Aldehydes and Ketones: Preparation and Nucleophilic Addition

Vanillin gives vanilla its scent, cinnamaldehyde gives cinnamon its own, and salicylaldehyde comes from meadowsweet, three genuinely pleasant fragrances that all trace back to the same functional group: a carbon-oxygen double bond, the carbonyl group. Formaldehyde, in its aqueous formalin solution, preserves biological specimens and builds bakelite, one of the very first synthetic plastics; acetone serves as an everyday industrial solvent; acetaldehyde starts the industrial route to acetic acid itself. What separates an aldehyde from a ketone is simply what else sits on that carbonyl carbon: an aldehyde carries at least one hydrogen alongside the double-bonded oxygen, while a ketone carries two carbon groups instead, never a hydrogen. This chapter follows the carbonyl group through how it is made and, above all, through the one reaction type that defines its entire chemistry: nucleophilic addition, the exact mechanistic mirror of what alkenes do with electrophiles.

Common names for aldehydes borrow from their corresponding acid's own name, swapping -ic acid for -aldehyde, with Greek letters marking substituent position starting from the carbon next to the carbonyl; common names for ketones simply name the two attached groups and add the word ketone, dimethyl ketone carrying the older, still-common name acetone. IUPAC names are more systematic: an aldehyde's parent alkane loses its final -e for -al, a ketone's for -one, and each numbering starts from whichever end reaches the functional group first, the carbonyl carbon itself becoming carbon one for a ketone on a ring. The carbonyl carbon itself is sp2 hybridised, forming three sigma bonds that all lie in one plane at roughly 120 degrees to each other, with the fourth electron left in an unhybridised p-orbital forming a pi bond with oxygen; oxygen's own greater electronegativity polarises that pi bond so strongly that the real molecule is better described as a resonance hybrid of a neutral structure and a dipolar one, leaving the carbonyl carbon genuinely electrophilic and the carbonyl oxygen genuinely nucleophilic, a polarity sharper than an ether's own and the entire reason for everything that follows in this chapter.

Oxidation or dehydrogenation of an alcohol supplies the most direct route, exactly the same chemistry already established for alcohols themselves: a primary alcohol gives an aldehyde, a secondary alcohol gives a ketone, and passing alcohol vapour over a hot silver or copper catalyst achieves the same outcome industrially, at scale. Two further routes start from hydrocarbons directly: ozonolysis of an alkene, followed by treatment with zinc dust and water, splits the double bond cleanly into two carbonyl fragments, aldehydes, ketones or a mixture, depending entirely on the alkene's own substitution pattern; and hydration of an alkyne, under mercuric sulphate and sulphuric acid catalysis, adds water according to Markovnikov's rule to give a ketone in every case except one, ethyne itself, whose hydration gives acetaldehyde specifically, since the triple bond there has nothing but hydrogens on either side to begin with.

Acyl chloride, hydrogenated over palladium supported on barium sulphate, stops cleanly at the aldehyde rather than over-reducing to an alcohol, the Rosenmund reduction, the poisoned catalyst deliberately weakened just enough to halt at the right stage. Nitriles offer two related routes to the same destination: reduced with stannous chloride and hydrochloric acid, or with diisobutylaluminium hydride, DIBAL-H, a nitrile forms an imine intermediate first, and only hydrolysis of that imine releases the aldehyde, the tin route called the Stephen reaction. Aromatic aldehydes need their own dedicated methods, since oxidising toluene directly tends to run straight past the aldehyde to benzoic acid: chromyl chloride converts a methyl group to an intermediate chromium complex that hydrolyses cleanly to benzaldehyde, the Etard reaction, while chromic oxide in acetic anhydride instead gives benzylidene diacetate, itself hydrolysable to the same aldehyde; industrially, side-chain chlorination of toluene to benzal chloride followed by hydrolysis is the more common commercial route; and the Gatterman-Koch reaction builds a fresh -CHO group directly onto a benzene ring, treating it with carbon monoxide and hydrogen chloride under an anhydrous aluminium or cuprous chloride catalyst.

Ketones have their own dedicated routes too: dialkylcadmium, itself made from cadmium chloride and a Grignard reagent, reacts with an acyl chloride to give a ketone cleanly; a nitrile treated with a Grignard reagent and then hydrolysed gives a ketone as well; and treating benzene or a substituted benzene with an acyl chloride under anhydrous aluminium chloride, the Friedel-Crafts acylation already met in the aromatic hydrocarbons chapter, remains the standard route to aromatic ketones. Physically, methanal is a gas and ethanal a volatile liquid at room temperature, but every heavier aldehyde and ketone is liquid or solid; their boiling points sit above a hydrocarbon or ether of comparable mass, thanks to the dipole-dipole attraction a polar carbonyl group provides, yet below an alcohol's own boiling point, since a carbonyl group, unlike a hydroxyl, has no hydrogen to donate for hydrogen bonding, only oxygen to accept one. The smaller aldehydes and ketones, methanal, ethanal and propanone among them, mix with water in every proportion by accepting hydrogen bonds from water molecules, a solubility that fades quickly as the alkyl chain grows; and smell shifts right alongside size, the lightest aldehydes carrying a sharp, pungent odour that mellows into genuine fragrance as the molecule grows larger.

A nucleophile attacks the carbonyl carbon from a direction roughly perpendicular to the plane of its sp2 orbitals, and as the new bond forms, that carbon's own hybridisation shifts from sp2 to sp3, producing a tetrahedral alkoxide intermediate that then captures a proton from the surrounding medium to give a neutral product, the net result being addition of a nucleophile and a hydrogen across the carbon-oxygen double bond. Aldehydes react faster than ketones at essentially every one of these additions, for two compounding reasons: sterically, a ketone's two bulky substituents crowd the approach to its carbonyl carbon more than an aldehyde's single substituent does, and electronically, two electron-releasing alkyl groups reduce a ketone's own carbonyl-carbon electrophilicity more effectively than one does in an aldehyde. Aromatic aldehydes trail behind aliphatic ones for a related but distinct reason: the ring's own electron density delocalises into the carbonyl group by resonance, reducing the carbon's electrophilicity directly, which is exactly why benzaldehyde reacts more slowly than propanal in nucleophilic addition despite both being aldehydes.

Hydrogen cyanide, catalysed by base since pure HCN reacts too slowly on its own, adds across a carbonyl to give a cyanohydrin, a genuinely useful synthetic intermediate; sodium hydrogensulphite adds similarly, its equilibrium favouring the product for most aldehydes but tilting back toward starting material for most ketones due to steric crowding, which is precisely why the reaction, easily reversed with dilute acid or alkali, is useful for purifying aldehydes specifically. Alcohols add too, but in two distinct stages: one equivalent, under dry hydrogen chloride, gives a hemiacetal, and a second equivalent converts that hemiacetal into a full acetal, gem-dialkoxy in structure, dry HCl doing its job by protonating the carbonyl oxygen and boosting the carbon's own electrophilicity; ketones reacting with ethylene glycol under the same conditions give a cyclic ketal instead. Ammonia and its many derivatives round out the toolkit, each adding to the carbonyl and then losing water to leave a carbon-nitrogen double bond behind: ammonia itself gives an imine, an amine gives a substituted imine known as a Schiff's base, hydroxylamine gives an oxime, hydrazine gives a hydrazone, and 2,4-dinitrophenylhydrazine gives a 2,4-DNP derivative, a yellow, orange or red solid so reliably formed and so visually distinctive that it became the standard laboratory test for confirming a carbonyl group is present at all.

Reduction can stop at the alcohol stage, using sodium borohydride, lithium aluminium hydride or catalytic hydrogenation exactly as already established for carbonyl-to-alcohol chemistry, or it can go further still, all the way to a plain methylene group: zinc amalgam with concentrated hydrochloric acid achieves this directly, the Clemmensen reduction, while hydrazine followed by heating with strong alkali in a high-boiling solvent achieves the same deoxygenation by a different route, the Wolff-Kishner reduction. Oxidation, by contrast, is where aldehydes and ketones genuinely part ways: an aldehyde oxidises easily, even to strong agents like nitric acid or potassium permanganate, and even to two deliberately mild reagents used precisely because they leave everything else in the molecule untouched: Tollens' reagent, ammoniacal silver nitrate, deposits a bright silver mirror on the container wall as it reduces, and Fehling's reagent, alkaline copper sulphate with Rochelle salt, deposits a reddish-brown precipitate instead, though aromatic aldehydes do not respond to this particular test. Ketones resist both mild reagents entirely, oxidising only under genuinely vigorous conditions that cleave carbon-carbon bonds outright and scatter the molecule into smaller acids, which is exactly why Tollens' and Fehling's tests distinguish an aldehyde from a ketone so reliably. A methyl ketone, or any compound convertible to one, gives away its own identity a third way: sodium hypohalite oxidises the methyl group specifically to a haloform, leaving the rest of the carbon skeleton as a carboxylate one carbon shorter, the iodoform reaction, useful for detecting a stray CH3CO- group or a CH3CH(OH)- group anywhere in an unknown molecule.

A hydrogen on the carbon directly next to a carbonyl group, the alpha carbon, is measurably acidic, for a reason that should feel familiar by now: the carbonyl group's own strong electron-withdrawing pull, combined with resonance stabilisation of the resulting conjugate base, makes losing that particular proton far easier than losing an ordinary C-H hydrogen anywhere else in the molecule. Under dilute alkali, a carbonyl compound with at least one alpha-hydrogen can attack a second molecule of itself this way, its own resonance-stabilised carbanion acting as the nucleophile in an ordinary nucleophilic addition, producing a beta-hydroxy aldehyde, an aldol, or a beta-hydroxy ketone, a ketol, named for the aldehyde-plus-alcohol functional groups the product now carries. Left to react further, that aldol or ketol readily loses water to form an alpha,beta-unsaturated carbonyl compound, the aldol condensation proper, and mixing two different carbonyl compounds that each carry an alpha-hydrogen extends the same idea into a cross aldol condensation, generating up to four distinct products at once, since either partner can act as the nucleophile attacking the other.

An aldehyde with no alpha-hydrogen at all cannot undergo aldol condensation, since it has no acidic proton to lose in the first place, and heated instead with concentrated alkali, it takes a genuinely different path: disproportionation, one molecule reduced all the way to an alcohol while a second is simultaneously oxidised to a carboxylate salt, the Cannizzaro reaction, self-oxidation and self-reduction happening within the very same reaction flask. Aromatic aldehydes and ketones show one further behaviour worth noting: on their own aromatic ring, the carbonyl group acts as a deactivating, meta-directing substituent in electrophilic substitution, exactly the same category benzaldehyde's own reduced nucleophilic-addition reactivity already hinted at, an electron-withdrawing group behaving consistently whether the ring is attacking something or being attacked itself. All of this chemistry adds up to real industrial and everyday use: formalin preserves and builds bakelite, acetaldehyde feeds the manufacture of acetic acid and vinyl acetate, benzaldehyde scents perfumes and dyes, acetone and ethyl methyl ketone serve as everyday solvents, and a long list of naturally fragrant aldehydes and ketones, vanillin and camphor among them, do exactly what they do in nature: smell good.