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A Strong Acid that Does Not Protonate Water

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The nature of solvated acids in benzene and chlorinated hydrocarbon solvents is sensitively revealed by IR spectroscopy. Two similarly strong, structurally related acids, triflic acid (CF 3SO 3H) and the N-H acid bis-(trifluoromethylsulfonyl) imide ((CF 3SO 2) 2NH), behave quite differently toward water in. these media. Triflic acid protonates water at the one-equivalent level to give the hydronium ion, H 3O +. By contrast, bis(trifluoromethylsulfonyl) imide forms a simple hydrate, (CF 3SO 2) 2NH-OH 2, even with water at the two-equivalent level. The difference arises from the differing abilities of the conjugate base anions to engage in H-bonding and ion pairing. It illustrates how acidity in low-dielectric media is critically dependent on the nature of the conjugate base anion. νO-H and νN-H stretching frequencies indicate that the H-bonding acceptor ability for solvents toward acidic protons increases in the order carbon tetrachloride < benzene ∼ 1, 2-dichoroethane < water. With benzene, a marked tendency for acidic protons to H-bond to the π-face of the arene ring is noted and an IR signature of this solvation mode is observed.

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