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Alcohols, Phenols, and Ethers — Chemistry STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 ScienceChemistryAlcohols, Phenols, and Ethers4 Marks
Question
Read the passage given below and answer the following questions:
Both alcohols and phenols are acidic in nature, but phenols are more acidic than alcohols. Acidic strength of alcohols mainly depends upon the inductive effect. Acidic strength of phenols depends upon a combination of both inductive effect and resonance effects of the substituent and its position on the benzene ring. Electron withdrawing groups increases the acidic strength of phenols whereas electron donating groups decreases the acidic strength of phenols. Phenol is a weaker acid than carboxylic acid.
The following questions are multiple choice questions. Choose the most appropriate answer:
  1. Phenols are highly acidic as compare to alcohols due to:
  1. The higher molecular mass of phenols.
  2. The stronger hydrogen bonds in phenols.
  3. Alkoxide ion is a strong conjugate base.
  4. Phenoxide ion is resonance stabilised.
  1. The correct order of acidic strength among the following is:
  1. (III) > (IV) > (II) > (I)
  2. (IV) > (III) > (I) > (II)
  3. (IV) > (III) > (II) > (I)
  4. (I) > (II) > (IV) > (III)
  1. The correct decreasing order of $pK_a$ value is:
  1. II > IV > I > III
  2. IV > II > III > I
  3. II I> II > IV > I
  4. IV > I > II > III
  1. The compound that does not liberate $CO_2$, on treatment with aqueous sodium bicarbonate solution is:
  1. Benzoic acid.
  2. Benzenesulphonic acid.
  3. Salicylic acid.
  4. Carbolic acid.
  1. Most acidic amongst the following is:
✓

Answer

  1. (d) Phenoxide ion is resonance stabilised.
  2. (b) (IV) > (III) > (I) > (II)
Explanation:
The order of acidic strength is,
  1. (a) II > IV > I > III
Explanation:
Weaker acids have higher $pK_a. - OCH_3$ at meta-position exerts only - I effect, hence increases the acidity.
- I effect order: - $NO_2 > - OCH_3 > - Cl$.
- $CH_3$ has + I effect. So, order is (a).
  1. (d) Carbolic acid.
Explanation:
Phenol (Carbolic acid) is a weaker acid than carbonic acid $(H_2CO_3)$ and does not liberate $CO_2​​​​​​​$​​​​​​​ on treatment with aqueous sodium bicarbonate solution.
  1. (c)

​​​​​​​​​​​​​​Explanation:
- $NO_2$ exhibits both - I and - R influence to stabilise the corresponding phenoxide. ln ortho derivative, intermolecular H - bonding lowers the acidity.

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