Nomenclature, Structure and Reactivity of Phenols: MDCAT Chemistry notes

Structure and names

A phenol has an $-\mathrm{OH}$ group attached directly to a benzene ring. The carbon bearing the OH is part of the ring and is sp$^2$ hybridized. A lone pair of the oxygen overlaps with the ring's $\pi$ system, which gives the C–O bond some double-bond character.

Physical properties and uses

• Colourless, crystalline, deliquescent solid; melting point about 41$^\circ$C, so it is a solid at room temperature (unlike ethanol, methanol or butane).
• Turns pink on standing in air because of slow oxidation.
• Characteristic smell; poisonous and corrosive, blistering the skin.
• Used by Lister as one of the earliest antiseptics in surgery; also used for disinfectants, Bakelite, dyes and drugs.

Acidity of phenol

Phenol is a weak acid: $\mathrm{C_6H_5OH \rightleftharpoons C_6H_5O^- + H^+}$. The phenoxide ion is stabilized because its negative charge is delocalized into the ring (over the ortho and para carbons). An alkoxide ion cannot spread its charge, so alcohols are weaker acids. The acidity is thus due to the nature (resonance stabilization) of the phenoxide ion.

• Phenol reacts with NaOH to give sodium phenoxide and water.
• Phenol is weaker than carbonic acid, so it does not liberate $\mathrm{CO_2}$ from carbonates or bicarbonates (e.g. $\mathrm{CaCO_3}$, $\mathrm{NaHCO_3}$).
• Sodium phenoxide with HCl (a stronger acid) gives back phenol.

Reactions of phenol

Reactions of the ring (electrophilic substitution)

The $-\mathrm{OH}$ group strongly activates the ring and directs incoming groups to the ortho and para positions, so phenol reacts far more easily than benzene.

• Bromine water at room temperature gives a white precipitate of 2,4,6-tribromophenol.
• Dilute $\mathrm{HNO_3}$ at room temperature gives a mixture of o-nitrophenol and p-nitrophenol.
• Concentrated $\mathrm{HNO_3}$ (with conc. $\mathrm{H_2SO_4}$; 3 moles) nitrates positions 2, 4 and 6, giving 2,4,6-trinitrophenol, picric acid. Di-nitration (2,4-dinitrophenol) is an intermediate stage of this nitration of phenol, not of benzene.
• Sulphonation with conc. $\mathrm{H_2SO_4}$ gives o- and p-phenolsulphonic acids.
• With formaldehyde it condenses to Bakelite.

Other reactions

• Distillation with zinc dust removes oxygen: $\mathrm{C_6H_5OH + Zn \rightarrow C_6H_6 + ZnO}$, giving benzene.
• Hydrogenation with $\mathrm{H_2}$/Ni at high temperature gives cyclohexanol, not benzene.
• The electron-rich ring is easily oxidized, e.g. by chromic acid to benzoquinone, so phenols are very reactive towards oxidizing agents.
• Neutral $\mathrm{FeCl_3}$ gives a violet colour with phenol.

Common MDCAT traps

• 1,3-diol is resorcinol; 1,2-diol is catechol. Cresol is methylphenol.
• "Carbolic acid" is phenol; distractor spellings such as "carpolic" or "carbonylic" are wrong.
• Phenol + carbonate gives no $\mathrm{CO_2}$; carboxylic acids do.
• Benzene from phenol needs zinc dust; $\mathrm{H_2}$/Ni gives cyclohexanol.
• 3 moles conc. $\mathrm{HNO_3}$ give picric acid, not a mixture of mono-nitrophenols.

Quick revision

• C–OH carbon of phenol is sp$^2$.
• Phenoxide ion is resonance-stabilized.
• Phenol: deliquescent crystalline solid, m.p. about 41$^\circ$C.
• Picric acid = 2,4,6-trinitrophenol.
• Lister used phenol as an antiseptic.