Organic chemistry tricks that make aldehydes & ketones stick

Both tests distinguish aldehydes from ketones. Tollens' reagent (Ag(NH3)2⁺): aldehyde → silver mirror. Fehling's solution (Cu²⁺): aldehyde → brick red Cu2O precipitate. Ketones — no reaction with either.

NaBH4 (mild): reduces aldehydes and ketones → alcohols. LiAlH4 (strong): reduces aldehydes, ketones, carboxylic acids, esters, amides → alcohols/amines. Use NaBH4 when selectivity needed.

Requires alpha-hydrogen (H adjacent to C=O). Base removes alpha-H → enolate attacks carbonyl of second molecule → aldol product. Heating causes dehydration → alpha,beta-unsaturated carbonyl.

RMgX (Grignard) is a strong nucleophile/base. Attacks carbonyl carbon. Formaldehyde → primary alcohol. Aldehyde → secondary alcohol. Ketone → tertiary alcohol. Always do acidic workup (H3O⁺).

Acetal = product of carbonyl reacting with 2 equivalents of alcohol (acid catalyst). Reaction is reversible. Acetals are stable to base and nucleophiles — used to PROTECT carbonyls during synthesis.

Carbonyl carbon (C=O) is electrophilic. Nucleophile attacks C from above or below the pi bond → tetrahedral alkoxide intermediate. Protonation (workup) gives the product. Key: nucleophile always attacks carbon, not oxygen. Products depend on nucleophile: H⁻ → alcohol. RMgX → alcohol. CN⁻ → cyanohydrin. H₂O → hydrate (gem-diol). NH₂R → imine.

Primary amines (RNH₂) react with aldehydes/ketones → imines (Schiff bases, C=N). Reaction is reversible; remove water to drive forward. Acid catalysis (pH ~4–5) is optimal — enough acid to protonate intermediate, not too much to protonate amine. Secondary amines (R₂NH) + carbonyl with alpha-H → enamine (C=C-N). Imines and enamines are both useful in synthesis as nucleophilic equivalents.

HCN adds to aldehydes and ketones via CN⁻ nucleophile (use NaCN + HCN or KCN). Product: cyanohydrin R₂C(OH)CN. Useful in synthesis because: (1) adds one carbon, (2) CN group can be hydrolyzed to COOH or reduced to CH₂NH₂. Aldehydes react more readily than ketones. Thermodynamic product with formaldehyde/acetaldehyde; equilibrium disfavors it with hindered ketones.

Wittig converts a carbonyl to an alkene. Phosphorus ylide (Ph₃P=CHR, a stabilized carbanion) reacts with aldehyde or ketone → alkene product + triphenylphosphine oxide byproduct. Highly selective — no rearrangements, specific alkene formed. Non-stabilized ylides → Z (cis) alkene. Stabilized ylides → E (trans) alkene. Used in synthesis to introduce double bonds at specific positions.

Carbonyls with alpha-H exist in equilibrium with their enol form. Keto form is usually dominant (>99%). Enol has OH on the alpha carbon with C=C. Interconverted by acid (protonation of carbonyl O, then deprotonation of C=C) or base (removal of alpha-H → enolate, then protonation of O). Enols and enolates are nucleophilic at the alpha carbon — key to aldol, alkylation, and halogenation reactions.

Acid conditions: enol intermediate is halogenated at alpha carbon. Mono-selective (product is less enol-forming after halogenation). Base conditions (haloform reaction with methyl ketones): three halogenations occur → trihalomethyl group → cleaved by OH⁻ → carboxylate + CHX₃ (haloform). Iodoform test: CH₃COR + I₂/NaOH → yellow CHI₃ precipitate → confirms methyl ketone.

Aldehydes with NO alpha-H (formaldehyde, benzaldehyde, trimethylacetaldehyde) undergo Cannizzaro reaction with base: one molecule is oxidized (→ carboxylate) and one is reduced (→ alcohol). Hydride transfer from one molecule to another. Does NOT occur with ketones or aldehydes that have alpha-H (those undergo aldol instead). Crossed Cannizzaro: formaldehyde always gets oxidized, other aldehyde gets reduced.

Alpha,beta-unsaturated carbonyls (enones) have two electrophilic sites: carbonyl C (1,2-position) and beta-C (1,4-position). Hard nucleophiles (RMgX, RLi, NaBH₄) prefer 1,2-addition (kinetic, direct to carbonyl). Soft nucleophiles (organocuprates R₂CuLi, RSH, amines) prefer 1,4-conjugate addition (thermodynamic, to beta-C). Organocuprates = best reagents for conjugate addition.