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Phenols & Their Reactions

Chemistry: Cambridge International A Level, topic 3.4.7

The Structure and Bonding of Phenols

Phenols are a specific type of aromatic compound that we will look at.

Structure

Structure

  • Phenols consist of a benzene ring with an -OH side group.
  • They are of the formula C6H5OH
The -OH group

The -OH group

  • The oxygen atom has two lone pairs. One of these can overlap with the delocalised p-orbitals and donate a pair of electrons.
  • This increases the electron density in the ring, and makes it more nucleophilic.
  • The electrophilic substitution of phenol with bromine is therefore much faster than with benzene.
  • It does not need a catalyst.
Weak aciditiy

Weak aciditiy

  • Phenol is a weak acid.
  • It is more acidic than other alcohols.
  • It is acidic because the resulting anion is stabilised by donation of the lone pair into the delocalised pi system of the benzene ring.
  • This spreads the charge over the entire molecule and, in doing so, stabilises the negative charge.
  • As phenols are acidic they react with bases. An example reaction with sodium hydroxide is shown below:
  • C6H5OH + NaOH → C6H5ONa + H2O
Bromine water

Bromine water

  • The reaction of phenol with bromine is so easy that it occurs multiple times.
  • Adding phenol to bromine water results in 2,4,6-tribromophenol.
Other reactions of phenols

Other reactions of phenols

  • Phenols also undergo reactions involving diazonium ions ( -N2+).
  • This is covered in the section on amines.

Test yourself on Phenols & Their Reactions

  • Which of the following shows the formula for a phenol?

  • Which of the following best describes the acidic strength of a phenol?

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