Lewis salt formation

Second-addition example (path Adfj then Ep, Lewis salt formation with the aluminum species to create the nucleophilic aluminate, followed by a second AdN) ... 

If the L is poor as in amides, Lewis salt formation allows an aluminum oxide to be kicked out instead of the amine (path AdN, then An, followed by the Lewis salt serving as a hydride source, then Ep, and then AdN). 

A final point concerning Lewis complex possibilities should be mentioned. The adducts formed between Group III organometallics and alkali metals are somewhat remindful of Lewis salt formation except only one electron is involved (compare oxidation of R—M, Section IV.C.l) ... 

Salt formation with Brmnsted and Lewis acids and exhaustive alkylation to form quaternary ammonium cations are part of the rich derivati2ation chemistry of these amines. Carbamates and thiocarbamates are formed with CO2 and CS2, respectively the former precipitate from neat amine as carbamate salts but are highly water soluble. 

Salt formation as a criterion for an acid-base interaction has a long history (Walden, 1929). Rudolph Glauber in 1648 stated that acids and alkalis were opposed to each other and that salts were composed of these two components. Otto Tachenius in 1666 considered that all salts could be broken into an acid and an alkali. Boyle (1661) and the founder of the phlogistic theory, Stahl, observed that when an acid reacts with an alkali the properties of both disappear and a new substance, a salt, is produced with a new set of properties. Rouelle in 1744 and 1754 and William Lewis in 1746 clearly defined a salt as a substance that is formed by the union of an acid and a base. 

Aniline does not undergo Frledel-Crafts reaction (alkylation and acetylation) due to salt formation with aluminium chloride, the Lewis acid, which Is used as a catalyst. Due to this, nitrogen of aniline acquires positive charge and hence acts as a strong deactivating group for further reaction. 

Enzymatic enantioselectivity in organic solvents can be markedly enhanced by temporarily enlarging the substrate via salt formation (Ke, 1999). In addition to its size, the stereochemistry of the counterion can greatly affect the enantioselectivity enhancement (Shin, 2000). In the Pseudomonas cepacia lipase-catalyzed propanolysis of phenylalanine methyl ester (Phe-OMe) in anhydrous acetonitrile, the E value of 5.8 doubled when the Phe-OMe/(S)-mandelate salt was used as a substrate instead of the free ester, and rose sevenfold with (K)-maridelic acid as a Briansted-Lewis acid. Similar effects were observed with other bulky, but not with petite, counterions. The greatest enhancement was afforded by 10-camphorsulfonic acid the E value increased to 18 2 for a salt with its K-enanliomer and jumped to 53 4 for the S. These effects, also observed in other solvents, were explained by means of structure-based molecular modeling of the lipase-bound transition states of the substrate enantiomers and their diastereomeric salts. 

Figure 7. Formation of the sheet structure in inclusion crystals with a Lewis base guest or a Lewis acid guest, (a) Formation of one-dimensional ribbon by head-to-tail arrangement (salt formation) of the dipeptide molecules, (b) Two-dimensional arrangement of the ribbons under the influence of the guest.

FIGURE 7.4 Of the 16 chemistry topics examined (1-16) on the final exam, overall the POGIL students had more correct responses to the same topics than their L-I counterparts. Some topics did not appear on all the POGIL exams. Asterisks indicate topics that were asked every semester and compared to the L-I group. The topics included a solution problem (1), Lewis structures (2), chiral center identification (3), salt dissociation (4), neutralization (5), acid-base equilibrium (6), radioactive half-life (7), isomerism (8), ionic compounds (9), biological condensation/hydrolysis (10), intermolecular forces (11), functional group identification (12), salt formation (13), biomolecule identification (14), LeChatelier s principle (15), and physical/chemical property (16). 

Fig. 2. Principle of Lewis acid formation by photolysis of aryldiazonium salts, MX = PFj, BFj, SbFs etc.

The range of chemical-shifts differences. Ad, between those recorded in neutral and alkaline solutions fall into two distinct groups. If the Ah value is greater than +13, then the acid is behaving as a Lewis acid (scheme 9), but if the shift is to low field and is in the range of — 3 to — 7 then the acid behaves as a protic acid and salt formation has occurred (scheme 10). Consequently, phenyl and tolyl boronic acids with A8 oi + 25 and +19, respectively, behave as acceptor molecules whereas... 

The opening of the pyrrole ring leads to clean reactions in only a few cases, because Bronsted as well as Lewis acids initiate polymerization, and strong bases cause only salt formation. 

Synthesis of diaryl heteroarotinoids (11) and (12) [27,30,31] began with a Lewis acid-catalyzed cyclization of tertiary alcohol (34) to give dihydroben-zothiophene (36) as the sole isolated product. The chemistry of the ensuing steps was similar to that used to prepare (9) and (10) and other diaryl heteroarotinoids and involved (a) Friedel-Crafts acylation of a fused aromatic-heterocyclic system, (b) reduction of the resulting ketone to a benzylic carbinol, (c) phosponium salt formation, and finally (d), Wittig coupling to methyl 4-formylbenzoate. The free acids (13) and (14) were obtained by saponification. 

Steric hindrance by substituents at the nitrogen atom of amines has been determined by preparing certain Lewis salts and measuring their dissociation constants and heats of formation. Quinuclidine (98) was found to yield the most stable adduct with trimethylboron , while the adduct in the case of tiiethylamine is unstable . These results show that the approach to the nitrogen atom is hindered to a certain extent by the ethyl groups, which are free to rotate and flip... 

The type of substitution pattern as we see it with 541 and 544 obviously excludes tertiary amines, but it turned out that in these cases ammonium salt formation with Lewis adds also strongly modifies the electronic status of the nitrogen. 

Stabilization Mechanism. Zinc and cadmium salts react with defect sites on PVC to displace the labHe chloride atoms (32). This reaction ultimately leads to the formation of the respective chloride salts which can be very damaging to the polymer. The role of the calcium and/or barium carboxylate is to react with the newly formed zinc—chlorine or cadmium—chlorine bonds by exchanging ligands (33). In effect, this regenerates the active zinc or cadmium stabilizer and delays the formation of significant concentrations of strong Lewis acids. 

Nucleophilic Ring Opening. Opening of the ethyleneimine ring with acid catalysis can generally be accompHshed by the formation of an iatermediate ayiridinium salt, with subsequent nucleophilic substitution on the carbon atom which loses the amino group. In the foUowiag, R represents a Lewis acid, usually A = the nucleophile. 

Physical Properties. Both (1) and (2) are weak bases, showing 4.94 and 5.40, respectively. Their facile formation of crystalline salts with either inorganic or organic acids and complexes with Lewis acids is in each case of considerable interest. Selected physical data for quinoline and isoquinoline are given in Table 1. Reference 4 greatly expands the range of data treated and adds to them substantially. 

In the chlorination of 2,4-dichlorophenol it has been found that traces of amine (23), onium salts (24), or triphenylphosphine oxide (25) are excellent catalysts to further chlorination by chlorine ia the ortho position with respect to the hydroxyl function. During chlorination (80°C, without solvent) these catalysts cause traces of 2,4,5-trichlorophenol ( 500 1000 ppm) to be transformed iato tetrachlorophenol. Thus these techniques leave no 2,4,5-trichlorophenol ia the final product, yielding a 2,4,6-trichlorophenol of outstanding quaUty. The possibiUty of chlorination usiag SO2CI2 ia the presence of Lewis catalysts has been discussed (26), but no mention is made of 2,4,5-trichlorophenol formation or content. 

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