Good proton donors

Intermolecular forces may be caused by a solute molecule having a dipole moment, when it can interact selectively with other dipoles. If a molecule is a good proton donor or acceptor it can interact with other such molecules by hydrogen bonding. Molecules can also interact via much weaker dispersion forces which rely on a given molecule being polarised by another molecule. 

However, too few examples are available to allow a general prediction of the position for hydrogen attachment. Electrochemical reduction of 1,4-diphenylbutadiene gives products from both 1,2- and 1,4-attachment of hydrogen [6], The 1,2-addition product predominates in absence of a good proton donor because electro-chemically generated base catalyses isomerisation of the 1,4-addition product. Reduction of a steroid diene 4 gives the 1,4-addition product [17],... 

Reduction of geminal dibromocyclopropenes of the type 15 gives the endo-monobromo compound preferentially. In solvents containing a good proton donor, selectivity rises to 100% in favour of the eni/o-product [70]. The behaviour suggests that the bromocyclopropyl radical intermediates exist with a preference for-exo arrangement of the radical centre, which is then reduced and protonated on the exo-side. 

Hydrogen chloride (HCl) is a gas that readily dissolves in water. In water, HCl is called hydrochloric acid and the HCl molecule serves as a good proton donor, readily undergoing the reaction... 

The conjugate acid-base pairs have the same subscript and are bracketed together. This reaction goes almost to completion because HCI is a good proton donor and hence a strong acid. 

All good proton donors and acceptors are used in hydrogen bonding. 

The mobile phases used in normal-phase chromatography are based on nonpolar hydrocarbons, such as hexane, heptane, or octane, to which is added a small amount of a more polar solvent, such as 2-propanol.5 Solvent selectivity is controlled by the nature of the added solvent. Additives with large dipole moments, such as methylene chloride and 1,2-dichlor-oethane, interact preferentially with solutes that have large dipole moments, such as nitro- compounds, nitriles, amines, and sulfoxides. Good proton donors such as chloroform, m-cresol, and water interact preferentially with basic solutes such as amines and sulfoxides, whereas good proton acceptors such as alcohols, ethers, and amines tend to interact best with hydroxylated molecules such as acids and phenols. A variety of solvents used as mobile phases in normal-phase chromatography are listed in Table 2.2, some of which may need to be stabilized by addition of an antioxidant, such as 3-5% ethanol, because of the propensity for peroxide formation. 

This is the complete mechanism and we can even make a prediction about the reaction conditions from it. The second step needs a proton and water is not a very good proton donor. A weak acid as catalyst would help. 

Protonation is another common reaction in this category and reduction of the acceptor via protonation of the radical anion (e.g. by moisture present in the solvent) is often observed as a side process. It may become the main path either under acidic conditions or if the radical cation of the donor is a good proton donor (Scheme 3). As an example, the 1,2-dihydro derivative is the main product from 1,4-dicyanonaphthalene (DON) when irradiated in the presence of a donor and trifluoroacetic acid [33]. Such a reaction is more important when the radical cation is the proton source. Typically, the irradiation of naphthalene with tertiary amines leads to a mixture of 1,4-dihydro- and 1,2,3,4-tetrahydro-naphthalene, as well as tetrahydrobinaphthyl and l(l-diethylamino)ethyl-l,4-... 

Water molecules are both fairly good proton donors and proton acceptors contrary to hydroxide ions , which are only poor proton donors, but very strong acceptors. There-... 

In aprotic media complications of a different kind may arise. If the protected amine is primary, the resulting sulfonamide is fairly good proton donor that can protonate the resulting substituent amide ion, and = 1 is found on workup 50% of the sulfonamide is recovered [258] ... 

Elimination reactions such as those found in equations (11) and (12) occur readily when the substrate is a good proton donor. 

An acid is a substance that donates a proton (Brpnsted-Lowry). Acidic compounds have low pKa-values and are good proton donors, as the anions (or conjugate bases), formed on deprotonation, are relatively stable. 

The mutant in which Glu-212 has been replaced by Asp has a saturating turnover rate of 100 s which is close to that for native RCs (200 s" ). This is consistent with the good proton donor properties of Asp. 

The strong acidic behavior of carboxylic acids as good proton donors is the consequence of stabilization of the corresponding conjugated base (carboxylate anion). As can be seen from the resonance structures presented below the electrons in the carboxylate anion are delocalized. 

---