Assertion (A) : Alcohols act as Bronsted bases as well as Bronsted acids.
Reason (R) : It is due to the presence of unshared electron pairs on oxygen atom and presence of polar \( O - H \) bond.
Reason (R) : It is due to the presence of unshared electron pairs on oxygen atom and presence of polar \( O - H \) bond.
Show Hint
The lone pairs on Oxygen also make alcohols good nucleophiles in organic reactions.
- Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
- Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
- Assertion (A) is true, but Reason (R) is false.
- Assertion (A) is false, but Reason (R) is true.
The Correct Option is A
Solution and Explanation
• A Bronsted acid is a species that can donate a proton (\( H^{+} \)).
• A Bronsted base is a species that can accept a proton (\( H^{+} \)).
• Substances that can behave as both acids and bases are termed amphoteric.
• The reactivity of alcohols (\( R-OH \)) is centered around the Oxygen atom and the \( O-H \) bond.
Step 1: Evaluate the acidic behavior of alcohols
Alcohols have a polar \( O-H \) bond due to the high electronegativity of oxygen.
This polarity allows the hydrogen to be released as a proton (\( H^{+} \)) when reacting with a strong base.
\[ R-O-H + B: \rightarrow R-O^{-} + BH^{+} \] Thus, they act as Bronsted acids.
Step 2: Evaluate the basic behavior of alcohols
The oxygen atom in alcohols has two lone pairs (unshared electron pairs).
These lone pairs can be donated to a proton from a strong mineral acid to form an oxonium ion.
\[ R-O-H + H^{+} \rightarrow R-OH_{2}^{+} \] Thus, they act as Bronsted bases.
Step 3: Verify the link between Assertion and Reason
The Assertion states they are amphoteric (acting as both acid and base), which is true.
The Reason explains that the acidity is due to the polar \( O-H \) bond and basicity is due to the lone pairs on Oxygen, which is also true.
Since these chemical features directly enable the two types of Bronsted behavior, the Reason is the correct explanation for the Assertion. The final answer is (A).
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Alcohols undergo a number of reactions involving the cleavage of C – OH bond. However, phenols do not undergo reactions involving the cleavage of C – OH bond. Alcohols are weaker acids than water. Alcohols react with halogen acids to form the corresponding haloaklanes. Phenols are stronger acids than alcohols. A charac- teristic feature of phenols is that they undergo electrophilic substitution reactions such as halogenation, nitration, etc. Since –OH group is a strong activating group, phenol gives trisubstituted products during halogenation, nitration, etc.
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