Hydrogen-bonded amide protons

The principal hydrogen-bonding amide solvents are formamide, acetamide, iV-methylformamide (NMF), and A-methylacetamide (NMA). All four are basic solvents. For example, the medium effect for transfer of the proton from water to formamide (Appendix 3.3.2) is —1.2 the redox acidity function i o(H) is 4.7 in molten acetamide at 98°C. These solvents are most notable for their very high dielectric constants, which range from 109.5 for formamide to 182.4 for NMF at 25°C. Their autoprotolysis constants are comparable with those of water and the alcohols, e.g. pATs of formamide is 17.0 (Appendix 3.3.3), so that a wide range of acid strengths can be investigated. 

Tjandra N and Bax A 1997 Solution NMR measurement of amide proton chemical shift anisotropy in N-15-enriched proteins. Correlation with hydrogen bond length J. Am. Chem. Soc. 119 8076-82... 

FIGURE 6.5 A hydrogen bond between the amide proton and carbonyl oxygen of adjacent peptide groups. 

When two compounds whose molecules form hydrogen bonds with each other are both dissolved in water, the hydrogen bond between the two molecules is usually greatly weakened or completely removed, because the molecules generally form hydrogen bonds with the water molecules rather than with each other, especially since the water molecules are present in such great numbers. In amides, the oxygen atom is the preferred site of protonation or complexation with water. 

The P-N bond in phosphinous amides is essentially a single bond, so the lone pairs on N and P are available for electrophiUc reagents and for donor bonding towards metal atoms. Proton addition to the N atom of HjPNHj has been calculated to loosen the P-N bond, whereas protonation at P renders this bond stronger than in the parent molecule [26]. NH-Phosphinous amides are practically not associated by intermolecular hydrogen bonds [27]. 

Proton donors and acceptors from hydrogen bonds (alcohols, water, amines, orgartic acids, amides, etc.)... 

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