Replacement reaction

First, mention should be made of the metathetical reaction, replacing an anion of a pyrylium salt by another-, when the solubility of the latter salt is lower than that of the former, the conversion is easy. In the opposite case, one has to find a solvent in which the solubilities are reversed (perchlorates are less soluble in water than chloroferrates or iodides, but in concentrated hydrochloric or hydroidic acids, respectively, the situation is reversed For preparing chlorides which are usually readily soluble salts, one can treat the less soluble chloroferrates with hydrogen sulfide or hydroxylamine. Another method is to obtain the pseudo base in an organic solvent and to treat it with an anhydrous acid. 

A common feature of these intermediates is that they are of high energy, compared to structures with completely filled valence shells. Their lifetimes are usually very short. Bond formation involving carbocations, carbenes, and radicals often occurs with low activation energies. This is particularly true for addition reactions with alkenes and other systems having it bonds. These reactions replace a tt bond with a ct bond and are usually exothermic. 

Nickel halides and nickel complexes resulting from oxidative addition can also give rise to subsequent replacement and insertion reactions. Replacement reactions have been described mainly with arylnickel halide complexes (examples 23, 29, and 31, Table III). Carbanionic species replace halide ions and can undergo coupling or insertion reactions. An example of application of a carbanionic reaction to the synthesis of a natural product is the coupling step between an aromatic iodo-derivative and an active methylene group to form cephalotaxinone (example 23, Table III). 

The first examples of transition metal-catalyzed [5 + 2]-cycloadditions between vinylcyclopropanes (VCPs) and 7r-systems were reported in 1995 by Wender and co-workers.10 This [5 + 2]-reaction was based conceptually on the Diels-Alder reaction, replacing the four-carbon, four-7r-electron diene with a five-carbon, four-electron VCP (Scheme 1). Although the [5 + 2]-reaction of VCPs and 7r-systems can be thought of as a homolog of the Diels-Alder [4 +21-reaction, the kinetic stability of VCPs (activation barrier for the thermal isomerization of VCP to cyclopentene has been reported as 51.7 kcal mol-1)11 makes the thermal [5 + 2]-reactions involving VCPs and 7r-systems very difficult to achieve. A report of a thermal [5 + 2]-cycloaddition between maleic anhydride and a VCP has been published,12 but this reaction has not been reproduced by others.13 14 Based on the metal-catalyzed isomerization of VCPs to cyclopentenes and dienes,15-20 Wender and co-workers hypothesized that a metal might be used to convert a VCP to a metallocyclohexene which in turn might be trapped by a 7r-system to produce a [5 + 2]-cycloadduct. Based on its previous effectiveness in catalyzed [4 + 2]-21 and [4 + 4]-cycloadditions (Section 10.13.2.4), nickel(0) was initially selected to explore the potential of VCPs as four-electron, five-carbon components in [5 + 2]-cycloadditions. 

In the case of tertiary aldehydes, which cannot undergo the aldol condensation, the Cannizzaro reaction replaces it also in the aliphatic series. Thus glyoxylic acid is disunited into glycoUic and oxalic acids. 

Like chemistry, nuclear astrophysics is a combinatorial art. Nuclear reactions are written down like chemical reactions, replacing atoms with nuclei. 

Kulkarni and Doraiswamy [38] considered cases similar to that just described but with the simple first-order reaction replaced by that shown in eqn. (17). When Cb, = 0, the expressions obtained by these workers show how rg/—rA depends on, 2,over a wide range of conditions. 

When a chemical compound decomposes, it loses an ingredient and leaves a simpler substance behind. Combination is the opposite of decomposition. A simple substance gains a new ingredient in the course of a chemical reaction. Replacement... 

This single enzyme reaction replaces several chemical steps and the use of organic solvent in the pre-existing process. A reaction temperature close to ambient can be used. 

Since the early detection of amino acids with ninhydrin, many derivatization procedures have investigated these solutes. 4-Dimethylaminoazobenzene-4 -sulfonyl chloride (DABS-C1) is performing well, but the best seems to be FMOC (9-fluorenyl chloroformate) (63). More generally, each class of reactions replaces the active hydrogens of the OH, NH, and SH groups. 

Properties of Benzo[b]thiophenes Containing a Halo Atom in a Side Chain, and Their Derived Crignard Reagents Side-chain halobenzo[6]thiophenes undergo most of the expected nucleophilic replacement reactions. Replacement by the cyanide... 

Jencks and coworkers9 noted that a likely route for catalysis of carboxylation reactions (replacement of a proton by a carboxyl group) is the generation of low entropy carbon dioxide by a reaction of ATP and bicarbonate adjacent to Nl of biotin. This way of promoting carboxylation produces a situation which is precisely what is created at the stage of the initial formation of products in decarboxylation reactions. Since there is no directional momentum, the proximity of low entropy carbon dioxide and a nucleophile similarly will slow the reaction in the direction of decarboxylation. The same authors suggest that for decarboxylation reactions, nucleophilic addition to carbon dioxide in an enzyme s active site would prevent re-addition and promote the forward reaction if the addition product is itself sufficiently unstable. 

This reaction replaces any one of the hydrogens of C5H12 with a Cl. What are the structures of the C5H12 isomer and the three C5HnCl isomers produced from it ... 

First, identify the leaving group, the electrophilic carbon, and the nucleophile. Then decide whether the reaction follows the SN1 or SN2 mechanism because this determines the stereochemistry. If the leaving group is bonded to a tertiary carbon, then the reaction must occur by the SN1 mechanism. (Later we will learn other factors that control which substitution mechanism a reaction follows.) For an SNI reaction, replace the leaving group on the electrophilic carbon with the nucleophile with loss of stereochemistry at the reaction center. 

O Ammonia acts as the nucleophile in an Sn2 reaction, replacing the bromine. 

Anaplerotic reactions replace substances present in catalytic amounts, which are indispensable for the operation of biochemical cycles. Which is an anaplerotic reaction ... 

The Hell-Volhard-Zelinsky (HVZ) reaction replaces a hydrogen atom with a bromine atom on the a carbon of a carboxylic acid. The carboxylic acid is treated with bromine and phosphorus tribromide, followed by water to hydrolyze the intermediate a-bromo acyl bromide. 

ChlorinatiChlorination hydrochloric acid, 65 Chlwination reactions replacement of substituent groups, 67 solvents for, 66 Chlorine nascent, 9, 67 toxicity, 66... 

Sridhar and co-workers [192] have described a microwave-promoted synthesis of 2-aminothiophenes 146 by reaction of a ketone with a nitrile and elemental sulfur using KF-alumina as a base via multicomponent condensation reaction. This method offers an efficient and convenient modification to the Gewald reaction replacing organic base with KF-alumina (Scheme 111). 

The same authors also reported a modified sequential reaction, replacing the isocyanide partner with an alkyne (Scheme 61) [172]. Varied functionalized TMS-protected -allylic alcohols could be prepared. In addition, the phenyltellurium group final transfer to vinyl radical 191 afforded vinyl telluride 189, which could be further elaborated. Once again, this reaction proved highly versatile, as basically all kind of carbonyl compounds were suitable (aromatic and aliphatic ketones and aldehydes). Electron-poor alkynes did not give the desired products. 

The replacement of the diazonium group by chlorine or bromine is accomplished using the Sandmeyer reaction Replacement with fluorine and iodine can be achieved by variations of this reaction. 

Substitution at the ) -carbon has much smaller effects on the reaction rates and seem to be slightly enhanced by electron donating groups. Again, this is similar to the ester ene reactions. Replacement of methyl for hydrogen in the y-substi-tuted ester increased the rate by only a factor of 2.2. 

The formal similarity allows us to carry over the equations for mass and energy balances in the tubular reactor, Eqs. (3.4.11)-(3.4.14). The momentum equation has no meaning. Care must be taken however to distinguish between a batch reactor working at constant volume and one that works at constant pressure. The latter has the Eqs. (3.4.12) or (3.4.14) which were derived from an enthalpy balance. In the former case the heat added would be equated to the internal energy change. Thus in this case c should replace Cp and the internal energy of reaction replace the heat of reaction. These... 

Strong support for the operation of square-planar to 5-coordinate to square-planar reaction pathways comes from an unusual source negative ion mass spectrometry (37). Several reactions replacing X by Y" in frans-[PtX2L2l (X and Y are halides L is PEta) were examined in the gas phase. Equation (13) is an example. 

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