Aldehydes & Ketones — Nucleophilic Addition Meet the Carbonyl: Why It Is So Reactive The carbonyl group (C=O) is the heart of aldehydes (R–CHO) and ketones (R–CO–R'). Oxygen is much more electronegative than carbon. So the C=O bond is polarized: carbon carries partial positive ( +) and oxygen carries partial negative ( -). This makes the carbonyl carbon an electrophile — it attracts nucleophiles (electron-rich species), while the oxygen can be protonated by acids (electrophiles). The carbonyl carbon is sp 2 -hybridized and planar (around 120°), so nucleophiles can approach from either face. Carbonyl compound An organic compound containing the C=O group. Aldehydes (R–CHO) and ketones (R–CO–R') are key examples. R–CHO; carbonyl carbon bonded to one hydrogen and one alkyl/aryl group. Example: methanal (formaldehyde), SMILES: C=O. Aldehyde Ketone R–CO–R'; carbonyl carbon bonded to two alkyl/aryl groups. Example: propanone (acetone), SMILES: CC(=O)C. General nucleophilic addition to a carbonyl: polarized C=O, nucleophile attacks electrophilic carbon, tetrahedral alkoxide intermediate forms, then protonation. General nucleophilic addition Two-step picture: nucleophilic attack to make a tetrahedral intermediate, then protonation. Aldehyde vs Ketone: Properties and Reactivity Basics Physical properties: Aldehydes and ketones have higher boiling points than alkanes of similar molar mass (due to dipole–dipole interactions) but generally lower than comparable alcohols (they cannot donate hydrogen bonds). Lower members are miscible with water; solubility drops with longer alkyl chains. Reactivity trend in nucleophilic addition: formaldehyde (HCHO) > aliphatic aldehydes (R–CHO) > ketones (R2CO). Two reasons: sterics (a smaller group like H gives easier access) and electronics (alkyl groups are weakly electron-donating by +I, stabilizing the partial positive charge on the carbonyl carbon and making it less electrophilic). Aromatic carbonyls are even less reactive due to conjugation (resonance delocalization reduces electrophilicity). Why aldehydes are more reactive than ketones: less steric hindrance and less +I stabilization of the carbonyl carbon in aldehydes. Nucleophilic Addition Reactivity Order Carbonyl class Reasoning Relative reactivity Entry HCHO (formaldehyde) Zero alkyl groups; minimal steric and electronic donation Highest R–CHO (aliphatic aldehyde) One alkyl donates weakly; moderate steric effect High R2CO (ketone) Two alkyl groups donate (+I) and hinder approach Lower Ar–CHO / Ar–CO–R (aromatic) Ring conjugation delocalizes C=O; reduces electrophilicity Lowest (typical) Ketones are generally more reactive than aldehydes in nucleophilic addition. Aldehydes are more reactive than ketones due to less steric hindrance and less +I stabilization at the carbonyl carbon (trend: HCHO > RCHO > R2CO). Steric hindrance and +I donation from alkyl groups significantly slow ketones relative to aldehydes; aromatic conjugation slows them even more. Steric and electronic effects are minor, so all carbonyls add nucleophiles at similar rates. Preparations of Aldehydes (NEET-favorite named reactions) High-yield routes Oxidation of primary alcohols: mild PCC stops at aldehyde; KMnO4/K2Cr2O7 over-oxidize to carboxylic acids. Rosenmund reduction: acid chloride (R–COCl) to aldehyde using H2, Pd/BaSO4 (poisoned, e.g., quinoline) — prevents over-reduction. Stephen reaction: nitrile (R–C≡N) with SnCl2/HCl to iminium (R–CH=NH·HCl), hydrolysis gives R–CHO. DIBAL-H (Diisobutylaluminium hydride): reduces esters or nitriles to aldehydes at about −78°C (low temperature). Etard reaction: benzylic methyl (Ar–CH3) to Ar–CHO using chromyl chloride (CrO2Cl2) in CS2 (via chromyl complex). Gattermann–Koch formylation: Ar–H + CO + HCl, AlCl3/CuCl catalyst → Ar–CHO (aromatic ring formylation). Ozonolysis: Alkenes + O3 followed by Zn/H2O (reductive workup) → aldehydes/ketones depending on substituents. Primary alcohol to aldehyde with PCC; strong oxidants over-oxidize. Acid chloride to aldehyde; Pd/BaSO4 poisoned catalyst. Nitrile to aldehyde via iminium chloride; hydrolysis gives R–CHO. Chromyl chloride oxidizes Ar–CH3 to Ar–CHO. Direct formylation of aromatic rings to Ar–CHO. Alkene cleavage with O3; reductive workup yields carbonyls. gpt-image-2 At-a-glance aldehyde syntheses: Rosenmund, Stephen, DIBAL-H, Etard, Gattermann–Koch, ozonolysis. 2026-05-26T17:05:36.664Z Flowchart of aldehyde preparations on white background. Boxes: PCC on 1° alcohol → aldehyde; Rosenmund (H2, Pd/BaSO4, quinoline) on RCOCl → RCHO; Stephen (SnCl2/HCl then H2O) on RCN → RCHO; DIBAL-H at − 78 C on ester/nitrile → aldehyde; Etard (CrO2Cl2/CS2) on Ar–CH3 → Ar–CHO; Gattermann–Koch (CO + HCl, AlCl3/CuCl) on Ar–H → Ar–CHO; Ozonolysis (O3, Zn/H2O) on alkene → carbonyl fragments. Clean vector style, arrows in red, labels crisp, no text captions inside. Preparations of Ketones (quick survey) Reliable routes Oxidation of secondary alcohols (KMnO4, K2Cr2O7, PCC) → ketones. Friedel–Crafts acylation: Ar–H + R–COCl, AlCl3 → Ar–CO–R (aryl alkyl ketones). Acyl chloride + lithium dialkylcuprate (Gilman reagent, R2CuLi) → ketone (single acyl substitution). Nitrile + Grignard (R–MgX) → ketimine magnesium salt; hydrolysis → ketone. Acylation introduces –CO–R on aromatics under AlCl3. Grignard reagents add to nitriles and carbonyls to form alcohols or ketones after workup. Mechanism Deep-Dive: Nucleophilic Addition Steps Think of the nucleophile as an aggressive tennis server aiming at the electron-poor carbonyl carbon. Step 1: the nucleophile attacks the + carbonyl carbon to form a tetrahedral alkoxide intermediate. Step 2: the alkoxide gets protonated (usually by an acid or water) to give the addition product (an alcohol or derivative). Acidic conditions often protonate the carbonyl oxygen first, enhancing electrophilicity; basic conditions generate a stronger nucleophile. neet-alert Do not mix up nucleophilic addition to C=O with electrophilic addition to C=C alkenes. In carbonyls, the electrophile is the carbonyl carbon. Signature Additions You Must Master HCN / NaCN Aldehydes, ketones Cyanohydrin (R2C(OH)CN) Builds a C–C bond; route to -hydroxy acids NaHSO3 (bisulfite) Many aldehydes, some ketones Bisulfite adduct (R2C(OH)SO3Na) Crystalline adduct for purification; reversible Grignard (R–MgX) Aldehydes, ketones, formaldehyde Alcohols HCHO → 1°; R'CHO → 2°; R'2CO → 3° alcohols H2O Aldehydes (esp. HCHO); some ketones Gem-diol (hydrate, R2C(OH)2) Stable mainly for HCHO and chloral (CCl3CHO) ROH (acid-catalyzed) Aldehydes, ketones Hemiacetal → acetal Protecting group; stable in base, hydrolyzes in acid 1° amine (R'NH2) Aldehydes, ketones Imine (Schiff base) Reversible condensation; water by-product NH2OH / NH2NH2 / PhNHNH2 / H2N–NH–CONH2 Aldehydes, ketones Oxime / hydrazone / phenylhydrazone / semicarbazone Useful derivatives for identification NaC≡CR' Aldehydes, ketones Propargyl alcohol From acetylide nucleophile (terminal alkyne) Case Reagent Carbonyl substrate Product Key point Important Nucleophilic Additions to C=O General C=O additions/reductions framework. Cyanohydrins: Building C–C Bonds Equilibrium addition of HCN across the C=O of an aldehyde. Cyanohydrin formation A base (e.g., NaCN) generates CN - from HCN (or supplies CN - directly). CN - attacks the + carbonyl carbon, forming a tetrahedral alkoxide. The alkoxide is protonated by HCN or water to give the cyanohydrin RCH(OH)CN. electrophile alkan al aldehyde (generic) hydrogen cyanide hydrogen cyanide nucleophile source nucleophile cyanide cyanide ion CN - adds to the carbonyl carbon; the resulting alkoxide is protonated to give the cyanohydrin. Base-catalyzed cyanohydrin formation Mechanism panels for cyanohydrin formation with curved arrows and charge labeling. gpt-image-2 3-panel vector mechanism: Panel 1: NaCN generates CN− from HCN; Panel 2: CN− attacks carbonyl C of R–CHO, showing curved arrows and tetrahedral alkoxide; Panel 3: protonation to give RCH(OH)CN. Labels: electrophilic C, nucleophile, intermediate. Clean textbook diagram, arrows in red. 2026-05-26T17:05:36.974Z Cyanohydrins are valuable synthons: hydrolyzing the –CN group gives -hydroxy acids used in synthesis problems. remember Bisulfite Adducts: Smart Purification Trick Sodium bisulfite (NaHSO3) adds to many aldehydes and some ketones to form crystalline, water-soluble adducts (R2C(OH)SO3Na). These adducts can be filtered, then decomposed (with dilute acid or base) to regenerate the carbonyl compound — a classic purification method for aldehydes. Reversible formation of the bisulfite adduct; useful for purification. 2026-05-26T17:05:37.402Z Schematic: Flask 1: aldehyde + NaHSO3 → crystalline adduct; Funnel: filtration of crystals; Flask 2: treat crystals with dilute acid/base → regenerate aldehyde. Minimalist vector chemistry style, step labels A–C, arrows in red. gpt-image-2 How bisulfite adducts help purify aldehydes: formation, crystallization, and regeneration steps. Grignard Reagents: Predict 1°, 2°, 3° Alcohols Grignard addition to ketone Addition to a ketone followed by hydrolytic workup gives a tertiary alcohol. Grignard reagents (R–MgX) are strong nucleophiles/bases prepared in dry ether. They add to carbonyls to form alkoxides; aqueous workup (NH4Cl/ H3O + ) gives alcohols. Mapping is high-yield: • Formaldehyde (H–CHO) + R–MgX → 1° alcohol (CH2OH–R). • Aldehyde (R'–CHO) + R–MgX → 2° alcohol. • Ketone (R'2CO) + R–MgX → 3° alcohol. Anhydrous conditions are essential; water destroys Grignards. gpt-image-2 Grignard additions: formaldehyde → 1°, aldehyde → 2°, ketone → 3° alcohols, with workup. 2026-05-26T17:05:37.573Z Four mini-panels: (1) H–CHO + R–MgX → R–CH2–O−MgX+ → H3O+ → 1° alcohol; (2) R'–CHO case → 2°; (3) R'2CO case → 3°; (4) crossed red X over water on Grignard bottle (moisture sensitive). Clean vector, arrows in red, alcohols labeled 1°/2°/3°. Water Addition: When Are Hydrates Stable? Adding water across C=O gives a geminal diol (hydrate, R2C(OH)2). Most aldehyde/ketone hydrates are unstable and revert to the carbonyl. Two classic stable cases: formaldehyde (methanediol predominates in water) and chloral (CCl3–CHO) forms chloral hydrate (CCl3–CH(OH)2), historically known as a sedative. Most hydrates are unstable; formaldehyde and chloral are notable exceptions with relatively stable hydrates. All carbonyl compounds form and keep stable hydrates in water. Alcohol Addition: Hemiacetals and Acetals (Protecting Groups) Hemiacetal formation Acetal formation Under acid catalysis, one equivalent of alcohol adds to give a hemiacetal (contains –OH and –OR’ on the same carbon). With excess alcohol and removal of water, this converts to an acetal (two –OR’ groups). Acetals are stable in base but hydrolyze back to the carbonyl in acid — that is why they are excellent protecting groups for aldehydes/ketones in basic conditions. Biological hemiacetal: open-chain D-glucose cyclizes intramolecularly to a cyclic hemiacetal (pyranose form). In biology, sugars exist mostly as cyclic hemiacetals/hemiketals. Glucose cyclization is an intramolecular nucleophilic addition of an –OH to an aldehyde — central to carbohydrate chemistry. remember Amines: Imines and Diagnostic Derivatives Primary amines (R'NH2) condense with aldehydes/ketones (acid-catalyzed) to form imines (Schiff bases, R2C=NR') with loss of water. Related nucleophiles give characteristic derivatives used in identification: hydroxylamine (NH2OH) → oximes, hydrazine (NH2NH2) → hydrazones, phenylhydrazine (PhNHNH2) → phenylhydrazones, semicarbazide (H2N–NH–CONH2) → semicarbazones. 2,4-Dinitrophenylhydrazine (2,4-DNP; Brady’s reagent) gives yellow to orange-red precipitates with aldehydes and ketones — a staple qualitative test. Acid-catalyzed imine formation (reversible condensation). 2,4-DNP test: formation of orange-red hydrazone precipitates with carbonyl compounds. gpt-image-2 Side-by-side vials: left clear (no carbonyl), right showing orange-red precipitate after adding 2,4-DNP to a carbonyl solution. Include small structural inset: carbonyl + 2,4-DNP → hydrazone + H2O. Clean lab illustration style. 2026-05-26T17:05:37.320Z tip Remember the family: NH2OH (oxime), NH2NH2 (hydrazone), PhNHNH2 (phenylhydrazone), H2N–NH–CONH2 (semicarbazone). Acetylide Addition: Propargyl Alcohols Terminal alkynes are deprotonated by strong base (e.g., NaNH2) to give acetylide anions (R'–C≡ C − ), strong nucleophiles that add to aldehydes and ketones forming propargyl alcohols after protonation. Useful C–C bond-forming step in synthesis. Formation of acetylide anions from terminal alkynes. Lab Tests and Everyday Relevance Test Test Positive for Observation/Color Main reagent Classic Tests for Carbonyls 2,4-DNP (Brady’s reagent) Aldehydes and ketones Yellow/orange to red precipitate (hydrazone) 2,4-Dinitrophenylhydrazine in acid Tollens’ test Aldehydes Silver mirror/black Ag [Ag(NH3)2] + (ammoniacal AgNO3) Fehling’s test Aliphatic aldehydes Brick-red Cu2O precipitate Cu 2 + in alkaline tartrate solution Schiff’s reagent Aldehydes Magenta/pink color reappears Fuchsin–sulfite reagent Industry links: formaldehyde → phenol–formaldehyde (Bakelite) and urea–formaldehyde resins (adhesives); acetone is a common solvent (e.g., nail-polish remover); benzaldehyde for almond fragrance; vanillin and cinnamaldehyde are key flavor molecules. Brady’s test (2,4‑DNP) is a classic undergraduate lab assay for carbonyls. remember NEET Traps and Quick Recap Grignard outcome: H–CHO → 1°, R–CHO → 2°, R2C=O → 3°. Think: more R on carbonyl, higher-degree alcohol. neet-alert Acetals are base-stable but acid-labile (reverse to carbonyl + alcohol in acid). Use them as protecting groups during base-mediated steps — then deprotect with acid. Aromatic carbonyls add nucleophiles more slowly than aliphatic ones due to resonance delocalization reducing electrophilicity. neet-alert Key Terms (Quick Glossary) The C=O functional group. Carbonyl Aldehyde R–CHO; carbonyl carbon bonded to H and R. R–CO–R'; carbonyl carbon bonded to two R groups. Ketone Addition product of HCN to carbonyl: R2C(OH)CN. Cyanohydrin Bisulfite adduct R2C(OH)SO3Na; reversible adduct with NaHSO3 used for purification. Hemiacetal R2C(OH)OR'; formed by addition of one ROH to carbonyl in acid. R2C(OR')2; formed from hemiacetal + ROH (acid-catalyzed). Acetal Imine (Schiff base) R2C=NR'; from R'NH2 + carbonyl (acid-catalyzed). R2C=NOH; from NH2OH. Oxime Hydrazone / phenylhydrazone R2C=NNH2 / R2C=NNHPh; from NH2NH2 / PhNHNH2. Semicarbazone R2C=NNHCONH2; from semicarbazide. 2,4-dinitrophenylhydrazine; gives colored precipitates with carbonyls. 2,4-DNP (Brady’s reagent) Rosenmund reduction RCOCl → RCHO using H2, Pd/BaSO4 (poisoned). RCN → RCHO via SnCl2/HCl then hydrolysis. Stephen reaction Etard reaction Ar–CH3 → Ar–CHO using CrO2Cl2/CS2. Gattermann–Koch Ar–H + CO + HCl, AlCl3/CuCl → Ar–CHO. Schiff’s reagent Fuchsin–sulfite solution; turns magenta with aldehydes.