HX protonations

HX protonates the ether oxygen, thus making a good leaving group. HX provides a source of X for nucleophilic attack. 

In the event, spin polarization of one (see 46) of the three Hx protons in 37b was achieved using a selective 180°-pulse-delay-observe sequence. As is evident from Fig. 17, the triptycene rotates clockwise and counterclockwise to the same extent. Control experiments polarizing the spin of each of the other two Hx protons give equivalent results. 

HB 0X0 acids HX protonic acids other than oxo acids... 

Electrophilic addition of HX to an alkene involves a two-step mechanism, the overall rate being given by the rate of the initial protonation step. Differences in protonation energies are usually explained by considering differences in carbocation stability, but the relief or buildup of strain can also be a factor. One of the following alkenes protonates much more easily than the other. 

Alcohols react with metal hydrides, MH, and with hydrogen halides, HX, but in very different ways. Proton transfer is involved in both reactions, but different molecules act as the proton donor and acceptor. 

In addition to the above mentioned 4J couplings, we have now observed several examples of couplings across five bonds—i.e. 5J. For example, the ribopyranose derivative (13) shows a coupling of ca. 0.1 Hz between Hx and H4. Further examples occur in the spectrum of 2,3,4-tri-0-acetyl-l,6-anhydro-/ -D-glucopyranose (12), where Ji,4 = ca. 0.1 Hz and J2,5 = 0.5 Hz. Whether it is significant that these couplings all occur between equatorially oriented protons must await further studies. 

Markovnikov s rule can be restated by saying that, in the addition of HX to an aikene, the more stable carbocation intermediate is formed. This result is explained by the Hammond postulate, which says that the transition state of an exergonic reaction step structurally resembles the reactant, whereas the transition state of an endergonic reaction step structurally resembles the product. Since an aikene protonation step is endergonic, the stability of the more highly substituted carbocation is reflected in the stability of the transition state leading to its formation. 

Water adds to alkenes to yield alcohols, a process called hydration. The reaction takes place on treatment of the alkene with water and a strong acid catalyst (HA) by a mechanism similar to that of HX addition. Thus, protonation of an alkene double bond yields a carbocation intermediate, which reacts with water to yield a protonated alcohol product (ROH2+). Loss of H+ from this protonated alcohol gives the neutral alcohol and regenerates the acid catalyst (Figure 7.2). 

The reaction of a tertiary alcohol with HX takes place by an Sf l mechanism when acid protonates the hydroxyl oxygen atom, water is expelled to generate... 

This is the first example of a proton transfer process to a hydride complex with a second-order dependence. Theoretical calculations indicate that the role of the HX molecules is the formation of W-H H-Cl- H-Cl adducts that convert into W-Cl, H2 and HCl2 in the rate-determining state through hydrogen complexes as transition states. 

Figure 1 General dehydrogenase mechanism. In this example, the A hydride of NAD(P)H is transferred to the carbonyl substrate, which is activated by interaction with a Lewis acid (LA). A proton is donated to the developing oxyanion by a general acid (HX).

In molecular chemistry, the reaction of [Re(= CtBu)(= CHfBu)(CH2tBu)2l with Bronsted acids (HX, X = C1, OCeFs, BF4, OTf...) gives [Re(sCfBu) (CH2tBu)3Xj as a stable product resulting exclusively from the protonation... 

Thermodynamically it can be stated, if the differences of solvation of the compounds X" and HX between two solvents are neglected, that the difference in the pK values of compound HX in the two solvents is completely determined by the difference in the proton affinities of the two solvents80 hence a comparison of the pjfifj, values of various compounds in the solvents 1,2-DCE, m-cresol, acetic acid, pyridine and water is worth considering (see Table 4.5)80. 

In view of the fact that complete methylation of F N- HX to give (CH3)3N- -HX leads to an increased extent of proton transfer from HX to the base when X is Cl and essentially complete transfer when X is I, it seemed reasonable to seek a more significant contribution from the ionic valence bond structure [(CH3)3NC1] + - F in (CT N- ClF by examining properties similarly derived from its rotational spectrum [68]. 

As expected, the high reactivity of the polar Si=P bond favors protolysis reactions with HX (X = OH, OMe, Cl), in which the phosphorus is protonated and the silicon center bonds to X (Eq. 5).IOa... 

As before, the chiral centre renders Ha and Hb non-equivalent and for the reasons already covered, Hx will couple to both with all three couplings (Ha-Hb, Ha-Hx and Hb-Hx) likely to be different. So the classical presentation of an ABX system is that of three multiplets, each of four lines. (Note that in Spectrum 6.2, the size of the Ha-X and the Hb-X couplings are almost identical so the X proton appears as an approximate triplet. This is quite common.). The AB part indicates that the geminal pair are likely to be relatively close in terms of chemical shift, whilst the X proton is someway distant from both. Obviously, the scope for variation in the appearance of ABX systems is enormous. The difference in chemical shift between Ha and Hb is a major factor in this but we have also come across ABX systems... 

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