Aldehydes, Ketones and Carboxylic Acids
Bite into a vinegar-dressed salad, smell vanilla in a cake, or watch a silver mirror bloom inside a warmed test tube during a Tollens' test, and you are meeting the same functional group in three different costumes. Aldehydes, ketones and carboxylic acids are all built around carbon joined to oxygen, and almost everything this chapter teaches follows from how that one bond behaves. A carbon-oxygen double bond, the carbonyl group >C=O, is planar and sp2 at carbon, and because oxygen is far more electronegative than carbon it pulls the bonding electrons toward itself. That leaves the carbonyl carbon short of electrons — an electrophile begging a nucleophile to attack it — and the oxygen rich in electrons — a base and a hydrogen-bond acceptor. Every reaction in this chapter, from cyanohydrin formation to the aldol condensation to esterification, is a variation on a nucleophile approaching that positively polarised carbon. Aldehydes (R-CHO) and ketones (R-CO-R') share the carbonyl group but differ in what flanks it, and that difference in sterics and electronics explains why aldehydes are more reactive, easier to oxidise, and give a silver mirror while ketones do not. Push the oxidation state one notch further and the carbonyl carbon picks up a hydroxyl group to become the carboxyl group, -COOH, defining carboxylic acids — compounds acidic enough to fizz with sodium bicarbonate, a test no alcohol or phenol passes as readily. This chapter builds outward from that shared electronic story: how to name and draw these three families under IUPAC rules; how to make them from alcohols, hydrocarbons, nitriles and Grignard reagents; why their boiling points and solubilities sit between hydrocarbons and alcohols; the full toolkit of carbonyl chemistry — nucleophilic addition, reduction, oxidation, aldol condensation, Cannizzaro disproportionation; and why carboxylic acids are among the most acidic organic compounds you will meet, their strength tunable by every electron-withdrawing or electron-donating group nearby. Master the polarised carbonyl and the rest of the chapter falls into place as its logical consequence.
- The carbonyl group >C=O is planar, sp2 at carbon, and polarised (Cδ+=Oδ−) by resonance between a neutral and a dipolar structure — this drives nearly every reaction in the chapter.
- Aldehydes (one H on the carbonyl carbon) are more reactive than ketones (two alkyl/aryl groups) in nucleophilic addition, for both steric and electronic reasons.
- IUPAC names end in -al for aldehydes, -one for ketones, -oic acid for carboxylic acids; the functional carbon is always numbered 1 for aldehydes and acids.
- Boiling points rank hydrocarbons/ethers lowest, aldehydes/ketones (dipole-dipole forces only) next, alcohols higher, and carboxylic acids highest, since acids hydrogen-bond so strongly they exist as dimers even in vapour.
- Nucleophilic addition is the master reaction: HCN gives cyanohydrins, NaHSO3 gives bisulphite adducts, alcohols give hemiacetals then acetals, and H2N-Z gives imines, oximes, hydrazones and 2,4-DNP derivatives after acid-catalysed dehydration.
- Aldehydes are oxidised by mild reagents (Tollens', Fehling's) that ketones cannot reduce — the classic test to tell the two apart; methyl ketones and CH3CH(OH)- compounds give a positive iodoform test.
- Alpha-hydrogens are acidic because the resulting carbanion is resonance-stabilised by the carbonyl; this powers aldol condensation, and, for aldehydes lacking an alpha-hydrogen, the Cannizzaro disproportionation instead.
- Carboxylic acids ionise to a resonance-stabilised, symmetric carboxylate, making them far more acidic (pKa about 3 to 5) than alcohols (about 16) or phenols (about 10); electron-withdrawing substituents raise acid strength further, electron-donating groups lower it.
- Named reactions to keep straight: Rosenmund, Stephen, Etard, Gattermann-Koch, Friedel-Crafts acylation, Clemmensen, Wolff-Kishner, Cannizzaro, Hell-Volhard-Zelinsky, Kolbe electrolysis.
- Key idea: the electronegativity-driven polarisation of the carbonyl carbon (Cδ+=Oδ−) makes it an electrophile for every nucleophilic addition reaction here, and oxidising that carbon one step further to -COOH creates a carboxylate conjugate base stable enough to make carboxylic acids some of the most acidic organic compounds you will study.
Because oxygen is more electronegative than carbon, the carbonyl group >C=O is polarised into an electrophilic carbon and a nucleophilic oxygen, driving all nucleophilic addition chemistry of aldehydes and ketones, and when that carbon gains an extra -OH to form -COOH, the resulting carboxylate anion is doubly resonance-stabilised, making carboxylic acids markedly more acidic than alcohols or phenols.
What this chapter covers
This companion follows the NCERT sequence. First, nomenclature and electronic structure of the carbonyl and carboxyl groups (8.1, 8.6), so later reactions read as attacks on a known electron distribution. Next, preparation of aldehydes and ketones (8.2) — oxidation and dehydrogenation of alcohols, ozonolysis, hydration of alkynes, Rosenmund reduction, the Stephen and DIBAL-H routes to aldehydes, the Etard and Gattermann-Koch routes to aromatic aldehydes, and Friedel-Crafts acylation for ketones. Physical properties (8.3) explain the boiling-point and solubility ladder you should be able to rank on sight. The core of the chapter, Section 8.4, catalogues carbonyl chemistry: nucleophilic addition and its mechanism, reduction (to alcohols, or to methylene groups), oxidation and the tests that distinguish aldehydes from ketones, and alpha-hydrogen reactions — aldol condensation, cross aldol, Cannizzaro. Section 8.5 lists uses. The article then turns to carboxylic acids: nomenclature and structure (8.6), preparation (8.7), physical properties (8.8), and the three reaction classes of Section 8.9 — reactions at the O-H bond (acidity, esterification), at the C-OH bond (anhydride and acid-chloride formation), and of the -COOH group and its chain (reduction, decarboxylation, Hell-Volhard-Zelinsky, ring substitution). Three derivation blocks then walk through the nucleophilic addition mechanism, the Fischer esterification mechanism, and the acidity trend as substituents change — the organic-chemistry analogue of a physical-chemistry derivation.
The core ideas
Naming and drawing the carbonyl and carboxyl groups
Aldehydes carry the carbonyl group bonded to one carbon (or hydrogen) and one hydrogen, -CHO; ketones carry it bonded to two carbon substituents, >C=O flanked by two R groups; carboxylic acids fuse a hydroxyl onto the carbonyl carbon to give -COOH. IUPAC names replace the terminal -e of the parent alkane with -al for aldehydes (methanal, ethanal, propanal), -one for ketones (propan-2-one, butan-2-one), or -oic acid for acids (methanoic, ethanoic, propanoic acid), always numbering from the functional carbon, which is carbon-1 for aldehydes and acids (ketones number from whichever end gives the carbonyl the lower locant). An aldehyde on a ring takes the suffix -carbaldehyde after the full ring name (cyclohexanecarbaldehyde), the ring carbon bearing it being numbered one. Common names persist heavily — formaldehyde, acetaldehyde, acetone, formic acid, acetic acid, benzaldehyde, benzoic acid — and aldehyde common names are usually built from the corresponding acid's common name by swapping -ic acid for -aldehyde, with Greek letters marking substituent position starting next to the carbonyl carbon. All three families share one electronic story. The carbonyl carbon is sp2, its three sigma bonds coplanar at about 120°, with the fourth valence electron in a p-orbital forming a pi bond with oxygen; the pi cloud sits above and below this plane, and oxygen keeps two lone pairs. Because oxygen is more electronegative, the true structure is a resonance hybrid of a neutral form (A) and a dipolar form (B) with full charges on carbon and oxygen.
where: the true carbonyl is a weighted blend of A and B, giving carbon electrophilic (Lewis acid) character and oxygen nucleophilic (Lewis base) character; this is why carbonyl compounds are more polar, with larger dipole moments, than ethers of similar mass.
The carboxyl carbon is actually less electrophilic than an isolated carbonyl carbon, because a third resonance structure lets the hydroxyl oxygen's lone pair delocalise into the pi system, partly neutralising the carbon's positive character — a fact that matters once the group ionises, discussed under acidity below.
Physical properties: the boiling-point and solubility ladder
Aldehydes and ketones cannot hydrogen-bond to themselves (no O-H or N-H sits on the carbonyl carbon), so they associate only through weaker dipole-dipole forces. Carboxylic acids carry both a donor (O-H) and acceptor sites, so they self-associate strongly, often existing as hydrogen-bonded dimers even in the vapour phase. This gives a clean four-tier ladder for compounds of comparable mass: hydrocarbons/ethers boil lowest, aldehydes/ketones next, alcohols higher, carboxylic acids highest. NCERT's own comparison near molecular mass 58 to 60 shows this numerically: n-butane 273 K, methoxyethane 281 K, propanal 322 K, acetone 329 K, propan-1-ol 370 K. Solubility follows the same logic. Lower members — methanal, ethanal, propanone, methanoic and ethanoic acid — are miscible with water because they hydrogen-bond to it directly; solubility falls rapidly as the hydrocarbon chain lengthens and cannot hydrogen-bond, though all remain soluble in organic solvents such as benzene, ether and chloroform.
Nucleophilic addition: the master reaction of the carbonyl group
Alkenes, electron-rich, attract electrophiles; the carbonyl group is the electronic mirror image, its electron-poor carbon attracting nucleophiles instead. A nucleophile approaches roughly perpendicular to the sp2 plane, carbon rehybridises sp2 to sp3, and a tetrahedral alkoxide intermediate forms; protonation from the medium then gives the neutral product. Overall, Nu and H add across the C=O bond. Aldehydes react faster than ketones. Sterically, a ketone's two flanking groups crowd the approaching nucleophile more than an aldehyde's one substituent. Electronically, alkyl groups donate electron density by hyperconjugation and induction, so a ketone's two alkyl groups damp electrophilicity more than an aldehyde's one; conjugation (as in benzaldehyde) reduces reactivity further by donating electron density into the carbonyl by resonance. Several nucleophiles add across C=O. Base-generated cyanide gives a cyanohydrin, a useful synthetic intermediate. Sodium hydrogensulphite gives a water-soluble bisulphite adduct, favoured for aldehydes and methyl ketones and reversible with dilute acid or alkali, useful for purification. Alcohols under dry HCl give a hemiacetal, then, with a second equivalent, an acetal (gem-dialkoxy compound); ketones give the cyclic ketal with ethylene glycol. Ammonia derivatives H2N-Z add and then dehydrate under acid catalysis to give >C=N-Z: an imine for Z = H, a Schiff's base for Z = alkyl/aryl, an oxime for Z = -OH, a hydrazone for Z = -NH2, a phenylhydrazone or 2,4-DNP derivative for the corresponding hydrazines (2,4-DNP derivatives are brightly coloured solids used to characterise unknowns), and a semicarbazone for semicarbazide. Grignard reagents add to give, after acidic workup, alcohols.
Reduction and oxidation of aldehydes and ketones
Reduction can stop at the alcohol or go to a methylene group. NaBH4, LiAlH4 or catalytic hydrogenation reduce the carbonyl to a hydroxyl, giving a primary alcohol from an aldehyde and a secondary alcohol from a ketone. To remove the oxygen entirely, two named methods apply: Clemmensen reduction (Zn-Hg with concentrated HCl, acid-stable substrates) and Wolff-Kishner reduction (hydrazine, then KOH or NaOH in a high-boiling solvent, base-stable substrates). Oxidation is where aldehydes and ketones part company, giving the standard test to distinguish them. An aldehyde's oxidation merely adds oxygen to an existing C-H bond and proceeds even with mild reagents; a ketone's oxidation requires vigorous conditions that cleave a C-C bond, giving a mixture of smaller acids, so mild tests below are effectively negative for ketones. Tollens' reagent (ammoniacal AgNO3) deposits a silver mirror as Ag+ is reduced while the aldehyde oxidises to a carboxylate in alkaline medium. Fehling's reagent (CuSO4 plus alkaline sodium potassium tartrate) gives a reddish-brown Cu2O precipitate; aromatic aldehydes fail this test though they pass Tollens'. The haloform (iodoform) reaction is structurally selective: any carbonyl with a methyl group directly on the carbonyl carbon (a methyl ketone, or acetaldehyde) is cleaved by sodium hypohalite to a carboxylate with one fewer carbon, the methyl group becoming CHX3; secondary alcohols CH3-CH(OH)-R also give a positive test since they oxidise to the methyl ketone first.
Alpha-hydrogen reactions: aldol condensation and Cannizzaro
Hydrogens on the carbon next to a carbonyl — the alpha-carbon — are unusually acidic (pKa around 20) because removing one gives a carbanion whose charge delocalises onto the electronegative carbonyl oxygen by resonance. Under dilute base this powers the aldol reaction: two molecules of an aldehyde or ketone with at least one alpha-hydrogen combine, one as nucleophile (its enolate attacking) and one as electrophile, to give a beta-hydroxy aldehyde (aldol) or beta-hydroxy ketone (ketol). These readily dehydrate to an alpha,beta-unsaturated carbonyl compound — the aldol condensation proper, a name kept for ketone reactions too. Cross aldol condensation between two different alpha-hydrogen-bearing partners gives up to four products; it becomes synthetically clean when one partner has no alpha-hydrogen and can act only as electrophile. Aldehydes with no alpha-hydrogen at all — formaldehyde, benzaldehyde, and other aryl or fully substituted aldehydes — cannot enolise, so under concentrated alkali they instead disproportionate: the Cannizzaro reaction, in which one molecule is reduced to the alcohol and a second oxidised to the carboxylate salt, via hydride transfer from a tetrahedral intermediate. Aromatic carbonyl compounds also undergo ordinary electrophilic aromatic substitution; the carbonyl group deactivates the ring and directs incoming electrophiles meta, opposite to an alkyl or -OH substituent's ortho/para direction.
The acidity of carboxylic acids
Carboxylic acids dissociate in water to give a hydronium ion and a carboxylate anion.
where: larger (smaller p) means a stronger acid. Acids with p less than 1 are strong, 1 to 5 moderately strong, 5 to 15 weak, above 15 extremely weak; HCl sits near p -7, trifluoroacetic acid (strongest common carboxylic acid) at 0.23, benzoic acid at 4.19, acetic acid at 4.76, while ethanol sits near 16 and phenol near 10. Carboxylic acids out-acid both alcohols and phenols because their conjugate base spreads charge best. The carboxylate ion has two exactly equivalent resonance structures, each placing negative charge on an equally electronegative oxygen, giving a genuinely delocalised, low-energy anion. Phenoxide's resonance structures are not equivalent — most place charge on ring carbons, far less electronegative than oxygen — and an alkoxide from a simple alcohol has no resonance stabilisation at all. More delocalised charge means a more stable, lower-energy anion and a stronger acid, ranking alcohols weakest, phenols intermediate, carboxylic acids strongest. Substituents tune this further through the inductive effect. An electron-withdrawing group (EWG) pulls density away through sigma bonds, dispersing and stabilising the carboxylate's negative charge, raising ; an electron-donating group (EDG) does the opposite, lowering . The effect fades quickly with distance — each extra bond roughly halves it — so alpha-substitution matters far more than beta or gamma.
Preparation and further reactions of carboxylic acids
Carboxylic acids are made by pushing oxidation as far as it will go: primary alcohols and aldehydes are oxidised by KMnO4, K2Cr2O7 or CrO3 in acidic media; alkylbenzenes are side-chain oxidised (regardless of chain length, given a benzylic hydrogen) by chromic acid or permanganate to the aromatic acid directly; nitriles hydrolyse through the amide to the acid under H+ or OH- catalysis; and Grignard reagents react with dry CO2 to give, after acidification, an acid with one carbon more than the parent alkyl halide. Acid chlorides hydrolyse with water or aqueous base, and anhydrides with water, both back to the acid. Beyond ionisation, three further reaction classes matter. Reactions cleaving the C-OH bond include esterification (mechanism below), anhydride formation on heating with P2O5 or hot concentrated H2SO4, conversion to acid chlorides with PCl5, PCl3 or SOCl2 (preferred, since its by-products SO2 and HCl simply escape as gases), and reaction with ammonia to an ammonium salt that loses water on strong heating to give an amide. Reactions of the -COOH group itself include reduction, by LiAlH4 or more selectively by diborane (which spares esters, nitro and halo groups), to a primary alcohol, and decarboxylation — loss of CO2 from the sodium salt heated with sodalime (NaOH plus CaO, 3:1), or on electrolysis of the aqueous salt (Kolbe electrolysis), coupling two alkyl radicals to a hydrocarbon with twice the carbon count of the acid's alkyl group. Substitution on the chain includes alpha-halogenation with Cl2 or Br2 and a trace of red phosphorus — the Hell-Volhard-Zelinsky reaction, needing an alpha-hydrogen — and, for aromatic acids, ring electrophilic substitution where -COOH is deactivating and meta-directing; Friedel-Crafts fails outright because AlCl3 simply complexes with the carboxyl oxygen instead of activating the ring.
Mechanism deep dives
The goal here is to write out, step by step, how a small anionic nucleophile converts a carbonyl compound into a stable tetrahedral product, because every other nucleophilic addition in this chapter — bisulphite addition, hemiacetal/acetal formation, imine and oxime formation — follows the same three-stage pattern (nucleophile attacks carbon, negative charge parks on oxygen, oxygen is protonated) with only the nucleophile and workup changed. Cyanohydrin formation is the cleanest place to learn it, since cyanide is a simple, strong nucleophile and the product cannot react further.
Step 1. Generate the active nucleophile. Pure HCN is a weak acid and barely dissociated, so the uncatalysed reaction is slow; a trace of base deprotonates HCN to cyanide, CN-, a far stronger and more available nucleophile than neutral HCN — hence the reaction is described as base-catalysed even though no base appears in the final product.
Step 2. Nucleophilic attack on the carbonyl carbon. Cyanide's carbon lone pair attacks the electrophilic carbonyl carbon roughly perpendicular to the sp2 plane, pushing the C=O pi electrons fully onto oxygen. The carbon rehybridises sp2 to sp3, giving a tetrahedral alkoxide intermediate carrying a full negative charge on oxygen and a new -CN group.
Step 3. Protonation of the alkoxide. The alkoxide oxygen, now strongly basic, removes a proton from the solvent, regenerating catalytic base and delivering the neutral cyanohydrin, R2C(OH)(CN). The carbon is sp3 and tetrahedral, with no good leaving group left, so the reaction stops here.
Step 4. Generalise. Every carbonyl addition in the chapter follows Steps 2 and 3 with the nucleophile swapped: HSO3- gives the bisulphite adduct; ROH (with the carbonyl oxygen first protonated by acid to boost electrophilicity) gives a hemiacetal, then an acetal on losing water and adding a second ROH; H2N-Z gives, after addition and acid-catalysed dehydration of the resulting amino-alcohol, the product >C=N-Z. Recognising this shared skeleton is the fastest way to predict any carbonyl addition product.
Go deeper · Derivation B · 1 minMechanism of acid-catalysed Fischer esterification
The goal is to derive why carboxylic acids need an acid catalyst to react efficiently with alcohols, why the reaction is an equilibrium, and why removing water (or the ester) as it forms drives the reaction toward completion, as NCERT's own note implies. This is a nucleophilic acyl substitution at the carboxyl carbon, so the catalyst's first job is simply to make that carbon a better electrophile.
Step 1. Protonate the carbonyl oxygen. Catalytic H+ adds to a carbonyl oxygen lone pair, delocalising the positive charge over both oxygens by resonance and making the carbon considerably more electrophilic than in the neutral acid, offsetting the resonance donation from the -OH oxygen that normally weakens its electrophilicity.
Step 2. Nucleophilic addition of the alcohol. The alcohol's oxygen lone pair attacks this activated carbon, converting it from sp2 to a tetrahedral sp3 intermediate carrying three oxygen groups: the original -OH, the new, protonated -O+HR', and the former carbonyl oxygen (now neutral -OH, the charge having spread by resonance).
Step 3. Proton transfer within the intermediate. A proton hops from the positively charged -O+HR' group onto the original hydroxyl oxygen, converting it to -OH2+. This turns a poor leaving group (-OH) into an excellent one (neutral water) without needing a strong base, which the acidic medium would simply neutralise anyway.
Step 4. Loss of water, then deprotonation to the ester. The C-OH2+ bond breaks, water leaves as a neutral molecule, and the resulting cation is deprotonated by another alcohol or water molecule, regenerating catalytic H+ and delivering the neutral ester, R-CO-OR'. Because every step is reversible under acidic conditions, the whole esterification is itself an equilibrium; by Le Chatelier's principle, continuously removing water or the ester as it forms pulls the equilibrium toward product and improves yield.
Go deeper · Derivation C · 2 minTrend in the acid strength of substituted carboxylic acids
The goal is to explain, from electronegativity and bond polarity, why substituted acetic acids follow the observed order CF3COOH greater than CCl3COOH greater than NO2CH2COOH greater than FCH2COOH greater than ClCH2COOH greater than HCOOH greater than CH3COOH, and why a substituent's effect fades sharply with distance from the carboxyl carbon — the organic analogue of a periodic trend, governed entirely by the inductive effect transmitted through sigma bonds.
Step 1. Identify what varies: the conjugate base's stability. Since measures how favourably RCOOH sits relative to RCOO- plus H+, and the acid side of the equilibrium is roughly constant across the series, the whole trend in reduces to how stable the resulting carboxylate anion is; a more stable anion means a larger and smaller p.
Step 2. Rank the inductive pull. Electronegativity decides how strongly a group pulls sigma density toward itself: fluorine (3.98) greater than chlorine (3.16) greater than bromine (2.96) greater than iodine (2.66); -NO2 or -CN withdraw even more strongly, reinforced by resonance as well as induction. This explains the order Ph less than I less than Br less than Cl less than F less than CN less than NO2 less than CF3 in withdrawing power, and why CF3COOH, with three fluorines on one carbon, tops the series.
Step 3. Connect withdrawal to anion stabilisation. Each electron-withdrawing substituent pulls density away from the carboxylate oxygens through the sigma-bond chain, spreading the negative charge over more of the molecule; a more spread-out charge is lower in energy, exactly as in resonance delocalisation. Three fluorines withdraw more than one, so CF3COO- is more stabilised than FCH2COO-, which is more stabilised than CH3COO-, matching the observed p order (0.23, 2.66, 4.76).
Step 4. Show the distance fall-off and the phenyl exception. The inductive effect weakens roughly by half with each extra bond, so ClCH2COOH (p 2.86, chlorine on the alpha-carbon) is far more acidic than ClCH2CH2COOH (p around 4.0, chlorine on the beta-carbon) despite carrying the same one chlorine. A phenyl or vinyl group attached directly to -COOH also raises acidity, though naive resonance reasoning suggests it should donate density and weaken the acid; the resolution is that this ring or double-bond carbon is itself sp2 and more electronegative than an ordinary sp3 alkyl carbon, so its inductive pull wins over any resonance donation — exactly why benzoic acid (p 4.19) beats acetic acid (p 4.76) in acid strength.
Worked examples
Board level - Identifying an unknown methyl ketone (extending NCERT Example 8.4)
An organic compound A, molecular formula C8H8O, forms an orange-red precipitate with 2,4-DNP reagent, gives a yellow precipitate with iodine and NaOH, but does not reduce Tollens' or Fehling's reagent, and does not decolourise bromine water or Baeyer's reagent. Drastic oxidation with chromic acid gives a carboxylic acid B, formula C7H6O2. Identify A and B.
Step 1. A positive 2,4-DNP test means A has a carbonyl group — an aldehyde or a ketone.
Step 2. No reduction of Tollens' or Fehling's rules out an aldehyde, since only aldehydes give that result; A must be a ketone.
Step 3. A positive iodoform test requires a methyl group directly on the carbonyl carbon, so A is a methyl ketone, CH3-CO-R.
Step 4. C8H8O has degree of unsaturation 5, yet A does not decolourise bromine water or Baeyer's reagent, so none of it is an isolated alkene; it must be an aromatic ring (4 degrees) plus the carbonyl (1 degree).
Step 5. A monosubstituted aromatic methyl ketone of formula C8H8O is phenyl methyl ketone (acetophenone). Vigorous oxidation of a methyl ketone's aryl-attached skeleton cleaves at the carbonyl, converting the ring's attachment point to -COOH, giving benzoic acid, C7H6O2, matching B.
A is acetophenone, C6H5COCH3, and B is benzoic acid, C6H5COOH.
Board level - Naming reagents for functional group conversions (extending NCERT Example 8.1)
Give reagent(s) for: (i) hexan-1-ol to hexanal without over-oxidation; (ii) cyclohexanol to cyclohexanone; (iii) ethanenitrile to ethanal; (iv) toluene to benzaldehyde avoiding over-oxidation to the acid.
Step 1. A primary alcohol oxidised by strong KMnO4 or K2Cr2O7 goes straight to the acid, so a mild, selective oxidant is needed: pyridinium chlorochromate (PCC) in anhydrous CH2Cl2 stops cleanly at hexanal.
Step 2. A ketone cannot be over-oxidised without breaking a C-C bond, so an ordinary oxidant works: anhydrous CrO3 (or acidified K2Cr2O7/KMnO4) converts cyclohexanol to cyclohexanone.
Step 3. Full reduction with LiAlH4 would overshoot to the amine, so a reagent stopping at the imine is needed: DIBAL-H reduces ethanenitrile to the imine, hydrolysed on workup to ethanal.
Step 4. Direct strong oxidation of toluene overshoots to benzoic acid, so a two-step, carbon-protecting route is used: CrO2Cl2 oxidises the methyl group to a chromium complex (Etard reaction), hydrolysed with water to give benzaldehyde directly.
(i) PCC in CH2Cl2; (ii) anhydrous CrO3; (iii) DIBAL-H, then aqueous hydrolysis; (iv) CrO2Cl2 followed by hydrolysis (Etard reaction).
Competitive level (JEE / NEET) - pH of a weak carboxylic acid from its Ka
Calculate the pH of a 0.1 M aqueous solution of acetic acid, given .
Step 1. For , let be the degree of dissociation. Starting concentration ; at equilibrium , .
Step 2. For small , Ostwald's dilution law gives , so
Step 3. Since (about 1.34 percent) is much smaller than 1, the approximation holds (a common trap is skipping this check when exceeds about 5 percent, which then needs the full quadratic). So
Step 4. Taking the negative logarithm,
The pH of 0.1 M acetic acid is approximately 2.87, with only about 1.3 percent of the acid ionised — a direct numerical demonstration that acetic acid is a weak acid even though it is far more acidic than any alcohol. Timing note: recognise instantly that "weak acid plus given Ka plus find pH" always means Ostwald's dilution law, — do not set up a full quadratic unless exceeds roughly 5 percent; this recognition should keep the problem under 90 seconds.
Common traps
A carboxylic acid reacting with NaHCO3 to release CO2 is often confused with a phenol doing the same — phenols are not acidic enough to react with bicarbonate usefully, so this test cleanly separates the two even though both react with NaOH. Ketones can never give a positive Tollens' or Fehling's test, no matter how electron-poor; only the aldehydic C-H bond is oxidisable this way. The iodoform test is misapplied often — it needs a methyl ketone or a CH3-CH(OH)-R that oxidises to one in situ; ethanol and acetaldehyde pass, propan-1-ol and propanal do not. Aldol condensation is wrongly attempted on carbonyls with no alpha-hydrogen (formaldehyde, benzaldehyde, benzophenone), which instead undergo Cannizzaro (aldehydes) or no reaction at all (ketones like benzophenone). Numbering errors are common too: the carbonyl or carboxyl carbon is always carbon-1, never assigned by generic lowest-locant rules. Finally, do not assume electron-donating groups near -COOH always weaken acidity — a directly attached phenyl or vinyl group strengthens the acid through its own sp2-carbon inductive pull, so benzoic acid is stronger than acetic acid, not weaker.
You should be able to…
- Draw and name any aldehyde, ketone or carboxylic acid by IUPAC and common rules, numbering correctly from the functional carbon.
- Explain, via resonance between a neutral and a dipolar structure, why the carbonyl carbon is electrophilic and the oxygen nucleophilic.
- List named preparation methods for aldehydes or ketones (Rosenmund, Stephen, Etard, Gattermann-Koch, Friedel-Crafts acylation) and state what each converts.
- Predict the product of any nucleophilic addition (HCN, NaHSO3, an alcohol, an ammonia derivative) and name the product class.
- Rank aldehydes, ketones, alcohols and hydrocarbons of similar mass by boiling point, explaining it through hydrogen bonding and dipole forces.
- Choose the right test (Tollens', Fehling's, iodoform) to distinguish two carbonyl compounds and predict the result.
- Work through aldol condensation and Cannizzaro mechanisms, deciding which applies from the presence or absence of an alpha-hydrogen.
- Explain, using resonance and the inductive effect, why carboxylic acids beat phenols and alcohols in acidity, and how a substituent shifts pKa.
- Write the mechanism for acid-catalysed Fischer esterification and explain how to drive the equilibrium toward ester.
- Predict the product of decarboxylation, Hell-Volhard-Zelinsky halogenation, or Kolbe electrolysis from a given acid.
Ask the tutor: if you are unsure which named reaction applies to a given conversion, or want to practise predicting nucleophilic addition products and aldol versus Cannizzaro outcomes step by step, ask the tutor to walk through it with you.
Why do aldehydes reduce Tollens and Fehling reagents but ketones do not, even though both have a carbonyl group?
Both tests work by attacking a hydrogen sitting directly on the carbonyl carbon, converting the C-H bond into a C-OH bond as part of forming the carboxylate product. An aldehyde, R-CHO, has exactly that hydrogen. A ketone, R-CO-R', has two carbon substituents on the carbonyl carbon and no hydrogen there, so the mild oxidant has nothing to grab. Oxidising a ketone at all needs much harsher reagents that break a carbon-carbon bond instead, which Tollens' and Fehling's are far too mild to do. That is why this pair of tests reliably distinguishes an aldehyde from a ketone.
In a cross aldol condensation, how do I know which molecule is the nucleophile and which is the electrophile?
Under dilute base, an alpha-hydrogen is removed from one molecule to give an enolate; that carbon, now carrying negative charge, is the nucleophile. It attacks the carbonyl carbon of a second, un-deprotonated molecule, which stays the electrophile throughout. Any molecule with an alpha-hydrogen can in principle play either role, which is why a cross aldol between two such partners gives up to four products. To get one clean product, chemists deliberately pick a partner with no alpha-hydrogen, so it can only ever be the electrophile, and pair it with a partner that has alpha-hydrogens.
Why does a carboxylic acid react with sodium bicarbonate to release carbon dioxide, but phenol does not, even though both react with sodium hydroxide?
Both are acidic enough to be fully deprotonated by the strong base hydroxide, so both react with NaOH. Bicarbonate is a much weaker base, and only an acid strong enough to protonate it appreciably will react. Carboxylic acids, pKa typically 3 to 5, are acidic enough to protonate bicarbonate and drive off CO2 as gas. Phenol, pKa around 10, is not acidic enough to do this usefully, so no bubbling occurs. This difference is exactly why the NaHCO3 test confirms a -COOH group and distinguishes it from a phenolic -OH.
Why does the Cannizzaro reaction only work for aldehydes with no alpha-hydrogen?
Under concentrated alkali, hydroxide first adds to one aldehyde's carbonyl carbon, forming a tetrahedral intermediate carrying the original aldehydic hydrogen alongside a negatively charged oxygen. This electron-rich intermediate delivers that hydrogen as a hydride to a second aldehyde molecule: the donor is oxidised to a carboxylate, the acceptor reduced to an alkoxide, protonated on workup to the alcohol. If the aldehyde has an alpha-hydrogen, base instead removes that far more accessible hydrogen almost instantly, triggering aldol condensation long before the slower hydride-transfer pathway can compete, so Cannizzaro is only seen cleanly for formaldehyde, benzaldehyde, and other aldehydes with no alpha-hydrogen.
Why does Fischer esterification need an acid catalyst, and why is it reversible?
The carboxyl carbon is a poor electrophile to start with, because its own hydroxyl oxygen donates into the carbonyl by resonance and quenches some of the positive character a nucleophile would need to attack. Protonating the carbonyl oxygen removes that problem, making the carbon electrophilic enough for the alcohol to add efficiently. Every subsequent step is an ordinary acid-base or addition-elimination equilibrium, so the whole sequence runs equally well backwards — which is exactly how acid-catalysed ester hydrolysis works. Because it is a true equilibrium, esterification never completes on its own; you must shift it, usually by distilling off water as it forms or using excess reactant, to get a good yield.