Rings all around you

Section 4 of 4

Put functional groups from lesson 5 on a benzene ring and you get molecules from your kitchen and your medicine cupboard.

In the 1800s, chemists called benzene and its relatives aromatic compounds, because many of them have strong smells. Chemists still use the word for molecules built on a benzene-like ring, whatever they smell like. In a skeletal formula, each group sits on a corner of the ring in place of a hydrogen.

Vanillin, C₈H₈O₃A benzene ring carrying three groups, an aldehyde group (-CHO), an -OH group and an oxygen atom joined to a methyl group.OOHO
Vanillin, C₈H₈O₃, the main flavour of vanilla. The aldehyde group is at the bottom right, the OH on the left, and an O joined to a methyl group at the top.
Cinnamaldehyde, C₉H₈OA benzene ring joined to a three-carbon chain with a C=C double bond, ending in an aldehyde group (-CHO).O
Cinnamaldehyde, C₉H₈O, the taste and smell of cinnamon. A short chain with a C=C joins the ring to an aldehyde group.

Vanillin gives vanilla its taste and smell. Its ring carries an aldehyde group, -⁠CHO, an -⁠OH group, and an oxygen joined to a methyl group, -⁠OCH₃. Vanilla pods are expensive, so most vanillin is made in factories. It is the same molecule as the vanillin in the pod.

Cinnamaldehyde gives cinnamon its taste and smell, and its name says what it is: the aldehyde from cinnamon. A chain of 3 carbons with a C=C double bond hangs off the ring, and its last carbon is the aldehyde group, -⁠CHO.

Aspirin, C₉H₈O₄A benzene ring carrying a -COOH group and, on the next corner, an oxygen joined to a two-carbon piece with a double-bonded oxygen.OOHOO
Aspirin, C₉H₈O₄. The -⁠COOH group and the ester link, -⁠COO⁠-, sit on neighbouring corners of the ring.

Aspirin, a medicine for pain and fever, has a carboxylic acid group and an ester link on its ring, two groups you already know. For thousands of years people chewed willow bark to ease pain. It contains salicin, which the body turns into salicylic acid. In 1897, chemists at the Bayer company joined an ethanoic acid piece to the -⁠OH group of salicylic acid, making an ester, and the result was aspirin.

Benzene rings turn up in many more places. Paracetamol, another painkiller, is built on one. Indigo, the blue dye in denim jeans, has two.

The rings at a glance

Ring molecules in this lesson
MoleculeFound in or used for
cyclohexanemaking nylon
benzenemaking other chemicals
vanillinvanilla
cinnamaldehydecinnamon
aspirinmedicine

Quick check

Which of these molecules is built on a benzene ring?

That's the end of lesson 6

In this lesson you found out:

  • How a carbon chain closes into a ring with 2 fewer hydrogens
  • Why cyclohexane's ring folds into a chair
  • Benzene's flat ring and the six electrons it shares
  • Where benzene rings turn up in flavours, medicines and dyes
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