Step 1: Understanding the Concept:
Acidity is determined by the stability of the conjugate base formed after releasing an $H^+$ ion. Resonance (-M) and inductive (-I) electron-withdrawing groups stabilize the negative charge, increasing acidity. Electron-donating groups (+M, +I) destabilize the conjugate base, decreasing acidity. Furthermore, carboxylic acids are inherently far more acidic than phenols. Step 2: Key Formula or Approach:
Acidity order: Carboxylic acids $\gg$ Phenols.
Substituent effects: $-M, -I$ increase acidity; $+M, +I$ decrease acidity. Step 3: Detailed Explanation:
Let's categorize the given molecules:
A. Phenol
B. p-Nitrophenol (contains strongly electron-withdrawing $-NO_2$ group; $-M, -I$)
C. p-Methoxyphenol (contains electron-donating $-OCH_3$ group; $+M, -I$)
D. p-Nitrobenzoic acid (Carboxylic acid with $-NO_2$ group; $-M, -I$)
E. Benzoic acid (Unsubstituted carboxylic acid)
First, separate the carboxylic acids (D, E) from the phenols (A, B, C). Carboxylic acids are much stronger acids because their conjugate base (carboxylate) features equivalent resonance over two highly electronegative oxygen atoms.
So, {D, E}>{A, B, C}.
Evaluate the Carboxylic Acids (D, E):
The $NO_2$ group in D withdraws electron density, heavily stabilizing the carboxylate anion.
Thus, D (p-nitrobenzoic acid)>E (benzoic acid).
Evaluate the Phenols (A, B, C):
- B has a $-NO_2$ group (-M effect), drastically stabilizing the phenoxide ion $\implies$ Most acidic phenol.
- A is the baseline phenol.
- C has an $-OCH_3$ group. Though it has a -I effect, its resonance donating effect (+M) heavily outweighs it at the para position, destabilizing the phenoxide ion $\implies$ Least acidic phenol.
Thus, B>A>C.
Combining the two series:
D>E>B>A>C. Step 4: Final Answer:
The descending order is D>E>B>A>C.
Was this answer helpful?
0
Top Questions on Acid-base concepts and principles
