Phenol Acidity: Nature, Resonance, Properties and Factors

Phenols are organic compounds and the simplest hydroxyl derivative of benzene. Phenol is used both as a common name and an IUPAC name. They are also referred to as carbolic acids due to their acidic nature. The terms ortho (1,2-disubstituted), meta (1,3 disubstituted), and para (1,4 disubstituted) can be used in substituted forms of phenol, as its structure involves a benzene ring.

• Phenoxide is created when phenols combine with active metals like salt and potassium. 
• Phenols react with aqueous sodium hydroxide to produce phenoxide ions. 
• This reaction indicates that the acidity of phenol is greater than alcohol and water molecules.

Read More: Saturated Solutions

Key Terms: Hybridised carbon, Phenol, Hybridization, Phenoxide, Resonance, Electronegativity.

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Acidic Nature of Phenol

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The acidity of phenol is attributed to its ability to lose hydrogen ions and thus form phenoxide ions.

• In the phenol molecule, the carbon attached directly to the hydroxy group is sp² hybridized.
• This sp² hybridized carbon atom attached to the hydroxy group acts as an electron-withdrawing group as it has higher electronegativity compared to the hydroxyl group.
• The oxygen atom's electron density drops as a result of the greater electronegativity of the sp2 hybridized carbon atom. 
• This increases the polarity of the O-H bond and results in the ionization of phenols.
• The resultant phenoxide ion is formed. 
• The resonance of the benzene ring causes the negative charge to delocalize, stabilizing the phenoxide ion that results.
• The phenoxide ion has greater stability than phenols and a few special properties: a) Well-established due to resonance and b) The oxygen is connected to an sp2 atom which has a higher electronegativity. 
• The carbon atom will pull an electron from oxygen, which makes the phenoxide ion stable due to the distribution of electronegative charge.

Since phenoxide ion is stable, the phenol readily loses hydrogen ions and shows its acidic behavior.

Read More: Benzene or Substituted Benzenes

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Effect of Substituents on Acidity of Phenol

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However, there is a drawback: the stability of the phenoxide ion will be compromised if the benzene ring is substituted in any way. The substituent at the ortho and para position has a more significant influence on acidity compared to the meta position. 

1. Electron-donating groups

• The stability of the phenoxide ion is decreased if electron-donating groups are replaced on phenols because they push those electrons onto the negatively charged oxygen.
• This reduces the acidic nature of phenol, this is the reason for cresol being less acidic than phenol. In cresol, methyl is an electron-donating group that reduces the acidity of phenol.

2. Electron-withdrawing groups

• If electron-withdrawing groups are substituted with phenol, they pull the electrons from the negatively charged oxygen atom, this increases the stability of the phenoxide ion.
• This increases the acidity of phenol, this is the reason nitrophenol is more acidic than phenol. In nitrophenol, NO₂ is an electron-withdrawing group that increases the acidity of phenol. 

Read More: Unsaturated Solutions

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Resonance of Phenol

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A structure is said to have resonance when more than one Lewis Structure can be drawn.

• In resonance, electrons are delocalized over 3 or more atoms of a compound or molecule. 
• A Lewis It is impossible to describe the structure of the molecule as a simple, uniform structure.
• The oxygen atom in the molecule has a positive charge in three of the four contributing configurations. 
• Therefore, the true hybrid structure must have a partial positive charge. 
• Since hydrogen is a partly positive element and oxygen is an electronegative element, the electrons in the oxygen-hydrogen bond orbital are drawn to the oxygen atom.
• The loss of a hydrogen ion to a base creates a phenoxide ion, which is completely resonance stabilized.
• The phenoxide anion results from the removal of hydroxy hydrogen by a base. 
• This anion is resonance stabilized by the delocalization of an electron pair all over the molecule.

Structure of Phenol

Read More: Structure of Amines

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Physical Properties of Phenol

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Below mentioned are the properties of phenol as an acid

• State: Phenol is a colorless liquid or solid. They turn reddish-brown due to oxidation in the atmosphere.
• Boiling Point: The boiling point of phenol increases with an increase in the number of carbon atoms due to an increase in Van Der Waals's force.
• Solubility: The solubility of phenols is due to their ability to form hydrogen bonds with water molecules. The solubility decreases with an increase in hydrophobic groups.
• Reaction with O–H bond cleavage: Phenols react with metals like sodium, potassium, and aluminum to form phenoxide.

Read More: Classification of Organic Compounds

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Properties of Phenol as an Acid

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Below mentioned are the properties of phenol as an acid

1. With Indicators: The litmus paper is blue at pH 8 and red at pH 5. 

• Anything in between is neutral. 
• According to its content, a typical diluted solution of phenol in water is probably between 5 and 6.

2. With Sodium Hydroxide solution: Phenol reacts with sodium hydroxide to give a colorless solution called sodium phenoxide. 

• The hydrogen ion here is removed by the strongly basic hydroxide ion in sodium hydroxide.

3. With sodium carbonate or sodium hydrogen carbonate: Phenol is not acidic enough, as it does not give out carbon dioxide when one mixes it with sodium carbonate or sodium hydrogen carbonate. This lack of CO₂ formation is useful as it makes phenol

• Fairly insoluble in water.
• Reacts with NaOH to give a colorless solution (must be acidic).
• It does not produce carbon dioxide with sodium carbonate or hydrogen carbonate solution (must be only weakly acidic)

4. With metallic sodium: Acid reacts with more reactive metals to give hydrogen gas. 

• Phenol is also not an exception although, the reaction of phenol is comparatively slow. 
• Phenol is warmed in a dry tube until it is molten and a small piece of sodium is added. 
• The hydrogen gas escapes with a frizz. 
• The mixture left in the tube contains sodium phenoxide.

Read More: Aliphatic Hydrocarbons

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Factors Governing the Acidity of Phenol

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Below mentioned are the factors affecting the acidity of Phenol

1. Element effects: The compounds are more acidic when the element to which the acidic hydrogen is bound has a higher atomic number within either a row or group of the periodic table. 

• The effect within a row (period) of the periodic table is dominated by relative electronegativities (or electron affinities). 
• Thus, water is much more acidic than methane, and phenol is much more acidic than toluene because oxygen (higher atomic number) is more electronegative than carbon (lower atomic number). 
• Relative bond energies dominate the impact inside a column of the periodic table. 
• Thus, thiols are more acidic than alcohols because the SLH bond is weaker than the OLH bond. 

2. Charge effects: A positive charge on the atom to which the acidic proton is attached enhances acidity. For example, H3O| is more acidic than H2O. 
3. Resonance effects: Enhanced delocalization of electrons in the conjugate base enhances acidity. 
4. Polar effects: Stabilization of charge in the conjugate base enhances the acidity.

Read More: Organic Solvent

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Things to Remember

• Phenol's acidic nature can be contributed to its sp² hybridized carbon, resonance, phenoxide ion, and polar effects.
• A structure is said to be in resonance when more than one Lewis Structure can be drawn.
• Electron-donating and electron-withdrawing groups also affect the acidity of Phenol.

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