Question

The correct order of decreasing acid strength of trichloroacetic acid (A), trifluoroacetic acid (B), acetic acid (C) and formic acid (D) is:
A) $A > B > C > D$
B) $A > C > B > D$
C) $B > A > D > C$
D) $B > D > C > A$

Answer 1

An acid can be defined as a chemical which releases hydrogen ions in water thereby forming salts by integrating with specific metals. $pH$ scale can be used to measure acidity. On the pH scale, value of 7 is considered to be neutral, whereas, a value from 7 to 0 indicates increasing acidity.

Complete step by step answer:
In general, the acidic strength in an organic compound is directly related to the stability of the acid's conjugate base. In other words, an acid having a more stable conjugate base will be more acidic in comparison to an acid possessing a less stable conjugate base. In the given question, all four given acids are carboxylic acid ($- COOH$) whose conjugate base ($- CO{O^ - }$) includes a negative charge. This conjugate base can be stabilized if its carbon gets connected to an electron withdrawing group. Now let us look at the four given acids one by one:
(A) Trichloroacetic acid: The structure is given below:

Here three chloride groups attached are the electron withdrawing groups which impose the$-I$ effect thereby increasing the acidic strength. Thus, trichloroacetic acid is highly acidic.
(B) Trifluoroacetic acid: The structure is given below:

Here three fluoride groups attached are the electron withdrawing groups which impose the$-I$ effect thereby increasing the acidic strength. Thus, trifluoroacetic acid is highly acidic. Since$-I$ effect of fluorine is more than that of chlorine, trifluoroacetic acid is more acidic in comparison to trichloroacetic acid.
(C) Acetic acid: The structure is given below:

Here the methyl group attached is the electron donating group and thus will impose$+ I$ effect thereby decreasing the acidic strength. Thus, acetic acid will be a weak acid.
(D) Formic acid: The structure is given below:

Here the hydrogen attached will impose none of the effect thereby does not either increase or decrease the acidic strength.
As a result, the correct order of decreasing acid strength will be$B > A > D > C$.

So, the correct answer is Option C.

Note: There are certain factors that influence acidity which are: (i) Electronegativity: Acidity goes on increasing as you move from left to right in a periodic chart. (ii) Anion size: Acidity goes on increasing as you move from top to down in a periodic chart. (iii) Resonance: Anions having resonance structures are more acidic in comparison to anions which don't have.