Phenols

In a nutshell

Phenols are more reactive than benzene due the increased electron density in the $$\pi$$-system. For this reason they can undergo many reactions easily such as bromination and nitration. Substituted benzenes influence the electron density of each carbon in the ring.​

Reactions

These are specific reactions you need to know.

​$$phenol \ + \ bromine \rightarrow 2,4,6-tribromophenol \ + \ hydrogen \ bromide \\ \ \\phenol \ + \ nitric \ acid \rightarrow 2-nitrophenol \ and \ 4-nitrophenol \\ \ \\phenol \ + \ sodium \ hydroxide \rightarrow sodium \ phenoxide \ + \ water \\$$​​

Phenols

Phenols are benzene rings with an $$-OH$$ group directly attached to it. The simplest phenol (no substituents) has the molecular formula $$C_6H_5OH$$.

Example

Reactivity of phenols

The hydroxyl group attached to the benzene increases the electron density of the benzene ring. A lone pair on the oxygen is partially delocalised in to the $$\pi$$-system of electrons in the benzene ring. This makes phenols more reactive than benzene and more prone to electrophilic substitution. 

Substituted benzene

Electrophiles can substitute any hydrogen atom in an unsubstituted benzene ring as all the carbons have the same electron density. However, the electron density on each carbon changes in a substituted benzene. Some carbons are more likely to react than others, depending on the electrophile.

Electron donating groups (EDGs)

Electron donating groups (EDGs) have $$\pi$$-electrons which activate the ring as they become partially delocalised into the benzene ring. They increase the electron density of carbons $$2,4$$ and $$6$$ so electrophiles tend to react at these positions.

Example

Examples of EDGs are $$-NH_2$$ and $$-OH$$ groups.

Electron withdrawing groups (EWGs)

​Electron withdrawing groups (EWGs) tend to be electronegative therefore they withdraw electron density away from the ring, which deactivates the ring. They decrease the electron density of carbons $$2,4$$​ and $$6$$​ so electrophiles do not tend to react at these positions. EWGs direct substitution to occur at carbons $$3$$ and $$5$$.

Example

An example of EWG is the $$-NO_2$$ group.

Predicting reactions

You can predict the product formed from a substituted benzene.

Example

Name the product formed for the following reaction.

​$$C_6H_5NO_2\ + \ CH_3Cl \xrightarrow{FeCl_3} \ ?$$​​

Firstly, determine whether the substituent is EDG or EWG.

$$-NO_2 \rightarrow EWG$$​ 

Nitro group is an EWG which means substitution will occur at positions $$3$$ and $$5$$. As there is only one substituent, positions $$3$$​ and $$5$$ are the same. 

Therefore, the product is $$\underline{3-methylnitrobenzene}$$​​

Bromination

The greater reactivity of phenol compared to benzene means they can react with bromine without a catalyst. As $$-OH$$ is an EDG, substitution occurs at positions $$2,4$$​ and $$6$$. Phenol decolourises bromine water forming the product $$2,4,6$$-tribromophenol, which precipitates out of the solution.

​$$phenol \ + \ bromine \rightarrow 2,4,6-tribromophenol \ + \ hydrogen \ bromide$$​​

Nitration

Phenols can react with dilute nitric acid giving rise to two isomers, $$2-$$nitrophenol and $$4-$$nitrophenol. 

$$phenol \ + \ nitric \ acid \rightarrow 2-nitrophenol \ and \ 4-nitrophenol$$​​​

Forming salts

Phenols are weakly acidic therefore they are able to undergo acid-base reactions, forming a salt. They typically react with sodium hydroxide at room temperature.

​$$phenol \ + \ sodium \ hydroxide \rightarrow sodium \ phenoxide \ + \ water \\ \ \\C_6H_5OH \ + \ NaOH \rightarrow C_6H_5O^-Na^+ \ +\ H_2O$$​​

Synthesising aspirin

Phenols and their derivatives can undergo esterification. This is how aspirin is made in a lab.

​

procedure