Phenol

• Aim:• To introduce phenol and its properties• To describe the reactions of phenol • (i) With aqueous alkalis and sodium

to form salts.• (i) With bromine to form 2,4,6-

tribromophenol.• To explain the relative ease of

bromination of phenol compared with benzene.

What is phenol?

• Phenol is an aromatic alcohol.

• The formula is C6H5OH

• It has a sweet smell.• It is sparingly soluble in water.• It is extremely corrosive and causes white

blisters on the skin.

Solubility?

• The OH group hydrogen bonds to water.

• The ring can only form weak Van der Waal’s forces since it is a large hydrocarbon section of the molecule.

• When a large mass of phenol is added to water 2 liquid layers form, one , the lower layer is a solution of water in phenol and the other, top layer is a solution of phenol in water.

Reactions of Phenol• Phenol was originally called carbolic

acid.• It is a weak acid- a proton donor.• Strength of acid?• Compare ethanol, water and phenol

reactions with sodium metal.• All react to give off hydrogen gas in a

similar reaction where the OH bond breaks.

Comparing alcohols as acids

Water Ethanol Solution of phenol in ethanol

Melts, fizzes, floats. Hydrogen gas will burn if ignited.

Sinks. Steady evolution of hydrogen gas. Does not melt.

Sinks, rapid production of hydrogen gas. Does not melt.

Order of acidity?

• Phenol>water>ethanol• Why?• Phenol a weak solution in ethanol so

does not melt.• Wouldn’t want it to because if it was

hot enough to melt it would ignite the ethanol.

• Explain the observations.

Water as an acid.

• In the reaction :

• 2Na +2H2O → 2NaOH + H2

• The water is behaving as H-OH.• The H-O bond breaks, producing H+

ions and OH- ions.• Reduction takes place:

• 2Na + 2H+ → 2Na+ + H2

• When evaporated the OH- ions and Na+ ions form the ionic sodium hydroxide.

Ethanol as an acid

• Since ethanol also liberates hydrogen the reaction is the same.

• H H• | |• H-C-C-O-H The O-H bond breaks .• | |• H H• This liberates H+ as before.• 2Na + 2C2H5OH→ 2C2H5O-Na+ + H2

Why is ethanol a weaker acid than water?

• This is written as an equilibrium:• C2H5OH + H2O C2H5O- + H3O+

• C2H5OH C2H5O- + H+

• is the simpler alternative.• This equilibrium lies LEFT because

there is a high electron density on the O- which attracts the proton back to recombine.

• The ethoxide ion is a good base due to the +I effect of the chain.

Phenol as an acid

• Phenol is a stronger acid than water because, in the equilibrium the phenoxide ion is a weaker base than the OH- ion because it is a –I group.

• The ion is stabilised by the partial delocalisation of the negative charge over the ring by the oxygen p orbitals interacting with the ring Π cloud.

• This causes the equilibrium to lie RIGHT so that the H+ ion does not recombine.

• The phenoxide ion is a weak base.

2C6H5OH + 2Na → 2C6H5O-Na+ +H2

• C6H5OH C6H5O- + H+

• However all three are weak acids compared with other weak acids ( e.g. ethanoic acid).

• Only phenol will affect UI – but even then it has a pH of only 6.6.

Reaction with alkali

• Only phenol is a strong enough acid (of the three) to react with sodium hydroxide solution.

• C6H5OH + NaOH → C6H5O-Na+ + H2O

• The salt dissolves completely in water (why?) so a colourless solution of sodium phenoxide forms.

• If strong acid is added hydrolysis occurs and phenol reforms as a milky emulsion (why?).

• C6H5O-Na+ + H+ → C6H5OH + Na+

Reactions of the benzene ring

• Undergoes electrophilic substitution reactions MORE READILY than benzene.

• The ring is said to be activated.• It has a higher electron density in the

ring than benzene due to partial delocalisation of the lone pair electrons on the O atom into the ring via the py orbitals.

Wheland Intermediates (not real but an attempt to explain

activation)

OH OH OH

Electrons delocalise into the ring and repel electron density away progressively around the ring as shown, leading to extra electron density at 2,4 and 6 positions of the ring which are therefore more available for electrophilic attack by bromine which does not need a halogen carrier as a result.