Protons on an Oxygen Atom

1 Alcohols Depending on concentration, the hydroxylic peak in alcohols is found between S 0.5 and S 4.0. A change in temperature or solvent will also shift the peak position. 

Intermolecular hydrogen bonding explains why the shift depends on concentration, temperature, and polarity of solvent. Hydrogen bonding decreases the electron density around the hydroxylic proton, thus moving the proton peak to higher frequency. 

Decrease in concentration in a nonpolar solvent disrupts such hydrogen bonding, and the peak appears at lower frequency—i.e., the alcohol molecules become less polymeric. Increased temperature has a similar effect. 

The use of dry, deuterated dimethyl sulfoxide (DMSO-d6) or deuterated acetone has the same effect as the above treatments. In addition, the OH proton peak is moved to higher frequency by hydrogen bonding between the solute and the solvent, thus providing 

FIGURE 3.39 CH3CH2OH run in dry deuterated DMSO at 300 MHz. From left to right, the peaks represent OH, CH2, CH3.The small absorption at 8 2.5 represents the H impurity in DMSO-d6 (see appendix G). 

Intramolecular hydrogen bonds are less affected by their environment than are intermolecular hydrogen 

FIGURE 4.32. CH3CH2OH in CDC13 at 300 MHz, allowed to stand at room temperature overnight exposed to air. The CH2 peaks are broadened by residual coupling to OH. 

At intermediate rates of exchange, the hydroxylic multiplet merges into a broad band, which progresses to a singlet at higher exchange rates (Fig. 4.32).  

The spectrum of a compound containing rapidly exchangeable protons can be simplified, and the exchangeable proton absorption removed, simply by shaking the solution with excess deuterium oxide or by obtaining a spectrum in deuterium oxide solution if the compound is soluble. A peak resulting from HOD will appear, gen- 

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Amides, too, require a special acidity function ( A)b ecause they protonate on the oxygen atom (Yates and Stevens, 1965). The use of this new scale has shown amides to be weaker bases than the use of the//o scale had suggested. Not surprisingly, heterocycles which protonate on an oxygen atom require the // scale, for example pyridine-W-oxides (Johnson, et al, 1967) and pyrid-3-ones (Bellingham, c/fl/., 1967). 

A fairly strong base is required to deprotonate an alcohol. By removing the proton, we are forming a negative charge on an oxygen atom (an alkoxide ion). Therefore, in... 

Furthermore, the amine part of the amide group is unlike any normal amine group. Most amines are easily protonated. However, since the lone pair on the amide s nitrogen is tied up in the 7t system, it is less available for protonation or, indeed, reaction with any electrophile. As a result, an amide is preferentially protonated on the oxygen atom but it is difficult to protonate even there (see next chapter, p. 201). Conjugation affects reactivity. 

Since the O-H bond is much stronger than the M-H bond, hydrogen atoms are more likely to be adsorbed as protons on the oxygen atoms. When the oxide is not reducible, in the H2 dissociation, protons are equilibrate by an equivalent amount of hydrides. In the case of reducible oxides, there is a better possibility the excess of electrons should be transferred on the oxide, reducing the Ti+4... 

Protonation of an enolate ion can occur via two competing exothermic reactions, as shown. Protonation on the oxygen atom of the enolate ion to form the enol is faster than protonation on the carbon atom to form the ketone however, the ketone is thermodynamically more stable than the enol. 

Tbp.-The electrons in the 0—H bonds of H2O molecules are attracted to the oxygen atoms, leaving the positively charged protons partially exposed. A proton on one water molecule is attracted to a lone pair on an oxygen atom in another water molecule.Soffom Lewis structures of water with hydrogen bonding between molecules. 

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