Aromatic carbonyl compounds and choosing a route

Carbonyl groups on a benzene ring

An aldehyde or ketone with the carbonyl group on a benzene ring, or next to one, is an aromatic carbonyl compound. Benzaldehyde, acetophenone and benzophenone are the usual ones.

Aromatic aldehydes

1. Etard reaction. Toluene is oxidised with chromyl chloride, $\text{CrO}_{2}\text{Cl}_{2}$, in carbon disulphide. The methyl group becomes a chromium complex, and this is not oxidised further. Dilute acid or water then hydrolyses it to benzaldehyde.

$\text{C}_{6}\text{H}_{5}\text{CH}_{3} \xrightarrow{1.\ \text{CrO}_{2}\text{Cl}_{2}\ \text{CS}_{2}\ 2.\ \text{H}_{3}\text{O}+} \text{C}_{6}\text{H}_{5}\text{CHO}$

2. Side-chain chlorination. Toluene reacts with chlorine in sunlight or at high temperature. The three hydrogens of the methyl group are replaced one at a time. Stopping at two chlorine atoms gives benzal chloride, $\text{C}_{6}\text{H}_{5}\text{CHCl}_{2}$. It is hydrolysed by water at $373\ \text{K}$, and the unstable gem-diol loses water to give benzaldehyde.

$\text{C}_{6}\text{H}_{5}\text{CH}_{3} \xrightarrow{\text{Cl}_{2}\ \text{light}} \text{C}_{6}\text{H}_{5}\text{CHCl}_{2} \xrightarrow{\text{H}_{2}\text{O}\ 373\ \text{K}} \text{C}_{6}\text{H}_{5}\text{CHO}$

If the chlorination goes on to three chlorine atoms, the product is benzotrichloride, and hydrolysis then gives benzoic acid.

3. Gattermann-Koch reaction. Benzene reacts with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride and a little copper(I) chloride. The electrophile is the formyl cation, $\text{HCO}^{+}$. The product is benzaldehyde. It works for benzene and for alkylbenzenes. It does not work well for phenols and their ethers.

$\text{C}_{6}\text{H}_{6} + \text{CO} + \text{HCl} \xrightarrow{\text{AlCl}_{3}\ \text{CuCl}} \text{C}_{6}\text{H}_{5}\text{CHO}$

Aromatic ketones: Friedel-Crafts acylation

Benzene reacts with an acyl chloride (or an acid anhydride) in the presence of anhydrous aluminium chloride. An acyl group replaces a hydrogen of the ring.

$\text{C}_{6}\text{H}_{6} + \text{CH}_{3}\text{COCl} \xrightarrow{\text{AlCl}_{3}} \text{C}_{6}\text{H}_{5}\text{COCH}_{3} + \text{HCl}$

Three things make this reaction a good one:

• The acylium ion, $\text{RCO}^{+}$, does not rearrange, as an alkyl cation can.
• The ketone is less reactive than benzene. So only one acyl group goes on, and there is no polyacylation.
• The ketone binds to aluminium chloride. A little more than one mole of aluminium chloride is needed for each mole of acyl chloride.

Benzene and benzoyl chloride give benzophenone, $\text{C}_{6}\text{H}_{5}\text{COC}_{6}\text{H}_{5}$.

Move the yield to $100\ \%$ and read the theoretical mass. Then move to $60\ \%$. The aluminium chloride readout does not change with the yield. It depends only on the acyl chloride.

Practise with new numbers

Try it in Python

The program takes a mass of benzene and a mass of ethanoyl chloride. It finds the limiting reagent and gives the mass of acetophenone and the aluminium chloride.

Choosing a route

A route is a plan. Ask these questions in order.

1. What is the product? An aldehyde, a ketone, a ring or a chain?
2. How many carbons are there? The carbon count of the starting material must match, unless the reaction adds or removes carbon. A nitrile has one more carbon than the alkyl halide that it came from. An acyl chloride has the same carbon count as the aldehyde it gives.
3. Is the starting material aromatic? Toluene goes to benzaldehyde by Etard or by side-chain chlorination. Benzene goes to a ketone by Friedel-Crafts acylation.
4. Can the reagent attack the product? A strong oxidant destroys an aldehyde. A strong reducing agent reduces the carbonyl to an alcohol.

Take the question: how can pentan-2-one be made from a compound with five carbons? Pent-1-yne with water and mercury(II) sulphate does it. A different answer is pentan-2-ol with an oxidant, or ethanoyl chloride with dipropylcadmium, $\text{CH}_3\text{COCl} + (\text{C}_3\text{H}_7)_2\text{Cd}$.

Common mistakes

• Using Friedel-Crafts alkylation to put an acyl group on. The reagent is an acyl chloride, not an alkyl chloride.
• Forgetting the catalyst must be anhydrous. Water destroys aluminium chloride.
• Using too little aluminium chloride. The ketone ties up the first mole.
• Trying acylation on nitrobenzene. Strongly deactivated rings do not react.
• Writing the Etard reagent as chromic acid. It is chromyl chloride in carbon disulphide.

Remember these

• Toluene to benzaldehyde: Etard (CrO$_2$Cl$_2$, CS$_2$, then H$_3$O$^+$), or Cl$_2$ in light then H$_2$O at $373\ \text{K}$.
• Benzene to benzaldehyde: CO, HCl, anhydrous AlCl$_3$ and CuCl (Gattermann-Koch).
• Benzene to a ketone: acyl chloride and anhydrous AlCl$_3$ (Friedel-Crafts acylation).
• The overall yield of a route is the product of the yields of its steps.

Check yourself

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

1. What does toluene give with chromyl chloride in carbon disulphide, followed by water?

2. Why can Friedel-Crafts acylation not put two acyl groups on benzene easily?

3. Two steps give yields of $80\ \%$ and $50\ \%$. What is the overall yield?

4. Name the reaction that turns benzene, CO and HCl into benzaldehyde.

References

• OpenStax, Chemistry 2e, Chapter 20, section 20.3 Aldehydes, Ketones, Carboxylic Acids, and Esters.
• NCERT, Chemistry Part II for Class XII, Chapter 8, the section on the preparation of aldehydes and ketones.