How Chemists Stop an Oxidation Exactly at the Aldehyde

The smell of cinnamon, the taste of vanilla and the solvent in nail-polish remover all come from compounds with a carbonyl group, C=O. Aldehydes carry it at the end of a carbon chain; ketones hold it between two carbon groups.

This part covers naming and the carbonyl group, general preparation methods, selective routes to aldehydes, and physical properties.

**Aldehydes are named with the ending -al and ketones with -one; the carbonyl carbon is $s{p}^2$ hybridised and trigonal planar, and oxygen's higher electronegativity makes C=O polar, with a partial positive charge on carbon that attracts nucleophiles.

Naming:

- HCHO — formaldehyde; IUPAC methanal**
- CH${}_3$CHO — acetaldehyde; ethanal
- CH${}_3$COCH${}_3$ — acetone; propanone
- CH${}_3$COCH${}_2$CH${}_3$ — ethyl methyl ketone; butan-2-one
- C${}_6$H${}_5$COCH${}_3$ — acetophenone; 1-phenylethanone

The carbonyl group:

- The carbon forms three $\sigma$ bonds at about $120^{\circ }$ and one $\pi$ bond with oxygen
- The bond is polar: $\stackrel{\delta +}{C}=\stackrel{\delta -}{O}$
- The carbon is electrophilic and the oxygen nucleophilic

Worked example. For (CH${}_3$)${}_2$CHCH${}_2$CHO, numbering starts at the CHO carbon, so the methyl branch sits on C3 of a four-carbon chain: 3-methylbutanal.

An everyday example. Nail-polish remover is mostly propanone, the simplest ketone, which dissolves the dried polish.

The substance. The aldehyde carbon is always C1, so the locant is never written in names such as butanal.

**Oxidising or dehydrogenating primary alcohols gives aldehydes and secondary alcohols gives ketones, ozonolysis splits alkenes into carbonyl compounds, and hydration of alkynes with dilute H${}_2$SO${}_4$ and HgSO${}_4$ gives ethanal from ethyne and ketones from other alkynes.

From alcohols:

- Oxidation** with acidified K${}_2$Cr${}_2$O${}_7$ — primary alcohols to aldehydes, secondary to ketones; PCC stops primary alcohols at the aldehyde
- Dehydrogenation — alcohol vapour over hot copper at $573$ K

Ozonolysis of alkenes. Ozone adds across C=C, and zinc with water splits the ozonide into two carbonyl compounds:

$${\text{(CH}}_3{)}_2{\text{C=CHCH}}_3\stackrel{{\text{O}}_3,{\text{Zn/H}}_2\text{O}}{\to }{\text{CH}}_3{\text{COCH}}_3+{\text{CH}}_3\text{CHO}$$

Hydration of alkynes:

- Ethyne gives ethanal
- Other alkynes give ketones by Markovnikov addition — propyne gives propanone

Worked example. An alkene C${}_5$H${}_{10}$ gives propanone and ethanal on ozonolysis. Joining the two carbonyl carbons by a double bond rebuilds it as (CH${}_3$)${}_2$C=CHCH${}_3$, 2-methylbut-2-ene — and $3+2=5$ carbons confirms it.

An everyday example. Wine turning sour into vinegar passes through ethanal, formed as ethanol is oxidised on the way to ethanoic acid.

The substance. Only ethyne gives an aldehyde on hydration — every other alkyne gives a ketone.

Aldehydes over-oxidise or over-reduce easily, so selective routes stop exactly at the aldehyde: Rosenmund reduction of acyl chlorides over poisoned palladium, Stephen reduction of nitriles through an imine, Etard oxidation of toluene with chromyl chloride, and Gatterman-Koch formylation of benzene with CO and HCl.

The reactions:

- Rosenmund reduction — $\text{RCOCl}+{\text{H}}_2\to \text{RCHO}+\text{HCl}$ over Pd on BaSO${}_4$; the barium sulphate poisons the catalyst so the aldehyde is not reduced further
- Stephen reaction — a nitrile is reduced by SnCl${}_2$ and HCl to an imine, which hydrolyses to the aldehyde; DIBAL-H does the same job for nitriles and esters
- Etard reaction — toluene with chromyl chloride, CrO${}_2$Cl${}_2$, forms a complex that hydrolyses to benzaldehyde
- Gatterman-Koch reaction — benzene with CO and HCl, with anhydrous AlCl${}_3$ and CuCl, gives benzaldehyde

Aromatic ketones come from Friedel-Crafts acylation: benzene with ethanoyl chloride and AlCl${}_3$ gives acetophenone.

Worked example. Rosenmund reduction of $15.45$ g of benzoyl chloride, C${}_6$H${}_5$COCl ($M=140.5$ g mol${}^{-1}$), gives at most

$$\frac{15.45}{140.5}=0.110\text{ mol},0.110\times 106\approx 11.7\text{ g of benzaldehyde}$$

An everyday example. Benzaldehyde gives almonds their characteristic smell, and it is exactly the kind of aldehyde these routes produce.

The substance. The poisoned catalyst is the whole point of Rosenmund reduction — an active catalyst would carry on to the alcohol.

Aldehydes and ketones boil higher than alkanes and ethers of similar molar mass because of dipole–dipole attractions between polar C=O groups, but lower than alcohols, which hydrogen-bond; the smaller members dissolve in water because water hydrogen-bonds to the carbonyl oxygen.

Physical state. Methanal is a gas, ethanal a volatile liquid, and other small aldehydes and ketones are liquids.

Boiling points follow the strength of intermolecular attraction:

- Alkanes and ethers — only weak van der Waals forces
- Aldehydes and ketones — dipole–dipole attractions
- Alcohols — hydrogen bonding

Worked example. Four compounds with molar masses of $58$ to $60$ g mol${}^{-1}$:

- Butane — $273$ K
- Methoxyethane — $281$ K
- Propanal — $322$ K
- Propan-1-ol — $370$ K

Propanal boils $49$ K above butane but still $48$ K below propan-1-ol.

Solubility. Methanal, ethanal and propanone mix with water in all proportions, but solubility falls quickly as the chain lengthens.

An everyday example. Formalin, a solution of methanal in water, preserves biological specimens — possible only because methanal dissolves so well.

The substance. Aldehydes and ketones cannot hydrogen-bond with each other, having no O–H or N–H, which is why they boil below alcohols.