The reactions of phenols


Phenol bottle and sample of phenol	A closer look!

Phenols are compounds in which an OH group is directly attached to an arene ring. Phenol itself is a pale pink solid with very corrosive properties. Contact with the skin leads quickly to the formation of unpleasant blisters. Consequently, we shall be using a less corrosive alternative called methyl 4-hydroxybenzoate:


phenol	methyl 4-hydroxybenzoate

Watch the videos in each of the following sections. Try to answer all of the questions and then check your conclusions about phenolic compounds using the links below.

1 Solubility in water

Phenol itself is only sparingly soluble in water. When added to water the solid forms as an oily liquid under the water. Try to think why this happens. You might like to look at the phenol label above.

Look at the structure of methyl 4-hydroxybenzoate and try to decide whether it will dissolve in hot and/or cold water:

Video - the solubility and pH of methyl 4-hydroxybenzoate in water

You will see from the pH of the resulting solution that this compound is weakly acidic. Phenol itself is around pH 6 in solution. This is more acidic than neutral alcohols but much less acidic than the weak carboxylic acids. The reason for this can be found in the section on solubility and pH of phenol.

2 Phenolic compounds as acids

Video - action of sodium on phenols

What is the gas seen produced by this reaction? Which bond is broken in the phenolic compound? Try to write an equation for the reaction which takes place. You might like to remind yourself of the reactions with alcohols.

Video - action of sodium hydroxide solution on phenols

The phenolic compound is clearly much more soluble in sodium hydroxide solution than it was in water. Try to write an equation for the reaction which takes place and explain the much increased solubility. What is the explanation for your observation on adding concentrated hydrochloric acid to the newly formed solution?

Video - action of sodium carbonate on phenols

Did you notice any effervescence of carbon dioxide gas in this experiment? What does this tell you about the acidic strength of this phenolic compound?

Theory - phenols as acids

3 Reaction with an organic acid

You might want to check out the theory of the formation of esters by this general reaction:

ACID + ALCOHOL ⇒ ESTER + WATER

Phenols are insufficiently reactive to react readily with an acid. However, they will react with the more reactive acid anhydrides, like ethanoic anhydride, as seen in the video. Try to write an equation for the reaction seen and suggest why phenols are less effective nucleophiles than alcohols.

Video - the formation of an ester

Theory - phenols in ester formation

4 Properties of the benzene ring

Apart from possessing a phenolic functional group, phenols also possess a benzene ring. In this section we investigate whether the presence of the phenolic group influences the properties of the benzene ring. You might like to review the arene practical which we saw earlier.

Combustion

Video - combustion of a phenolic compound

Describe the type of flame that you see and explain it in terms of the molecular formula for the compound. We have used this type of argument before.

Reaction with bromine

Video - the reaction of methyl 4-hydroxybenzoate with bromine

With phenol an electrophilic substitution reaction occurs in which three atoms of bromine become attached to the ring producing a symmetrical compound. Try to write an equation for the reaction between phenol and bromine. Explain why there is a difference in the reactivity of the phenol towards bromine compared to that of the simple arenes. Suggest an equation for the reaction between methyl 4-hydroxybenzoate and bromine.

Reaction with nitric acid

Video - the reaction of nitric acid with methyl 4-hydroxybenzoate

In this reaction the addition of the nitro group reduces the reactivity of the ring so that only one substitution takes place. Try to write an equation for the reaction between phenol and nitric acid and explain how many isomers are possible. The same explanation as that with bromine should explain the difference in reactivity as compared to the reaction of a simple arene.

Theory - the properties of the phenolic benzene ring