Named reactions of the carbonyl group

Names are labels

A named reaction is a reaction that is common enough to need a short label. Each name hides a reagent, a substrate and a limit. When you meet one, ask three questions. What must the substrate have? What does the product have? How many carbons does the reaction add or remove?

Reactions that keep the carbon count

• Rosenmund. An acyl chloride to an aldehyde with H$_2$ and poisoned Pd-BaSO$_4$.
• Stephen. A nitrile to an aldehyde with SnCl$_2$ and HCl, then water.
• Etard. Toluene (any methyl on a ring) to the aldehyde with CrO$_2$Cl$_2$ in CS$_2$.
• Clemmensen. C=O to CH$_2$ with Zn-Hg and conc. HCl. The molecule must survive acid.
• Wolff-Kishner. C=O to CH$_2$ with NH$_2$NH$_2$ and then KOH at about $473\ \text{K}$. The molecule must survive base. The hydrazone anion loses nitrogen, and the carbon takes a proton.
• Hell-Volhard-Zelinsky. An acid to its 2-halo acid with Br$_2$ and red P. It needs an alpha hydrogen.

Reactions that add carbons

• Friedel-Crafts acylation. Benzene and an acyl chloride, with anhydrous AlCl$_3$, give an aryl ketone. It does not work on a ring with a strong withdrawing group.
• Gattermann-Koch. Benzene, CO and HCl, with AlCl$_3$ and CuCl, add one carbon as CHO.
• Aldol. Two molecules with alpha hydrogens, in dilute base, join. The product has the carbons of both.
• Claisen-Schmidt. A crossed aldol in which the aldehyde has no alpha hydrogen. Benzaldehyde and propanone in base give 4-phenylbut-3-en-2-one. With a second benzaldehyde, the result is dibenzylideneacetone, $\text{C}_{17}\text{H}_{14}\text{O}$.
• Perkin. An aromatic aldehyde, ethanoic anhydride and sodium ethanoate, heated to about $453\ \text{K}$, give a cinnamic acid. Benzaldehyde gives 3-phenylprop-2-enoic acid. It adds two carbons.

$\text{C}_{6}\text{H}_{5}\text{CHO} + (\text{CH}_{3}\text{CO})_{2}\text{O} \xrightarrow{\text{CH}_{3}\text{COONa}\ 453\ \text{K}} \text{C}_{6}\text{H}_{5}\text{CH}{=}\text{CHCOOH} + \text{CH}_{3}\text{COOH}$

• Benzoin condensation. Two molecules of benzaldehyde, with cyanide as the catalyst, join to give benzoin, $\text{C}_{6}\text{H}_{5}\text{CH}(\text{OH})\text{COC}_{6}\text{H}_{5}$. The cyanide adds to one aldehyde, and the carbon that carried the C-H becomes a nucleophile. Benzoin has an alpha-hydroxy ketone group. It reduces Tollens' reagent.

Reactions that remove carbons

• Haloform. A methyl ketone or ethanal, with a halogen and base, gives a carboxylate and a haloform, CHX$_3$. The three halogenations are fast, because each halogen makes the next hydrogen more acidic. Then hydroxide attacks, and the $\text{CX}_3^-$ group leaves.

$\text{C}_{6}\text{H}_{5}\text{COCH}_{3} + _{3}\text{I}_{2} + _{4}\text{NaOH} \longrightarrow \text{C}_{6}\text{H}_{5}\text{COONa} + \text{CHI}_{3} + _{3}\text{NaI} + _{3}\text{H}_{2}\text{O}$

• Decarboxylation with soda lime. The sodium salt of an acid gives the alkane with one carbon fewer.
• Kolbe electrolysis. Two carboxylate ions lose CO$_2$, and the two alkyl groups join.

Cannizzaro, once more

Cannizzaro is a redox reaction of one compound. No carbons are added. Half becomes acid and half becomes alcohol.

The Wolff-Kishner reduction gives off one molecule of nitrogen for each ketone molecule. Choose acetophenone and $1.2\ \text{g}$. At $298\ \text{K}$ the gas is $244\ \text{mL}$. Raise the temperature to $373\ \text{K}$, and the same amount of gas takes more room.

Practise with new numbers

Try it in Python

The program lists what each named reaction does to the number of carbons, for a compound with 7 carbons.

Common mistakes

• Mixing up Clemmensen and Wolff-Kishner. One needs acid, the other needs base.
• Calling every crossed aldol a Claisen-Schmidt reaction. It needs an aldehyde with no alpha hydrogen.
• Using the haloform reaction on an ethyl ketone. It needs a CH$_3$CO group.
• Counting the carbons of the haloform acid wrongly. The acid has one carbon fewer than the methyl ketone.
• Forgetting that Kolbe doubles the alkyl group. The product has $2(n - 1)$ carbons.

Remember these

• Same carbons: Rosenmund, Stephen, Etard, Clemmensen, Wolff-Kishner and HVZ.
• More carbons: Friedel-Crafts (plus 2), Gattermann-Koch (plus 1), Perkin (plus 2), aldol, Claisen-Schmidt and benzoin (two molecules join).
• Fewer carbons: haloform and soda lime (minus 1). Kolbe: $2(n - 1)$.
• Each of these has a limit, and the limit is what an exam question tests.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which named reaction turns benzaldehyde into cinnamic acid?

2. What do pentan-2-one and iodine in base give?

3. A substrate has an acid-sensitive tert-butyl ether and a ketone. Which reduction of C=O to CH$_2$ should be used?

4. Why is the haloform reaction not given by pentan-3-one?

References

• Clayden, Greeves and Warren, Organic Chemistry, the chapters on the aldol reaction and on enolates, and on nucleophilic addition to the carbonyl group.
• Morrison and Boyd, Organic Chemistry, the chapters on aldehydes and ketones.
• NCERT, Chemistry Part II for Class XII, Chapter 8, the sections on named reactions.