The correct acidic order of following is
- I > II > III
- III > I > II
- II > III > I
- I > III > II
The Correct Option is B
Solution and Explanation
(a) Due to resonance (which is not possible in alcohols), the oxygen atom of the - OH group acquires a positive charge which helps in the release of a proton.
(b) In the dissociation of phenol to phenoxide ion and a proton the equilibrium lies mainly towards the right hand side as the resulting phenoxide ion is more stabilised by resonance as compared to phenol. The acidic strength of phenols depends on the nature of substituents present in the benzene nucleus.
Electron withdrawing groups like - $NO_2$, - CN, -CHO, - COOH, etc, when present at the ortho and para-positions with respect to phenolic group increases the acidity of phenol due to greater stabilisation of phenoxide ion. While the presence of electron releasing group like - $NH_2,- CH_3,$ etc, decrease the acidity of phenols. This explains the
following order of acidity $\, \, \, \, \, \, \, $p-nitrophenol > phenol > p-cresol.
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Concepts Used:
Phenols
Phenol is a different class of organic compounds that are usually found in wide usage in an extensive range of industries and also for domestic purposes. Phenol is formed when a hydrogen atom in a benzene molecule is replaced by the chemical -OH group.
Phenol is also called as Carbolic acid. It is an aromatic organic compound having the molecular formula C6H5OH. It is a volatile white crystalline solid. The molecule consists of a phenyl group (−C6H5) bonded to a hydroxy group (−OH). Phenol is mildly acidic, it requires careful handling because it can cause chemical burns.
Classification of Phenol
On the basis of the different number of hydroxyl groups attached, the phenols can be classified further into three types.
- Monohydric phenols: These contain one -OH group.
- Dihydric phenols: These contain two -OH groups. These may be ortho-, meta- or para- derivative.
- Trihydric phenols: These contain three -OH groups.
Properties of Phenol
- Phenol is an organic compound that is appreciably soluble in water, with about 84.2 g dissolving in 1000 mL (0.895 M).
- It is a weak acid.
- is highly reactive toward electrophilic aromatic substitution.