Carbonyl Compounds — Intuition Note

1. Definition — The Electrophilic Trap

The carbonyl group ($C=O$) is the defining feature of aldehydes and ketones. General formula: $C_nH_{2n}O$.

The carbonyl carbon is sp2 hybridised — trigonal planar geometry, one $\sigma$ bond and one $\pi$ bond to oxygen. Oxygen is more electronegative, so the bond is polarised:

$$\delta+ \quad \delta-$$ $$C=O$$

The carbon is electrophilic. The oxygen is nucleophilic. This polarity drives everything.

MM: The carbonyl is a mousetrap. The carbon is the spring-loaded bar, the oxygen is the bait (electronegative, pulling electrons). Every nucleophile that walks into the room wants to snap that trap.

Aldehydes vs Ketones

Aldehyde Ketone
Structure $RCHO$ (terminal) $RCOR'$ (internal)
IUPAC suffix -al -one
Carbonyl neighbours 1 H + 1 R 2 R groups
Reactivity More reactive Less reactive
Oxidation Easily oxidised Resistant
Example Ethanal ($CH_3CHO$) Propanone ($CH_3COCH_3$)

Why aldehydes are more reactive than ketones — two reasons:

  1. Steric: Aldehydes have only one R group — less bulky, easier for nucleophiles to reach the carbonyl carbon.
  2. Electronic: Alkyl groups are electron-donating (+I). Ketones have two alkyl groups donating electron density to the carbonyl carbon, making it less $\delta+$ (less electrophilic). Aldehydes only have one — the H doesn't donate.

MM: Aldehydes are the keen younger sibling who volunteers for everything. Ketones are the older one wearing two layers of armour (two R groups) — tougher to get through.

2. Physical Properties — The Polarity Story

Compound Type BP Why?
Propan-1-ol Alcohol 97 °C H-bonding
Propanone Ketone 56 °C Dipole-dipole only
Propanal Aldehyde 49 °C Dipole-dipole only
Ethyl methyl ether Ether 11 °C Weak dipole-dipole

Key observations:

  • Carbonyl compounds have higher BP than alkanes/ethers (dipole-dipole > vdW) but lower than alcohols (no H-bonding — carbonyls have no O-H bond).
  • Lower members are water-soluble (carbonyl oxygen can H-bond with water).

3. Identification Tests — The Exam Goldmine

This is the highest-yield section for exams. If you memorise one table, memorise this one.

Test Reagent Aldehyde Ketone
Tollens' $[Ag(NH_3)_2]^+$ Silver mirror (Ag↓) No reaction
Fehling's/Benedict's $Cu^{2+}$ (alkaline) Brick-red $Cu_2O$↓ No reaction
2,4-DNP (Brady's) $C_6H_3(NO_2)_2NHNH_2$ Orange/red ppt Orange/red ppt
Iodoform $I_2/NaOH$ Only if $CH_3CHO$ Only if methyl ketone
Schiff's Schiff's reagent Pink colour No reaction

What Each Test Actually Detects

Tollens': Aldehydes are oxidised to carboxylic acids. $Ag^+$ is reduced to $Ag$ (silver mirror). $$RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow RCOO^- + 2Ag\downarrow + 4NH_3 + 2H_2O$$

Fehling's: Same logic — aldehydes oxidised, $Cu^{2+}$ reduced to $Cu_2O$ (brick-red). Aromatic aldehydes give negative Fehling's (benzaldehyde is stubborn).

2,4-DNP (Brady's reagent): Detects any carbonyl — both aldehydes and ketones. The orange/red precipitate is a hydrazone derivative. This is the universal carbonyl test.

Iodoform (CHI3 yellow precipitate): Specific for $CH_3CO-$ groups (methyl ketones) or $CH_3CH(OH)-$ that can be oxidised to methyl ketones. Only ethanal among aldehydes.

Schiff's: Aldehydes restore the pink colour to the decolourised magenta dye. Ketones don't.

Quick Decision Tree

Unknown compound
       |
  + 2,4-DNP
  |
  Orange/red ppt? ———— No → Not a carbonyl
       |
      Yes
       |
  + Tollens'
       |
  Silver mirror? ———— Yes → Aldehyde
       |
       No
       |
  + Iodoform
       |
  Yellow ppt? ———— Yes → Methyl ketone (CH3COR)
       |
       No → Other ketone

Key Exam Traps

  • Aromatic aldehydes give positive Tollens' but negative Fehling's
  • Ethanal is the only aldehyde that gives positive iodoform
  • Ethanol gives positive iodoform (oxidised to ethanal first) — the only 1° alcohol that does
  • Methanol gives negative iodoform (oxidises to formic acid, no CH3CO-)

4. Conditions — The Fine Print

Reactivity Order (Nucleophilic Addition)

$$Formaldehyde > Aliphatic\ aldehydes > Aromatic\ aldehydes > Aliphatic\ ketones > Aromatic\ ketones$$

Why benzaldehyde is less reactive than aliphatic aldehydes: The phenyl ring donates electron density via resonance, reducing the $\delta+$ on carbonyl carbon.

Condition Summary

Reaction Conditions Notes
HCN addition Base-catalysed (CN⁻ is nucleophile) One-carbon extension
Grignard Anhydrous ether, separate hydrolysis step Water destroys RMgX
NaHSO3 addition Conc. NaHSO3 solution Crystalline addition compound — purification
Aldol condensation Dilute base (OH⁻), then heat for dehydration Need $\alpha$-H
Cannizzaro Conc. base (no $\alpha$-H allowed) Disproportionation
Haloform I2 + NaOH, room temp Only for CH3CO-
Tollens' AgNO3 + NH3, mild base, warm Clean glass essential for silver mirror
2,4-DNP 2,4-DNPH in acidic methanol Works on all carbonyls
Reduction (NaBH4) Methanol/ethanol, RT Mild — only reduces C=O
Reduction (LiAlH4) Anhydrous ether Strong — reduces C=O, COOH, esters

5. Preparation — Making Carbonyls

Aldehydes

Method Starting material Reagent Notes
1° alcohol oxidation $RCH_2OH$ PCC/CH2Cl2 (only choice) Strong oxidants over-oxidise to acid
Ozonolysis Alkene with =CHR O3 then H2O/Zn Aldehyde from monosubstituted C=C
Hydroformylation Alkene CO/H2, catalyst Industrial

Gold quote: PCC is the only reagent that stops at the aldehyde. Everything else barrels through to carboxylic acid.

Ketones

Method Starting material Reagent Notes
2° alcohol oxidation $R_2CHOH$ Any oxidant (PCC, K2Cr2O7, KMnO4) Won't over-oxidise
Ozonolysis Alkene with =CR2 O3 then H2O/Zn Ketone from disubstituted C=C
Friedel-Crafts acylation Benzene + RCOCl AlCl3 (Lewis acid) Aromatic ketones
Alkyne hydration $RC\equiv CR'$ H2O, Hg2+/H+ Markovnikov

6. Reactions — What They Do

6a. Nucleophilic Addition (The Core Reaction)

The mechanism every carbonyl question builds on:

$$\ce{R2C=O + Nu^- ->[slow] R2C(Nu)O^- ->[H3O^+] R2C(Nu)OH}$$

The tetrahedral intermediate is the key. The carbonyl carbon goes from sp2 (flat, trigonal planar) to sp3 (tetrahedral, bulging). Then protonation locks the addition.

HCN → Cyanohydrin

$$R_2C=O + HCN \xrightarrow{OH^-} R_2C(OH)CN$$ One carbon extension. The CN group can be hydrolysed to COOH later.

Grignard → Alcohol

Carbonyl Product
Formaldehyde Primary alcohol
Aldehyde Secondary alcohol
Ketone Tertiary alcohol

Grignard is a carbon nucleophile — it forms new C-C bonds. This is the most powerful tool for making complex alcohols.

NaHSO3 → Addition Compound

$$R_2C=O + NaHSO_3 \rightarrow R_2C(OH)SO_3Na$$ Crystalline solid. Used for purification — precipitate the carbonyl, filter, then regenerate with acid.

Alcohols → Acetals/Ketals (Protecting Groups)

$$R_2C=O + 2ROH \xrightarrow{H^+} R_2C(OR)_2 + H_2O$$ Acetals are stable to base and nucleophiles but cleave in acid. Used to protect carbonyls during reactions elsewhere in the molecule.

6b. Addition-Elimination (Nitrogen Nucleophiles)

These go through a tetrahedral intermediate that eliminates water:

Reagent Product Test?
$NH_2OH$ (hydroxylamine) Oxime ($C=NOH$) —
$NH_2NH_2$ (hydrazine) Hydrazone ($C=NNH_2$) —
2,4-DNPH (Brady's) 2,4-DNP derivative Orange/red ppt ✅
$RNH_2$ (primary amine) Imine/Schiff base ($C=NR$) —

6c. Oxidation

Only aldehydes oxidise. Ketones resist. $$RCHO \xrightarrow{[O]} RCOOH$$

This is why Tollens' and Fehling's distinguish them.

6d. Reduction

Reagent Product Notes
NaBH4 1° alcohol (from aldehyde), 2° alcohol (from ketone) Mild, selective
LiAlH4 Same but stronger Also reduces acids/esters
H2/metal catalyst Same Catalytic hydrogenation

6e. Aldol Condensation

Two carbonyl compounds with $\alpha$-hydrogens combine:

$$2CH_3CHO \xrightarrow{OH^-} CH_3CH(OH)CH_2CHO \xrightarrow{\Delta} CH_3CH=CHCHO + H_2O$$

  • Step 1: Enolate forms (requires $\alpha$-H)
  • Step 2: Enolate attacks another carbonyl → $\beta$-hydroxy carbonyl
  • Step 3: Heat → dehydration to $\alpha,\beta$-unsaturated carbonyl

Crossed aldol: Two different carbonyls → mixture of 4 products (unless one has no $\alpha$-H).

MM: The aldol is like a conga line. One carbonyl grabs the $\alpha$-H from another, then links arms. Heat makes them let go of water and hold tighter with a double bond.

6f. Cannizzaro Reaction

Only for aldehydes with NO $\alpha$-H (e.g. formaldehyde, benzaldehyde).

$$2HCHO \xrightarrow{conc.\ OH^-} CH_3OH + HCOO^-$$

One aldehyde is reduced (to alcohol), one is oxidised (to carboxylate). Disproportionation.

MM: If an aldehyde has no $\alpha$-H, it can't do the aldol dance. Its only option in strong base is to cannibalise itself.

6g. Haloform (Iodoform) Reaction

$$CH_3COR + 3I_2 + 4NaOH \rightarrow CHI_3\downarrow + RCOONa + 3NaI + 3H_2O$$

Specific to methyl ketones. The three $\alpha$-hydrogens each get replaced by I, then the CI3 group leaves as CHI3 (yellow).

6h. Full Synthesis Flow

                    Carbonyl (C=O)
                    /           \
              Aldehyde          Ketone
              /      \              \
        Oxidation  Reduction      Reduction
           ↓           ↓              ↓
      Carboxylic   1° alcohol      2° alcohol
         acid
              \
           + HCN → Cyanohydrin → α-hydroxy acid
           + RMgX → 2° alcohol (aldehyde) / 3° alcohol (ketone)
           + NH2OH → Oxime
           + 2,4-DNP → Orange/red precipitate

7. Exam Watch — Carbonyl + Stereochemistry

From exam leaks: Carbonyl compounds appear in Section A (structured, ~7% weight) and can combine with stereochemistry.

Watch for:

  • Cyanohydrin formation creates a new chiral centre at the former carbonyl carbon (now sp3 with 4 different groups)
  • Grignard addition creates chiral alcohols
  • Iodoform test combined with structure determination puzzles (e.g. "Compound C4H8O gives positive iodoform but negative Tollens' → methyl ketone")
  • Reaction schemes from past year papers: alkene → ozonolysis → aldehyde → Grignard → secondary alcohol → oxidation → carboxylic acid → ester

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