Protons on Oxygen Alcohols

330 Nuclear Magnetic Resonance Spectroscopy Part Four 

FIGURE 6.1 Stacked plot of NMR spectra of methanol determined at a range of temperatures from 290 K to 200 K. 

The coupling constant for the methylene-hydroxyl interaction is found to be V (CH2, OH) = 5 Hz. The methyl triplet is found to have a different coupling constant, V (CH3, CH2) = 7 Hz, for the methylene- nethyl coupling. The methylene protons are not split into a quintet by their four neighbors as the coupling constants for hydroxyl-methylene and methyl-methylene are different. As discussed in Chapter 5, the n +1 Rule does not apply in such an instance each coupling interaction is independent of the other, and a graphical analysis is required to approximate the correct pattern. 

FIGURE 6.3 The NMR spectrum of an ultrapure sample of ethanol. Expansions of the splitting patterns are included. 

If the rate of exchange in an alcohol can be slowed down to the point at which it approaches the time scale of the NMR (i.e., 10 to 10 exchanges per second), then coupling can be observed. For instance, the NMR spectrum of methanol at 25°C (ca. 300 K) consists of only two peaks, both unsplit singlets, integrating for one proton and three protons, respectively. However, at temperatures below -33°C (less than 240 K) the spectrum changes dramaticaUy. The one-proton O—H resonance becomes a quartet ( /= 5 Hz), and the three-proton methyl resonance becomes a doublet 5 Hz). Clearly, at 

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We saw m Chapter 1 especially m Table 1 7 that alcohols resemble water m respect to their Brpnsted acidity (ability to donate a proton/rom oxygen) They also resemble water m their Brpnsted basicity (ability to accept a proton on oxygen) Just as proton transfer... 

The first direct NMR spectroscopic evidence for the existence of primary alkylox-onium ions (protonated alcohols) in superacid solutions was found in 1961 by MacLean and Mackor.50 The NMR spectrum of ethanol in HF-BF3 solution at —70°C gave a well-resolved triplet at about 81 H 9.90 for the protons on oxygen coupled to the methylene protons. In HS03F this fine structure is not observed, even at 95°C, due to the fast proton exchange.51... 

Similar to alcohols, aliphatic ethers [54], thiols [55], and sulfides are also protonated on oxygen or sulfur, respectively, at -60 °C in magic acid carbocations are subsequently formed upon raising the temperature. Promoted sulfides, excluding tertiary alkyl, are resistant to cleavage up to -i-70 °C [56]. 

Similar to alcohols, aliphatic ethers, thiols, and sulfides are also protonated on oxygen or sulfnr, respectively, at — 60 °C in... 

Current ideas about acids and bases as applied to alcohols, enols, and phenols, as noted earlier (Chapter 5), are divided into the main categories of proton donors/ acceptors (Brpnsted acids/bases) and electron acceptors/donors (Lewis acids/bases). Because alcohols, enols, and phenols all bear protons on oxygen, which can be donated and have sufficient electron density on oxygen to permit protonation, they readily fit the Brpnsted criteria. Flowever, since the electron density on oxygen can also be donated to nonprotonaceous species (e.g., the empty orbital on BFj [Chapter 1]), alcohols, enols, and phenols also qualify as Lewis bases. 

When substitution occurs at oxygen, the alcohol, enol, or phenol is acting as the nucelophile and the proton on oxygen is replaced (e.g., C-O-H C-O-G [G H]) by the electrophile. These reactions are generally referred to as addition reactions (the alcohol having added to something). 

Step 1 The carboxylic acid is protonated on its carbonyl oxygen The proton donor shown in the equation for this step is an aUtyloxonium ion formed by proton transfer from the acid catalyst to the alcohol... 

The reaction is reversible, the reverse being the perhaps better known acid-catalysed dehydration of alcohols (p. 247). Protonation can also occur on oxygen in a carbon-oxygen double bond,... 

Exactly the same considerations apply to the esterification of hindered acids (182) in the reverse direction. It will be noticed that this mechanism requires protonation on the less favoured (cf. p. 240) hydroxyl oxygen atom (185) to allow the formation of the acyl carbocationic intermediate (184). Apart from a number of R3C types, a very well known example is 2,4,6-trimethylbenzoic (mesitoic) acid (186), which will not esterify under ordinary acid-catalysis conditions—and nor will its esters (187) hydrolyse. Dissolving acid or ester in cone. H2S04 and pouring this solution into told alcohol or water, respectively, is. found to effect essentially quantitative esterification or hydrolysis as required the reaction proceeds via the acyl cation (188) ... 

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