Chemists often modify molecular structure to tune acid strength
CHEMISTRY · Multi-part · AP · Chemistry · Acids and Bases · Acids and Bases
Problem
Chemists often modify molecular structure to tune acid strength. Consider the following series of related molecules, where chlorine is successively substituted by highly electronegative fluorine atoms bonded directly to the chlorine center.
1. HOCl (no F substituents) pKa = 7.5
2. HOClF (one F substituent) pKa = 5.5
3. HOClF2 (two F substituents) pKa = ?)
(The O–Cl–F bonds are single bonds; the O atom is also singly bonded to H.)
Answer the prompts below.
(a) Draw plausible Lewis structures for HOCl, HOClF, and HOClF2, clearly indicating all lone pairs and formal charges that best satisfy the octet rule.
(b) Rank the three acids in order of increasing strength and explain, at the atomic level, how introducing one or more fluorine atoms influences the ability of the molecule to donate a proton.
(c) Using a linear-free-energy assumption that each fluorine substituent decreases the pKa by approximately the same amount, estimate the pKa of HOClF2 and calculate the ratio of Ka(HOClF2) to Ka(HOCl). Show all reasoning.
(d) Critically evaluate the limitations of using only electronegativity considerations to predict acid strength in this series, citing at least one additional structural factor that could affect the accuracy of your estimate in part (c).