Ch 12Carbonyl Compounds and Carboxylic Acids — Class 12 Chemistry, concept-first.

Oxidation of a primary alcohol gives an aldehyde, and oxidation of a secondary alcohol gives a ketone (Unit 11).

Reductive ozonolysis of an alkene, learned in Class XI, gives aldehydes and/or ketones: the alkene first reacts with ozone to form an unstable cyclic ozonide, which is then cleaved -- without isolatin…

Hydration of an alkyne, in the presence of dilute (40%) sulphuric acid and catalytic (1%) HgSO4, gives the corresponding aldehyde or ketone (Class XI).

Aldehydes and ketones can be obtained by the dry distillation of calcium salts of carboxylic acids -- an older, largely historical preparative method still tested for its mechanistic clarity.

Aldehydes can be prepared by the catalytic hydrogenation of an acid chloride in the presence of palladium supported on barium sulphate (Pd/BaSO4) -- this specific transformation is called the Rosenmun…

When an alkyl cyanide (nitrile) is reduced with stannous chloride in the presence of hydrochloric acid (SnCl2/HCl), an imine salt is formed as an intermediate, which on subsequent hydrolysis with wate…

Diisobutylaluminium hydride (DIBAL-H) selectively reduces alkyl cyanides to imines, which on subsequent hydrolysis give the corresponding aldehyde -- a modern, milder alternative to Stephen's reaction…

Strong oxidising agents such as KMnO4 oxidise the methyl side chain of toluene all the way through to benzoic acid, so a milder, more selective oxidant is needed to stop cleanly at the aldehyde stage.…

The Gattermann-Koch reaction is a variant of Friedel-Crafts acylation used specifically to install a -CHO group directly onto an aromatic ring.

Side-chain (free-radical) chlorination of toluene, using Cl2 gas with light (Cl2/h-nu) rather than a Lewis-acid catalyst, substitutes two hydrogens on the methyl group to give benzal chloride (benzyli…

Ketones can be prepared by treating an acid chloride with a dialkyl cadmium reagent, R2Cd (itself typically made from a Grignard reagent and CdCl2).

Friedel-Crafts acylation is the best general method for preparing alkyl aryl ketones or diaryl ketones, and succeeds only with benzene and activated (electron-rich) benzene derivatives, since the ring…

Nucleophilic addition is the most common reaction of aldehydes and ketones. The carbonyl carbon carries a small degree of positive charge; a nucleophile such as CN- attacks that carbon, donating its l…

Attack of the cyanide ion (CN-) on the carbonyl carbon, followed by protonation of the resulting alkoxide, gives a cyanohydrin.

Aldehydes and methyl ketones (the less hindered carbonyls) add sodium bisulphite to give a crystalline bisulphite addition compound, via nucleophilic attack of the sulphur atom on the carbonyl carbon…

When an aldehyde or ketone is treated with TWO equivalents of an alcohol in the presence of an acid catalyst (dry HCl gas), an acetal (or, for a ketone, a ketal) is formed, with loss of one molecule o…

A whole family of nitrogen nucleophiles, generically written H2N-G, add to a carbonyl compound the same way: nucleophilic addition of the NH2 nitrogen to the carbonyl carbon, protonation of the carbon…

Plain ammonia (not one of its substituted derivatives) reacts differently with each of four representative carbonyl compounds, and each case is separately named.

Aldehydes are easily oxidised to the carboxylic acid containing the SAME number of carbon atoms as the parent aldehyde, since the oxidation simply converts the aldehyde's own C-H into a C-OH (then tau…

Ketones are NOT easily oxidised under ordinary conditions, precisely because a ketone carbon has no C-H of its own left to remove (unlike an aldehyde).

Aldehydes and ketones are easily reduced to primary and secondary alcohols respectively; the most commonly used reducing agents are lithium aluminium hydride (LiAlH4) and sodium borohydride (NaBH4), b…

The carbonyl group of an aldehyde or ketone can be reduced all the way down to a methylene (-CH2-) group, giving a hydrocarbon directly, using either of two named methods that remove the oxygen entire…

Ketones, on reduction with magnesium amalgam (Mg-Hg) and water, are reduced not to a simple secondary alcohol but to a SYMMETRICAL 1,2-diol known as a pinacol -- two ketone molecules are joined togeth…

Acetaldehyde and methyl ketones -- any carbonyl compound containing the CH3-CO- group -- when treated with a halogen and alkali, give the corresponding haloform (CHX3); this specific halogenation-plus…

The carbon directly attached to the carbonyl carbon is called the alpha-carbon, and a hydrogen atom on that alpha-carbon is called an alpha-hydrogen.

Aldol condensation can also take place between two DIFFERENT aldehydes, two different ketones, or one aldehyde and one ketone -- this is called crossed (or mixed) aldol condensation.

Benzaldehyde -- having no alpha-hydrogen of its own -- condenses with an aliphatic aldehyde or a methyl ketone (which DOES have an alpha-hydrogen) in the presence of dilute alkali at room temperature…

In the presence of CONCENTRATED aqueous or alcoholic alkali, an aldehyde that has NO alpha-hydrogen atom undergoes self-oxidation-and-reduction (disproportionation) to give a mixture of an alcohol and…

Treatment of an aromatic aldehyde with aqueous-alcoholic potassium cyanide (KCN) gives a hydroxy ketone -- this specific carbon-carbon-bond-forming reaction is called the benzoin condensation.

When an aromatic aldehyde is heated with an aliphatic acid anhydride in the presence of the SODIUM SALT of the acid corresponding to that anhydride, condensation takes place to give an alpha,beta-unsa…

Benzaldehyde condenses with malonic acid (a compound whose central CH2 is flanked by TWO electron-withdrawing -COOH groups, making its alpha-hydrogens unusually acidic) in the presence of pyridine, wh…

Aromatic aldehydes react with primary amines (aliphatic or aromatic) in the presence of an acid catalyst to form a Schiff's base -- an imine, C=N-R, formed by the same nucleophilic-addition-then-dehyd…

Benzaldehyde condenses with a TERTIARY aromatic amine such as N,N-dimethylaniline (two equivalents) in the presence of strong acid (concentrated H2SO4) to form a triphenylmethane dye -- Malachite Gree…

Because the -CHO group is a deactivating, meta-directing substituent (its carbonyl carbon withdraws electron density from the ring by resonance, exactly like -COOH and -COR), electrophilic aromatic su…

Primary alcohols and aldehydes are both readily oxidised straight through to the corresponding carboxylic acid, using oxidising agents such as potassium permanganate (in either acidic or alkaline medi…

Nitriles (alkyl cyanides) yield carboxylic acids when subjected to hydrolysis with either an acid or an alkali catalyst -- the nitrile carbon, already at the correct (carboxylic) oxidation level, is s…

Esters, on hydrolysis with dilute mineral acid, yield the corresponding carboxylic acid together with the parent alcohol -- the exact reverse of the esterification reaction of Section 12.12.B.2, and,…

A Grignard reagent reacts with carbon dioxide (typically as solid dry ice, to keep the reaction anhydrous and well-controlled) to form the magnesium salt of a carboxylic acid, which on subsequent acid…

(a) Acid chlorides, hydrolysed with water, give carboxylic acids directly: acetyl chloride + H2O gives acetic acid + HCl.

Aromatic carboxylic acids can be prepared by the vigorous oxidation of an alkylbenzene, using chromic acid or acidic/alkaline potassium permanganate.

Carboxylic acids react with active metals such as Na, Mg or Zn to form the corresponding metal salt, with liberation of hydrogen gas -- exactly the same acid-metal reaction any Bronsted acid gives, si…

Carboxylic acids react with alkalies such as NaOH or KOH in a straightforward acid-base neutralisation to form the corresponding salt and water, exactly as any Bronsted acid neutralises any strong bas…

Carboxylic acids decompose carbonates and bicarbonates, evolving carbon dioxide gas with visible effervescence -- because a carboxylic acid (pKa around 3-5) is a stronger acid than carbonic acid, it r…

All carboxylic acids turn blue litmus red in aqueous solution -- the simplest and most direct qualitative confirmation of their acidic character, reflecting straightforward ionisation to release H3O+…

The hydroxyl group of a carboxylic acid behaves, in this one respect, like the hydroxyl of an ordinary alcohol: it is readily replaced by a chlorine atom on treatment with PCl5, PCl3, or SOCl2, giving…

When a carboxylic acid is heated with an alcohol in the presence of concentrated H2SO4 (or dry HCl gas), an ester is formed; the reaction is reversible and is called esterification.

(i) PARTIAL reduction to a primary alcohol: carboxylic acids are reduced to primary alcohols by LiAlH4, or by hydrogen gas in the presence of copper chromite as catalyst; sodium borohydride, notably,…

Removal of CO2 from a carboxyl group is called decarboxylation. Carboxylic acids lose carbon dioxide to form a hydrocarbon (one carbon shorter) when their SODIUM salt is heated with soda lime -- a mix…

The aqueous solution of the sodium or potassium salt of a carboxylic acid, on electrolysis, gives an ALKANE at the anode -- this specific electrolytic route is called Kolbe's electrolysis (or Kolbe's…

Carboxylic acids react with ammonia to form the ammonium salt (a simple acid-base reaction), which on further heating at high temperature loses water to give an AMIDE.

A carboxylic acid, heated in the presence of a strong dehydrating agent such as P2O5, forms the corresponding acid anhydride, with loss of one molecule of water between two acid molecules.

Carboxylic acids that possess an alpha-hydrogen are halogenated specifically AT the alpha-position, on treatment with chlorine or bromine in the presence of a small, catalytic amount of red phosphorus…

Aromatic carboxylic acids undergo the ring's usual electrophilic substitution reactions, but the -COOH group is itself strongly DEACTIVATING and META-directing (its resonance-withdrawing carbonyl pull…

Formic acid (HCOOH) is structurally unique among carboxylic acids: it contains BOTH an aldehyde-type C-H directly on the carboxyl carbon AND the acid group itself, so, like any true aldehyde, it is ea…

(i) ELECTRON-RELEASING alkyl groups DECREASE acidity. A +I (electron-donating, inductive) substituent pushes extra electron density onto the already-negative carboxylate ion, further destabilising tha…

Acid chlorides are prepared from carboxylic acids by treating them with any one of three chlorinating agents: SOCl2, PCl5, or PCl3 (the same three reagents already introduced for exactly this transfor…

Acid chlorides emit pale, visible fumes of hydrogen chloride gas when exposed to open air, on account of their ready reaction with atmospheric water vapour.

Acid chlorides react readily with a range of weak nucleophiles -- water, alcohols, ammonia, and amines -- each giving a different corresponding derivative product, since the chloride is an excellent l…

(1) Heating a carboxylic acid with P2O5: this is the SAME dehydration already introduced in Section 12.12.C.5, restated here from the anhydride's own point of view -- 2 acetic acid + P2O5 gives acetic…

An acid anhydride reacts with the same family of nucleophiles as an acid chloride (water, alcohols, ammonia, PCl5), but more SLOWLY, since -OCOR is a poorer leaving group than -Cl (Section 12.14's rea…

(1) Esterification: as already covered in Section 12.12.B.2, treatment of an alcohol with a carboxylic acid in the presence of a mineral-acid catalyst gives an ester; because the reaction is reversibl…

Esters are colourless liquids or solids with a characteristic, often pleasant fruity smell -- a property exploited directly in food flavouring and perfumery, since many natural fruit and flower scents…

(1) HYDROLYSIS: as already covered (Section 12.10.3), acidic hydrolysis of an ester gives back the parent alcohol and carboxylic acid.

(1) Ammonolysis of acid derivatives: amides are prepared by the action of ammonia on either an acid chloride or an acid anhydride (both reactions already introduced, Sections 12.14.2.3 and 12.14.3.2)…

(1) AMPHOTERIC CHARACTER: amides behave as both a weak base AND a weak acid. As a base, acetamide reacts with hydrochloric acid to form a salt: acetamide + HCl gives acetamide hydrochloride.