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Aldehydes, Ketones and Carboxylic Acids

Nucleophilic addition, the Grignard reagent, reductions, the tests, aldol and Cannizzaro, and acid strength. Short reaction schemes with a product to name.

Questions in the bank
165
q/paper in 2025–26
1.12
Calculation
10%
Notes pages
8

Strand: Reactions

When you’ll see it

A C=O compound meets a reagent in a short scheme, or a list is ranked by carbonyl reactivity, α-H acidity or acid strength.

How this chapter is tested

Almost every question is a short reaction scheme: a starting compound, one or more reagents, and a product to name or pick. The work is knowing exactly how far each reagent goes, which carbon a nucleophile or a base attacks, and which group a test detects.

Reagent reach decides most marks. PCC stops at the aldehyde, the Jones reagent goes on to the acid; NaBH₄ reduces only aldehydes and ketones, LiAlH₄ almost everything; Clemmensen and Wolff–Kishner go all the way to CH₂. Carbon counts matter too: CO₂ on a Grignard adds one carbon, the haloform reaction removes one.

Rankings come often, of carbonyl reactivity, α-hydrogen acidity and acid strength, and each ranks by one effect at a time. The numeric answers are counts, such as how many compounds give the iodoform or Tollens' test, and small mole calculations on the same reactions.

The sub-skills

The distinct skills inside the chapter, in the order to learn them.

  • Preparing aldehydes and ketones

    Rosenmund (RCOCl, H₂, Pd/BaSO₄), Stephen (RCN, SnCl₂/HCl, then H₃O⁺), Etard (CrO₂Cl₂ on toluene), Gattermann–Koch (CO, HCl, AlCl₃); PCC and DIBAL-H stop at the aldehyde.

  • Nucleophilic addition and derivatives

    HCHO > RCHO > R₂CO > aryl ketones; HCN gives a racemic cyanohydrin; acetals are stable to base; H₂N–Z gives C=N–Z at pH about 4 to 5; semicarbazide bonds through its NH-side NH₂.

  • Grignard reagents

    HCHO → 1°, RCHO → 2°, ketone → 3° alcohol; an ester takes two equivalents; a nitrile gives a ketone after hydrolysis; CO₂ gives an acid one carbon longer; each acidic H uses one more equivalent.

  • Reductions

    Clemmensen (Zn–Hg, conc. HCl) and Wolff–Kishner (N₂H₄, KOH, glycol) give CH₂; NaBH₄ leaves esters and acids; LiAlH₄ reduces amides and nitriles to amines; DIBAL-H must be cold.

  • Oxidation and identification tests

    Tollens' for any aldehyde, Fehling's for aliphatic aldehydes only; 2,4-DNP for any aldehyde or ketone; iodoform for CH₃CO– or CH₃CH(OH)– joined to H or C.

  • Enols and self-aldol

    α-H between two C=O is most acidic; the α-carbon of one molecule joins the C=O carbon of the other; a 1,4-diketone closes a five-membered ring, a 1,5-diketone a six.

  • Crossed aldol and Cannizzaro

    Two enolisable aldehydes give four products, one non-enolisable partner gives two; Claisen–Schmidt gives chalcone; concentrated alkali on an aldehyde with no α-H gives acid salt + alcohol.

  • Carboxylic acids

    −I groups strengthen, closer and more of them strengthen more; HCOOH > CH₃COOH; derivative reactivity RCOCl > anhydride > ester > amide; HVZ halogenates the α-carbon; soda lime removes one carbon.

Traps to expect

Distractor shapes this chapter reuses. The Traps page covers the ones that cut across chapters.

  • Fehling's counted like Tollens'

    Aromatic aldehydes give a silver mirror but no red Cu₂O. An α-hydroxy ketone passes both tests although it is a ketone, so count the test, not the class name.

  • CH₃CO joined to oxygen

    Acetic acid, its esters and amides contain CH₃CO yet give no iodoform. Ethanol and ethanal are the only positives in their classes; methanol and methanal are negative.

  • The reduction that stops at the alcohol

    Clemmensen and Wolff–Kishner give CH₂, never CHOH. Choose between them by what else survives: acid dehydrates a 3° alcohol, hot base removes a chlorine.

  • Enolate and acceptor swapped

    In a crossed aldol the enolate gives the α-carbon and the acceptor gives the carbon that ends up double-bonded to it. A partner with no α-H, such as benzaldehyde or methanal, is never the enolate.

  • Dilute base for Cannizzaro

    Dilute base with an enolisable aldehyde gives an aldol. Cannizzaro needs concentrated alkali and no α-hydrogen, and the hydride moves from carbon, so in D₂O the CH₂ of the alcohol carries no D.

Learn it before you drill it

This chapter has full teaching notes — foundations, worked examples, self-checks and a mastery check for each page. Read the notes once, then drill page by page below.

Aldehydes, Ketones and Carboxylic Acids notes

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