Lewis base transfer reaction

In reactions with HX, alkenes behave more or less as Brpnsted-Lowry bases. Alkenes can also function as Lewis bases in reactions with electrophilic halogens. Symmetrical reagents such as bromine or chlorine are not usually polarized, but when they are brought into proximity with a n bond, a dipole is induced in the X—X bond [C=C- X+X ]. This type of structure is sometimes called a charge transfer complex. It is important to note that this occurs only with reagents such as diatomic chlorine, bromine, or iodine that are polarizable. Molecules that are not polarizable, such as diatomic hydrogen, do not react in this way, and actually require... 

In a complexation reaction, a Lewis base donates a pair of electrons to a Lewis acid. In an oxidation-reduction reaction, also known as a redox reaction, electrons are not shared, but are transferred from one reactant to another. As a result of this electron transfer, some of the elements involved in the reaction undergo a change in oxidation state. Those species experiencing an increase in their oxidation state are oxidized, while those experiencing a decrease in their oxidation state are reduced, for example, in the following redox reaction between fe + and oxalic acid, H2C2O4, iron is reduced since its oxidation state changes from -1-3 to +2. 

A1C13, or S02 in an inert solvent cause colour changes in indicators similar to those produced by hydrochloric acid, and these changes are reversed by bases so that titrations can be carried out. Compounds of the type of BF3 are usually described as Lewis acids or electron acceptors. The Lewis bases (e.g. ammonia, pyridine) are virtually identical with the Bransted-Lowry bases. The great disadvantage of the Lewis definition of acids is that, unlike proton-transfer reactions, it is incapable of general quantitative treatment. 

An interesting and stereoselective synthesis of 1,3-diols has been developed which is based on Lewis acid promoted reactions of /f-(2-propenylsilyloxy (aldehydes. Using titanium(IV) chloride intramolecular allyl transfer takes place to give predominantly Ag/r-l,3-diols, whereas anti-1,3-diols, formed via an / / /-molecular process, are obtained using tin(IV) chloride or boron trifluoride diethyl ether complex71. 

C17-0128. Pure sulfuric acid (H2 SO4) is a viscous liquid that causes severe bums when it contacts the skin. Like water, sulfuric acid is amphiprotic, so a proton transfer equilibrium exists in pure sulfuric acid, (a) Write this proton transfer equilibrium reaction, (b) Construct the Lewis stmcture of sulfuric acid and identify the features that allow this compound to function as a base, (c) Perchloric acid (HCIO4) is a stronger acid than sulfuric acid. Write the proton transfer reaction that takes place when perchloric acid dissolves in pure sulfiaric acid. 

It was G. N. Lewis who extended the definitions of acids and bases still further, the underlying concept being derived from the electronic theory of valence. It provided a much broader definition of acids and bases than that provided by the Lowry-Bronsted concept, as it furnished explanations not in terms of ionic reactions but in terms of bond formation. According to this theory, an acid is any species that is capable of accepting a pair of electrons to establish a coordinate bond, whilst a base is any species capable of donating a pair of electrons to form such a coordinate bond. A Lewis acid is an electron pair acceptor, while a Lewis base is an electron pair donor. These definitions of acids and bases fit the Lowry-Bronsted and Arrhenius theories, and cover many other substances which could not be classified as acids or bases in terms of proton transfer. 

In summary, there now exists a body of data for the reactions of carbocations where the values of kjkp span a range of > 106-fold (Table 1). This requires that variations in the substituents at a cationic center result in a >8 kcal mol-1 differential stabilization of the transition states for nucleophile addition and proton transfer which have not yet been fully rationalized. We discuss in this review the explanations for the large changes in the rate constant ratio for partitioning of carbocations between reaction with Bronsted and Lewis bases that sometimes result from apparently small changes in carbocation structure. 

In the reaction with a strong Lewis acid such as AsF5, ClF behaves as a Lewis base with the fluorine atom being the electron pair donor resulting in transfer of the F ion. 

For simplicity (and to avoid strong net change in dipolar character), we examine the simple proton-transfer reaction between two strong anionic Lewis bases, H ... 

As shown in Fig. 5.36, / R and vy become equal near s 0.6, whereas the charge transfer is half-complete near s — 0.3, and the energetic TS is at 5 = 0. Different criteria therefore lead to different estimates of the progress of reaction, but by all criteria the shifts of partial charge and covalency are quite pronounced in the H-bonded complexes. The net charge transfer gH to the evolving hydride Lewis base is seen to parallel bow closely, as the resonance picture (5.68a) suggests. 

In 1923 the American chemist G.N. Lewis provided a broad definition of acids and bases, which covered acid-base reactions not involving the traditional proton transfer an acid is an electron-pair acceptor (Lewis acid) and a base is an electron-pair donor (Lewis base). The concept was extended to metal-ligand interactions with the ligand acting as donor, or Lewis base, and the metal ion as acceptor, or Lewis acid. 

Despite of the common reaction mechanism, peroxo complexes exhibit very different reactivities - as shown by the calculated activation energies -depending on the particular structure (nature of the metal center, peroxo or hydroperoxo functionalities, type and number of ligands). We proposed a model [72, 80] that is able to qualitatively rationalize differences in the epoxidation activities of a series of structurally similar TM peroxo compounds CH3Re(02)20-L with various Lewis base ligands L. In this model the calculated activation barriers of direct oxygen transfer from a peroxo group... 

A Lewis base ligand has been developed as the chiral catalyst. Dahmen, Brase and coworkers reported that the [2.2]paracyclophane-based chiral N,O-ligand 8 (2 mol%) affords (V-formylamine with 84-95% (equation 34)134. In this system, the phenyl transfer reaction also proceeds in a catalytic manner134b. 

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