Which of the following alcohols is expected to have a lowest pKa value?
A
Ethanol
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B
1-Propanol
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C
2,2,2-trifluoroethanol
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D
2-chloroethanol
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Solution
The correct option is C 2,2,2-trifluoroethanol Acidic strength ∝Ka∝1pKa∝−I of groups attached.
Option a : CH3CH2O− destabilized by +I effect of alkyl group attached.
Option b : CH3CH2CH2O− destabilized by +I effect of alkyl group attached.
Option c :
−I effect of fluorine stabilizes the conjugate base.
Option D :
−I effect of chlorine stabilizes the conjugate base.
Since −I effect of fluorine is greater than that of chlrone, so pKa value of 2,2,2-trifluorethanol is the least.