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PCAT Chemical Processes – Proton Acidity on Organic Molecules

The acidity of a given proton on a molecule decreases as the strength of the covalent bond connecting that hydrogen to the rest of the molecule increases. With this information, which of the following ethanol derivatives has the most acidic proton?

A. 2,2-dimethylpropan-1-ol
B. 2,2,2-tribromoethanol
C. 2,2,2-trichloroethanol
D. 2,2,2-trifluoroethanol

Click for Explanation

D is correct. Due to the inductive effect, some of the electron density that comprises the bond between hydrogen and oxygen is drawn toward the electronegative fluorine atoms, away from the carbon chain. This weakens the hydrogen-oxygen bond and increases the acidity of the hydrogen atom. This electron-withdrawing effect also stabilizes the conjugate base, making it more favorable for the proton to leave. This effect is stronger in 2,2,2-trifluoroethanol than in the chlorinated and brominated derivatives because fluorine is more electronegative than chlorine or bromine.

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