A Strong Acid that Does Not Protonate Water

Abstract

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 3 SO 3 H) and the N-H acid bis-(trifluoromethylsulfonyl) imide ((CF 3 SO 2 ) 2 NH), behave quite differently toward water in these media. Triflic acid protonates water at the one-equivalent level to give the hydronium ion, H 3 O + . By contrast, bis(trifluoromethylsulfonyl) imide forms a simple hydrate, (CF 3 SO 2 ) 2 NH‚‚‚OH 2 , even with water at the twoequivalent 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.

Brønsted acids in ionic liquids: how acidity depends on the liquid structure

“…Further addition of acid leads to a solution of ionic liquid in acid. Although the existence of higher-order clusters was postulated also in triflate systems, 15 data in Figure 12 show no evidence for larger clusters in either mesylate or triflate system. It appears that, as the acid content increases beyond χ HA = 0.67, the equilibrium shown in Eqn 5 shifts to the left, liberating free acid.…”

“…Distances between hydrogen and oxygen atoms partaking in 15 [NTf 2 ] -is the poorest H-bond acceptor studied here; in addition, there is a steric hindrance, as well as fluorinefluorine and oxygen-oxygen repulsion, hindering the formation of N-H … N bonds, as shown in Figure 5. This analysis ties in the empirical basicity of the anions 30 with their structural consequences and resulting acidity of their solutions.…”

“…This can be rationalised by the formation of [OTf(HOTf)] -, which had been found in both ionic liquids 10 and in molecular solvents. 15 Afterwards, AN values increased linearly up to χ HOTf = 0.86 (Eqn 4), suggesting formation of weakly-bound oligonuclear clusters, 10 In the bistriflamide system (Figure 4, ◆), as in the triflate one, the addition of a small amount of acid leads to a spike in acidity, up to a very high level of AN = 124.3±0.8. However, in contrast to other systems, this value remains constant throughout the measurable composition range 0.05 < χ NTf2 ≤ 0.80.…”

“…1,12,13 Poorly ionic mixtures of 1-methylpyrrolidine and acetic acid were shown to contain polymeric [OAc(HOAc) x ] -anions. 13 Such clusters are not specific to ionic liquids environment; they were observed in aqueous solutions, 14 non-polar solvents, 15 and are often found in crystal structures. 16 …”

“…In contrast, when equimolar amounts of water and HNTf 2 were dissolved in a noncoordinating solvent, such as CCl 4 or benzene, HNTf 2 did not protonate water, behaving like a weak acid. 15 …”