Question

Arrange the following in the decreasing order of their \(pK_a\) values.

• A. \(A>C>D>B\)
• B. \(C>A>B>D\)
• C. \(B>D>C>A\)
• D. \(D>C>A>B\)

Hint:

Electron donating groups decrease the acidity of phenols and increase \(pK_a\), while electron withdrawing groups increase acidity and decrease \(pK_a\).

Answer 1

The Correct Option is A

Step 1: Understanding the relation between acidity and \(pK_a\).
The \(pK_a\) value is inversely related to acidic strength.
\[ \text{Higher acidity} \Rightarrow \text{Lower } pK_a \] \[ \text{Lower acidity} \Rightarrow \text{Higher } pK_a \] So, to arrange in decreasing order of \(pK_a\), we arrange compounds from least acidic to most acidic.

Step 2: Basic idea for substituted phenols.
Phenols are acidic because after losing \(H^+\), they form phenoxide ion.
\[ ArOH \rightarrow ArO^- + H^+ \] If the substituent stabilizes phenoxide ion, acidity increases and \(pK_a\) decreases.
If the substituent destabilizes phenoxide ion, acidity decreases and \(pK_a\) increases.

Step 3: Effect of electron donating groups.
Electron donating groups such as \(-NH_2\) and \(-OCH_3\) increase electron density on the benzene ring.
They destabilize the phenoxide ion.
Therefore, they decrease acidity and increase \(pK_a\).
Among them, \(-NH_2\) is a stronger electron donating group than \(-OCH_3\).
So, compound \(A\) has higher \(pK_a\) than compound \(C\).
\[ A>C \]

Step 4: Effect of halogen group.
Compound \(D\) contains chlorine group.
Chlorine shows \(-I\) effect, which withdraws electron density and stabilizes the phenoxide ion.
Due to this, compound \(D\) is more acidic than compounds having electron donating groups.
So, its \(pK_a\) is lower than \(A\) and \(C\).
\[ C>D \]

Step 5: Effect of nitro group.
Compound \(B\) contains \(-NO_2\) group.
The nitro group is a very strong electron withdrawing group due to both \(-I\) and \(-R\) effects.
It strongly stabilizes the phenoxide ion.
Therefore, compound \(B\) is the most acidic and has the lowest \(pK_a\).
\[ D>B \]

Step 6: Combining all effects.
The decreasing order of \(pK_a\) values will be:
\[ A>C>D>B \] Here, \(A\) has the highest \(pK_a\) due to strong electron donation by \(-NH_2\), while \(B\) has the lowest \(pK_a\) due to strong electron withdrawal by \(-NO_2\).

Step 7: Final Answer.
Thus, the correct decreasing order of \(pK_a\) values is:
\[ \boxed{A>C>D>B} \] Hence, the correct answer is option (A).