Hydrides, acid strengths

For nonmetal hydrides, acid strength increases across a period, with the electronegativity of the nonmetal (E), and down a group, with the length of the E—H bond. 

Since the hydrogen-element bond energy decreases from sulphur to tellurium they are stronger acids than hydrogen sulphide in aqueous solution but are still classified as weak acids—similar change in acid strength is observed for Group Vll hydrides. 

Let us add here that despite the general similarities of selenium and sulfur in their chemical properties, the chemistry of selenium differs from that of sulfur in two important aspects their oxoanions are not similarly reduced, and their hydrides have different acid strengths. For example, Se(-HlV) tends to undergo reduction to Se(-II), whereas S(-hIV) tends to undergo oxidation. This difference is evidenced by the ability of selenous acid to oxidize sulftirous acid ... 

Although the oxoacids of selenium and sulfur have comparable acid strengths (pITai 2.6 vs. p fa 1-9, respectively, for the quadrivalent species pA a 3 for both the hexavalent species), the hydride H2Se is much more acidic than H2S (pATai 3.9 vs. 7.0). Thus, while thiols such as cysteine are mainly protonated at physiological pH, selenols such as selenocysteine are predominantly dissociated under the same conditions. 

MR donor bonds, is o-bond metathesis. Alternatively, as the Lewis-acid strength of M increases, the tendency toward agostic or bridging interactions can finally result in H—H bond scission and formal migration of hydride to the metal atom,... 

In the case of alkenes, 1-pentene reactions were studied over a catalyst with FAU framework (Si/Al2 = 5, ultrastable Y zeoHte in H-form USHY) in order to establish the relation between acid strength and selectivity [25]. Both fresh and selectively poisoned catalysts were used for the reactivity studies and later characterized by ammonia temperature programmed desorption (TPD). It was determined that for alkene reactions, cracking and hydride transfer required the strongest acidity. Skeletal isomerization required moderate acidity, whereas double-bond isomerization required weak acidity. Also an apparent correlation was established between the molecular weight of the hard coke and the strength of the acid sites that led to coking. 

This analogy is plausible on energetic grounds, since the decreased base strength of the proton acceptor should be approximately compensated by the increased acid strength of the proton donor. In view of the different species involved, however, it is reasonable to expect appreciable differences in the configurations of the transition states and hence in the activation barriers for the two paths. Therefore, the failure to observe an acid-catalyzed exchange reaction cannot be taken as conclusive evidence in favor of the alternative (hydride ion) mechanism. 

Acid strengths of the hydrides of the elements of a period increase from left to right across the period in the same order that electronegativity increases in the periodic table. A highly electronegative element repels an electron of hydrogen atom, and releases it as hydrogen (H" ) ion. Let us consider N, O and F. 

The acid strength of the elements hydrides within a group increases with increasing size of the central atom. 

In 1958, Parry and Edwards predicted the acidic character of the bridge hydrogens in the lower boron hydrides and proposed that the acid strength should vary directly with the size of the boron framework for a given analogous series D. This has been confirmed for the series B5H9, BeHio, B10H14 by proton competition reactions 18) which proceed to... 

I Reducing nature of Thernmal stability of HX hydrides (HX) -DHdlss of H X At Acid strength of HOX ns2 Molecular state X2... 

Table 12.2 shows that SAPO-34 has significantly better stability due to a lower rate of coke formation than SSZ-13 samples with comparable tetrahedral-atom substitutions and acid site densities. SSZ-13 also shows greater production of light paraffins which is consistent with accelerated hydride transfer for the catalyst with higher acidic strength. 

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