Question

Given below are two statements, one is labeled as Assertion (A), and the other one is labeled as Reason (R).
Assertion (A): Alcohols are weaker acids than water.
Reason (R): Water is a better proton donor than alcohol.
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A).
• B. Both (A) and (R) are correct but (R) is NOT the correct explanation of (A).
• C. (A) is correct but (R) is not correct.
• D. (A) is not correct but (R) is correct.

Hint:

To compare the acidity of organic compounds, look at the inductive effect of the attached groups.
Alkyl groups are electron-donating ($+\text{I}$ effect) and destabilize the conjugate alkoxide ion, making alcohols weaker acids than water.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The acidic strength of a chemical species in a Brønsted-Lowry framework is defined by its ability to donate a proton ($\text{H}^+$) to a base.
When an acid donates a proton, it is converted into its corresponding conjugate base.
The stability of this conjugate base directly dictates the equilibrium position of the dissociation reaction.
A more stable conjugate base corresponds to a stronger starting acid, as the system favors the forward dissociation pathway.
For water and aliphatic alcohols, the acidic strength is compared by analyzing the stability of the hydroxide ion ($\text{OH}^-$) relative to the alkoxide ion ($\text{R-O}^-$).

Step 2: Detailed Explanation:
Let us analyze the chemical structure and electron density of both species to evaluate the statements:
Assertion (A) is correct:
Alcohols ($\text{R-OH}$) are generally weaker acids than water ($\text{H}_2\text{O}$).
In an alcohol molecule, the hydroxyl group is attached to an alkyl group ($\text{R}$), which is electron-donating in nature due to the inductive effect ($+\text{I}$ effect).
This $+\text{I}$ effect increases the electron density on the oxygen atom of the alcohol, which in turn destabilizes the resulting conjugate base, the alkoxide ion ($\text{R-O}^-$).
Furthermore, because the oxygen atom in the alkoxide ion has a higher electron density, it holds onto the proton more tightly, making it more basic than the hydroxide ion ($\text{OH}^-$).
In water, the hydroxyl group is bonded only to a hydrogen atom, which does not exhibit an inductive effect.
Therefore, the conjugate base of water, the hydroxide ion ($\text{OH}^-$), is more stable than the alkoxide ion ($\text{R-O}^-$) in solution.
Consequently, the equilibrium for water dissociation lies further to the right compared to that of alcohols, making water a stronger acid than almost all aliphatic alcohols (with the exception of methanol).
Reason (R) is correct:
Since water is a stronger acid than alcohol, it has a higher tendency to release its proton.
By definition, a stronger Brønsted-Lowry acid is a more efficient and better proton donor.
Therefore, water is indeed a better proton donor than alcohol.
Furthermore, the Reason directly explains why alcohols are weaker acids than water, as acidic strength is fundamentally defined by the ease of proton donation.

Step 3: Final Answer:
Thus, both (A) and (R) are correct, and (R) is the correct explanation of (A).