Polarity aprotic solvents

Rate increases with increasing po larity of solvent as measured by its dielectric constant e (Section 8 12) Polar aprotic solvents give fastest rates of substitution solvation of Nu IS minimal and nucleophilicity IS greatest (Section 8 12)... [Pg.356]

HCN(CH3)2 DMF IS a polar aprotic solvent (Section 8 12) and an excellent medium for Sm2 reactions... [Pg.930]

This IS an example of an Sn2 reaction in a polar aprotic solvent... [Pg.1008]

Synthesis and Properties. Several methods have been suggested to synthesize polyimides. The predominant one involves a two-step condensation reaction between aromatic diamines and aromatic dianhydrides in polar aprotic solvents (2,3). In the first step, a soluble, linear poly(amic acid) results, which in the second step undergoes cyclodehydration, leading to an insoluble and infusible PL Overall yields are generally only 70—80%. [Pg.530]

New heat-resistant polymers containing -iiitrophenyl-substituted quinoxaline units and imide rings as well as flexible amide groups have been synthesi2ed by polycondensation reaction of a dianainoquinoxaline derivative with diacid dichlorides (80). These polymers are easily soluble in polar aprotic solvents with inherent viscosities in the range of 0.3—0.9 dL/g in NMP at 20°C. AH polymers begin to decompose above 370°C. [Pg.537]

Aromatic Radical Anions. Many aromatic hydrocarbons react with alkaU metals in polar aprotic solvents to form stable solutions of the corresponding radical anions as shown in equation 8 (3,20). These solutions can be analyzed by uv-visible spectroscopy and stored for further use. The unpaired electron is added to the lowest unoccupied molecular orbital of the aromatic hydrocarbon and a... [Pg.237]

Aluminum chloride dissolves readily in chlorinated solvents such as chloroform, methylene chloride, and carbon tetrachloride. In polar aprotic solvents, such as acetonitrile, ethyl ether, anisole, nitromethane, and nitrobenzene, it dissolves forming a complex with the solvent. The catalytic activity of aluminum chloride is moderated by these complexes. Anhydrous aluminum chloride reacts vigorously with most protic solvents, such as water and alcohols. The ability to catalyze alkylation reactions is lost by complexing aluminum chloride with these protic solvents. However, small amounts of these "procatalysts" can promote the formation of catalyticaHy active aluminum chloride complexes. [Pg.147]

Sodium and magnesium do not react with tetrachlorosilane at room temperature, but do so at elevated temperatures and ia the presence of polar aprotic solvents at moderately elevated temperatures. The Wurtz-Fittig coupling of organosilanes to form disilanes (168) and polysdanes (169) is usually accomphshed usiag molten sodium ia toluene or xylene. [Pg.31]

The polarity of the polymer is important only ia mixtures having specific polar aprotic solvents. Many solvents of this general class solvate PVDC strongly enough to depress the melting temperature by more than 100°C. SolubiUty is normally correlated with cohesive energy densities or solubiUty parameters. For PVDC, a value of 20 0.6 (J/cm (10 0.3 (cal/cm ) has been estimated from solubiUty studies ia nonpolar solvents. The value... [Pg.433]

The degradation of VDC polymers in nonpolar solvents is comparable to degradation in the soHd state (101,125,129,130). However, these polymers are unstable in many polar solvents (131). The rate of dehydrochlorination increases markedly with solvent polarity. In strongly polar aprotic solvents, eg, hexamethylphosphoramide, dehydrochlorination proceeds readily (129,132). This reaction is cleady unlike thermal degradation and may well involve the generation of ionic species as intermediates. [Pg.438]

Traditionahy, the reactions to be considered here are performed in homogenous medium, either in hydroxyUc solvents or in polar aprotic solvents. In comparison, PTC has the foUowing advantages no need for expensive aprotic solvents simpler work-up shorter reaction time and or lower reaction temperature use of aqueous alkaU hydroxides instead of other expensive bases. [Pg.186]

It is prepared from the polycondensation of the disodium salt of bisphenol A and 4,4-dichlorodiphenyl sulfone in a polar aprotic solvent such as dimethyl sulfoxide (26). [Pg.39]

Instead of Hquid ammonia, aqueous ammonia is also used together with a polar aprotic solvent such as formamide (48). It is also prepared by sulfonating... [Pg.312]

The realization that die nucleophilicity of anions is strongly enhanced in polar aprotic solvents has led to important improvements of several types of synthetic processes that involve nucleophilic substitutions or additions. [Pg.241]

Neopentyl (2,2-dimethylpropyl) systems are resistant to nucleo diilic substitution reactions. They are primary and do not form caibocation intermediates, but the /-butyl substituent efiTectively hinders back-side attack. The rate of reaction of neopent>i bromide with iodide ion is 470 times slower than that of n-butyl bromide. Usually, tiie ner rentyl system reacts with rearrangement to the /-pentyl system, aldiough use of good nucleophiles in polar aprotic solvents permits direct displacement to occur. Entry 2 shows that such a reaction with azide ion as the nucleophile proceeds with complete inversion of configuration. The primary beiuyl system in entry 3 exhibits high, but not complete, inversiotL This is attributed to racemization of the reactant by ionization and internal return. [Pg.303]

Sodium acetate reacts with /p-nitrophenyl benzoates to give mixed anhydrides if the reaction is conducted in a polar aprotic solvent in the presence of a crown ether. The reaction is strongly accelerated by quartemary nitrogen groups substituted at the orthc position. Explain the basis for the enhanced reactivity of these compounds. [Pg.500]

Terminally unsaturated fluonnated alkenoic acids can be obtained from poly-fluorocycloalkenes by reaction with potassium hydroxide m rert-butyl alcohol [24] (equation 26) The use of a tertiary alcohol is critical because primary and secondar y alcohols lead to ethers of the cycloalkenes The use of a polar aprotic solvent such as diglyme generates enols of diketones [26] (equation 27) The compound where... [Pg.429]

The SfjAr reaction between thiolates and monofluorobenzme and its derivatives requires high temperatures and polar aprotic solvents [19, 20, 21. Polyfluoroaro-matics show very little selectivity for fluoiine replacement [22, 2J] (equation 13). [Pg.504]

Both the dipolymers and terpolymers have excellent resistance to hydrocarbons found m petroleum-based fuels and lubricants The 69 5% F terpolymer resists swellmg m blended fuels that contain metlianol and can be used in contact with certain phosphate ester-based hydraulic fluids Terpolymers are preferred for contact with aromatic solvents, although either type performs well in higher alcohols VDF-based elastomers dissolve m polar aprotic solvents such as ketones, esters, amides, and certam ethers These elastomers are therefore not suitable for contact with fluids that contain substantial amounts of these solvents because of excessive swell and consequent loss of mechanical properties... [Pg.1113]

The large rate enhancements observed for bimolecular- nucleophilic substitutions in polar- aprotic solvents are used to advantage in synthetic applications. An example can be seen in the preparation of alkyl cyanides (nitriles) by the reaction of sodium cyanide with alkyl halides ... [Pg.347]

We consider first the Sn2 type of process. (In some important Sn2 reactions the solvent may function as the nucleophile. We will treat solvent nucleophilicity as a separate topic in Chapter 8.) Basicity toward the proton, that is, the pKa of the conjugate acid of the nucleophile, has been found to be less successful as a model property for reactions at saturated carbon than for nucleophilic acyl transfers, although basicity must have some relationship to nucleophilicity. Bordwell et al. have demonstrated very satisfactory Brjinsted-type plots for nucleophilic displacements at saturated carbon when the basicities and reactivities are measured in polar aprotic solvents like dimethylsulfoxide. The problem of establishing such simple correlations in hydroxylic solvents lies in the varying solvation stabilization within a reaction series in H-bond donor solvents. [Pg.358]

The reaction of 3,4-dinitrofurazan with pentaerythrol in polar aprotic solvents in the presence of a base gave the mono-, di-, tri-, and tetrafurazanyl ethers... [Pg.136]

Aromatic denitrocyclizations have been used for many years in some well-known synthetic reactions. Probably the best known example is the Turpin synthesis of phenoxazines and similar synthesis of phenothiazines. The classical setup used usually base-catalyzed reactions in polar protic solvents, very often alcohols. In many cases using polar aprotic solvents was found advantageous. Besides the mentioned influence of the H-bonding, better ionization and lower solvation of the nucleophile are also important. Sf Ar reactions proceed through strongly polarized complexes, which are well soluble and highly polarized in polar aprotic solvents. [Pg.190]

ILs are considered to be polar solvents, but can be non-coordinating (mainly depending on the IL s anion). Solvatochromatic studies indicate that ILs have polarities similar to those of short-chain alcohols and other polar, aprotic solvents... [Pg.68]

Synthesis of all these heterocycle activated polyethers is carried out in polar aprotic solvents, such as NMPs, by the K2CO3 method. The effective displacement reactions are reported at varied temperatures (140-190°C) and durations (3-24 h). [Pg.42]

Interests in the phase transfer catalysis (PTC) have grown steadily for the past several years [68-70]. The use of PTC has recently received industrial importance in cases where the alternative use of polar aprotic solvents would be prohibitively expensive [71-74]. Thus, the potential application of the phase transfer catalyzed aromatic nucleophilic displacement reactions between phenoxide or thiophenoxide and activated systems has... [Pg.42]

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