Question

The correct decreasing order for acid strength is?

• A. ${\mathrm{FCH}}_2\mathrm{COOH}>{\mathrm{NCCH}}_2\mathrm{COOH}>{\mathrm{NO}}_2{\mathrm{CH}}_2\mathrm{COOH}>{\mathrm{ClCH}}_2\mathrm{COOH}$
• B. ${\mathrm{CNCH}}_2\mathrm{COOH}>{\mathrm{O}}_2{\mathrm{NCH}}_2\mathrm{COOH}>{\mathrm{FCH}}_2\mathrm{COOH}>{\mathrm{ClCH}}_2\mathrm{COOH}$
• C. ${\mathrm{NO}}_2{\mathrm{CH}}_2\mathrm{COOH}>{\mathrm{NCCH}}_2\mathrm{COOH}>{\mathrm{FCH}}_2\mathrm{COOH}>{\mathrm{ClCH}}_2\mathrm{COOH}$
• D. ${\mathrm{NO}}_2{\mathrm{CH}}_2\mathrm{COOH}>{\mathrm{FCH}}_2\mathrm{COOH}>{\mathrm{CNCH}}_2\mathrm{COOH}>{\mathrm{ClCH}}_2\mathrm{COOH}$

Answer 1

The correct option is C

${\mathrm{NO}}_2{\mathrm{CH}}_2\mathrm{COOH}>{\mathrm{NCCH}}_2\mathrm{COOH}>{\mathrm{FCH}}_2\mathrm{COOH}>{\mathrm{ClCH}}_2\mathrm{COOH}$

The explanation for the correct option:

Option(C):${\mathrm{NO}}_2{\mathrm{CH}}_2\mathrm{COOH}>{\mathrm{NCCH}}_2\mathrm{COOH}>{\mathrm{FCH}}_2\mathrm{COOH}>{\mathrm{ClCH}}_2\mathrm{COOH}$

• The acidic strength of a compound is directly propertional to the electron withdrawing attached to the given compound.
• Here the order of electron-withdrawing effect can be given as: $-{\mathrm{NO}}_2>-\mathrm{CN}>-\mathrm{F}>-\mathrm{Cl}$.
• Thus, the acidic strength of the given acids will be :

${\mathrm{NO}}_2{\mathrm{CH}}_2\mathrm{COOH}>{\mathrm{NCCH}}_2\mathrm{COOH}>{\mathrm{FCH}}_2\mathrm{COOH}>{\mathrm{ClCH}}_2\mathrm{COOH}$

The explanation for the incorrect option:

Option(A):${\mathrm{FCH}}_2\mathrm{COOH}>{\mathrm{NCCH}}_2\mathrm{COOH}>{\mathrm{NO}}_2{\mathrm{CH}}_2\mathrm{COOH}>{\mathrm{ClCH}}_2\mathrm{COOH}$

• The given order is not correct according to the order of electron-withdrawing group effect attached to given acids.
• Hence, the given option does not coincide with the correct concept.

Option(B):${\mathrm{CNCH}}_2\mathrm{COOH}>{\mathrm{O}}_2{\mathrm{NCH}}_2\mathrm{COOH}>{\mathrm{FCH}}_2\mathrm{COOH}>{\mathrm{ClCH}}_2\mathrm{COOH}$

• As the electron-withdrawing strength of the group $-{\mathrm{NO}}_2>-\mathrm{CN}$, the acidic strength order of ${\mathrm{NO}}_2{\mathrm{CH}}_2\mathrm{COOH}$ should be greater than ${\mathrm{NCCH}}_2\mathrm{COOH}$
• Hence, the given option is the incorrect one.

Option(D):${\mathrm{NO}}_2{\mathrm{CH}}_2\mathrm{COOH}>{\mathrm{FCH}}_2\mathrm{COOH}>{\mathrm{CNCH}}_2\mathrm{COOH}>{\mathrm{ClCH}}_2\mathrm{COOH}$

• The given order is not correct according to the order of the electron-withdrawing group effect attached to given acids.
• Hence, the given option does not represent the correct concept.

Hence, the correct decreasing order for acid strength is ${\mathrm{NO}}_2{\mathrm{CH}}_2\mathrm{COOH}>{\mathrm{NCCH}}_2\mathrm{COOH}>{\mathrm{FCH}}_2\mathrm{COOH}>{\mathrm{ClCH}}_2\mathrm{COOH}$, option (C) is the correct answer.