Reaction in other Solvents

Several acetylenic derivatives are prepared by dehydrohalogenation of vicinal dibromo compounds, which are obtained by adding bromine to olefinic compounds. 

In many reactions with organolithium- or Grignard intermediates tetrahydrofuran has distinct advantages over diethyl ether, as the solubility in THF of these intermediates is generally much better (see Table I, p. 37). 

For the alkylation of tnetallated acetylenes, diethyl ether and tetrahydrofuran are unsuitable solvents, unless a certain amount of dimethyl sulfoxide (DMSO) or hexamethyl-phosphoric triamide (HMPT) is added. Alkylations in liquid ammonia with higher alkyl bromides are also slow, though addition of DMSO or HMPT followed by evaporation of (pan of) the ammonia in most cases gives rise to a smooth conversion. For methods of purifying, solvents, one should consult our previous book [ I, Chapter I). 

A practical container for holding coolants is the cylindrical Dewar vessel (fig. 9, diameter -15 cm, height -20 cm). For reactions in flasks with a volume greater than 1 1, the cooling liquid may be placed in a pan which is insulated by straw or cotton wool contained in a larger pan (fig. 10). 

The choice of methods for working up and isolation is the result of careful consideration, taking into account inter alia the thermal stability and the volatility of the product. In many procedures water (or ice) is added after the conversion is terminated. Salts and water-soluble (co-)solvents such as DMSO and HMPT are separated in this manner from the product or organic solution of the product 

---

Returning to our two definitions of an acid, the first, the operational definition, gives clearcut instructions on how to decide whether a given substance is an acid. Dissolve it in water and see if it has certain properties. The second (conceptual) definition, however, has the deeper significance since it includes our knowledge of why an acid has these particular properties. It provides a basis for finding hidden likenesses between acid-base reactions in water and other reactions in other solvents. Each type of definition has its merit neither is the definition. 

The SjvAr reactions with amines in chloroform show a peculiar behaviour and the rates cannot usually be correlated with reactions in other solvents. It has been observed in the reaction of 2,4-dinitrochlorobenzene with piperidine480 and in the reaction of 1,2-DNB with butylamine115 that chloroform exerts a special solvent effect due to its known hydrogen-bond donor ability. Thus, an association between the solvent and the nucleophile can be postulated as a side-reaction to the S Ar115. Associations of chloroform with amines are known122 and the assumption of a partial association between piperidine or butylamine and chloroform as the cause of the downward curvature in the plots of k vs [amine] seems plausible. 

It is not possible to extrapolate the mechanistic findings of Dessy et al.28 to reactions in other solvents, as will be evident from the activation parameters for reaction (21) (R = Et), viz. 

The solvolysis reactions of cis and tram f-butylcyclohexyl tosylates also have been studied in NMA238. In both cases the primary product was 4-f-butylcyclohexene (no other cyclohexene products were detected) and much smaller quantities of cyclohexyl acetates and cyclohexanols were also recovered. The reaction rate was first order with respect to the tosylates. Similar to results of studies of the reaction in other solvents, the c/s-tosylate was solvolyzed more readily than was the tram compound. The stereochemical distribution of the minor products was significantly altered by small amounts of water (<1%) added to the NMA solvent. 

The yield of high molecular polysilanes is usually low (10-25%). Many attempts have been made to increase the yield of the high molecular mass polysilane by variation of the reaction conditions including inverse addition of a sodium suspension to the dichlorosilane, reactions in other solvents such as THF and at lower temperatures [33], addition of crown ethers [34] to increase the solubility of the alkaline metal, and the use of ultrasound [35] to accelerate the reaction. 

Enthalpies and entropies of activation for the decarboxylation of oxalic, malonic, and acetic acids are listed in Table 1 and are shown separately on the isokinetic plots in Fig. 8. Linear trends are observed for (1) aqueous acetic acid and sodium acetate in the presence of various catalysts (2) aqueous oxalic acid at several pH values (3) oxalic acid in different solvents and (4) malonic acid in different solvents and in aqueous solutions having a different pH. Note that the isokinetic trend for the decarboxylation of malonic acid in aqueous solutions at various pH is identical to that for the reaction in nonaqueous solvents, i.e., there is one isokinetic trend for malonic acid. Moreover, the effect of pH on the activation parameters for the decarboxylation of malonic acid in aqueous solution is minimal. On the other hand, the activation data for the decarboxylation of oxalic acid in aqueous solutions determined by Crossey (1991) do not follow the same isokinetic trend as do the corresponding data for this reaction in other solvents. By contrast, activation data calculated from the rate constants determined by Dinglinger and Schroer (1937) for oxalic acid in water (pH 0.5) fall on the isokinetic trend set by the decarboxylation of oxalic acid in nonaqueous solvents, as well as the rate data determined by Lapidus et al. (1964) in the vapor phase. The cause of the disparity between the isokinetic relationships determined by Crossey (1991) and the remainder of the oxalic acid results requires further investigation. The reaction could have been surface-catalyzed, but this is doubtful because some of the oxalic acid... 

Typical properties of acids and bases are usually taken to be those that are observed in water solutions of acids and bases. Most of them are due to the increased concentration of solvent cation or solvent anion caused by the presence of the acid or base. The most familiar reactions dependent upon this effect probably are the reactions between the free elements and solutions of acids and bases electrolysis and the reactions of amphoteric substances. The first is the only one that requires further discussion before similar reactions in other solvents are considered. 

The rate of the Lewis-acid catalysed Diels-Alder reaction in water has been compared to that in other solvents. The results demonstrate that the expected beneficial effect of water on the Lewis-acid catalysed reaction is indeed present. However, the water-induced acceleration of the Lewis-add catalysed reaction is not as pronounced as the corresponding effect on the uncatalysed reaction. The two effects that underlie the beneficial influence of water on the uncatalysed Diels-Alder reaction, enforced hydrophobic interactions and enhanced hydrogen bonding of water to the carbonyl moiety of 1 in the activated complex, are likely to be diminished in the Lewis-acid catalysed process. Upon coordination of the Lewis-acid catalyst to the carbonyl group of the dienophile, the catalyst takes over from the hydrogen bonds an important part of the activating influence. Also the influence of enforced hydrophobic interactions is expected to be significantly reduced in the Lewis-acid catalysed Diels-Alder reaction. Obviously, the presence of the hydrophilic Lewis-acid diminished the nonpolar character of 1 in the initial state. 

Dramatic rate accelerations of [4 + 2]cycloadditions were observed in an inert, extremely polar solvent, namely in5 M solutions oflithium perchlorate in diethyl ether(s 532 g LiC104 per litre ). Diels-Alder additions requiring several days, 10—20 kbar of pressure, and/ or elevated temperatures in apolar solvents are achieved in high yields in some hours at ambient pressure and temperature in this solvent (P.A. Grieco, 1990). Also several other reactions, e.g, allylic rearrangements and Michael additions, can be drastically accelerated by this magic solvent. The diastereoselectivities of the reactions in apolar solvents and in LiClO EtjO are often different or even complementary and become thus steerable. 

Analogous reactions take place in other solvents which like water contain an —OH group Solvolysis in methanol (methanolysis) gives a methyl ether... 

Economic Aspects. Lithium metal is available commercially in ingots, special shapes, shot, and dispersions. Ingots are sold in 0.11-, 0.23-, 0.45-, and 0.91-kg sizes. Special shapes include foil, wire, and rod. Lithium is available in hermetically sealed copper cartridges and in sealed copper tubes for use in treating molten copper and copper-base alloys. Shot is sold in 1.19—4.76 mm (16—4 mesh) sizes. Lithium dispersions (30% in mineral oil) of 10—50-p.m particle size are used primarily in organic chemical reactions. Dispersions in other solvents and of other size fractions can be suppHed. 

On distillation at atmospheric pressure, vanillin undergoes partial decomposition with the formation of pyrocatechol. This reaction was one of the first to be studied and contributed to the elucidation of its stmcture. Exposure to air causes vanillin to oxidize slowly to vanillic acid. When vanillin is exposed to light in an alcohoHc solution, a slow dimerization takes place with the formation of dehydrodivanillin. This compound is also formed in other solvents. When fused with alkaU (eq. 3), vanillin (I) undergoes oxidation and/or demethylation, yielding vanillic acid [121 -34-6] (8) and/or protocatechaic acid (2). 

EPM and EPDM mbbers are produced in continuous processes. Most widely used are solution processes, in which the polymer produced is in the dissolved state in a hydrocarbon solvent (eg, hexane). These processes can be grouped into those in which the reactor is completely filled with the Hquid phase, and those in which the reactor contents consist pardy of gas and pardy of a Hquid phase. In the first case the heat of reaction, ca 2500 kJ (598 kcal)/kg EPDM, is removed by means of cooling systems, either external cooling of the reactor wall or deep-cooling of the reactor feed. In the second case the evaporation heat from unreacted monomers also removes most of the heat of reaction. In other processes using Hquid propylene as a dispersing agent, the polymer is present in the reactor as a suspension. In this case the heat of polymerisation is removed mainly by monomer evaporation. 

The real world of Sn reactions is not quite as simple as the discussion has so far suggested. The preceding treatment in terms of two clearly distinct mechanisms, SnI and Sn2, implies that all substitution reactions will follow one or the other of these mechanisms. This is an oversimplification. The strength of the dual mechanism hypothesis and its limitations are revealed by these relative rates of solvolysis of alkyl bromides in 80% ethanol methyl bromide, 2.51 ethyl bromide, 1.00 isopropyl bromide, 1.70 /er/-butyl bromide, 8600. Addition of lyate ions increases the rate for the methyl, ethyl, and isopropyl bromides, whereas the tert-butyl bromide solvolysis rate is unchanged. The reaction with lyate ions is overall second-order for methyl and ethyl, first-order for tert-butyl, and first- or second-order for the isopropyl member, depending upon the concentrations. Similar results are found in other solvents. These data show that the methyl and ethyl bromides solvolyze by the Sn2 mechanism, and tert-butyl bromide by the SnI mech-... 

This procedure, in contrast to previous methods, comprises only one step and is readily adapted to large-scale preparative work. Furthermore dibromination is very slow in methanol and hence the crude reaction products contain only traces of dibromo ketones. This contrasts with the behavior in other solvents such as ether or carbon tetrachloride, where larger amounts of dibromo ketones are always present, even when one equivalent of bromine is used. Methanol is thus recommended as a brominating solvent even when no orientation problem is involved. It should be noted that a-bromomethyl ketals are formed along with x-bromoketones and must be hydrolyzed during the workup (Note 8).7... 

Repeating the reaction in HMDS without BSTFA gave 78% assay yield. The reaction could also be performed with BSTFA in other solvents. Of the solvents screened, dioxane gave the highest yields and cleanest reaction. The oxidation was also successful with amide 10 and ketone 31, as shown in Scheme 3.10. Reactions... 

SYil MisSIS The polymerization can be run in any one of several solvents, including dimethylsulfoxide, 1,4-dioxacyclohexane, dimethylformamide, and dimethylacetamide. Dimethylsulfoxide or mixtures based on dimethylsulfoxide have been used as the solvent for all reactions reported here. In other solvents, the product often precipitates as the reaction proceeds. This reaction can be successfully run with mole ratios of the reactants in the following ranges 1. hydroperoxide to calcium chloride 0.25 to 32> and 2. hydroperoxide to lignin (M ) 21 to 115 ... 

We have reported the first example of a ring-opening metathesis polymerization in C02 [144,145]. In this work, bicyclo[2.2.1]hept-2-ene (norbornene) was polymerized in C02 and C02/methanol mixtures using a Ru(H20)6(tos)2 initiator (see Scheme 6). These reactions were carried out at 65 °C and pressure was varied from 60 to 345 bar they resulted in poly(norbornene) with similar conversions and molecular weights as those obtained in other solvent systems. JH NMR spectroscopy of the poly(norbornene) showed that the product from a polymerization in pure methanol had the same structure as the product from the polymerization in pure C02. More interestingly, it was shown that the cis/trans ratio of the polymer microstructure can be controlled by the addition of a methanol cosolvent to the polymerization medium (see Fig. 12). The poly(norbornene) prepared in pure methanol or in methanol/C02 mixtures had a very high trans-vinylene content, while the polymer prepared in pure C02 had very high ds-vinylene content. These results can be explained by the solvent effects on relative populations of the two different possible metal... 

---