Question

Arrange the following in increasing order of their acidic strength.

• A. \(a\lt c\lt d\lt b\)
• B. \(a\lt c\lt b\lt d\)
• C. \(d\lt c\lt a\lt b\)
• D. \(a\lt b\lt c\lt d\)

Hint:

Electron-withdrawing groups increase the acidic strength of carboxylic acids by stabilizing the carboxylate ion: \[ -NO_2 \gt -Br \gt -PhCH_2 \gt -CH_3 \] Greater \(-I\) effect means stronger acid.

Answer 1

The Correct Option is D

Step 1: Identify the given acids.
The given compounds are carboxylic acids. \[ (a)\ CH_3COOH \] \[ (b)\ PhCH_2COOH \] \[ (c)\ BrCH_2COOH \] \[ (d)\ O_2NCH_2COOH \]

Step 2: Recall the effect of substituents on acidic strength.
The acidic strength of carboxylic acids increases when the conjugate base is stabilized.
Electron-withdrawing groups show \(-I\) effect and stabilize the carboxylate ion. Therefore, \[ \text{Stronger } -I \text{ effect} \Rightarrow \text{Greater acidic strength} \]

Step 3: Compare the substituent effects.
In \(CH_3COOH\), the \(CH_3\) group shows \(+I\) effect and decreases acidity.
In \(PhCH_2COOH\), the benzyl group is slightly electron-withdrawing compared to methyl and hence increases acidity slightly.
In \(BrCH_2COOH\), bromine shows strong \(-I\) effect and increases acidity more.
In \(O_2NCH_2COOH\), the nitro group shows a very strong \(-I\) effect and stabilizes the carboxylate ion the most.

Step 4: Write the increasing order of acidic strength.
Thus, the increasing order of acidic strength is \[ CH_3COOH \lt PhCH_2COOH \lt BrCH_2COOH \lt O_2NCH_2COOH \] That is, \[ a\lt b\lt c\lt d \]

Step 5: Final conclusion.
Therefore, the correct order is \[ \boxed{a\lt b\lt c\lt d} \] Hence, option (4) is correct.