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Question: The correct order of decreasing acid strength of trichloroacetic acid (A), trifluoroacetic acid (B),...

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>DA > B > C > D
B) A>C>B>DA > C > B > D
C) B>A>D>CB > A > D > C
D) B>D>C>AB > D > C > A

Explanation

Solution

An acid can be defined as a chemical which releases hydrogen ions in water thereby forming salts by integrating with specific metals. pHpH 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 - COOH) whose conjugate base (COO - 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 theI-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 theI-I effect thereby increasing the acidic strength. Thus, trifluoroacetic acid is highly acidic. SinceI-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 + 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 beB>A>D>CB > 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.