Question

i. ${{F}_{3}}C-COOH$
ii. $C{{H}_{3}}COOH$
iii. ${{C}_{6}}{{H}_{5}}COOH$
iv. $C{{H}_{3}}C{{H}_{2}}COOH$
Correct order of $p{{K}_{a}}$ value is?
A. 1>3>2>4
B. 4>2>3>1
C. 4>3>2>1
D. 1>2>4>3

Answer 1

Hint: The $p{{K}_{a}}$value gives us nothing but the acidic strength of the compounds. The compound which has more value of $p{{K}_{a}}$is less acidic and vice versa. So the order of $p{{K}_{a}}$ is the same as that of the basic strength as it is inversely proportional to the acidic strength.

Complete step by step solution:
-Acids are the compounds that donate ${{H}^{+}}$ ion to become anion. Bases are compounds that accept the ${{H}^{+}}$ ion and convert into cation. Their strength can be measured by a constant called $p{{K}_{a}}$.
-The strength of acidic nature is inversely related to the strength of the basic nature of a compound. It means that strong acid has a weak base and vice versa. So we can solve this question in two ways, either by finding the basic strength directly or by inverting the order of the acidic strengths.
-The basic strength in a compound depends on the electron density of the compound, size of the compound, electronegativity of the atoms or ions, number of delocalizations possible in the compound, aromaticity of the compound and the +I and +M effects of the compound.
-The strength is directly proportional to the electron density and the +I/+M effect and is inversely proportional to all the other mentioned properties.
-So the increment in the size of the molecules decreases the basic strength of the compounds. But the addition of the methyl groups increases the +I effect on the compound which increases the basic strength of the compound.
-In the given options we can see that the first option shows –I effect and so it will have the least basic strength. The third option has more delocalisations possible due to benzene ring and so it will be less basic than the aliphatic compounds.
-So the order of the basic strength will be 4>2>3>1 as the increase in the number of carbon chains increases the +I effect in the molecule and thus increases the basic strength.

Therefore the correct option is B.

Note: The anions are generally more basic than the neutral molecules. This is because the electron density is more in the anions compared to the neutral molecules. So always check for that before any other property to check for the strength of basicity.