Aqueous versus Nonaqueous Acids. Acid Strengths

One of the more confusing features of organic chemistry is the multitude of conditions that are used to carry out a given kind of reaction, such as the electrophilic addition of proton acids to different alkenes. Strong acids, weak acids, water, no water - Why can't there be a standard procedure? The problem is that alkenes have very different tendencies to accept protons. In the vapor phase, ΔH0 for addition of a proton to ethene is about 35kcal more positive than for 2-methylpropene, and although the difference should be smaller in solution, it still would be large. Therefore we can anticipate (and we find) that a much more powerful proton donor is needed to initiate addition of an acid to ethene than to 2-methylpropene. But why not use in all cases a strong enough acid to protonate any alkene one might want to have a proton acid add to? Two reasons: First, strong acids can induce undesirable side reactions, so that one usually will try not to use a stronger acid than necessary; second, very strong acid may even prevent the desired reaction from occurring!

In elementary chemistry, we usually deal with acids in more or less dilute aqueous solution and we think of sulfuric, hydrochloric, and nitric acids as being similarly strong because each is essentially completely disassociated in dilute water solution:

HCl+H₂O←⟶H₃O^⊕+Cl^⊖   (10.4.2)     

This does not mean they actually are equally strong acids. It means only that each of the acids is sufficiently strong to donate all of its protons to water. We can say that water has a "leveling effect" on acid strengths because as long as an acid can donate its protons to water, the solution has but one acid "strength" that is determined by the H₃O^⊕ concentration, because H₃O^⊕ is where the protons are.

Now, if we use poorer proton acceptors as solvent we find the proton-donating powers of various "strong" acids begin to spread out immensely. Furthermore, new things begin to happen. For example, ethene is not hydrated appreciably by dilute aqueous acid; it just is too hard to transfer a proton from hydronium ion to ethene. So we use concentrated sulfuric acid, which is strong enough to add a proton to ethene. But now we don't get hydration, because any water that is present in concentrated sulfuric acid is virtually all converted to H₃O^⊕, which is non-nucleophilic!

H₂SO₄+H₂O→H₃O^⊕+HSO₄⁻  (10.4.3)

However, formation of H3O^⊕ leads to formation of HSO4−, which has enough nucleophilic character to react with the CH3CH₂⁺ to give ethyl hydrogen sulfate and this is formed instead of the conjugate acid of ethanol. The epitome of the use of stronger acid and weaker nucleophile is with liquid SO2 (bp ∼10o) as the solvent and HBF6 as the acid. This solvent is a very poor proton acceptor (which means that its conjugate acid is a very good proton donor) and SbF6⁻ is an extremely poor nucleophile. If we add ethene to such a solution, a stable solution of CH3CH2⁺SbF6− is formed. The reason is that there is no better proton acceptor present than CH2=CH2CH2=CH2 and no nucleophile good enough to combine with the cation.