHO \xrightarrow{[O]} R–COOH$
• Aldehydes give positive Tollens’, Fehling’s, and Benedict’s tests
• Ketones generally resist oxidation (do NOT give Tollens’ or Fehling’s)
• Haloform reaction: Methyl ketones ($CH_3CO–R$) react with $I_2/NaOH$ to give iodoform ($CHI_3$, yellow precipitate). Acetone gives a positive iodoform test. This is a TEST for methyl ketones.

Aldol Condensation: In the presence of dilute base, aldehydes/ketones with α-hydrogen undergo aldol condensation:

• Step 1: Enolate ion formation (α-hydrogen removal)
• Step 2: Nucleophilic attack on another carbonyl
• Step 3: Aldol product (β-hydroxy carbonyl)
• Step 4 (with heating): Dehydration → α,β-unsaturated carbonyl compound

Example: $2CH_3CHO \xrightarrow{NaOH} CH_3CH(OH)–CH_2–CHO \xrightarrow{\Delta} CH_3CH=CH–CHO$ (crotonaldehyde)

⚡ Common mistakes: Confusing Clemmensen reduction (Zn/Hg, HCl) with Wolf-Kishner (N$_2$H$_4$, KOH) — both reduce C=O to CH$_2$. For Tollens’ test, aldehydes are oxidised to carboxylic acids while Ag$^+$ is reduced to Ag$^0$ (silver mirror). Ketones do NOT give Tollens’. For aldol condensation, the dehydration product is more stable due to conjugation.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Aldehydes and Ketones — Advanced Notes

Mechanism of Nucleophilic Addition: The carbonyl group is planar (sp² hybridisation). The nucleophile attacks from either face with equal probability (unless chiral environment). The intermediate is tetrahedral (sp³).

The reactivity order for nucleophilic addition: Aldehydes > Ketones (because: 1) ketones have two electron-donating alkyl groups that reduce electrophilicity; 2) steric hindrance in ketones)

Specific order: HCHO > RCHO > RCOR’ (aromatic ketones are even less reactive due to resonance with aryl group)

Enolisation and Tautomerism: Carbonyl compounds with α-hydrogen can exist in keto-enol tautomeric forms: $$R–CH_2–C(=O)–R’ \rightleftharpoons R–CH=C(OH)–R’$$

The enol form has the C=C double bond and the OH group. The equilibrium lies heavily toward the keto form for simple aldehydes and ketones. However, enols are important intermediates in acid- and base-catalysed reactions.

Mechanism of Base-Catalysed Aldol Condensation:

1. Base abstracts α-hydrogen → enolate anion (stabilised by carbonyl)
2. Enolate attacks carbonyl carbon of another molecule → alkoxide intermediate
3. Protonation → β-hydroxy aldehyde/ketone (aldol product)
4. Under heated conditions: dehydration → α,β-unsaturated carbonyl

Crossed Aldol (Mixed Aldol): When two different carbonyl compounds are used:

• If both have α-hydrogens → mixture of 4 products (generally useless)
• Useful crossed aldol: one has NO α-hydrogen (e.g., benzaldehyde) + other has α-hydrogen
• Example: Benzaldehyde + acetone → benzylideneacetone (condensation product)

Cannizzaro Reaction: A disproportionation reaction of aldehydes with NO α-hydrogen in the presence of concentrated NaOH: $$2HCHO + NaOH \rightarrow CH_3OH + HCOONa$$ One molecule is oxidised (formic acid salt), one is reduced (alcohol). This is a mutual oxidation-reduction reaction. Only aldehydes without α-hydrogen undergo this reaction.

Important Named Reactions:

1. Rosenmund reduction: $R–COCl + H_2/Pd–BaSO_4 \rightarrow R–CHO$ (acyl chloride to aldehyde)
2. Stephen reduction: $R–CN + SnCl_2/HCl \rightarrow R–CHO$ (nitrile to aldehyde)
3. Friedel-Crafts acylation: $R–COCl + ArH \xrightarrow{AlCl_3} Ar–CO–R$ (ketone synthesis)
4. Knoevenagel condensation: Active methylene compound (malonic ester, ethyl acetoacetate) + aldehyde/ketone → α,β-unsaturated compound

Polymers from Aldehydes:

• Bakelite: Phenol + formaldehyde (condensation polymer, thermosetting)
• Formaldehyde resins: Urea + formaldehyde → urea-formaldehyde polymer

MDCAT Question Patterns: MDCAT Pakistan questions frequently test: (1) Tollens’ vs Fehling’s test — which aldehydes give which, (2) aldol condensation products, (3) distinguishing tests between aldehydes and ketones, (4) IUPAC naming, (5) crossed aldol products when one partner has no α-H, (6) reduction products (alcohol type). 2–3 questions per paper. Aldol condensation mechanisms and Cannizzaro reactions are high-yield for written-answer questions.

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