Covalent bonds substitution reactions

Fiber-Reactive Dyes. These dyes can enter iato chemical reaction with the fiber and form a covalent bond to become an iategral part of the fiber polymer. They therefore have exceptional wetfastness. Thein main use is on ceUulosic fibers where they are appHed neutral and then chemical reaction is initiated by the addition of alkaH. Reaction with the ceUulose can be by either nucleophilic substitution, using, for example, dyes containing activated halogen substituents, or by addition to the double bond in, for example, vinyl sulfone, —S02CH=CH2, groups. 

The stereochemistry of the most fundamental reaction types such as addition, substitution, and elimination are described by terms which specify the stereochemical relationship between the reactants and products. Addition and elimination reactions are classified as syn or anti, depending on whether the covalent bonds which are made or broken are on the same face or opposite faces of the plane of the double bond. 

Blasius and coworkers have offered a somewhat different approach to systems of this general type. In the first of these, shown in Eq. (6.20), he utilizes a hydroxymethyl-substituted 15-crown-5 residue as the nucleophile. This essentially similar to the Mon-tanari method. The second approach is a variant also, but more different in the sense that covalent bond formation is effected by a Friedel-Crafts alkylation. In the reaction... 

Step 2, another priming reaction, involves a further exchange of thioester linkages by another nucleophilic acyl substitution and results in covalent bonding of the acetyl group to a cysteine residue in the synthase complex that will catalyze the upcoming condensation step. 

As a recent result an example of C- and 0-covalently bonded tin substituted ylides (respectively 90 and 91) has been reported, the adducts resulting from the reaction of a stabilized yldiide with tin derivatives (RjSnCl, R2SnCl2, or SnCl2) (Scheme 28) [64]. 

In this review, CPOs constructed by covalent bonds are mainly focused on however, stable coordination bonds comparable to the stability of the covalent bonds have potential for future enhanced molecular design of novel CPOs. One representative is the bond between pyridine-type nitrogen and metal, which is widely used in supramolecular chemistry, that is, the cyclic supramolecular formation reaction between pyridine-substituted porphyrin and metal salts (Fig. 6d) [27,28]. Palladium salts are frequently used as the metal salts. From the viewpoint of the hard and soft acid and base theory (HSAB), this N-Pd coordination bond is a well-balanced combination, because the bonds between nitrogen and other group X metals, N-Ni and Ni-Pt coordination bonds, are too weak and too strong to obtain the desired CPOs, respectively. For the former, the supramolecular architectures tend to dissociate into pieces in the solution state, and for the latter. 

It is clear that reactions suitable for use in titrimetric procedures must be stoichiometric and must be fast if a titration is to be carried out smoothly and quickly. Generally speaking, ionic reactions do proceed rapidly and present few problems. On the other hand, reactions involving covalent bond formation or rupture are frequently much slower and a variety of practical procedures are used to overcome this difficulty. The most obvious ways of driving a reaction to completion quickly are to heat the solution, to use a catalyst, or to add an excess of the reagent. In the last case, a hack titration of the excess reagent will be used to locate the stoichiometric point for the primary reaction. Reactions employed in titrimetry may be classified as acid-base oxidation-reduction complexation substitution precipitation. 

In substitution reactions, a new covalent bond is formed and an old one is broken, as in the hydrolysis of tertiary-butyl bromide ... 

Soil A reversible equilibrium is quickly established when aniline covalently bonds with humates in soils forming imine linkages. These quinoidal structures may oxidize to give nitrogen-substituted quinoid rings. The average second-order rate constant for this reaction in a pH 7 buffer at 30 °C is 9.47 x 10 L/g-h (Parris, 1980). In sterile soil, aniline partially degraded to azobenzene, phenazine, formanilide, and acetanilide and the tentatively identified compounds nitrobenzene and jD-benzoquinone (Pillai et ah, 1982). 

Soil. 4-Chloroaniline covalently bonds with humates in soils to form quinoidal structures followed by oxidation to yield a nitrogen-substituted quinoid ring. A reaction half-life of 13 min was determined with one humic compound (Parris, 1980). Catechol, a humic acid monomer, reacted with 4-chloroaniline yielding 4,5-bis(4-chlorophenylamino)-3,5-cyclohexadiene-l,2-dione (Adrian et al., 1989). 

One explanation for the alpha effect is ground-state de-stabilization Repulsive electronic interactions between the alpha atom s lone-pair and the nucleophile occur in the ground-state, and such destabilization is expected to be relieved as a covalent bond is forming in the transition-state of a nucleophilic substitution reaction. Reduced solvation in molecules exhibiting the alpha effect may also play a role in the increased nucleophilicity for example, OH2 displays no effect in the gas phase, but a substantial effect is observed in solution. Another factor may be the abihty of the alpha lone-pair to stabilize any partially positive group formed in the transition state. 

A nonunity ratio (sometimes called a thermodynamic isotope effect) of the equilibrium constants ( ught/ heavy) for two reactions differing only in the isotopic composition at one or more positions of their otherwise chemically identical substances . If the equilibrium isotope effect is attributable to a covalent bond making/breaking, then the effect is often referred to as a primary equilibrium isotope effect. If isotopic substitution at a position other than the scissile bond results in an equilibrium isotope effect, the term secondary equilibrium istope effect is used. 

A. 1.1. Covalently Functionalized Dendrimers Applied in a CFMR The palladium-catalyzed allylic substitution reaction has been investigated extensively in the preceding decades and provides an important tool for the formation of carbon—carbon and carbon—heteroatom bonds 14). The product is formed after attack of a nucleophile to an (f/ -allyl)Pd(II) species, formed by oxidative addition of the unsaturated substrate to palladium(0) (Scheme 1). To date several nucleophiles have been used, mostly resulting in the formation of carbon—carbon and... 

Sulfonamide groups incorporated in rotaxanes enable the construction of new topological assemblies provided with mechanically and covalently bonded subunits. Methylation of a [2]rotaxane containing a sulfonamide unit in the axle revealed that the substitution reaction is not sterically hindered by the macrocycle. Similar to the synthesis of the pretzelane 96, the two sulfonamide groups of rotaxane 80m were bridged with 95 to form 100 in 71% yield (Figure 39) [46]. The additional covalent bond converts the former [2]rotaxane into a [l]rotaxane and reduces the mobility of the wheel along the axle. Rotaxanes 80m and 100 are his-... 

In what follows we will be concerned with the rates of ionic reactions under nonequilibrium conditions. We shall use the term nucleophile repeatedly and we want you to understand that a nucleophile is any neutral or charged reagent that supplies a pair of electrons, either bonding or nonbonding, to form a new covalent bond. In substitution reactions the nucleophile usually is an anion, Y 0 or a neutral molecule, Y or HY . The operation of each of these is illustrated in the following equations for reactions of the general compound RX and some specific examples ... 

Their approach in looking into the problem further was to find structures in which specific covalent bonding to the back side of the carbon undergoing substitution is difficult or impossible. As models for reactions at tertiary carbon they chose bridgehead substitutions. We have seen in Section 5.2 that rates in these systems are retarded, in some cases by many powers of ten, because of the increase in strain upon ionization. But the important point in the present context is that it is impossible for a solvent molecule to approach from the back side of a bridgehead carbon the only possibilities are frontside attack, known to be strongly disfavored (Section 4.2), or limiting SW1 solvolysis with nonspecific solvation. 

Comparisons of structurally related hydroxy- and methoxy-substituted cations show that hydroxy is more stabilizing by between 4 and 5 log units. This difference was recognized 20 years ago by Toullec who compared pifas for protonation of the enol of acetophenone and its methyl ether145 (-4.6 and 1.3, respectively) based on a cycle similar to that of Scheme 15, but with the enol replacing the hydrate, and a further cycle relating the enol ether to a corresponding dimethyl acetal and methoxycarbocation.146 Toullec concluded, understandably but incorrectly, that there was an error in the pA a of the ketone (over which there had been controversy at the time).147,148 In a related study, Amyes and Jencks noted a difference of 105-fold in reactivity in the nucleophilic reaction with water of protonated and O-methylated acetone and concluded that the protonated acetone lacked a full covalent bond to... 

The CSPs based on chiral crown ethers were prepared by immobilizing them on some suitable solid supports. Blasius et al. [33-35] synthesized a variety of achiral crown ethers based on ion exchangers by condensation, substitution, and polymerization reactions and were used in achiral liquid chromatography. Later, crown ethers were adsorbed on silica gel and were used to separate cations and anions [36-39]. Shinbo et al. [40] adsorbed hydrophobic CCE on silica gel and the developed CSP was used for the chiral resolution of amino acids. Kimura et al. [41-43] immobilized poly- and bis-CCEs on silica gel. Later, Iwachido et al. [44] allowed benzo-15-crown-5, benzo-18-crown-6 and benzo-21-crown-7 CCEs to react on silica gel. Of course, these types of CCE-based phases were used in liquid chromatography, but the column efficiency was very poor due to the limited choice of mobile phases. Therefore, an improvement in immobilization was realized and new methods of immobilization were developed. In this direction, CCEs were immobilized to silica gel by covalent bonds. 

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