Aldehydes, Ketones and Carboxylic Acids: Class 12 Chemistry · Chapter 8

Open a bottle of nail-polish remover and the sharp smell is propanone (acetone). Warm some ghee and part of its rich, buttery aroma is butane-2,3-dione. A stalk of lemongrass in masala chai owes its lemony note to citral, and the sourness of the vinegar in a jar of achaar is ethanoic acid. All four molecules are built around the same small unit: a carbon atom joined to an oxygen atom by a double bond, the carbonyl group, $>\mathrm{C}=\mathrm{O}$.

Unit 7 dealt with compounds that have a C–O single bond: alcohols, phenols and ethers. The C=O double bond behaves very differently. It is flat, strongly polar and open to attack. That is why carbonyl compounds sit at the centre of organic synthesis and of the chemistry of living cells (sugars are aldehydes and ketones, fats are esters). Industries that make solvents, plastics, perfumes and medicines depend on them too.

What else is bonded to the carbonyl carbon decides the family (Fig. 8.1). An aldehyde has at least one hydrogen there, a ketone has two carbon groups, and a carboxylic acid has a hydroxyl group. Replace that –OH by another group and you get the acid derivatives: acyl chlorides (–Cl), esters (–OR′), amides (–NH₂) and acid anhydrides (–OCOR′).

8.1 Nomenclature and Structure of Carbonyl Group

8.1.1 Nomenclature

Aldehydes and ketones have two kinds of names. The common names are older and are still heard in laboratories and industry. IUPAC names are systematic, so you can build one from the structure alone, and the structure from the name.

Common names

Most aldehydes borrow their common names from the carboxylic acid with the same carbon skeleton: drop -ic acid and add aldehyde. Formic acid gives formaldehyde, acetic acid gives acetaldehyde and butyric acid gives butyraldehyde. Positions along the chain are labelled with Greek letters, starting from the carbon next to the –CHO group: that carbon is α, the next one β, then γ and δ. So $\mathrm{C}\mathrm{l}\mathrm{C}\mathrm{H}{\phantom{X}}_2\mathrm{C}\mathrm{H}{\phantom{X}}_2\mathrm{C}\mathrm{H}\mathrm{O}$ is β-chloropropionaldehyde.

Ketones are named by listing the two groups on the carbonyl carbon, alphabetically, and adding ketone: $\mathrm{C}\mathrm{H}{\phantom{X}}_3\mathrm{C}\mathrm{O}\mathrm{C}\mathrm{H}{\phantom{X}}_2\mathrm{C}\mathrm{H}{\phantom{X}}_3$ is ethyl methyl ketone and $(\mathrm{C}\mathrm{H}{\phantom{X}}_3){\phantom{X}}_2\mathrm{C}\mathrm{H}\mathrm{C}\mathrm{O}\mathrm{C}\mathrm{H}{\phantom{X}}_3$ is isopropyl methyl ketone. Two traditional names break this pattern and are still everywhere: acetone for dimethyl ketone, and the ending -phenone for ketones with a benzene ring on the carbonyl carbon, as in acetophenone, $\mathrm{C}{\phantom{X}}_6\mathrm{H}{\phantom{X}}_5\mathrm{C}\mathrm{O}\mathrm{C}\mathrm{H}{\phantom{X}}_3$, and butyrophenone, $\mathrm{C}{\phantom{X}}_6\mathrm{H}{\phantom{X}}_5\mathrm{C}\mathrm{O}\mathrm{C}\mathrm{H}{\phantom{X}}_2\mathrm{C}\mathrm{H}{\phantom{X}}_2\mathrm{C}\mathrm{H}{\phantom{X}}_3$. To mark a substituent, count Greek letters outwards from the C=O on each side, α and α′ for the two neighbouring carbons, then β and β′, so $\mathrm{C}\mathrm{l}\mathrm{C}\mathrm{H}{\phantom{X}}_2\mathrm{C}\mathrm{O}\mathrm{C}\mathrm{H}{\phantom{X}}_3$ is α-chloroacetone.

IUPAC names

Four steps give the IUPAC name of an open-chain aldehyde or ketone:

1. Find the parent chain. It is the longest chain that contains the carbonyl carbon (for an aldehyde, the –CHO carbon itself).
2. Number it. The aldehyde carbon is always C1, so its locant is never written. For a ketone, number from the end that gives the C=O carbon the lower locant.
3. Choose the ending. Replace the final -e of the alkane name by -al (aldehyde) or -one (ketone); keep -en- or -yn- for a C=C or C≡C, as in but-3-en-2-one.
4. Add substituents as prefixes in alphabetical order, each with its locant.
5. Rings. A ring ketone counts its C=O carbon as C1, as in 3-methylcyclohexanone. A –CHO group on a ring is not part of the ring, so it is shown by the suffix -carbaldehyde, numbering from the ring carbon that carries it: cyclopentanecarbaldehyde, 2-methylcyclohexanecarbaldehyde.
6. Benzene rings. $\mathrm{C}{\phantom{X}}_6\mathrm{H}{\phantom{X}}_5\mathrm{C}\mathrm{H}\mathrm{O}$ keeps its familiar name, benzaldehyde, which IUPAC accepts in place of benzenecarbaldehyde; ring substituents are numbered from the carbon carrying –CHO, as in 4-chlorobenzaldehyde.

8.1.2 Structure of the Carbonyl Group

Measure the bond angles around a carbonyl carbon and they come out close to 120°, with the carbon and the three atoms bonded to it lying in one plane. That geometry points to sp² hybridisation: three hybrid orbitals in a plane make the σ bonds, to oxygen and to the two other groups. The carbon's remaining electron occupies an unhybridised p orbital standing upright from that plane. Oxygen lines up a p orbital of its own alongside, and the sideways overlap of the two is the π bond, with electron density in two lobes, one above and one below the plane (Fig. 8.2).

The angles are not quite equal. In methanal the H–C–H angle is about 116° and each H–C=O angle about 122°: the electron-rich double bond pushes the other two bonds slightly together. Oxygen also carries two lone pairs, which become important when acids and Lewis acids attack it.

Oxygen (electronegativity 3.4) holds shared electrons much more tightly than carbon (2.6), and the loosely held π electrons are pulled furthest. The carbonyl carbon is left with a partial positive charge and the oxygen with a partial negative charge. Two resonance structures describe this: a covalent form with a full C=O double bond, and a charge-separated form with $\mathrm{C}{\phantom{X}}^{+}$ and $\mathrm{O}{\phantom{X}}^{-}$. The real molecule is a hybrid that is mostly the first but has a real share of the second.

Two predictions follow. The δ+ carbon is electrophilic (a Lewis-acid site), so electron-rich reagents, nucleophiles, attack it. The δ− oxygen, with its lone pairs, is nucleophilic and weakly basic (a Lewis-base site), so it is where protons and Lewis acids attach. And because the bond is so polar, carbonyl compounds have large dipole moments: from about 2.3 D for methanal to 2.9 D for propanone, more than twice those of ethers such as methoxymethane (1.3 D).

8.2 Preparation of Aldehydes and Ketones

There are many ways to make a C=O group. Some give an aldehyde or a ketone depending only on the starting material; others are special to one family. Making an aldehyde always involves the same challenge: stop the reaction before the aldehyde is oxidised further to an acid, or reduced further to an alcohol.

8.2.1 Preparation of Aldehydes and Ketones

By oxidising alcohols. As you saw in Unit 7, a primary alcohol is oxidised to an aldehyde and a secondary alcohol to a ketone. For an aldehyde a mild oxidant such as pyridinium chlorochromate (PCC) is chosen, so that the reaction ends there; a ketone resists further oxidation, so acidified dichromate works well.

By dehydrogenating alcohols. In industry the vapour of a volatile alcohol is passed over hot copper (about 573 K) or silver. Hydrogen is stripped off and no oxidant is needed. A primary alcohol gives an aldehyde and a secondary alcohol a ketone.

From hydrocarbons. Ozonolysis splits an alkene at its C=C. After work-up with zinc dust and water, each carbon of the old double bond carries a C=O, so the products are aldehydes, ketones or one of each, depending on what was attached to those carbons. 2-Methylbut-2-ene, for example, gives propanone and ethanal.

Alkynes add water when warmed with dilute sulphuric acid and a mercury(II) salt. The first product, an enol, rearranges at once into a carbonyl compound. Because the oxygen goes to the carbon predicted by Markovnikov's rule, only ethyne itself ends up as an aldehyde (ethanal); any other alkyne gives a ketone, so propyne gives propanone.

8.2.2 Preparation of Aldehydes

An aldehyde is the middle rung of an oxidation ladder, with the alcohol below it and the acid above. Each method here is designed to land on that middle rung.

From acyl chlorides (Rosenmund reduction). Hydrogen over ordinary palladium would reduce an acyl chloride right down to the alcohol. Spreading the palladium on barium sulphate poisons it, leaving it just active enough to swap Cl for H, so the aldehyde survives.

From nitriles (Stephen reaction). Tin(II) chloride and hydrochloric acid reduce a nitrile only as far as an imine, which is held as its salt. Warm water then hydrolyses the C=NH group to C=O.

A more modern reagent, diisobutylaluminium hydride (DIBAL-H, $[(\mathrm{C}\mathrm{H}{\phantom{X}}_3){\phantom{X}}_2\mathrm{C}\mathrm{H}\mathrm{C}\mathrm{H}{\phantom{X}}_2]{\phantom{X}}_2\mathrm{A}\mathrm{l}\mathrm{H}$), hands just one hydride ion to a nitrile or an ester when used cold; water then releases the aldehyde. An ester such as methyl hexanoate gives hexanal.

From toluene and benzene. A strong oxidant such as hot $\mathrm{K}\mathrm{M}\mathrm{n}\mathrm{O}{\phantom{X}}_4$ turns the methyl group of toluene straight into –COOH, overshooting the aldehyde. Three methods get round this by first converting –$\mathrm{C}\mathrm{H}{\phantom{X}}_3$ into a group that the reagent cannot oxidise further, then releasing –CHO in water (Fig. 8.4):

• Etard reaction. Chromyl chloride, $\mathrm{C}\mathrm{r}\mathrm{O}{\phantom{X}}_2\mathrm{C}\mathrm{l}{\phantom{X}}_2$, in $\mathrm{C}\mathrm{S}{\phantom{X}}_2$ converts the methyl group into a brown chromium complex; hydrolysis gives benzaldehyde.
• Chromic oxide in ethanoic anhydride. $\mathrm{C}\mathrm{r}\mathrm{O}{\phantom{X}}_3$ oxidises the methyl group while the anhydride traps it, in the cold, as the diacetate $\mathrm{C}{\phantom{X}}_6\mathrm{H}{\phantom{X}}_5\mathrm{C}\mathrm{H}(\mathrm{O}\mathrm{C}\mathrm{O}\mathrm{C}\mathrm{H}{\phantom{X}}_3){\phantom{X}}_2$; warming with aqueous acid releases the aldehyde.
• Side-chain chlorination. Chlorine in light replaces two H atoms of the methyl group, giving benzal chloride, $\mathrm{C}{\phantom{X}}_6\mathrm{H}{\phantom{X}}_5\mathrm{C}\mathrm{H}\mathrm{C}\mathrm{l}{\phantom{X}}_2$. Boiling with water turns the $\mathrm{C}\mathrm{H}\mathrm{C}\mathrm{l}{\phantom{X}}_2$ group into –CHO. Much commercial benzaldehyde is made this way.

Gattermann–Koch reaction. Benzene itself can be formylated: carbon monoxide and hydrogen chloride, with anhydrous aluminium chloride and a little copper(I) chloride, put a –CHO group directly onto the ring. Toluene gives mainly 4-methylbenzaldehyde in the same way.

8.2.3 Preparation of Ketones

From acyl chlorides and dialkylcadmium. A Grignard reagent attacks an acyl chloride, but it would go on to attack the ketone it makes. Treating it with cadmium chloride first gives a dialkylcadmium, a gentler reagent that reacts with the acyl chloride and leaves the ketone alone.

From nitriles and Grignard reagents. The carbon of RMgX, which behaves like a carbanion, adds to the carbon of C≡N. The imine salt formed is hydrolysed by aqueous acid, and =NMgX is replaced by =O. Ethanenitrile and ethylmagnesium bromide give butanone.

From arenes (Friedel–Crafts acylation). An acyl chloride and anhydrous $\mathrm{A}\mathrm{l}\mathrm{C}\mathrm{l}{\phantom{X}}_3$ generate the acylium ion, $\mathrm{R}\mathrm{C}\equiv \mathrm{O}{\phantom{X}}^{+}$, an electrophile that substitutes onto the ring. The product is an aryl ketone. Because its C=O group deactivates the ring, the reaction stops after one acyl group goes in, unlike Friedel–Crafts alkylation. (Acid anhydrides can replace the acyl chloride.)

8.3 Physical Properties

Only the two smallest members are awkward to keep in a bottle: methanal boils at 254 K, so it is sold dissolved in water, and ethanal at 293 K, just below the temperature of a warm room. Every larger aldehyde or ketone is a liquid or a solid at room temperature.

When molar masses are almost equal, the forces between molecules decide the boiling point (Fig. 8.5). A hydrocarbon such as hexane has only weak London forces. An ether is slightly polar, but its oxygen is tucked between two carbon groups. Aldehydes and ketones have a strongly polar C=O, so neighbouring molecules line up δ+ to δ−, and this dipole–dipole attraction lifts the boiling point by more than 30 K. They cannot hydrogen-bond to each other, though, because they have no hydrogen on oxygen. An alcohol of similar mass can, so it boils higher still.

A carbonyl oxygen cannot donate a hydrogen bond, but its lone pairs can accept one from water. That is enough to make methanal, ethanal and propanone dissolve in water in any proportion. Solubility then drops quickly as the chain grows, because more and more non-polar $\mathrm{C}\mathrm{H}{\phantom{X}}_2$ groups must be fitted into the water. About 7 g of butanal dissolves in 100 mL of water, but well under 1 g of hexanal.

Your nose can follow the same series. Methanal and ethanal are sharp and irritating, but aldehydes with eight to ten carbon atoms smell of citrus peel; perfumers use them, along with ketones such as jasmone (jasmine) and muscone (musk), in tiny amounts.

8.4 Chemical Reactions

Because aldehydes and ketones share the carbonyl group, they share most of their reactions. They fall into five groups: nucleophilic addition, reduction, oxidation, reactions of the α-hydrogens, and a few others (the Cannizzaro reaction and substitution on an aromatic ring).

Nucleophilic addition reactions

The C=C bond of an alkene is non-polar and rich in electrons, so its first attacker is an electrophile. A C=O bond is the other way round: its carbon is short of electrons, so the first attack comes from a nucleophile, and the overall change is an addition across the double bond. This nucleophilic addition is the signature reaction of aldehydes and ketones.

Mechanism

Follow the electrons (Fig. 8.6):

1. Attack (slow). A lone pair of the nucleophile starts a bond to the carbonyl carbon. It arrives from above or below the molecular plane, where the π orbital is, along a line roughly 107° from the C=O axis.
2. Push. The π electron pair moves onto oxygen, which holds it well because it is electronegative. Carbon now has four groups: it has gone from flat (sp²) to tetrahedral (sp³), and the oxygen is an alkoxide, –$\mathrm{O}{\phantom{X}}^{-}$.
3. Protonation (fast). The alkoxide oxygen picks up $\mathrm{H}{\phantom{X}}^{+}$ from the solvent or from an acid.

So the nucleophile ends up on carbon and a hydrogen on oxygen.

Reactivity

Line up methanal, ethanal and propanone (Fig. 8.7). Each time a hydrogen on the carbonyl carbon is replaced by a methyl group, two things change. First, room: a methyl group is much bulkier than H, so the nucleophile finds the carbon harder to reach. The tetrahedral product, with four groups packed round one carbon, is also more crowded. Second, charge: alkyl groups release electron density towards the carbonyl carbon (+I effect), so its δ+ shrinks. Both changes slow the reaction. That is why aldehydes generally react faster than ketones, and methanal, with no alkyl group at all, fastest of all.

Conjugation lowers reactivity too. When a C=O group is joined directly to a benzene ring, the ring's π electrons are delocalised into it, which feeds electron density towards the carbonyl carbon. Aromatic aldehydes and ketones are therefore less reactive than similar aliphatic ones, while electron-withdrawing groups on the ring, such as –$\mathrm{N}\mathrm{O}{\phantom{X}}_2$, make them more reactive.

Some important addition reactions

(a) Hydrogen cyanide. The product is a cyanohydrin, with –OH and –CN on the former carbonyl carbon. HCN is a weak acid, so pure HCN contains hardly any cyanide ions, and on its own it adds very slowly. The real attacker is $\mathrm{C}\mathrm{N}{\phantom{X}}^{-}$: a trace of base (or of KCN) supplies it, the alkoxide formed then takes a proton back from HCN, and that regenerates $\mathrm{C}\mathrm{N}{\phantom{X}}^{-}$ for the next molecule. Chemists value cyanohydrins because the new –CN carbon can later become –COOH (by hydrolysis) or $-\mathrm{C}\mathrm{H}{\phantom{X}}_2\mathrm{N}\mathrm{H}{\phantom{X}}_2$ (by reduction), adding one carbon to the chain.

(b) Sodium hydrogensulphite. Suppose an aldehyde has to be separated from other organic liquids. Shaking the mixture with saturated aqueous $\mathrm{N}\mathrm{a}\mathrm{H}\mathrm{S}\mathrm{O}{\phantom{X}}_3$ turns the aldehyde into a crystalline addition compound that dissolves in water. The other liquids are washed away, and warming the adduct with dilute acid or alkali gives back the pure aldehyde. This works because the addition is reversible. It suits aldehydes and a few unhindered methyl ketones; most ketones, crowded by two carbon groups, form very little adduct.

(c) Grignard reagents. As in Unit 7, RMgX adds to the C=O group and hydrolysis then gives an alcohol: methanal gives a primary alcohol, other aldehydes secondary alcohols, and ketones tertiary alcohols.

(d) Alcohols. An alcohol is a weak nucleophile, so on its own it barely touches a C=O group. Dry HCl gas changes that by protonating the carbonyl oxygen, and the protonated C=O is attacked readily. One alcohol molecule gives a hemiacetal, a carbon carrying both –OH and –OR. Under the same acidic conditions that –OH leaves as water and a second –OR takes its place, giving an acetal, with two –OR groups on one carbon. Ketones are usually treated with ethane-1,2-diol instead: its two –OH groups bond to the same carbon and close a five-membered ring, a cyclic (ethylene glycol) ketal.

Acetals and ketals survive bases and nucleophiles, but warm aqueous acid splits them back into the carbonyl compound and the alcohol. That makes them useful temporary caps on a C=O group while another part of the molecule is being changed.

(e) Ammonia and its derivatives. With a nitrogen nucleophile of the type $\mathrm{H}{\phantom{X}}_2\mathrm{N}-\mathrm{Z}$, addition is followed by elimination: the addition product (a carbinolamine) loses water, and C=O is replaced by C=N–Z. Every step is reversible and speeded up by acid, but the loss of water is fast, so the C=N product wins. Mildly acidic conditions (about pH 4–5) work best: enough acid to activate the C=O, not so much that the nitrogen lone pair is tied up as $\mathrm{H}{\phantom{X}}_3\mathrm{N}{\phantom{X}}^{+}-\mathrm{Z}$.

The 2,4-DNP derivatives crystallise as brightly coloured solids, anywhere from yellow to deep red, with sharp melting points. A coloured precipitate with 2,4-DNP reagent (Brady's reagent) is therefore a quick test for an aldehyde or a ketone, and the melting point of the purified solid helps to identify which one it is.

Reduction

To alcohols. Sodium borohydride, $\mathrm{N}\mathrm{a}\mathrm{B}\mathrm{H}{\phantom{X}}_4$, or lithium aluminium hydride, $\mathrm{L}\mathrm{i}\mathrm{A}\mathrm{l}\mathrm{H}{\phantom{X}}_4$, delivers a hydride ion to the carbonyl carbon (another nucleophilic addition), and work-up adds $\mathrm{H}{\phantom{X}}^{+}$ to oxygen. Hydrogen over nickel, platinum or palladium does the same job. Aldehydes give primary alcohols and ketones give secondary alcohols.

To hydrocarbons. Sometimes the oxygen has to go completely, turning C=O into $\mathrm{C}\mathrm{H}{\phantom{X}}_2$. Two named reactions do this, one in acid and one in base, so a chemist can choose the one that the rest of the molecule will survive:

Oxidation

Here aldehydes and ketones part company. An aldehyde still has a hydrogen on its carbonyl carbon, and that C–H bond is easily oxidised. Almost any oxidant turns an aldehyde into a carboxylic acid: acidified dichromate, permanganate, even the oxygen of the air (an open bottle of benzaldehyde slowly fills with crystals of benzoic acid). Gentle reagents that leave ketones untouched oxidise aldehydes too, and these make useful tests.

A ketone has no such hydrogen. Only strong oxidants at high temperature attack it, and then a C–C bond next to the C=O has to break. The break can happen on either side, so a ketone usually gives a mixture of acids, each with fewer carbon atoms than the ketone. Hexan-3-one, for instance, can give ethanoic acid plus butanoic acid, or two molecules of propanoic acid.

The difference gives simple tests that tell an aldehyde from a ketone (Fig. 8.8).

Tollens' test. The reagent is silver nitrate dissolved in excess aqueous ammonia, so the silver is present as the complex ion $[\mathrm{A}\mathrm{g}(\mathrm{N}\mathrm{H}{\phantom{X}}_3){\phantom{X}}_2]{\phantom{X}}^{+}$ in an alkaline solution. An aldehyde reduces this complex: metallic silver plates out on the glass as a mirror, while the aldehyde ends up as its carboxylate ion. The reagent is made in small amounts just before use (see the caution below), and ketones leave it unchanged.