An Introduction to Phenol Acidity
Phenol is a common name for a compound. It is attached to a hydroxyl group to an atom number of an atomic ring.The IUPAC name of phenol is benzonal. The substitution of phenol with either the Ortho meta para or the numbering system can be employed. In either of the cases, the parent molecule is to be referred to as phenol. Common names and given to certain phenols for example phenols are known as cresols. Due to their high acidity, phenols are also known as carbolic acids.
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 the 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.
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Similarly, phenols can also lose H+ from the -OH group by showing acidic behavior.
When phenol loses ion, they form a phenoxide ion.
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The phenoxide structure forms as,
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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.
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But, if the electron-withdrawing groups are substituted with 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 for phenol, it increases its acidity. Because of this, nitrophenol is more acidic than phenol.
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Thereby, the position of the substituent group also affects the acidity of phenol. The substituent at ortho and para position has a more significant influence on acidity compared to the meta position.
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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 meta positions.
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.
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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.
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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
A 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.
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 acidic 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.
Physical Properties of Phenol Acidity
Physical state: Phenols are colourless solids or liquids. However, due to oxidation they mostly turn reddish-brown in the atmosphere.
Boiling point: An increase in the number of carbon atoms due to Van Der Waals forces, increases the boiling point of phenol.
Solubility in water: Phenols are readily soluble in water due to their ability to form hydrogen bonding but the solubility decreases due to the addition of other hydrophobic groups and the ring.
Reactions involving O–H bond cleavage: Phenols react with metals such as Na, K, and AI, etc with the release of hydrogen gas to form phenoxide.
Conclusion
This is all about the structural, physical, and chemical properties of phenol. Its unique properties due to the resonance of the constituent atoms of the molecule make it different. Focus on the conceptual description here and understand how it behaves in different chemicalreactions.
FAQs on Phenol Acidity
1. Why is phenol considered acidic in nature?
Phenol is considered acidic because it can donate a proton (H⁺ ion) from its hydroxyl (-OH) group to form the stable phenoxide ion. This acidic character arises due to the unique interaction between the hydroxyl group and the aromatic benzene ring.
2. How does resonance contribute to the acidity of phenol?
The acidity of phenol is greatly enhanced by resonance. After phenol donates a proton, it forms the phenoxide ion (C₆H₅O⁻). The negative charge on the oxygen atom is not localised; instead, it is delocalised across the entire benzene ring through resonance. This delocalisation of the negative charge creates several stable resonance structures, which significantly stabilises the phenoxide ion. A more stable conjugate base results in a stronger acid.
3. Why are phenols significantly more acidic than alcohols?
Phenols are more acidic than alcohols due to the difference in the stability of their conjugate bases.
- Phenol: Forms a resonance-stabilised phenoxide ion, where the negative charge is spread over the benzene ring, making it very stable.
- Alcohol: Forms an alkoxide ion (RO⁻), where the negative charge is localised on the oxygen atom. The alkyl group (-R) is electron-donating, which further intensifies the negative charge and destabilises the alkoxide ion.
Because the phenoxide ion is much more stable than the alkoxide ion, phenol is more willing to donate a proton and is therefore a stronger acid.
4. How do different groups attached to the benzene ring affect the acidity of phenol?
Substituents on the benzene ring have a profound effect on phenol's acidity:
- Electron-withdrawing groups (EWGs): Groups like -NO₂, -CN, and -X (halogens) increase the acidity of phenol. They pull electron density away from the ring, which helps to further delocalise and stabilise the negative charge on the phenoxide ion.
- Electron-donating groups (EDGs): Groups like -CH₃ (alkyl), -OCH₃ (alkoxy), and -NH₂ push electron density into the ring. This intensifies the negative charge on the phenoxide ion, making it less stable and thus decreasing the acidity of the phenol.
5. How would you arrange phenol, p-nitrophenol, and p-cresol in increasing order of their acidic strength?
The correct increasing order of acidic strength is: p-cresol < phenol < p-nitrophenol. The reasoning is based on the substituent effects:
- p-Cresol: Has an electron-donating methyl group (-CH₃), which destabilises the phenoxide ion, making it the weakest acid.
- Phenol: Has no substituent, serving as the baseline for comparison.
- p-Nitrophenol: Has a strong electron-withdrawing nitro group (-NO₂), which significantly stabilises the phenoxide ion through resonance, making it the strongest acid of the three.
6. What is the role of the sp² hybridised carbon in phenol's acidity?
The hydroxyl group in phenol is attached to an sp² hybridised carbon of the benzene ring. This type of carbon is more electronegative than the sp³ hybridised carbon found in alcohols. Due to its higher electronegativity, the sp² carbon pulls electron density from the oxygen atom of the -OH group. This electron-withdrawing inductive effect makes the O-H bond more polar and facilitates the release of the H⁺ ion, contributing to phenol's acidic character.
7. If phenol is acidic, why is it considered a weak acid compared to carboxylic acids?
While phenol is acidic, it is a much weaker acid than carboxylic acids (like benzoic acid). This is because the conjugate base of a carboxylic acid, the carboxylate ion (RCOO⁻), is significantly more stable than the phenoxide ion. In the carboxylate ion, the negative charge is delocalised over two highly electronegative oxygen atoms, which is a more stable arrangement than the delocalisation over one oxygen and several less electronegative carbon atoms in the phenoxide ion.
8. What are some important uses of phenol related to its chemical properties?
Phenol and its derivatives are crucial in various industries. Based on its properties, some key uses include:
- Antiseptics and Disinfectants: Dilute solutions of phenol (carbolic acid) were one of the first surgical antiseptics and are still used in disinfectants like Dettol (chloroxylenol).
- Polymer Manufacturing: It is a key monomer for producing polymers like Bakelite (a phenol-formaldehyde resin) and Nylon.
- Pharmaceuticals: It serves as a precursor for synthesising many drugs, including aspirin and phenacetin.
- Chemical Synthesis: It is used to prepare other important chemicals like picric acid (an explosive) and various dyes.
