Aldehydes, Ketones, Carboxylic Acids and their Derivatives

The carbonyl group is organic chemistry's busiest crossroads, and this chapter is worth about 4 questions. One polarized C=O explains why aldehydes and ketones accept nucleophiles, why only some carbonyl compounds reduce Tollens' reagent, why α-hydrogens open the aldol pathway, and why replacing –OH of a carboxylic acid by Cl, OCOR, OR or NH₂ creates a ladder of derivative reactivity. Keep asking: is the carbonyl carbon being attacked, is the α-carbon forming an enolate, or is a group leaving from an acyl carbon?

19.1 Aliphatic aldehydes and ketones: nomenclature, preparation and reactions; benzaldehyde

The carbonyl group and names

In C=O, carbon and oxygen are sp² hybridized. The σ framework is planar with angles near 120°, while the π bond sits above and below that plane. Oxygen's electronegativity makes the carbon electron-poor. A nucleophile therefore attacks carbon, the π electrons move to oxygen, and the resulting tetrahedral alkoxide is protonated — the standard two-step script of carbonyl addition.

An aldehyde has at least one H on the carbonyl carbon and is terminal in a chain: CH₃CH₂CHO is propanal; HCHO is methanal (formaldehyde). A ketone has two carbon groups on C=O: CH₃COCH₂CH₃ is butan-2-one. Number a ketone to give C=O the lowest locant; in an aldehyde the –CHO carbon is automatically C-1 and needs no locant. Lower aldehydes/ketones are water-soluble carbonyl acceptors, but they cannot donate O–H hydrogen bonds, so their boiling points lie between ethers and alcohols of comparable mass.

Preparation map

Target	Reliable preparations	Control point
Aldehyde from 1° alcohol	PCC or another controlled oxidant; Cu-catalysed dehydrogenation	Stop before RCO₂H; vigorous aqueous dichromate overshoots
Aldehyde from acyl chloride	Rosenmund: H₂, Pd/BaSO₄ poisoned with sulfur/quinoline	Poisoned catalyst must not reduce RCHO to RCH₂OH
Aldehyde from nitrile	Stephen: SnCl₂/HCl, then hydrolysis of the iminium salt	Formal one-carbon retention from RCN
Aldehyde/ketone from alkene	Reductive ozonolysis	Each alkene carbon becomes a carbonyl carbon
Ketone from 2° alcohol	K₂Cr₂O₇/H⁺, PCC, Cu dehydrogenation	No aldehydic H, so no easy further oxidation
Ketone from alkyne	HgSO₄/H₂SO₄ hydration	Terminal alkyne gives a methyl ketone; ethyne gives ethanal
Aryl ketone	Friedel–Crafts acylation: arene + RCOCl/AlCl₃	Installed acyl group deactivates the ring to a second acylation
Ketone from acyl chloride	R₂Cd or R₂CuLi	These softer organometallics stop where RMgX would add twice
Ketone from nitrile	RMgX, then acidic hydrolysis	The imine salt resists a second addition until workup
Simple symmetrical ketone	Dry-distil a calcium carboxylate: Ca(CH₃CO₂)₂ → CH₃COCH₃ + CaCO₃	Mixed salts give mixed ketone products

Addition reactions and carbonyl derivatives

Reduction with H₂/Ni, NaBH₄ or LiAlH₄ converts aldehydes to primary alcohols and ketones to secondary alcohols. Hydrogen cyanide (generated/used with great care because of toxicity) adds under base catalysis to a cyanohydrin, lengthening the carbon skeleton and opening routes to α-hydroxy acids. Concentrated aqueous NaHSO₃ gives crystalline α-hydroxy sulfonates with aldehydes and many methyl ketones; acid or alkali regenerates the carbonyl compound, so the adduct is both a test and a purification handle. Bulky diaryl or hindered ketones may fail for steric/equilibrium reasons.

Ammonia derivatives of the form H₂N–Z condense with C=O and lose water:

Reagent H₂N–Z	Product name
Hydroxylamine (Z = OH)	Oxime
Hydrazine (Z = NH₂)	Hydrazone
Phenylhydrazine (Z = NHC₆H₅)	Phenylhydrazone
2,4-Dinitrophenylhydrazine	2,4-Dinitrophenylhydrazone — yellow/orange/red crystals
Semicarbazide	Semicarbazone

The sharp melting points of these solids historically identified liquid carbonyl compounds. 2,4-DNP (Brady's reagent) therefore proves “a carbonyl”, not “an aldehyde”. Formaldehyde is anomalous with ammonia itself, condensing to cage-like hexamethylenetetramine (urotropine); with phenol under acid/base catalysis it polymerizes to Bakelite-type phenol–formaldehyde resin. Formalin is an approximately 37–40% aqueous formaldehyde solution, used as a disinfectant and biological preservative.

Oxidation tests that separate aldehydes from ketones

Tollens' reagent — ammoniacal silver nitrate, [Ag(NH₃)₂]⁺ — oxidizes an aldehyde to a carboxylate while Ag(I) is reduced to a silver mirror. Fehling's solution keeps Cu²⁺ in alkaline tartrate solution; many aliphatic aldehydes reduce it on warming to brick-red Cu₂O. Ketones lack an aldehydic C–H and would require C–C cleavage to oxidize, so they resist these mild reagents. α-Hydroxy ketones and reducing sugars can be exceptions in biochemical settings because base-catalysed isomerization creates oxidizable forms — do not transfer the school rule blindly to carbohydrates.

Formic acid also reduces Tollens/Fehling reagents because its structure, H–CO₂H, contains an aldehyde-like H on the carbonyl carbon. Schiff's reagent (decolourized magenta) regains a pink-violet colour with aldehydes. With PCl₅, carbonyl oxygen is replaced by two Cl atoms; with strong oxidants, ketones cleave to carboxylic acids of fewer carbons.

Reactions at the α-carbon: aldol, Cannizzaro and haloform

Hydrogens on the carbon adjacent to C=O are weakly acidic because removal gives a resonance- stabilized enolate. In dilute NaOH/KOH, the enolate of one molecule attacks the carbonyl carbon of another: aldol addition. Ethanal gives 3-hydroxybutanal (CH₃–CH(OH)–CH₂–CHO); heating dehydrates it to but-2-enal (crotonaldehyde), an α,β-unsaturated aldehyde stabilized by conjugation. Ketones react more sluggishly, and crossed aldols are cleanest when one partner has no α-H and acts only as the electrophile.

If an aldehyde has no α-hydrogen, concentrated KOH cannot divert it into enolate chemistry, so one molecule transfers hydride to another: Cannizzaro reaction. Formaldehyde gives methanol + potassium formate; benzaldehyde gives benzyl alcohol + potassium benzoate. It is a disproportionation — one molecule oxidized, one reduced.

A methyl group directly attached to C=O is exhaustively halogenated by I₂/NaOH, then cleaved: CH₃COCH₃ + 3I₂ + 4NaOH → CHI₃↓ + CH₃CO₂Na + 3NaI + 3H₂O. Yellow iodoform (m.p. about 119 °C, antiseptic smell) signals a CH₃CO– group or a CH₃CH(OH)– group that the reagent oxidizes to one; ethanal is the only aldehyde that responds.

Finally, deoxygenation C=O → CH₂ is achieved under either acidic Clemmensen conditions (Zn–Hg/conc. HCl) or basic Wolff–Kishner conditions (first form the hydrazone, then KOH in a high-boiling solvent with heat). The products match; compatibility with the rest of the molecule makes the choice.

Benzaldehyde, C₆H₅CHO

Benzaldehyde is the bitter-almond oil aldehyde. It is prepared by Etard oxidation of toluene with chromyl chloride (CrO₂Cl₂) followed by hydrolysis; by side-chain chlorination of toluene to benzal chloride and hydrolysis; by Rosenmund reduction of benzoyl chloride; and by Gattermann–Koch formylation of benzene with CO/HCl over AlCl₃/CuCl.

Conjugation with the ring makes benzaldehyde less reactive toward nucleophiles than ethanal. It restores Schiff's colour and reduces Tollens' reagent but does not reduce Fehling's solution. Exposed to air it autoxidizes to benzoic acid. It has no α-H on the side chain, so concentrated KOH gives Cannizzaro products. With ethanolic KCN, two molecules undergo benzoin condensation: cyanide temporarily converts one aldehyde carbon into a nucleophilic equivalent, yielding benzoin, C₆H₅–CH(OH)–CO–C₆H₅. With acetic anhydride/sodium acetate it gives cinnamic acid (Perkin reaction); with acetone/dilute alkali it gives crossed-aldol (Claisen–Schmidt) enones. Ring substitution is slowed and meta-directed by –CHO.

19.2 Monocarboxylic acids: preparation and properties

A carboxylic acid contains –CO₂H: a carbonyl and a hydroxyl on the same carbon. That carbon is C-1 in the suffix name (ethanoic acid), while common names persist for formic (methanoic), acetic (ethanoic), propionic and benzoic acids. Carboxylic acids dimerize through two hydrogen bonds in the vapour and in nonpolar solvents, raising boiling points; solubility in water is high for the first few members and then falls as the hydrocarbon part grows.

Preparation

• Oxidation: a primary alcohol or aldehyde with hot acidified K₂Cr₂O₇/KMnO₄ gives the acid.
• Nitrile hydrolysis: RX →(KCN) RCN →(H₃O⁺ or OH⁻, heat) RCO₂H; the product has one carbon more than RX, a favourite chain-extension sequence.
• Grignard carboxylation: RMgX + CO₂, then acid workup (also one carbon more).
• Derivative hydrolysis: esters, amides, anhydrides and acid chlorides all return the acid.
• Haloform reaction: a methyl ketone gives the carboxylate with one carbon fewer.
• Alkylbenzene oxidation: hot alkaline KMnO₄ oxidizes any side chain bearing a benzylic H all the way to benzoic acid; ethylbenzene and propylbenzene converge on the same product. tert-Butylbenzene, with no benzylic H, resists.

Acidity and reactions

Loss of H⁺ gives a carboxylate ion in which the negative charge is shared equally by two oxygens; both C–O bonds become equivalent. Electron-withdrawing substituents stabilize that anion and lower pKₐ (Ch. 16's chloroacetic acids), whereas electron-donating alkyl groups raise pKₐ modestly. Formic acid (pKₐ 3.75) is therefore stronger than acetic acid (4.76). Benzoic acid (pKₐ 4.20) is strengthened by the sp² ring carbon; ring substituents then tune it by the usual –I/+R logic, with ortho effects able to override simple predictions.

Sodium bicarbonate gives brisk CO₂ effervescence — the practical acid test that phenol fails. Metals and bases give salts. With an alcohol and acid catalyst, Fischer esterification reaches equilibrium; removing water or using excess alcohol drags it forward. SOCl₂ or PCl₅ converts –OH into the far more reactive acid chloride (§19.3). LiAlH₄ (or borane) reduces the acid to a primary alcohol; NaBH₄ is generally too mild. Soda lime decarboxylates the sodium salt to an alkane with one fewer carbon, and Kolbe electrolysis of a concentrated carboxylate couples two alkyl fragments with loss of CO₂.

The Hell–Volhard–Zelinsky (HVZ) reaction brominates the α-carbon with Br₂/red phosphorus. A trace of acyl bromide forms first and enolizes far more readily than the acid, so substitution lands specifically at α-C; an α-H is mandatory. α-Bromo acids are gateways to α-hydroxy and α-amino acids.

Methanoic (formic) acid is the family rebel. Because H–CO₂H still carries a formyl hydrogen, it reduces Tollens' and Fehling's reagents, turns HgCl₂ first white (Hg₂Cl₂) then grey (Hg), and decolourizes acidified KMnO₄, itself becoming CO₂. Concentrated H₂SO₄ dehydrates it neatly to CO + H₂O — the standard laboratory preparation of carbon monoxide in Ch. 13. Benzoic acid's ring, deactivated and meta-directed by –CO₂H, undergoes nitration and related substitutions mainly at the meta position.

19.3 Acid halides, anhydrides, amides and esters: preparation, properties and relative reactivity

Replace the –OH of RCO₂H and a family appears: RCOCl (acid/acyl chloride), (RCO)₂O (anhydride), RCO₂R′ (ester) and RCONH₂ (amide; N-substituted amides replace one or both N-hydrogens). A nucleophile still attacks the acyl carbon, but now the tetrahedral intermediate can expel a leaving group and restore C=O: nucleophilic acyl substitution. The observed order,

acid chloride > anhydride > ester > amide,

follows two reinforcing effects. The substituent donates its lone pair into C=O most strongly for nitrogen and least effectively for chlorine (poor 3p–2p overlap), so an amide carbonyl is least electrophilic. The leaving groups also become more basic across the series (Cl⁻, RCO₂⁻, RO⁻, NH₂⁻), and stronger bases leave more reluctantly. The pKₐ of each leaving group's conjugate acid compresses the point into one scale.

Preparation from the acid (and from one another)

• Acid chloride: RCO₂H + SOCl₂ (cleanest; SO₂/HCl gases escape), PCl₅ or PCl₃.
• Anhydride: heat the acid with P₂O₅, or react an acid chloride with a sodium carboxylate; suitable dicarboxylic acids cyclize on heating (Ch. 21's phthalic anhydride).
• Ester: Fischer esterification, or alcohol + acid chloride/anhydride when speed and yield matter. Diazomethane methylates acids to methyl esters almost quantitatively.
• Amide: acid chloride/anhydride/ester + NH₃ or an amine; heating an ammonium carboxylate; or carefully stopping nitrile hydrolysis at the amide stage.

Movement down the reactivity ladder is easy (an acid chloride will make the other three); movement up requires hydrolysis to the acid and reconversion, because an amide will not directly become an acid chloride under examination conditions.

Properties and signature reactions

Hydrolysis. Acid chlorides fume and react violently with water to the acid + HCl. Anhydrides hydrolyse on warming to two acid molecules. Acid-catalysed ester hydrolysis is the reverse of Fischer esterification, whereas NaOH hydrolysis (saponification) is driven to completion because the acid product is immediately trapped as carboxylate — the chemistry by which fats become soap plus glycerol. Amides need prolonged strong acid/alkali and heat; enzymes called proteases perform the same cleavage gently on peptide bonds.

Acid chlorides are acylation couriers: alcohols/phenols give esters, ammonia/amines give amides (Schotten–Baumann conditions in aqueous NaOH for benzoylation), arenes give ketones by Friedel–Crafts, Rosenmund reduction gives aldehydes, organocadmium/Gilman reagents stop at ketones, and LiAlH₄ reduces to the primary alcohol.

Anhydrides acylate similarly but waste one equivalent as carboxylic acid. Acetic anhydride converts salicylic acid to aspirin — perhaps the most medically important reaction in this section for a CEE candidate. Cyclic anhydrides open with alcohols/amines to half-esters and amic acids.

Esters are volatile, often fruity-smelling liquids. Besides hydrolysis, they undergo transesterification (exchange of the –OR group with another alcohol), reduction by LiAlH₄ to two alcohols (the acyl side becomes a primary alcohol), reaction with two equivalents of a Grignard reagent to a tertiary alcohol after workup, and Claisen condensation: an ester with α-H plus a strong base such as sodium ethoxide gives a β-ketoester (ethyl acetate → ethyl acetoacetate). The base must match the ester's alkoxy group to avoid transesterification scrambling — a detail examiners love.

Amides are high-melting, strongly hydrogen-bonded solids (except liquid formamide/DMF-type solvents); nitrogen's lone pair is busy conjugating with C=O, so amides are only feebly basic and the C–N bond has partial double-bond character (the planar peptide bond). They hydrolyse under forcing conditions, dehydrate with P₂O₅ to nitriles, and reduce with LiAlH₄ to amines with the carbonyl carbon retained as CH₂. A primary amide with Br₂/NaOH undergoes the Hofmann bromamide reaction: N-bromination, rearrangement to an isocyanate with loss of the carbonyl carbon's bond to nitrogen, then hydrolysis to a primary amine with one carbon fewer plus carbonate. Retention at the migrating group is a mechanistic jewel; the carbon count is the examination takeaway.

Formula and fact sheet

Topic	Must-know
Carbonyl tests	2,4-DNP: aldehyde + ketone. Tollens: aldehydes (mirror). Fehling: aliphatic aldehydes (red Cu₂O); benzaldehyde negative
Name reductions	Rosenmund RCOCl → RCHO; Stephen RCN → RCHO; Clemmensen (acid) and Wolff–Kishner (base) C=O → CH₂
α-Carbon fork	α-H present → aldol/haloform (if CH₃CO–). No α-H in RCHO + conc. OH⁻ → Cannizzaro
Benzaldehyde	Etard/Gattermann–Koch; benzoin with KCN; Perkin cinnamic acid; oxidizes in air
Acid making	Oxidize 1° ROH/RCHO; hydrolyse RCN; RMgX + CO₂; alkylbenzene side-chain oxidation → benzoic acid
Acid doing	NaHCO₃ fizz; Fischer esterification; SOCl₂ gateway; LiAlH₄ → 1° ROH; soda-lime decarboxylation; HVZ α-Br
Formic acid	Reducing acid: Tollens/Fehling/KMnO₄ positive; conc. H₂SO₄ → CO + H₂O
Derivative ladder	RCOCl > (RCO)₂O > RCO₂R′ > RCONH₂; leaving-group basicity and C=O donation both increase rightward
Name reactions	Saponification; Claisen β-ketoester; Hofmann bromamide amine with −1 C; aspirin from acetic anhydride

Rapid-revision tables and high-yield facts

Item	Rule or value
Aldehyde / ketone suffix	–al / –one
Rosenmund	RCOCl + H₂, Pd/BaSO₄ → RCHO
Étard	toluene + CrO₂Cl₂ → PhCHO
Gattermann–Koch	C₆H₆ + CO + HCl/AlCl₃ → PhCHO
Clemmensen / Wolff–Kishner	Zn–Hg/HCl / NH₂NH₂ then KOH
Acid strength	ClCH₂COOH > HCOOH > CH₃COOH
NaHCO₃ test	CO₂ fizz: acid, not phenol
Derivative reactivity	RCOCl > (RCO)₂O > RCOOR' > RCONH₂
CHI₃ molar mass	394 g/mol
Copper dehydrogenation	573 K

Chapter summary

• Aldehydes and ketones are distinguished by oxidation behaviour, not by 2,4-DNP: aldehydes reduce Tollens' reagent; ordinary ketones do not.
• Controlled stopping power is the theme of preparation — PCC stops at aldehyde, poisoned Pd stops Rosenmund at aldehyde, and soft organometallics stop at ketone.
• Enolate chemistry at the α-carbon gives aldol products and the haloform reaction; blocking α-H switches an aldehyde to Cannizzaro disproportionation.
• Benzaldehyde combines an aldehyde's Tollens response with aromatic specialities: no Fehling reaction, Cannizzaro, benzoin and Perkin condensations.
• Carboxylic acids owe their acidity and high boiling points to carboxylate resonance and hydrogen-bonded dimers; substituent effects and HVZ chemistry operate through that same functional group.
• Formic acid breaks the family pattern by reducing mild oxidants and dehydrating to CO.
• Acyl derivatives react by addition–elimination in the order chloride > anhydride > ester > amide; easy conversions run down that ladder.
• Saponification, Claisen condensation and Hofmann bromamide degradation are the derivative reactions most likely to be recognized by product and carbon count.

Practice questions

Set A — Recall (25 questions)

1. The carbonyl carbon in aldehydes and ketones isasp³ and tetrahedralbpositively charged fullycsp and lineardsp² and planar
2. The IUPAC suffix for an aldehyde isa-alb-onec-old-oic acid
3. In naming a ketone, the chain is numbered to give the carbonyl carbon theahighest locantbmiddle positionclowest possible locantdlocant 1 always
4. Rosenmund reduction converts an acyl chloride into anaalkanebaldehydecketonedester
5. Stephen reduction prepares an aldehyde from aanitrilebketonecalkene onlydphenol
6. Oxidation of a secondary alcohol ordinarily gives aaaldehydebalkene onlyccarboxylic aciddketone
7. A yellow/orange precipitate with 2,4-dinitrophenylhydrazine indicates an aldehyde or aaalcoholbketonecetherdalkane
8. Tollens' reagent is reduced by aldehydes to metallicacopperbmercury onlycsilverdiron
9. The red precipitate in a positive Fehling test isaCuObCu₂OcCu(OH)₂dCu metal only
10. Benzaldehyde gives a negative test withaFehling solutionbTollens reagentc2,4-DNPdSchiff reagent
11. Addition of HCN to a carbonyl compound gives aahydrazonebacetal onlycoximedcyanohydrin
12. Crystalline sodium-bisulfite addition compounds are formed readily by aldehydes and most methylaalcoholsbetherscketonesdamides
13. Aldol condensation requires a carbonyl compound having at least oneaα-hydrogenbringchalogend–OH group
14. Cannizzaro reaction is shown by aldehydes havingatwo α-hydrogensba triple bondcno α-hydrogenda carboxyl group
15. Clemmensen reduction uses zinc amalgam with concentratedaNaOHbHClcHNO₃ solution onlydammonia
16. Wolff–Kishner reduction is carried out with hydrazine and strongaacidbbrominecozonedbase (heat)
17. The yellow solid in the iodoform test isaCHBr₃bCHCl₃cCHI₃dCI₄
18. Benzoin condensation of benzaldehyde is catalysed by ethanolicaHClbKCNcNaHCO₃dFeCl₃
19. Formalin is an aqueous solution of about 37–40%aformaldehydebmethanolcformic aciddphenol
20. A carboxylic acid liberates CO₂ briskly fromaNaClbNaNO₂ onlycNa₂SO₄dNaHCO₃
21. The Hell–Volhard–Zelinsky reaction uses Br₂ with redaleadbphosphorusccopperdsulfur only
22. Concentrated H₂SO₄ dehydrates formic acid mainly toaCO₂ and H₂bformaldehydecCH₄ and O₂dCO and H₂O
23. The most reactive carboxylic-acid derivative toward nucleophiles is generally theaamidebestercacid chloridedcarboxylate salt
24. The least reactive among acid chloride, anhydride, ester and amide is theaamidebacid chloridecanhydridedester
25. Hofmann bromamide reaction converts a primary amide into a primary amine withaone carbon morebthe same carbonscone carbon fewerdno nitrogen

Answers and explanations — Set A

1. (d) — planar sp² geometry leaves the π bond open to nucleophilic attack from either face.

2. (a) — methanal, ethanal, propanal; the aldehyde carbon is C-1.

3. (c) — principal-group low locant outranks substituent preferences.

4. (b) — the poisoned catalyst stops hydrogenation at RCHO.

5. (a) — SnCl₂/HCl reduces RCN to an iminium salt hydrolysed to RCHO.

6. (d) — oxidation removes the carbinol H and the O–H proton equivalent.

7. (b) — both families condense with the hydrazine reagent.

8. (c) — a silver mirror (or grey Ag precipitate if the glass is not clean).

9. (b) — Cu(I) oxide; do not confuse the Cu₂O of cuprite/Ch. 14 contexts.

10. (a) — aromatic aldehydes are the textbook Fehling exception while still reducing Tollens'.

11. (d) — the product bears –OH and –CN on the former carbonyl carbon.

12. (c) — sterically accessible methyl ketones; many hindered ketones fail.

13. (a) — enolate formation is impossible without an α-H.

14. (c) — HCHO and C₆H₅CHO are the standard examples in concentrated alkali.

15. (b) — strongly acidic deoxygenation; protect acid-sensitive groups by choosing Wolff–Kishner instead.

16. (d) — the hydrazone is decomposed by KOH at high temperature with N₂ loss.

17. (c) — triiodomethane, yellow crystals of m.p. about 119 °C.

18. (b) — cyanide is both nucleophile and leaving group in the umpolung cycle.

19. (a) — stabilized with some methanol; it is a preservative/disinfectant, not “formic acid solution”.

20. (d) — RCO₂H is stronger than carbonic acid; phenol cannot do this.

21. (b) — PBr₃ generated in situ creates the enolizable acyl bromide form.

22. (d) — the laboratory source of CO from Ch. 13.

23. (c) — Cl⁻ is the weakest base/best leaving group in the series and donates least into C=O.

24. (a) — amide resonance is strongest and –NH₂ would be a very poor leaving group.

25. (c) — the carbonyl carbon is expelled as carbonate through an isocyanate.

Set B — Understanding (15 questions)

1. Aldehydes generally undergo nucleophilic addition faster than ketones because ketones are more sterically hindered and have electron-donating alkyl groups onaoxygen onlybnitrogencboth sides of carbonyl carbonda ring always
2. Ordinary ketones do not reduce Tollens' reagent because their oxidation would require breaking aaC–H bond on oxygenbπ bond only reversiblychydrogen bonddC–C bond
3. Many bulky ketones fail to give a bisulfite adduct mainly because addition is reversible and the tetrahedral adduct isasterically crowdedbaromaticcionic in carbonddehydrated
4. Heating an aldol addition product often dehydrates it because the resulting C=C is conjugated withaan alkyl halidebthe carbonyl groupcwaterda benzene solvent
5. Formaldehyde undergoes Cannizzaro rather than aldol in concentrated KOH because it cannot form anaalkoxidebhydrogen bondchydratedenolate (no α-carbon/H)
6. Benzaldehyde is less reactive than ethanal toward many nucleophiles mainly because the ring conjugates with and stabilizes thealeaving groupbcarbonyl π systemcsolventdproton only
7. In benzoin condensation, cyanide's key role is to convert an aldehyde carbon temporarily into aacarbocationbfree radical onlycnucleophilic (umpolung) centredcarboxylate
8. Carboxylate ion has two equal C–O bond lengths because the negative charge and π bond areadelocalized over both oxygensblocalized on carboncremoved by resonance forms reactingdshared with hydrogen
9. Chloroethanoic acid is stronger than ethanoic acid because chlorine stabilizes the carboxylate by aa+I effectb−I effectcsteric effect onlydcolour effect
10. Formic acid is stronger than acetic acid mainly because acetic acid's methyl group is electronadonating (+I), destabilizing acetate slightlybwithdrawing by resonancecneutral in charge alwaysdbonded to oxygen
11. Alkaline ester hydrolysis goes essentially to completion because the carboxylic acid first formed is converted into resonance-stabilizedaacylium ionbcarbon dioxidecenolate onlydcarboxylate
12. Amides are much less reactive than esters toward hydrolysis mainly because nitrogen donates strongly into C=O and an amino group is a pooranucleophilebsolventcleaving groupdacid
13. Acetyl chloride reacts violently with water because its carbonyl is highly electrophilic and chloride is aagood leaving groupbstrong basecreducing agentdbulky group
14. An anhydride is more reactive than an ester mainly because a carboxylate leaving group is a weaker base than anaalkyl groupbprotonchydridedalkoxide
15. In Hofmann bromamide degradation, the product amine has one carbon fewer because the carbonyl carbon is lost (as carbonate) through an intermediateacarbene onlybisocyanatecnitrile oxideddiazonium salt

Answers and explanations — Set B

1. (c) — two +I groups make ketone carbon less δ+ and block the approach trajectory.

2. (d) — there is no aldehydic C–H to convert into C–OH; the carbon skeleton itself must be cleaved.

3. (a) — the equilibrium stops favouring a congested sp³ adduct.

4. (b) — an α,β-unsaturated carbonyl gains conjugation energy on losing water.

5. (d) — methanal has no α-carbon at all, so hydride-transfer disproportionation takes over.

6. (b) — ground-state conjugation lowers carbonyl electrophilicity relative to a small aliphatic aldehyde.

7. (c) — after CN addition and proton transfer, the former electrophilic carbon attacks a second benzaldehyde.

8. (a) — two equivalent resonance contributors produce bond order 1.5 for each C–O.

9. (b) — the same substituent argument quantified by pKₐ in Ch. 16.

10. (a) — HCO₂⁻ receives no comparable alkyl donation.

11. (d) — acid–base trapping removes a product and defeats the Fischer equilibrium.

12. (c) — amide resonance also flattens nitrogen and lowers carbonyl electrophilicity.

13. (a) — HCl's low pKₐ is the quick numerical proxy (see the pKₐ figure).

14. (d) — compare conjugate-acid pKₐ values near 5 (carboxylic acid) and 16 (alcohol).

15. (b) — R–N=C=O hydrolyses to RNH₂ + CO₂/carbonate under the alkaline conditions.

Set C — Application (10 questions)

1. A C₃H₆O compound gives a silver mirror but no iodoform precipitate. It isapropanonebpropanalcpropan-1-oldmethoxyethane
2. A C₄H₈O compound gives a 2,4-DNP precipitate, no silver mirror, and a positive iodoform test. It isabutanalbbutan-1-olcbutan-2-onedethoxyethane
3. Ethanal with dilute NaOH, followed by heating the aldol product, gives mainlyabut-2-enalbbutan-1-olcethanoic aciddbutane
4. Benzaldehyde heated with concentrated KOH gives benzyl alcohol together with potassiumaformatebcarbonate onlycphenoxidedbenzoate
5. Propanone treated with Zn–Hg and concentrated HCl givesapropan-2-olbpropenecpropanedpropanal
6. Toluene subjected to Etard oxidation (CrO₂Cl₂, then hydrolysis) givesabenzaldehydebbenzoic acidcbenzyl alcoholdphenol
7. Benzaldehyde with ethanolic KCN givesabenzoic acidbhydrobenzamide onlyccinnamic aciddbenzoin
8. Ethylbenzene boiled with alkaline KMnO₄ and then acidified givesaphenylacetic acidbbenzoic acidcacetophenonedphenol
9. Ethyl ethanoate boiled with aqueous NaOH gives ethanol and sodiumaethanoatebformatecpropanoatedcarbonate
10. Ethanamide treated with Br₂/NaOH givesaethylaminebmethylaminecethanenitrileddimethylamine

Answers and explanations — Set C

1. (b) — the same evidence pattern as Worked example 19.1: Tollens demands an aldehyde; iodoform excludes the methyl-ketone rival.

2. (c) — a ketone by Tollens, and specifically a CH₃CO– ketone by iodoform; the C₄ skeleton leaves butan-2-one.

3. (a) — aldol addition gives 3-hydroxybutanal and dehydration conjugates the new double bond with –CHO.

4. (d) — Cannizzaro: one C₆H₅CHO is reduced to the alcohol, the other oxidized to benzoate.

5. (c) — Clemmensen replaces C=O by CH₂ without changing the three-carbon skeleton.

6. (a) — Etard stops side-chain oxidation at the aldehyde stage, unlike hot KMnO₄.

7. (d) — C₆H₅CH(OH)COC₆H₅, the α-hydroxy ketone from two benzaldehyde molecules.

8. (b) — a benzylic side chain with benzylic H is degraded to –CO₂H regardless of its length.

9. (a) — saponification: CH₃CO₂C₂H₅ + NaOH → CH₃CO₂Na + C₂H₅OH.

10. (b) — Hofmann degradation removes the amide carbonyl carbon: CH₃CONH₂ → CH₃NH₂.