Phenol Acidity

Phenols are the organic compounds having benzene ring bonding to a hydroxyl group, which are also known as carbolic acids (phenol carbolic acid). Phenols usually react with active metals such as potassium, sodium and forms phenoxide. Happening such reactions of phenols with metals indicates it is acidic in nature.

Phenols also react with aqueous sodium hydroxide to produce phenoxide ions. It shows the acidity of phenols is higher compared to alcohol and water molecules as well.

Explaining acidity of phenol

The acidity of phenol is because of its ability to lose the hydrogen ion forming phenoxide ions.

  • All alcohols have a common property. They can lose H+ from the OH group in the presence of a suitable base providing an acidic character to alcohol.

(image will be uploaded soon)

Similarly, phenols can also lose H+ from the -OH group by showing the acidic behavior.

  • When phenol loses ion, they form a phenoxide ion.

(image will be uploaded soon)

  • The phenoxide structure forms as,

(image will be uploaded soon)

This phenoxide ion structure has a few special properties that are:

  • Phenoxide ion is well established due to the resonance

  • The oxygen is connected to sp2 carbon, which has a high electronegativity.

 So, the carbon will pull e- from the oxygen. And, this makes the phenoxide ion stable due to the distribution of the electronegative charge. 

  • Since the phenoxide ion is completely stable, phenol readily loses a hydrogen ion and shows the acidic character

  • However, if any substituent is attached to the benzene ring, the stability of the phenoxide ion will be affected

Let’s look at the effect of substituents on the acidity of Phenols.

  • If electron-donating groups are substituted on phenol, they push those electrons on the negative charged O. And, this reduces the phenoxide ion’s stability.

  • So, if the electron-donating groups are substituted on phenol, resultantly, its acidity reduces. Due to this reason, cresol is less acidic than phenol.

(image will be uploaded soon)

  • But, if the electron-withdrawing groups are substituted phenol, they pull the electrons from the negatively charged O, which increases the stability of the phenoxide ion.

  • So, if the electron-withdrawing groups are substituted on phenol, it increases its acidity. Because of this, nitrophenol is more acidic than phenol.

(image will be uploaded soon)

  • Thereby, the position of the substituent group also affects the acidity of phenol. Substituent at ortho and para position has more significant influence on acidity compared to the meta position. 

(image will be uploaded soon)

  • If a substituent is an EWG (Electron Withdrawing Group), delocalization of negative charge will be more when it lies in ortho and para position.So, EWG will cause an increased acidity rate when the group is at ortho and para positions compared to that of meta position.

Resonance of Phenol

When more than one Lewis structure can be drawn, either the ion or the molecule is said to have resonance.

Resonance is a concept where electrons are delocalized over three or more atoms of a compound or molecule and the Lewis structure of that molecule cannot be depicted as a single and straightforward structure. 

(image will be uploaded soon)

Observe that three of the four contributing structures possess a positive charge on the molecule's oxygen atom. Therefore, the true hybrid structure must have a partial positive charge. Since oxygen is an electronegative element, the electrons in the oxygen-hydrogen bond orbital attract to the oxygen atom, resulting in partially positive hydrogen.

The loss of a hydrogen ion to a base creates a phenoxide ion, which is completely resonance stabilized.

(image will be uploaded soon)

Also, observe that the phenoxide anion results upon the removal of hydroxy hydrogen by a base. This anion is resonance stabilized by delocalization of an electron pair all over the molecule, such as depicted by the contributing structures.

Properties of Phenol as an Acid

Few of the phenol’s properties by combining with different solutions are listed below.

With indicators

The pH value of a typical dilute solution of phenol in water is approximately to be of 5 - 6 depending on its concentration. It means a very dilute solution is not really acidic enough to turn a litmus paper ultimately to red. Whereas litmus paper will be blue at pH = 8, and at the same time, red at pH = 5. If anything in between exists, it will be shown with some shade of “neutral.” Phenol reacts with the sodium hydroxide solution resulting in a colorless solution with sodium phenoxide.

(image will be uploaded soon)

During this reaction, the hydrogen ion was removed by the strongly basic hydroxide ion in the sodium hydroxide solution.

With Sodium Carbonate or Sodium Hydrogen Carbonate

Phenol is not acid enough to react with any of these. Going towards another approach, carbonate and hydrogen carbonate ions are not solid enough to remove a hydrogen ion from phenol. Unlike most acids, phenol does not give carbon dioxide when you mix it with one of them. In addition, this lack of reaction is quite useful. You can also recognize phenol because of the reasons listed below.

  • It is favorably insoluble in water

  • It often reacts with sodium hydroxide solution to produce a colourless solution and must be acidic.

FAQ (Frequently Asked Questions)

Why is Ortho Hydroxybenzoic Acid more Acidic Compared to Para Hydroxybenzoic?

Let's have a look at the two figures listed below.

(image will be uploaded soon)

The hydroxyl group is located closer to the carboxylic acid group in salicylic acid compared to para-hydroxy benzoic acid.

This increases the acidity in two different ways.

1. The electronegative hydroxyl group stabilizes the ionized component of the carboxylic acid group. It happens with every carboxylic acid as adjacent hydrogen atoms are substituted with more electronegative atoms such as fluorine, chlorine, or Oxygen.

2. The negative partial charge of the oxygen atom weakly pulls the hydrogen ion away from the carboxylic acid group, enabling dissociation.

By all these observations, we can say that O-Hydroxybenzoic acid is more acidic when compared to P-Hydroxybenzoic acid because it has a more stable conjugate base.

How can we Decide the Acidic Nature of Phenols?

Phenols are acidic species. This means they can give H+ ions in the solution.

Let's look into why they do so.

We know that the acidic strength is directly proportional to the conjugate base's stability, which means more stable is the conjugate base, and more acidic is the compound.

In phenols, since H+ leaves Phenol, negative charge is absorbed by Oxygen, and there is no issue with that because Oxygen is nearly electronegative with a value of 3.5. So, if it has a negative charge on it, it won't create any problems.

Secondly, the most significant thing is the resonance. The negative charge on Oxygen and the π bond of Carbon in the ring is the conjugation, which is the origin of the resonance. Resonance decreases the negative charge of Oxygen by pulling the negative charge against itself as in the benzene ring. Carbon atoms may, therefore, easily retain the negative charge.

The negative charge is distributed in the ring and on Oxygen, reducing the compound's energy, adding to its stability.