Aldehydes, Ketones and Carboxylic Acids

Aldehydes and ketones both contain the carbonyl group, a carbon double-bonded to oxygen (C=O). In an aldehyde ($\text{R–CHO}$) the carbonyl carbon carries at least one hydrogen and sits at the end of a chain; in a ketone ($\text{R–CO–R'}$) it is flanked by two carbon groups. The carbonyl carbon is $sp^2$ hybridised and the group is planar, and because oxygen is far more electronegative than carbon the bond is strongly polar — carbon is electron-poor ($\delta+$) and oxygen electron-rich ($\delta-$). That polarity is the engine of the whole chapter: nucleophiles attack the carbon, electrophiles the oxygen.

The IUPAC names use the suffix -al for aldehydes (methanal, ethanal) and -one for ketones (propanone), with the chain numbered to give the carbonyl carbon the lowest locant; the aldehyde carbon is always C-1. Common names (formaldehyde, acetaldehyde, acetone, benzaldehyde) are still widely used and worth knowing for NEET.

The most reliable preparations start from alcohols: controlled oxidation of a primary alcohol gives an aldehyde (use a mild reagent such as PCC to stop there, since strong oxidants go on to the acid), while oxidation of a secondary alcohol gives a ketone. Two clean routes from unsaturated compounds are ozonolysis of alkenes (which cleaves C=C into two carbonyls) and acid-catalysed hydration of alkynes ($\text{H}_2\text{SO}_4$/$\text{HgSO}_4$ — ethyne gives ethanal, higher alkynes give ketones).

Several named methods make specific products. The Rosenmund reduction reduces an acyl chloride to an aldehyde over $\text{Pd}/\text{BaSO}_4$ (a poisoned catalyst that stops at the aldehyde). From nitriles, partial reduction or the Stephen reaction gives aldehydes. For aromatic carbonyls, Friedel–Crafts acylation makes aryl ketones, while the Gattermann–Koch reaction ($\text{CO}$ + $\text{HCl}$ with $\text{AlCl}_3$) puts a $-\text{CHO}$ onto benzene to give benzaldehyde. NEET often gives the reagent and asks for the carbonyl product, so linking each name to its outcome is high-yield.

Because the carbonyl carbon is electron-poor, the defining reaction of aldehydes and ketones is nucleophilic addition: a nucleophile adds to the carbon and a proton adds to the oxygen, opening the C=O to a tetrahedral product. Aldehydes are more reactive than ketones for two reasons — ketones have two electron-donating alkyl groups that reduce the positive charge on carbon ($+\text{I}$ effect), and those two groups also crowd the carbon (steric hindrance). This reactivity order is one of the most tested NEET facts.

Key additions include: HCN (giving a cyanohydrin, useful for making hydroxy acids), sodium bisulphite $\text{NaHSO}_3$ (giving a crystalline adduct used to purify carbonyl compounds), Grignard reagents (then water, building larger alcohols), and alcohols (giving hemiacetals and acetals, important as protecting groups). Reactions with ammonia derivatives give condensation products: hydroxylamine gives oximes, hydrazine gives hydrazones, and 2,4-dinitrophenylhydrazine (2,4-DNP, Brady's reagent) gives orange/yellow precipitates — the standard test for a carbonyl group.

Reduction converts carbonyls to alcohols ($\text{NaBH}_4$/$\text{LiAlH}_4$/$\text{H}_2$): aldehydes give $1^\circ$ alcohols, ketones give $2^\circ$ alcohols. To remove the oxygen entirely (C=O $\rightarrow$ CH$_2$) two named methods are used — Clemmensen reduction (Zn–Hg/HCl, acidic) and Wolff–Kishner reduction (hydrazine then strong base, basic). The complementary oxidation is hugely diagnostic: aldehydes are oxidised to acids very easily, so they give a positive Tollens' test (a silver mirror) and Fehling's/Benedict's test (a brick-red $\text{Cu}_2\text{O}$ precipitate); ketones do not react with these mild reagents — the classic way to tell the two apart.

Two condensation reactions hinge on the $\alpha$-hydrogen (a hydrogen on the carbon next to C=O, made acidic by the carbonyl). If at least one $\alpha$-H is present, a dilute base promotes the aldol condensation — two molecules join to give a $\beta$-hydroxy carbonyl that often dehydrates to an $\alpha,\beta$-unsaturated product. If there is no $\alpha$-H (as in HCHO or benzaldehyde), a concentrated base drives the Cannizzaro reaction, a disproportionation giving one alcohol and one carboxylate. Finally, the iodoform (haloform) test — $\text{I}_2$/NaOH gives yellow $\text{CHI}_3$ — detects a $\text{CH}_3\text{CO–}$ group (methyl ketones) or a $\text{CH}_3\text{CH(OH)–}$ group; these tests appear in almost every NEET paper.

Carboxylic acids contain the carboxyl group, $-\text{COOH}$ (a carbonyl and a hydroxyl on the same carbon). They are named with the suffix -oic acid (methanoic, ethanoic acid), and common names (formic, acetic, benzoic acid) are standard. The interaction of the C=O and O–H within the group gives carboxylic acids their hallmark property: they are the most acidic of the common organic compounds, far stronger than alcohols or phenols.

The reason is resonance stabilisation of the carboxylate ion. When $-\text{COOH}$ loses its proton, the resulting $-\text{COO}^-$ spreads its negative charge equally over two electronegative oxygen atoms, so the ion is very stable and the proton leaves readily. (Contrast a phenoxide, where the charge is shared less effectively with the ring, and an alkoxide, which has no resonance.) The acidity order is therefore carboxylic acid $\gt$ phenol $\gt$ water $\gt$ alcohol — a sequence NEET asks about repeatedly.

The strength of a particular acid is tuned by substituents. Electron-withdrawing groups (such as $-\text{Cl}$, $-\text{NO}_2$) stabilise the carboxylate and increase acidity — chloroacetic acid is stronger than acetic acid, and trichloroacetic acid stronger still; the effect weakens with distance from the $-\text{COOH}$. Electron-donating groups (alkyl) decrease acidity. Comparing relative strengths from substituent effects is a guaranteed NEET question.

The main preparations are: oxidation of primary alcohols or aldehydes (with $\text{KMnO}_4$ or $\text{K}_2\text{Cr}_2\text{O}_7$); oxidation of alkylbenzenes (any alkyl side-chain is oxidised right down to $-\text{COOH}$, giving benzoic acid); hydrolysis of nitriles (R–CN + water/acid $\rightarrow$ R–COOH, adding one carbon); the reaction of a Grignard reagent with carbon dioxide ($\text{RMgX} + \text{CO}_2$, then acid, again adding one carbon — a key chain-lengthening route); and hydrolysis of esters and amides. Physically, carboxylic acids form strong hydrogen bonds (even existing as dimers), so they have high boiling points and the lower members are very water-soluble.

The reactions of carboxylic acids fall into three groups: those that show their acidity, those that replace the $-\text{OH}$ to make derivatives, and reactions of the rest of the molecule. As acids, they react with active metals (liberating $\text{H}_2$) and with bases to form salts. A crucial NEET test lives here: carboxylic acids are strong enough to react with sodium bicarbonate ($\text{NaHCO}_3$), liberating brisk effervescence of $\text{CO}_2$, whereas phenols are not — so $\text{NaHCO}_3$ distinguishes a carboxylic acid from a phenol (both, however, react with the stronger base NaOH).

Replacing the $-\text{OH}$ of $-\text{COOH}$ gives the acid derivatives. With an alcohol and a trace of acid catalyst, esterification (Fischer esterification) gives a fragrant ester and water (a reversible reaction). With $\text{SOCl}_2$, $\text{PCl}_5$ or $\text{PCl}_3$, the acid becomes an acyl (acid) chloride. Heating with $\text{P}_2\text{O}_5$ (or from acid chlorides) gives anhydrides, and treatment with ammonia followed by heating gives amides. The reactivity order of these derivatives towards nucleophiles is acid chloride $\gt$ anhydride $\gt$ ester $\gt$ amide — useful for predicting interconversions.

The carbon framework also reacts. Reduction with $\text{LiAlH}_4$ (carboxylic acids resist mild $\text{NaBH}_4$) gives a primary alcohol. The Hell–Volhard–Zelinsky (HVZ) reaction introduces a halogen at the $\alpha$-carbon (using $\text{Cl}_2$ or $\text{Br}_2$ with a little red phosphorus). Decarboxylation — heating the sodium salt with soda lime ($\text{NaOH}$/$\text{CaO}$) — removes $-\text{COOH}$ as $\text{CO}_2$ and gives an alkane with one fewer carbon. For aromatic acids, the $-\text{COOH}$ group is meta-directing and deactivating in electrophilic substitution.

These derivatives connect this chapter to much of organic chemistry: esters give the smells of fruits and are the building blocks of fats and polyesters; amides are the linkages in proteins and nylons. For NEET, the practical takeaways are the $\text{NaHCO}_3$ test (acid vs phenol), the conversion of an acid into its chloride/ester/amide, and the carbon-count changes in decarboxylation (loses one C), Grignard/nitrile routes (gain one C) — these counting tricks frequently decide a multi-step synthesis question.