The hydride ion

The enthalpy change Aea (298K) (see Section 1.10) associated with the attachment of an electron to an H atom (reaction 10.2) is —73kJmor.  

All alkali metal hydrides (see Sections 10.7 and 11.4) crystallize with the NaCl-type structure. From diffraction data and the ionic radii of the metal ions (Appendix 6) the radius of H can be estimated using equation 10.3. It varies from 130 pm (in LiH) to 154 pm (in CsH) and can be considered similar to that of F (133 pm). 

Hydrides of the j-block metals (excluding Be) can be made by heating the metal with H2. 

When we compare A H for reaction 10.4 with those for the formations of F and Cl from F2 and CI2 (—249 and —228kJmoF, respectively), we understand why, since H is about the same size as F, ionic hydrides are relatively tmstable species with respect to dissociation into their constituent elements. Salt-like hydrides of metals in high oxidation states are most unlikely to exist. (There is more about binary hydrides in Section 10.7.) 

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As an example of a really strong base, the hydride ion H (for example in NaH) is unique it has one lone pair, a negative charge and a very small size. Like O , it is too strong a base to exist in water ... 

Since, generally, any base stronger than OH will react with water to produce OH we must use another solvent to observe very strong bases. The high base strengths of the hydride ion and the oxide ion can best be observed in molten salts as solvents, since hydrides and ionic oxides are either insoluble in ordinary solvents or attack them. 

These solid ionic hydrides (having an ionic lattice and containing the hydride ion H ) react with water, for example... 

The existence of the hydride ion is shown by electrolysis of the fused salt when hydrogen is evolved at the anode. If calcium hydride is dissolved in another fused salt as solvent, the amount of hydrogen evolved at the anode on electrolysis is 1 g for each Faraday of current (mole of electrons) passed, as required by the laws of electrolysis. 

This reaction is due to the very strong basic property of the hydride ion H" which behaves as a powerful proton acceptor and is therefore strongly basic, i.e. 

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

Bonding of Hydrogen to Other Atoms. The hydrogen atom can either lose the 1 valence electron when bonding to other atoms, to form the ion, or conversely, it can gain an electron in the valence shell to form the hydride ion, (see Hydrides). The formation of the ion is a very endothermic process ... 

A mechanism has been proposed to rationalize the results shown in Figure 23. The relative proportion of the A -pyrazolines obtained by the reduction of pyrazolium salts depends on steric and electronic effects. When all the substituents are alkyl groups, the hydride ion attacks the less hindered carbon atom for example when = Bu only C-5 is attacked. The smaller deuterohydride ion is less sensitive to steric effects and consequently the reaction is less selective (73BSF288). Phenyl substituents, both on the nitrogen atom and on the carbon atoms, direct the hydride attack selectively to one carbon atom and the isolated A -pyrazoline has the C—C double bond conjugated with the phenyl (328 R or R = Ph). Open-chain compounds are always formed during the reduction of pyrazolium salts, becoming predominant in the reduction of amino substituted pyrazoliums. 

The reaction has been applied to more complex enamines 13) and to dienamines 19). The reduction may be rationalized by initial protonation at the enamine carbon and subsequent decarboxylation of formate ion and addition of the hydride ion to the iminium cation. This mechanism has been given support by the reaction of the enamine (205) with deuterated formic acid 143) to give the corresponding amines. The formation of 206 on reaction with DCOOH clearly indicates that protonation at the enamine carbon is the initial step. 

In both cases, the hydride ion approaches the double bond from the sterically more accessible side of the molecule. Reduction of imines by metals and acids, electrolytically or by formic acid gives saturated secondary amines (38,255). 

This section briefly considers the proton H+, the hydride ion H, the hydrogen molecule ion H2, the triatomic 2-electron species H3+ and the recently established cluster species +... 

This is larger than the corresponding value for Li (57 kJ mol" ) but substantially smaller than the value for F (333kJmol" ). The hydride ion H" has the same electron configuration as helium but is much less stable because the single positive charge on the proton must now control the 2 electrons. The hydride ion is thus readily deformable and this constitutes a characteristic feature of its structural chemistry (see p. 66). 

The metals in Groups I and 2 of the periodic table react directly with hydrogen to form white, crystalline, stoichiometric hydrides of formula MX and MX2 respectively. The salt-like character of these compounds was recognized by G. N. Lewis in 1916 and he suggested that they contained the hydride ion H". Shortly thereafter... 

It is eommon knowledge that the reaetion is a two-step proeess, whieh ineludes the addition of a nueleophile at an unsubstituted earbon of an arene or hetarene followed by the aromatization of the intermediate cr -adduet (94MT). Elimination of the hydride ion seems to be an improbable step eompared to the elimination of a nueleofuge X from sueh cr -adduets. Thus the aromatization of a (T -adduet is the key step for the whole reaetion, and it ean proeeed by several general paths, as shown in Seheme 1. 

The Cannizzaro reaction takes place by nucleophilic addition of OH- to an aldehyde to give a tetrahedral intermediate, which expels hydride ion as a leaving group and is thereby oxidized. A second aldehyde molecule accepts the hydride ion in another nucleophilic addition step and is thereby reduced. Benzaldehyde, for instance, yields benzyl alcohol plus benzoic acid when heated with aqueous NaOH. 

Hydride ion-water reaction. As water is dropped onto solid calcium hydride, the hydride ion (H ) reacts immediately and vigorously to form H g) (which is ignited by the heat of the reaction) and OH-. 

When the hydride ion of lithium alanate is used as nucleophile, cyclohexa-2,4-dien-l-ol is obtained as a labile addition product which eliminates water on standing to give benzene.12 The reaction of an oxepin derivative that possesses a hexamethylene bridge across C3-C6 with sodium methoxide gives an addition product 5 in which the seven-membered heterocyclic system is retained.213 214... 

The hydride ion, H, can function as a ligand to tf-block metals. Compare the differences in bonding to metal ions that you would expect between H and the halide ions. 

The large effect of deformation of the hydride ion, whose mole refraction6 (deformability) is 25, is shown by the contraction in lithium hydride. 

This reaction corresponds to the basic process during the initiation of cationic polymerizations by RX/MtXn and when reversed is the termination reaction. It will be handled more in detail in part 4.2. When X = H, the reaction enthalpy of the previous equation is equal to the hydride ion affinity (HIA) which is shown for various relevant... 

That means The higher the proton affinity of the monomer, the lower the hydride ion affinity of its cation, that is, the more stable this cation (see part 4.1.3). 

If the hydride ion comes from 39, the final step is a rapid proton transfer. In the other case, the acid salt is formed directly, and the alkoxide ion acquires a proton from the solvent. Evidence for this mechanism is (1) The reaction can be first order in base and second order in substrate (thus going through 39) or, at higher base concentrations, second order in each (going through 40) and (2) when the reaction was run in D2O, the recovered alcohol contained no a deuterium, indicating that the hydrogen comes from another mole of aldehyde and not from the medium. 

This mechanism accounts for the positive value of p, namely, electron-withdrawing substituents increase the acidity of the hydrogen atom. If the reaction occurred by abstraction of the hydride ion, one would expect a negative value of p. 

Nonsolvated salts such as CsB3H8 and (CH3)4NB3H8 are the most stable. Closely related to B3H8 are the neutral base derivatives of B3H7, e.g., (CH3)3N-B3H7, which can be prepared from the B3H8 ion by displacement of the hydride ion 5... 

Another species that is a strong base is the hydride ion, H-. Metal hydrides react with water to produce basic solutions ... 

Reactions in which the hydride ion functions as a Bronsted base also result in the production of hydrogen. Typical reactions are the following. 

The most important characteristic of ionic hydrides is that they are strong Bronsted bases. The hydride ion will react with most molecules that contain a hydrogen atom bound to an atom of high... 

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