Phenol

What is phenol?

Phenol is an example of a benzene derivative. Benzene derivatives are formed when we take a benzene molecule and replace one or more of its hydrogen atoms with different substituents. Removing a hydrogen atom from benzene forms the phenyl group (C₆H₅-); the replacement substituent determines the identity of the benzene derivative. In phenol, we replace benzene's hydrogen atom with the hydroxyl group (-OH).

Like all benzene derivatives, phenol still contains benzene's characteristic ring of delocalised electrons. A benzene ring is also known as the arene functional group. This enables phenol to act similarly to benzene. However, it behaves in its own unique way thanks to its hydroxyl group. We'll explore this in more detail when we look at the properties and reactions of phenol

Phenol structure and formula

As we mentioned, phenol consists of a phenyl group (C₆H₅-) bonded to a hydroxyl group (-OH). Put the two together, and you end up with the formula for phenol: C₆H₅OH.

To show how phenol relates to benzene, we've shown their two structures below.

Fig. 1: The structures of benzene (left) and phenol (right), with their functional groups highlighted.StudySmarter Originals

Properties of phenol

Now that we know what phenol is, we can look at its properties. Importantly, we can look at the interactions between its two functional groups (the phenyl group and the hydroxyl group) and how they set phenol apart from similar molecules.

Melting and boiling point

You might know from articles such as Alcohols that molecules with the hydroxyl functional group (-OH) can form hydrogen bonds with each other. This is because of the large difference in electronegativity between oxygen and hydrogen. Hydrogen bonds are the strongest type of intermolecular force and require a lot of energy to overcome. As a result, molecules that experience hydrogen bonding have high melting and boiling points.

For example, compare methylbenzene (C₆H₅CH₃) and phenol. Both have the same number of electrons but very different boiling points. This is because phenol experiences hydrogen bonding, thanks to its -OH group, whereas methylbenzene does not.

Solubility

Many short-chain molecules with the hydroxyl functional group are soluble in water. This is because they can form hydrogen bonds with H₂O molecules. For example, ethanol (C₂H₅OH) and propanol (C₃H₇OH), both examples of alcohols, readily form aqueous solutions. But phenol, like other longer-chain alcohols, is only slightly soluble in water. This is because its non-polar phenyl group disrupts the hydrogen bonding between its hydroxyl group and water molecules, weakening the attraction between them.

Acidity

Phenol is weakly acidic. It loses a proton from its hydroxyl group when in aqueous solutions, forming a phenoxide ion (C₆H₅O^-).

Fig. 3: Phenol is weakly acidic and so ionises in aqueous solution.StudySmarter Originals

Directing effects

Phenol, like many benzene derivatives, takes part in electrophilic substitution reactions. These reactions swap one of the hydrogen atoms in the phenyl group's aromatic benzene ring for a different atom or group. However, substitution reactions aren't random, and the hydroxyl group plays a part in determining which hydrogen atom is replaced. The hydroxyl group encourages electrophiles to attack certain carbon atoms in the benzene ring; this is known as a directing effect.

In particular, hydroxyl groups are electron-donating groups. If we count phenol's C-OH bonded carbon as number 1 in the aromatic carbon ring, the hydroxyl group encourages electrophiles to attack carbons 2, 4, and 6. This means that substituents tend to bond to these carbons, replacing a hydrogen atom.

Fig. 5: The directing effects of phenol's hydroxyl group favour substitution of the hydrogen atoms bonded to carbons 2, 4, and 6.StudySmarter Originals

Production of phenol

Before we learn about the reactions of phenol, it is helpful to understand how we produce it.

Production of phenol using phenylamine

For your exams, you need to know about producing phenol from aromatic amines. This is a three-step procedure, although the first two steps happen in situ. Briefly put, we react phenylamine (C₆H₅NH₂) with nitrous acid (HNO₂) to produce an unstable diazonium salt. The salt then decomposes into phenol. Here's the reaction.

Forming nitrous acid

Our second reactant, besides phenylamine, is nitrous acid. Nitrous acid (HNO₂; also known as nitric(III) acid) is a very unstable acid and so needs making in situ by mixing sodium nitrite (NaNO₂) with an acid (typically hydrochloric acid, HCl). We carry out this step in an ice bath, keeping the temperature below 10 ^oC. The reaction also forms a salt, depending on the acid used, but here we have simply represented the acid using H⁺:

NaNO₂ + H⁺ → HNO₂ + Na⁺

Forming a diazonium salt

In next step of the reaction, the nitrous acid reacts with phenylamine and an acid. The reaction is once again cooled to below 10 ^oC using an ice bath. Overall, we produce a diazonium salt and water.

$$C_{6}H_{5}NH_{2} + HNO_{3} + H^{+} \rightarrow C_{6}H_{5}N^{+}\equiv N + 2H_{2}O$$

Forming phenol

Diazonium salts aren't very stable. Heating the solution we just formed causes the diazonium salt to react with water. It decomposes into phenol (C₆H₅OH) and nitrogen (N₂), and regenerates the acid (H⁺) used in step 2:

$$C_{6}H_{5}N^{+}\equiv N + H_{2}O \leftarrow C_{6}H_{5}OH + N_{2} + H^{+}$$

Reactions of phenol

Let's conclude by introducing you to the reactions of phenol. We can roughly break them down into two categories:

• Reactions involving the hydroxyl group (-OH).
• Reactions involving the phenyl group (C₆H₅-).

Reactions involving the hydroxyl group

Phenol's hydroxyl group reacts in much the same way as the hydroxyl group in water or alcohols. That's handy for you as a student, because it means you have fewer reactions to learn!

• Phenol's hydroxyl group is weakly acidic and so reacts with strong bases to produce a phenoxide salt and water. For example, reacting phenol with sodium hydroxide (NaOH) produces sodium phenoxide (C₆H₅O^-Na⁺) and water. Phenoxide salts dissolve readily in aqueous solution.
• Phenol's hydroxyl group also reacts with sodium itself, or in fact any other reactive metal. This forms a type of phenoxide salt once again, but this time the additional product is hydrogen gas (H₂). For example, reacting sodium metal with phenol produces sodium phenoxide and hydrogen.

Reactions involving the phenyl group

Phenol also takes part in some of the typical reactions of benzene, thanks to its aromatic phenyl group. These include electrophilic substitution reactions such as bromination.

• Phenol reacts with dilute nitric acid (HNO₃) at room temperature to form nitrophenol and water. Increasing the concentration of the acid causes multiple substitutions to occur. This is an example of nitration.
• Phenol also reacts with bromine water (Br₂) at room temperature to form a tribromophenol, in which three of phenol's hydrogen atoms have been substituted for bromine atoms. A similar reaction occurs if you use chlorine or iodine instead. This is an example of halogenation.
• Phenol also reacts with diazonium salts in alkaline conditions to form azo compounds.

Here's a mind-map summarising the reactions of phenol.

Fig. 6: A mind-map of the reactions of phenol. We've highlighted the changes in structure of the molecules formed compared to phenol itself.StudySmarter Originals