Question

Which of the following alcohols is expected to have a lowest $p{K_a}$ value?
A. Ethanol
B. $1 - propanol$
C. $2,2,2 - trifluoroethanol$
D. $2 - chloroethanol$

Answer 1

$p{K_a}$ is a way to determine the strength of an acid. The $p{K_a}$ gives the same information, just in a different way. The smaller the value of $p{K_a}$ , the stronger the acid. Weak acids have a $p{K_a}$ ranging from $2 - 14$ . In order to determine the alcohol with lowest $p{K_a}$ we should determine the most acidic alcohol from the above options.

Complete step by step solution
As we know that electronegativity is directly proportional to the acidity of the compound, that is if the compound is very electronegative then it must be more acidic.
From the above options, $2,2,2 - trifluoroethanol$ is most electronegative as it has three fluorines attached to it and fluorine is the most electronegative element in the periodic table. So this compound is most electronegative among all the given alcohols.
Now, as mentioned above that higher the electronegativity higher will be its acidity. So by applying this fact, we can say that $2,2,2 - trifluoroethanol$ is most acidic among all the given alcohols so it will have the lowest $p{K_a}$ value.
Because $p{K_a}$ value is inversely proportional to the strength of the acid that is acidity. In simple words higher the acidic strength lower will be the $p{K_a}$ value.
So, option C is correct.

Note
Mathematically, $p{K_a}$ value is equal to the $- \log {K_a}$ where ${K_a}$ is the acid dissociation constant. A large ${K_a}$ value indicates a strong acid because it means the acid is largely dissociated into its ions. A large ${K_a}$ value also means the formation of products in the reaction is favored. A small ${K_a}$ value means little of the acid dissociates, so you have a weak acid. The ${K_a}$ value for most weak acids ranges from ${10^{ - 2}} - {10^{ - 14}}$ .