Q67E - Organic Chemistry (Mcmurry)

Question

1,1,1-Trichloroethanol is an acid more than 1000 times stronger than ethanol, even though both have a conjugate base where the negative charge is on an oxygen. Provide an explanation for this observation.

Step-by-Step Solution

Answer

1, 1, 1-Trichloroethanol is 1000 times stronger acid than ethanol due to three attached chlorine atoms.

Step 1: Acidic Strength

Classification of acids as a weak acid or strong acid can be done on the basis of their acidic strength. When a substance dissociates completely into its ions then that substance is classified as strong acid and if a substance is unable to dissociate completely into ions then that substance is classified as a weak acid. We can describe acidic strength of any compound with the help of acid dissociation constant i.e. ${\text{K}}_{\text{a}}$ . Larger the ${\text{K}}_{\text{a}}$value stronger will be the acid and vice- versa. For our simplicity we can measure acidic strength with ${\text{pK}}_{\text{a}}$.

${\text{pK}}_{\text{a}}\text{ = } {\text{-logK}}_{\text{a}}$

Higher the ${\text{pK}}_{\text{a}}$value less will be the acidic strength. 

Step 2: Explanation

${\text{pK}}_{\text{a}}$ Value of 1, 1, 1-Trichloroethanol is less than that of ethanol so we can say that compound 1, 1, 1-Trichloroethanol is more acidic than ethanol. Also In 1, 1, 1-Trichloroethanol three chlorine atoms are attached that are highly electronegative atoms therefore, these will withdraw more electron density such that it will easily release its hydrogen ion and we will get a stable conjugate base and as we know that the more stable is our conjugate base, stronger will be the acid but in case of ethanol no such electronegative atoms are attached. Hence, 1, 1, 1-Trichloroethanol is 1000 times stronger acid than that of ethanol.