From formaldehyde to aspirin — the chemistry of the carbonyl group and beyond. With interactive distinguishing test predictor, acidity comparison, nucleophilic addition mechanisms, and 20+ real-life examples.
Before diving in, here's the entire chapter on one page. How the carbonyl group drives reactivity, and how it's converted to alcohols, acids, and beyond.
The carbonyl group (C=O) is polar (C δ+, O δ−) → undergoes nucleophilic addition. Aldehydes are more reactive than ketones (less steric, less electronic). α-H is acidic (stabilized enolate) → Aldol condensation, halogenation. Aldehydes reduce Tollens/Fehling reagents (ketones don't). Carboxylic acids are acidic (resonance-stabilized carboxylate) — much more than phenols or alcohols.
Carbonyl group: A carbon double-bonded to oxygen. Found in:
Hybridization: The carbonyl carbon is sp² — planar geometry, 120° bond angles.
Bonding: One σ bond + one π bond between C and O. The π bond is formed from p-orbitals perpendicular to the molecular plane.
Oxygen is more electronegative (3.44) than carbon (2.55). The C=O bond is highly polar:
This polarity makes the carbonyl carbon susceptible to nucleophilic attack — the core reaction of carbonyl chemistry.
Carbonyl group (C=O): sp² carbon, planar, polar (C δ+, O δ−). C=O is susceptible to nucleophilic attack. Aldehyde has terminal C=O with H; ketone has C=O between two carbons. Formaldehyde, acetaldehyde, acetone are the simplest examples.
IUPAC: alkane → alkanal (drop -e, add -al)
CHO carbon is always C1.
Examples:
IUPAC: alkane → alkanone (drop -e, add -one)
C=O gets the lowest possible locant.
Examples:
IUPAC: alkane → alkanoic acid (add -oic acid)
COOH carbon is C1.
Examples:
| Common Name | IUPAC Name | Formula |
|---|---|---|
| Formaldehyde | Methanal | HCHO |
| Acetaldehyde | Ethanal | CH₃CHO |
| Acetone | Propanone | CH₃COCH₃ |
| Benzaldehyde | Benzaldehyde | Ph-CHO |
| Formic acid | Methanoic acid | HCOOH |
| Acetic acid | Ethanoic acid | CH₃COOH |
| Oxalic acid | Ethanedioic acid | HOOC-COOH |
| Benzoic acid | Benzoic acid | Ph-COOH |
| Acetophenone | 1-Phenylethanone | Ph-CO-CH₃ |
| Benzophenone | Diphenylethanone | Ph-CO-Ph |
Aldehydes: alkanal. Ketones: alkanone (with lowest locant). Acids: alkanoic acid. CHO and COOH are always C1. Benzaldehyde, acetophenone, benzophenone are aromatic exceptions.
1. Oxidation of alcohols:
2. Ozonolysis of alkenes:
Alkene + O₃ then Zn/H₂O → aldehyde and/or ketone
Example: (CH₃)₂C=CH₂ → acetone + formaldehyde
3. Hydration of alkynes:
Alkyne + H₂O/HgSO₄/H₂SO₄ → enol → tautomerizes to ketone (Markovnikov)
Example: CH₃C≡CH + H₂O → CH₃COCH₃ (acetone)
For terminal alkynes only (internal give ketones too).
4. Rosenmund reduction (from acid chlorides):
R-COCl + H₂/Pd-BaSO₄ (poisoned catalyst) → R-CHO
Prevents further reduction to alcohol. Used for aldehydes.
5. Stephen reduction (from nitriles):
R-CN + SnCl₂/HCl then H₂O → R-CHO
6. DIBAL-H (from nitriles or esters):
R-CN + DIBAL-H → R-CHO
R-COOR' + DIBAL-H (at −78°C) → R-CHO
DIBAL-H = diisobutylaluminium hydride, selective reduction.
7. From Grignard reagent + nitrile:
R-CN + R'-MgX then H₃O⁺ → R-CO-R' (ketone)
8. Friedel-Crafts acylation (aryl ketones):
Benzene + R-COCl/AlCl₃ → Ph-CO-R + HCl
1. Oxidation of primary alcohols/aldehydes:
R-CH₂OH or R-CHO + KMnO₄/H⁺ → R-COOH
2. Hydrolysis of nitriles:
R-CN + H₂O/H⁺ (or OH⁻) → R-COOH
3. Grignard + CO₂ (carboxylation):
R-MgX + CO₂ → R-COOMgX → R-COOH (after H₃O⁺)
Adds one carbon — very useful for chain extension.
4. Hydrolysis of esters:
R-COOR' + H₂O (H⁺ or OH⁻) → R-COOH + R'-OH
5. From alkyl benzene (side-chain oxidation):
Ph-CH₃ + KMnO₄/H₂SO₄ → Ph-COOH (benzoic acid)
6. Carboxylation of phenol (Kolbe-Schmitt):
Ph-OH + CO₂ + NaOH → salicylic acid
Aldehydes: oxidation of 1° alcohols (controlled with PCC), ozonolysis, Rosenmund (acid chloride + H₂/Pd-BaSO₄), Stephen (nitrile), DIBAL-H. Ketones: oxidation of 2° alcohols, hydration of alkynes, Grignard + nitrile, Friedel-Crafts acylation. Acids: oxidation of 1° alcohols/aldehydes, hydrolysis of nitriles/esters, Grignard + CO₂.
Order for compounds of similar molecular mass:
Carboxylic acid > Alcohol > Ketone ≈ Aldehyde > Ether > Alkane
Why?
Example: Acetic acid (MW 60): BP 118°C. Ethanol (MW 46): BP 78°C. Propanone (MW 58): BP 56°C.
Note: Acids have MUCH higher BP than expected from molecular mass due to dimerization.
Lower aldehydes, ketones, and acids are soluble in water because they form H-bonds with water (via O of carbonyl and -OH of acid).
Solubility decreases with increasing carbon chain (hydrophobic effect).
Formaldehyde, acetaldehyde, acetone, acetic acid are completely miscible with water.
Higher members (>5-6 carbons) are sparingly soluble.
BP order: acid > alcohol > ketone/aldehyde > ether > alkane (for similar MW). Carboxylic acids form dimers via two H-bonds → very high BP. Aldehydes/ketones have dipole-dipole only (no H-bonding between themselves) → moderate BP. Solubility decreases with carbon chain length.
Nu⁻ + R₂C=O → R₂C(O⁻)Nu → (after H⁺ workup) → R₂C(OH)Nu
Step 1: Nu⁻ attacks the electrophilic carbonyl carbon.
Step 2: Protonation of the resulting alkoxide.
Result: addition across the C=O π bond (converted to C-O single bond).
Aldehydes > Ketones (aldehydes are more reactive)
Why?
Within aldehydes: HCHO > RCHO > Ar-CHO (aromatic aldehydes less reactive due to resonance stabilization of C=O by ring).
R₂C=O + HCN (NaCN + HCl) → R₂C(OH)-CN
Product: cyanohydrin (useful for synthesis of hydroxy acids, amino acids).
Base-catalyzed (OH⁻ or CN⁻ generates HCN → CN⁻ attacks).
R₂C=O + NaHSO₃ → R₂C(OH)-SO₃Na
Product: bisulfite adduct (white crystalline solid).
Used to:
R₂C=O + 2R'OH (dry HCl) ⇌ R₂C(OR')₂ + H₂O
Product: acetal (stable under basic conditions; protecting group for carbonyl).
Mechanism: first alcohol → hemiacetal (R₂C(OH)(OR')) → second alcohol → acetal (R₂C(OR')₂).
Reverse: hydrolysis (aqueous acid) regenerates the carbonyl.
Acetals are used as protecting groups — stable to base, reduces, oxidants (except strong acid).
R₂C=O + H₂N-R → R₂C=N-R + H₂O
Products: imines (Schiff bases) when R = alkyl/aryl.
Common examples:
These are crystalline solid derivatives — useful for identification of carbonyls.
R₂C=O + R'-MgX → R₂C(OMgX)(R') → (H₃O⁺) → R₂C(OH)R'
Product: alcohol.
Depending on starting carbonyl:
Nucleophilic addition: Nu⁻ attacks C=O, then protonation. Aldehydes more reactive than ketones (less steric, stronger δ+). Key additions: HCN (cyanohydrin), NaHSO₃ (bisulfite adduct — purification), ROH (acetal — protecting group), NH₂-R (Schiff base, oxime, hydrazone), Grignard (alcohols).
α-Hydrogen: H atom attached to the carbon adjacent to C=O.
These H atoms are acidic (pKa ~20) because the resulting carbanion (enolate) is resonance-stabilized by the C=O group:
R-CH₂-CHO + base ⇌ [R-CH-CHO]⁻ (enolate, resonance-stabilized) + BH⁺
The negative charge is delocalized onto the oxygen: R-CH⁻-CHO ↔ R-CH=C(O⁻)-H.
In presence of dilute base (NaOH, KOH), two aldehyde/ketone molecules react:
Step 1 — Aldol addition:
CH₃CHO + CH₃CHO → CH₃-CH(OH)-CH₂-CHO (aldol, β-hydroxy aldehyde)
Mechanism: α-H removed by base → enolate attacks the other C=O → protonation.
Step 2 — Dehydration (on heating):
CH₃-CH(OH)-CH₂-CHO → CH₃-CH=CH-CHO (α,β-unsaturated aldehyde, crotonaldehyde) + H₂O
Overall: 2CH₃CHO → CH₃-CH=CH-CHO + H₂O (aldol condensation)
Crossed aldol: Between two different carbonyls (both with α-H) → mixture of 4 products.
Directed aldol: One partner has no α-H (HCHO, Ph-CHO) → controlled product. Example: PhCHO + CH₃CHO → Ph-CH=CH-CHO (cinnamaldehyde).
Requirement: Aldehyde with NO α-H (e.g., HCHO, Ph-CHO, (CH₃)₃C-CHO).
In presence of conc. NaOH, two molecules undergo disproportionation:
2HCHO + NaOH → HCOONa (sodium formate) + CH₃OH (methanol)
One molecule is oxidized (to acid salt), one is reduced (to alcohol).
Mechanism: OH⁻ attacks C=O → hydride transfer from one aldehyde to the other → acid + alcohol.
Crossed Cannizzaro: HCHO + Ph-CHO → HCOONa + Ph-CH₂OH (HCHO oxidized, Ph-CHO reduced).
Requirement: Methyl ketone (CH₃-CO-R) or acetaldehyde (CH₃-CHO).
With X₂/NaOH (halogen in base), gives haloform (CHX₃) and carboxylate:
CH₃-CO-R + 3X₂ + 4NaOH → CHX₃ + R-COONa + 3NaX + 3H₂O
Iodoform reaction: I₂/NaOH gives CHI₃ (yellow precipitate) — used as test for methyl ketones and acetaldehyde.
Mechanism: All three α-H replaced by halogen → nucleophilic acyl substitution → CX₃⁻ (good leaving group) leaves → CHX₃ + carboxylate.
Choose a compound to see its test results (Tollens, Fehling, Iodoform, 2,4-DNP).
α-H is acidic (stabilized enolate). Aldol condensation: two carbonyls with α-H → β-hydroxy carbonyl (aldol) → α,β-unsaturated (on heating). Cannizzaro: aldehyde with NO α-H → disproportionation (acid + alcohol). Haloform (iodoform): CH₃-CO-R or CH₃-CHO + I₂/NaOH → CHI₃ (yellow ppt).
1. Aldehydes → Carboxylic acids (mild/strong oxidants):
Ketones are NOT oxidized by Tollens/Fehling — this is the key distinguishing test.
2. Ketone strong oxidation (C-C bond cleavage):
Under strong conditions (KMnO₄/HNO₃, high T), ketones cleave adjacent to C=O → mixture of carboxylic acids (usually of lower molecular mass).
1. Carbonyl → Alcohol:
Aldehydes → primary alcohols. Ketones → secondary alcohols.
2. Aldehyde/Ketone → Alkane (complete deoxygenation):
These are complementary — use Clemmensen if the molecule is stable to acid; Wolff-Kishner if stable to base.
3. Selective reduction:
1. Aldol condensation — covered above.
2. Haloform reaction — covered above.
3. Cannizzaro reaction — covered above.
4. Schiff base formation (with primary amine):
R₂C=O + H₂N-R' → R₂C=N-R' + H₂O
5. Baeyer-Villiger oxidation (ketones → esters):
Ketone + peracid (mCPBA) → ester (one R migrates)
6. Reformatsky reaction (with α-halo ester + Zn):
R₂C=O + Zn + Br-CH₂-COOC₂H₅ → β-hydroxy ester
7. Wittig reaction (with phosphonium ylide):
R₂C=O + Ph₃P=CHR' → R₂C=CHR' + Ph₃P=O
Forms C=C bond — very useful for building complex molecules.
Oxidation: aldehydes oxidized by Tollens/Fehling (Ag mirror, Cu₂O ppt); ketones not. Reduction: aldehyde → 1° alcohol, ketone → 2° alcohol (NaBH₄/LiAlH₄). Clemmensen (acid) and Wolff-Kishner (base) → alkane. NaBH₄ is chemoselective (C=O but not C=C).
Reagent: 2,4-dinitrophenylhydrazine (Brady's reagent) in acidic medium
Observation: Yellow/orange precipitate (2,4-dinitrophenylhydrazone) with ANY aldehyde or ketone.
Purpose: Confirms presence of a carbonyl group (>C=O).
Does not distinguish aldehyde from ketone. Does not react with alcohols, acids, esters.
Reagent: [Ag(NH₃)₂]⁺ (ammoniacal silver nitrate) in basic solution.
Observation: Silver mirror on the test tube walls.
Positive for: Aldehydes (including glucose, formic acid).
Negative for: Ketones, most other compounds.
Reaction: R-CHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → R-COO⁻ + 2Ag↓ + 4NH₃ + H₂O
Reagent: Fehling A (CuSO₄) + Fehling B (KNaC₄H₄O₆ + NaOH) — mixed just before use.
Observation: Red precipitate of Cu₂O.
Positive for: Aliphatic aldehydes, reducing sugars (glucose, fructose, maltose).
Negative for: Aromatic aldehydes (like benzaldehyde — its oxidation requires stronger conditions), ketones, sucrose.
Key distinction: Benzaldehyde gives Tollens but NOT Fehling (aromatic aldehyde — insensitive to Fehling).
Reagent: Modified Fehling (Cu²⁺ with citrate, milder)
Similar to Fehling — but milder and more stable. Used in clinical urine tests for reducing sugars.
Reagent: I₂ + NaOH (or KI + NaOCl)
Observation: Yellow precipitate of CHI₃.
Positive for: Acetaldehyde (CH₃CHO) and methyl ketones (CH₃-CO-R). Also ethanol (first oxidized to acetaldehyde by I₂/NaOH).
Negative for: Other aldehydes, other ketones.
Reagent: NaHSO₃ (sodium bisulfite)
Observation: White crystalline precipitate.
Positive for: Aldehydes and methyl ketones (CH₃-CO-R).
Common with the tests above — used for purification.
| Compound | Tollens | Fehling | Iodoform | 2,4-DNP |
|---|---|---|---|---|
| Formaldehyde (HCHO) | ✓ Silver mirror | ✓ Red ppt | ✗ | ✓ Yellow ppt |
| Acetaldehyde (CH₃CHO) | ✓ Silver mirror | ✓ Red ppt | ✓ Yellow CHI₃ | ✓ Yellow ppt |
| Benzaldehyde (PhCHO) | ✓ Silver mirror | ✗ No reaction | ✗ | ✓ Yellow ppt |
| Acetone (CH₃COCH₃) | ✗ | ✗ | ✓ Yellow CHI₃ | ✓ Yellow ppt |
| Acetophenone (PhCOCH₃) | ✗ | ✗ | ✓ Yellow CHI₃ | ✓ Yellow ppt |
| Benzophenone (PhCOPh) | ✗ | ✗ | ✗ | ✓ Yellow ppt |
| Ethanol (CH₃CH₂OH) | ✗ | ✗ | ✓ (oxidized first) | ✗ |
Mistake: "Both Tollens and Fehling are positive
for all aldehydes."
Correction: Tollens is
more sensitive — it works for both aliphatic AND aromatic
aldehydes. Fehling only works for aliphatic aldehydes (and
reducing sugars). Benzaldehyde gives silver mirror with Tollens
but NOT red ppt with Fehling. This is a classic exam question.
2,4-DNP: confirms ANY carbonyl (yellow ppt). Tollens: aldehydes (silver mirror) — works for both aliphatic and aromatic. Fehling: aliphatic aldehydes only (red Cu₂O ppt) — benzaldehyde negative. Iodoform: CH₃-CO-R and CH₃-CHO (yellow CHI₃). Use combination to distinguish specific compounds.
Carboxyl (-COOH): Carbonyl (C=O) + hydroxyl (-OH) on same carbon.
Resonance: Lone pair on -OH donates into C=O. Two equivalent resonance structures:
Result: C-O bond has partial double bond character (C-O shorter than typical).
Acidity: The -OH hydrogen is acidic. pKa ~4-5. Reason: carboxylate anion (RCOO⁻) is resonance-stabilized (two equivalent structures).
Choose a compound to see its pKa and compare acid strength.
Electron-withdrawing groups (EWG) increase acidity:
Example: Acetic acid (pKa 4.76) → chloroacetic acid (pKa 2.86) → dichloroacetic (pKa 1.48) → trichloroacetic (pKa 0.7). More Cl → more acidic.
Electron-donating groups (EDG) decrease acidity:
Example: Formic acid (HCOOH, pKa 3.75) > acetic acid (CH₃COOH, pKa 4.76). The extra -CH₃ decreases acidity.
Distance effect: The closer the EWG to COOH, the stronger the effect. α-Cl > β-Cl > γ-Cl in stabilizing effect.
1. Salt formation:
R-COOH + NaOH → R-COONa + H₂O
R-COOH + NaHCO₃ → R-COONa + CO₂↑ + H₂O
Evolution of CO₂ with NaHCO₃ is a distinguishing test for carboxylic acids (phenols don't react with NaHCO₃).
2. Esterification:
R-COOH + R'-OH (H⁺) ⇌ R-COOR' + H₂O
Reversible — need to remove water (using Dean-Stark) or use excess reactant.
With diazomethane (CH₂N₂): R-COOH + CH₂N₂ → R-COOCH₃ + N₂ — quantitative, gives methyl ester.
3. Formation of acid chlorides:
R-COOH + SOCl₂ → R-COCl + SO₂ + HCl (best — clean)
Or R-COOH + PCl₅ → R-COCl + POCl₃ + HCl
Or R-COOH + PCl₃ → 3R-COCl + H₃PO₃
4. Formation of anhydrides:
2R-COOH (P₂O₅) → (R-CO)₂O + H₂O
Or R-COOH + R-COCl (pyridine) → (R-CO)₂O + HCl
5. Formation of amides:
R-COOH + NH₃ → R-COONH₄ → R-CONH₂ (on heating, dehydration)
6. Reduction:
R-COOH + LiAlH₄ → R-CH₂OH (strong reducing agent needed)
NaBH₄ does NOT reduce carboxylic acids (too weak).
7. Decarboxylation:
R-COONa + NaOH (CaO, heat) → R-H + Na₂CO₃
Only works for acids with electron-withdrawing groups (β-keto acids, malonic acid) — otherwise requires high temperature.
8. Hunsdiecker reaction:
R-COOAg + Br₂ (CCl₄) → R-Br + CO₂ + AgBr
Decarboxylative halogenation — reduces chain length by one carbon.
9. Hell-Volhard-Zelinsky (HVZ) reaction:
R-CH₂-COOH + Br₂/PBr₃ → R-CH(Br)-COOH (α-halogenation)
α-Halo acid — useful for making amino acids.
Carboxylic acids: pKa ~4-5 (more acidic than phenol/alcohol/water). Reason: resonance-stabilized carboxylate. EWG (Cl, NO₂) increases acidity (α-Cl > β-Cl); EDG (CH₃) decreases. Reactions: salt with NaHCO₃ (CO₂ test), esterification, acid chlorides (SOCl₂), anhydrides (P₂O₅), amides (NH₃), reduction (LiAlH₄), decarboxylation (Hunsdiecker), α-halogenation (HVZ).
From strongest to weakest (typical pKa values):
| Compound | pKa | Relative Strength |
|---|---|---|
| HCl (hydrochloric acid) | −7 | Very strong |
| H₂SO₄ | −3 | Strong |
| Trichloroacetic acid (CCl₃COOH) | 0.7 | Very strong organic |
| Trifluoroacetic acid (CF₃COOH) | 0.2 | Very strong organic |
| Dichloroacetic acid (CHCl₂COOH) | 1.5 | Strong |
| Chloroacetic acid (ClCH₂COOH) | 2.9 | Moderate |
| Formic acid (HCOOH) | 3.75 | Moderate |
| Benzoic acid (PhCOOH) | 4.2 | Moderate |
| Acetic acid (CH₃COOH) | 4.76 | Weak |
| Carbonic acid (H₂CO₃) | 6.4 | Weak |
| p-Nitrophenol | 7.15 | Weak |
| Phenol | 10 | Very weak |
| Water | 15.7 | Neutral (reference) |
| Ethanol | 16 | Very weak |
| Acetone (α-H) | 20 | Extremely weak |
| Ammonia | 38 | Essentially non-acidic |
| Alkane (methane) | 50 | Non-acidic |
Overall order: Mineral acids > substituted carboxylic acids > simple carboxylic acids > carbonic acid > phenols > water > alcohols > α-H of ketones > amines > alkanes
Carboxylate anion (RCOO⁻): Two equivalent resonance structures, both with negative charge on oxygen. Very stable.
Phenoxide anion (PhO⁻): Resonance structures with negative charge distributed into the ring — but not all structures have negative charge on oxygen. Also, aromatic ring has its own electron density, so not all resonance forms are equally favored.
Result: carboxylate is more stable than phenoxide → carboxylic acids are stronger than phenols.
Additional factor: in carboxylic acid, the C=O and -OH are directly on the same carbon — the resonance is very effective. In phenol, the O is attached to the ring — less direct.
Phenoxide (PhO⁻): Resonance-stabilized — negative charge delocalized into the aromatic ring.
Alkoxide (RO⁻): Negative charge localized on O — no resonance stabilization.
Result: phenol (pKa 10) is ~10⁶ times more acidic than ethanol (pKa 16).
Cl is an electron-withdrawing group via inductive effect (-I). It pulls electron density AWAY from the carboxylate → disperses the negative charge → stabilizes the anion → makes it easier to form → increases acidity.
More Cl atoms → more withdrawal → more stable anion → stronger acid.
Trichloroacetic acid (pKa 0.7) > dichloroacetic (1.5) > chloroacetic (2.9) > acetic (4.76).
Distance matters: α-Cl is more effective than β-Cl, which is more than γ-Cl.
Acidity order: mineral acids > substituted acids > simple acids > carbonic acid > phenols > water > alcohols > α-H ketones. Carboxylic acids most acidic because carboxylate resonance is strongest. EWG (Cl, NO₂) increases acidity (α > β > γ). EDG (CH₃) decreases.
"Tollens Tolerates All aldehydes (aliphatic + aromatic)"
"Fehling Fails for aromatic"
Benzaldehyde: Tollens ✓ but Fehling ✗. Classic exam question!
"CH₃-CO-R → Yellow CHI₃"
Methyl ketones (CH₃-C=O) and acetaldehyde (CH₃-CHO). Also ethanol (oxidizes to CH₃CHO first).
"DNP = DETECTS Any Carbonyl"
Yellow/orange ppt with any aldehyde or ketone. Confirms C=O presence.
"Aldol needs Alpha-H"
"Cannizzaro has NO Alpha-H"
HCHO, PhCHO, (CH₃)₃C-CHO → Cannizzaro (no α-H). CH₃CHO, acetone, etc. → Aldol.
"Clemmensen is ACIDIC (Zn-Hg/HCl)"
"Wolff-Kishner is BASIC (NH₂NH₂/KOH)"
Use Clemmensen if molecule stable to acid; use Wolff-Kishner if stable to base.
"Mineral > Substituted Acid > Simple Acid > Phenol > Water > Alcohol > Ketone"
pKa: HCl (−7) > Cl₃CCOOH (0.7) > CH₃COOH (4.76) > Phenol (10) > Water (15.7) > Ethanol (16) > Acetone (20).
"NaBH₄ = Mild & selective (C=O, not C=C)"
"LiAlH₄ = Strong (everything)"
"H₂/Ni = Usually both"
"DIBAL-H = Stops at aldehyde"
"Rosenmund → ALDEHYDE (H₂/Pd-BaSO₄)"
"Stephen → ALDEHYDE (SnCl₂)"
"DIBAL-H → ALDEHYDE"
"Grignard + nitrile → KETONE"
"Grignard + CO₂ → ACID (adds 1 C)"
"Hell-Volhard-Zelinsky = α-HALOGENATION of acids"
R-CH₂-COOH + Br₂/PBr₃ → R-CH(Br)-COOH (α-bromo acid). Useful for making amino acids.
Use "Tollens Tolerates All, Fehling Fails for aromatic" for distinguishing tests. "Aldol needs Alpha-H, Cannizzaro has NO Alpha-H" for base reactions. "Clemmensen ACID, Wolff-Kishner BASE" for reductions.
Q1. Why are aldehydes more reactive than ketones toward nucleophilic addition?
Aldehydes have only one alkyl group attached to C=O; ketones have two. Two bulky groups create steric hindrance, making it harder for a nucleophile to approach the carbonyl carbon.
Alkyl groups are electron-donating (+I effect). Two alkyl groups in ketones donate more electron density to the carbonyl carbon, reducing its δ+ (electrophilicity). Aldehydes have only one +I group → stronger δ+ on carbon → more attractive to nucleophiles.
Q2. Give one test to distinguish benzaldehyde from acetaldehyde.
Fehling's solution: acetaldehyde gives red precipitate of Cu₂O (aliphatic aldehyde is oxidized).
Benzaldehyde does NOT react with Fehling (aromatic aldehyde is not oxidized under these conditions).
Acetaldehyde (CH₃CHO) has CH₃-CHO → gives yellow CHI₃ with I₂/NaOH.
Benzaldehyde (Ph-CHO) has no CH₃-CO group → no iodoform.
Q3. Write the mechanism of nucleophilic addition of HCN to a carbonyl compound.
NaCN ⇌ Na⁺ + CN⁻ (or from HCN + OH⁻ → CN⁻ + H₂O in basic medium)
CN⁻ attacks the electrophilic carbonyl carbon (δ+), pushing π electrons onto O to form an alkoxide intermediate:
R₂C=O + CN⁻ → R₂C(O⁻)-CN
Alkoxide abstracts proton from HCN (or H₂O):
R₂C(O⁻)-CN + HCN → R₂C(OH)-CN + CN⁻
R₂C=O + HCN → R₂C(OH)-CN (cyanohydrin)
Product: cyanohydrin — useful intermediate for synthesis of α-hydroxy acids (on hydrolysis of CN to COOH) and α-amino acids (on reduction of CN to CH₂NH₂).
Q4. Arrange the following in increasing order of acidity: formic acid, acetic acid, chloroacetic acid, trichloroacetic acid. Explain.
Acetic acid < formic acid < chloroacetic acid < trichloroacetic acid
pKa: 4.76 > 3.75 > 2.86 > 0.7
Acetic acid (CH₃COOH) has a methyl group (+I effect) that donates electron density, destabilizing the carboxylate anion → less acidic.
Formic acid (HCOOH) has H instead of CH₃ → no +I effect → more stable anion → more acidic.
Cl is electron-withdrawing (-I effect) — stabilizes carboxylate by dispersing negative charge.
More Cl atoms → stronger withdrawal → more stable anion → stronger acid: CH₃COOH (0 Cl) < ClCH₂COOH (1 Cl) < Cl₂CHCOOH (2 Cl) < Cl₃CCOOH (3 Cl).
Q5. How is acetaldehyde converted to: (a) ethanol, (b) acetic acid, (c) aldol, (d) 3-hydroxybutanal, (e) cyanohydrin?
CH₃CHO + NaBH₄ (or H₂/Ni or LiAlH₄) → CH₃CH₂OH
Reduction (aldehyde → primary alcohol)
CH₃CHO + KMnO₄/H⁺ (or K₂Cr₂O₇/H⁺) → CH₃COOH
Oxidation (aldehyde → carboxylic acid)
2CH₃CHO (dil. NaOH, room temp) → CH₃-CH(OH)-CH₂-CHO (3-hydroxybutanal, "aldol")
Aldol addition — one α-H removed, C-C bond forms.
3-Hydroxybutanal (on heating) → CH₃-CH=CH-CHO + H₂O (crotonaldehyde)
Dehydration of aldol to α,β-unsaturated aldehyde.
CH₃CHO + HCN (NaCN/HCl) → CH₃-CH(OH)-CN (acetaldehyde cyanohydrin, lactonitrile)
Nucleophilic addition of CN⁻ to C=O.
Q6. (a) What is Cannizzaro reaction? (b) Which compounds undergo this reaction? (c) Give one example with mechanism.
Disproportionation of aldehydes with NO α-H in the presence of concentrated NaOH (or KOH). One aldehyde is oxidized to carboxylate (salt), the other is reduced to alcohol.
Aldehydes with no α-H:
Aldehydes with α-H (like acetaldehyde, propanal) undergo aldol instead.
2HCHO + NaOH → HCOONa (sodium formate) + CH₃OH (methanol)
Mechanism:
Step 1: OH⁻ attacks C=O of formaldehyde → alkoxide H₂C(OH)O⁻
Step 2: This intermediate transfers hydride (H⁻) to another HCHO molecule. The first molecule is oxidized to formic acid (HCOOH) → HCOO⁻ (in base).
Step 3: The second HCHO gains hydride → becomes methanol (CH₃OH).
Overall: one molecule oxidized (formate), one reduced (methanol).
Q7. Discuss the reactions of carboxylic acids: (a) salt formation, (b) esterification, (c) acid chloride formation, (d) reduction, (e) decarboxylation.
R-COOH + NaOH → R-COONa + H₂O
R-COOH + NaHCO₃ → R-COONa + CO₂↑ + H₂O
The CO₂ evolution with NaHCO₃ is a distinguishing test for carboxylic acids (phenols don't give this).
R-COOH + R'-OH (H₂SO₄) ⇌ R-COOR' + H₂O
Reversible — acid-catalyzed Fischer esterification. With diazomethane: R-COOH + CH₂N₂ → R-COOCH₃ (methyl ester, quantitative).
R-COOH + SOCl₂ → R-COCl + SO₂ + HCl
SOCl₂ is best (gaseous byproducts, easy purification). Or PCl₅ → R-COCl + POCl₃. Or PCl₃ → 3R-COCl + H₃PO₃.
R-COOH + LiAlH₄ (dry ether) → R-CH₂OH
LiAlH₄ is strong enough to reduce carboxylic acids to primary alcohols. NaBH₄ does NOT reduce acids.
For selective reduction of COOH to CH₂OH, LiAlH₄ is the reagent of choice.
R-COONa + NaOH (CaO, heat) → R-H + Na₂CO₃
Removes CO₂ from the molecule. Requires β-keto acid or malonic acid for easy decarboxylation (otherwise needs very high temperature).
Hunsdiecker variant: R-COOAg + Br₂ → R-Br + CO₂ (decarboxylative bromination).
Selected exercise questions from NCERT Class 12 Chemistry Chapter 8. Click each to reveal the solution.
(a) CH₃CH₂CH₂CHO: Butanal (4-carbon chain, CHO is C1)
(b) CH₃COCH₂CH₂CH₃: Pentan-2-one (5-carbon chain, C=O at C2)
(c) CH₃CH(CHO)CH₂CH₃: 2-Methylbutanal (main chain = 4 carbons including CHO, methyl at C2)
(d) (CH₃)₃C-CHO: 2,2-Dimethylpropanal (3-carbon main chain, 2 methyls at C2)
(a) Aldol condensation: In dilute base, two molecules of aldehyde/ketone with α-H condense to form β-hydroxy carbonyl (aldol), which on heating loses water to form α,β-unsaturated compound. Example: 2CH₃CHO → CH₃-CH=CH-CHO.
(b) Cannizzaro reaction: Aldehydes with NO α-H undergo disproportionation in conc. NaOH. One molecule oxidized to acid salt, one reduced to alcohol. Example: 2HCHO → HCOONa + CH₃OH.
(c) Haloform reaction: Methyl ketones (CH₃-CO-R) or acetaldehyde treated with X₂/NaOH give haloform (CHX₃) + carboxylate. Example: CH₃COCH₃ + 3I₂ + 4NaOH → CHI₃↓ + CH₃COONa + 3NaI + 3H₂O.
(a) Alcohols: Carbonyl + NaBH₄ or LiAlH₄ or H₂/Ni → alcohol. Aldehyde → 1° alcohol; ketone → 2° alcohol. Grignard addition also gives alcohols (HCHO → 1°, RCHO → 2°, ketone → 3°).
(b) Acids: Aldehydes + KMnO₄/H⁺ or K₂Cr₂O₇/H⁺ → carboxylic acids. Ag(NH₃)₂⁺ or Cu²⁺-tartrate also give acids (mild oxidation).
(c) Amines: Reductive amination — carbonyl + NH₃ (or amine) → imine intermediate → reduction (H₂/Ni) → amine. Or via nitrile (via cyanohydrin → reduction to CH₂NH₂).
(a) HCN: CH₃CHO + HCN → CH₃CH(OH)CN (acetaldehyde cyanohydrin, lactonitrile)
(b) NH₂OH (hydroxylamine): CH₃CHO + NH₂OH → CH₃CH=N-OH (acetaldehyde oxime) + H₂O
(c) H₂N-NH₂ (hydrazine): CH₃CHO + H₂N-NH₂ → CH₃CH=N-NH₂ (hydrazone) + H₂O
(d) NaHSO₃: CH₃CHO + NaHSO₃ → CH₃CH(OH)-SO₃Na (bisulfite adduct, white crystalline solid)
(a) Aldehydes more reactive:
Steric factor: Ketones have 2 bulky R groups → block approach of Nu⁻. Aldehydes have only 1 R group → easier approach.
Electronic factor: 2 R groups in ketone donate electron density (+I) → reduce δ+ on C → less electrophilic. Aldehydes have only 1 +I group → stronger δ+ → more reactive.
(b) Acetic acid less acidic than chloroacetic acid:
CH₃ group is electron-donating (+I) → destabilizes carboxylate anion → decreases acidity (pKa 4.76).
Cl is electron-withdrawing (-I) → stabilizes carboxylate anion → increases acidity (pKa 2.86).
(a) Clemmensen reduction: R₂C=O + Zn-Hg/HCl (reflux) → R₂CH₂ + H₂O. (C=O → CH₂ under acidic conditions.)
(b) Wolff-Kishner reduction: R₂C=O + NH₂NH₂ (hydrazine) + KOH (ethylene glycol, heat) → R₂CH₂ + N₂ + H₂O. (C=O → CH₂ under basic conditions.)
(c) HVZ reaction: R-CH₂-COOH + Br₂/PBr₃ → R-CH(Br)-COOH (α-bromo acid) + HBr. (α-Halogenation of carboxylic acids.)
(d) Hunsdiecker reaction: R-COOAg + Br₂ (CCl₄) → R-Br + CO₂ + AgBr. (Decarboxylative bromination — reduces chain length by one carbon.)
(a) Propanal vs Propanone:
Tollens test: propanal (aldehyde) gives silver mirror, propanone (ketone) does not.
Fehling test: propanal gives red ppt, propanone does not.
Iodoform test: propanone does NOT have CH₃-CO-... wait, propanone is CH₃COCH₃ → yes, has CH₃-CO → iodoform positive. Propanal is CH₃CH₂CHO → no CH₃-CO → iodoform negative. So iodoform distinguishes (propanone positive).
(b) Benzaldehyde vs Acetophenone:
Fehling test: benzaldehyde (aromatic aldehyde) is negative, acetophenone is also negative. Not useful.
Iodoform: acetophenone (Ph-CO-CH₃) is positive (yellow CHI₃). Benzaldehyde is negative.
Tollens: benzaldehyde gives silver mirror, acetophenone does not.
Both work — Tollens (benzaldehyde positive) or iodoform (acetophenone positive).
(c) Ethanol vs Ethanal:
Tollens test: ethanal gives silver mirror, ethanol does not.
2,4-DNP: ethanal gives yellow ppt, ethanol does not.
Iodoform: BOTH positive (ethanol oxidizes to ethanal first). Not useful.
(a) Benzoic acids:
Order: 4-Methoxybenzoic < Benzoic < 4-Nitrobenzoic
4-NO₂ is strongly electron-withdrawing (-I, -M) → stabilizes carboxylate → strongest acid.
4-OCH₃ is electron-donating (+M) → destabilizes carboxylate → weakest acid.
pKa: 4-NO₂ (3.4) < benzoic (4.2) < 4-OCH₃ (4.5)
(b) Aliphatic acids:
Order: propanoic < acetic < formic
Propanoic (CH₃CH₂COOH) has ethyl group (+I) → destabilizes carboxylate → weakest.
Acetic (CH₃COOH) has methyl group (+I) → less destabilization.
Formic (HCOOH) has H (no +I) → most stable carboxylate → strongest acid.
pKa: propanoic (4.88) > acetic (4.76) > formic (3.75)
Overall reaction: R-COOH + R'-OH (H₂SO₄) ⇌ R-COOR' + H₂O
Mechanism (Fischer esterification):
Step 1: Protonation of carbonyl oxygen (H⁺ adds to C=O) → activates the carbon toward nucleophilic attack.
Step 2: Nucleophilic attack by alcohol O (lone pair) on the protonated carbonyl carbon → tetrahedral intermediate.
Step 3: Proton transfer — one -OH of the tetrahedral intermediate gets protonated to become a good leaving group (H₂O).
Step 4: Water leaves → oxonium ion intermediate (C=O⁺H-OR').
Step 5: Deprotonation → ester (R-COOR').
Note: The reaction is reversible. To push forward, remove water (Dean-Stark apparatus) or use excess of one reactant.
(a) Ethanol (1° alcohol, CH₃CH₂OH):
HCHO (formaldehyde) + CH₃MgBr → CH₃CH₂OMgBr → CH₃CH₂OH (after H₃O⁺)
Formaldehyde always gives primary alcohol.
(b) Propan-2-ol (2° alcohol, CH₃CHOHCH₃):
CH₃CHO (acetaldehyde) + CH₃MgBr → CH₃-CH(OMgBr)-CH₃ → CH₃CHOHCH₃ (after H₃O⁺)
Other aldehydes give secondary alcohols.
(c) 2-Methylpropan-2-ol (3° alcohol, (CH₃)₃COH):
CH₃COCH₃ (acetone) + CH₃MgBr → (CH₃)₃C-OMgBr → (CH₃)₃COH (after H₃O⁺)
Ketones give tertiary alcohols.
(a) Cannizzaro reaction:
2HCHO + NaOH (conc.) → HCOONa + CH₃OH (sodium formate + methanol)
Requires aldehyde with no α-H. Disproportionation — one oxidized, one reduced.
(b) Aldol condensation:
2CH₃CHO (dil. NaOH) → CH₃-CH(OH)-CH₂-CHO (aldol) --(heat)--> CH₃-CH=CH-CHO + H₂O
Requires aldehyde with at least one α-H. Forms β-hydroxy carbonyl → dehydrated to α,β-unsaturated.
(c) Crossed Cannizzaro:
HCHO + Ph-CHO + NaOH → HCOONa + Ph-CH₂OH
When two different aldehydes (no α-H) are used, the more reactive one (HCHO) is oxidized, the less reactive (Ph-CHO) is reduced. Product: sodium formate + benzyl alcohol.
| HCl | pKa ≈ −7 | Strong mineral acid |
| Trichloroacetic acid | pKa ≈ 0.7 | Strongest organic acid |
| Chloroacetic acid | pKa ≈ 2.86 | Cl stabilizes carboxylate |
| Formic acid | pKa ≈ 3.75 | Stronger than acetic (no +I) |
| Benzoic acid | pKa ≈ 4.2 | Aromatic acid |
| Acetic acid | pKa ≈ 4.76 | Classic weak acid |
| Carbonic acid | pKa ≈ 6.4 | Weaker than carboxylic |
| Phenol | pKa ≈ 10 | Weak acid |
| Water | pKa ≈ 15.7 | Reference neutral |
| Ethanol | pKa ≈ 16 | Very weak acid |
| Acetone (α-H) | pKa ≈ 20 | Extremely weak |
| R-CHO | Aldehyde |
| R-CO-R' | Ketone |
| R-COOH | Carboxylic acid |
| C=O | Carbonyl group |
| DIBAL-H | Diisobutylaluminium hydride |
| PCC | Pyridinium chlorochromate (mild oxidant) |
| 2,4-DNP | 2,4-Dinitrophenylhydrazine |
| HVZ | Hell-Volhard-Zelinsky (α-halogenation) |
| mCPBA | meta-Chloroperoxybenzoic acid (Baeyer-Villiger) |
| pKa | Acid dissociation constant (lower = stronger acid) |
(A) Both true, R explains A. (B) Both true, R does NOT explain A. (C) A true, R false. (D) A false, R true.
All key reactions & facts at one place — perfect for quick revision before exams.