Reaction competing

FIGURE 9.22 Elimination may compete with both 5, 1 and 5 2 reactions. 

FIGURE 9.23 Rearrangement during S l solvolysis of a secondary halide. 

Since methanol is a poor nucleophile, and the halide is secondary, this is likely to be an S l reaction. The initially formed secondary carbocation can react directly with methanol but also rearranges easily by migration of a hydride to give a tertiary cation. The tertiary cation reacts with methanol to give the major product  

The solution is to label either the hydrogens with deuterium or one of the carbon atoms with 

FIGURE 9.25 Deuterium labeling is used to study allylic substitution. 

We have already seen that the SN1 and SN2 reactions may be in competition under certain circumstances. In addition, other reactions also compete with these two. If we are trying to prepare a specific compound, these competing reactions often result in a lower yield of the desired compound and may also cause purification problems. 

0 Ethoxide ion acts as a nucleophile, attacking the Ethoxide ion acts as a base, removing a proton to carbon, resulting in the formation of the SN2 give the elimination product. The by-product is 

Show both the substitution and elimination products that are formed in these reactions  

This rearrangement is favorable because the initial carbocation is secondary while the product is a more stable tertiary carbocation. An example of a rearrangement involving a 1,2-alkyl shift is 

Section 11.5 A Simple Reaction Mechanism Two Consecutive Steps 

32 For the consecutive first-order reaction without reverse reactions, assume that k = 0.01000 s and 

In many syntheses a side reaction consumes part of the reactants but gives undesired products. We consider the simplest case that two competing reactions are first order with negligible reverse reaction. 

This equation is the same as Eq. (11.2-2) except that ki is replaced by k + k2, and its solution is 

The right-hand side of this equation does not contain [F], so we can multiply by dt and integrate to obtain 

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Nate 7. An excess of butyllithium is used, as some butyllithium is destroyed by the competing reaction with the THF. 

A species that can serve as both a proton donor and a proton acceptor is called amphiprotic. Whether an amphiprotic species behaves as an acid or as a base depends on the equilibrium constants for the two competing reactions. For bicarbonate, the acid dissociation constant for reaction 6.8... 

Resins. As mentioned above, both furfural and furfuryl alcohol are widely used in resin apphcations. Another resin former, 2,5-furandimethanol [1883-75-6] (BHME), is prepared from furfuryl alcohol by reaction with formaldehyde. It is usually not isolated because oligomerization occurs simultaneously with formation (competing reaction). Both the monomer and oligomers are very reactive owing to difuntionahty, and are used primarily as binders for foundry sand (72) and fiberglass insulation (147,148). 

Replacement of Labile Chlorines. When PVC is manufactured, competing reactions to the normal head-to-tail free-radical polymerization can sometimes take place. These side reactions are few ia number yet their presence ia the finished resin can be devastating. These abnormal stmctures have weakened carbon—chlorine bonds and are more susceptible to certain displacement reactions than are the normal PVC carbon—chlorine bonds. Carboxylate and mercaptide salts of certain metals, particularly organotin, zinc, cadmium, and antimony, attack these labile chlorine sites and replace them with a more thermally stable C—O or C—S bound ligand. These electrophilic metal centers can readily coordinate with the electronegative polarized chlorine atoms found at sites similar to stmctures (3—6). 

This reaction is slow and requires elevated temperatures of 120—150°C under pressure. The kinetics (93,94) and mechanism (95,96) of these reactions have been studied. An undesirable competing reaction is the further oxidation of hydrazine by chloramine ... 

Simple olefins do not usually add well to ketenes except to ketoketenes and halogenated ketenes. Mild Lewis acids as well as bases often increase the rate of the cyclo addition. The cycloaddition of ketenes to acetylenes yields cyclobutenones. The cycloaddition of ketenes to aldehydes and ketones yields oxetanones. The reaction can also be base-cataly2ed if the reactant contains electron-poor carbonyl bonds. Optically active bases lead to chiral lactones (41—43). The dimerization of the ketene itself is the main competing reaction. This process precludes the parent compound ketene from many [2 + 2] cyclo additions. Intramolecular cycloaddition reactions of ketenes are known and have been reviewed (7). 

Control of chromium penetration, essential to permit tannage of the center of the hide, is accompHshed by pH adjustment. At a pH > 3.0 the reactivity of the hide to the chromium complex is greatiy increased. The pH is therefore raised gradually to the desired point by addition of a mild alkah, usually sodium bicarbonate. The chemistry of chrome tanning involves competing reactions that must be controlled for satisfactory results. 

This is a particularly troublesome competing reaction when the olefin oxide, eg, ethylene oxide, produces the more reactive terminal primary hydroxy group. Glycol ethers are used as solvents ia lacquers, enamels, and waterborne coatings to improve gloss and flow. 

Metals and Metal Derivatives, Silane reacts with alkah metals dissolved in various solvents, forming as the chief product the silyl derivative of the metal, potassium being the most commonly studied, eg, KSiH [13812-63-OJ (27—30). When 1,2-dimethoxyethane or bis(2-methoxyethyl)ether are used as solvents, two competing reactions occur, where M is an alkah metal. 

In practice, two competing reactions combine to limit the process. 

Pyrolysis. Benzene undergoes thermal dehydrocondensation at high temperatures to produce small amounts of biphenyls and terphenyls (see Biphenyl AND terphenyls). Before the 1970s most commercial biphenyl was produced from benzene pyrolysis. In a typical procedure benzene vapors are passed through a reactor, usually at temperatures above 650°C. The decomposition of benzene iato carbon and hydrogen is a competing reaction at temperatures of about 750°C. Biphenyls are also formed when benzene and ethylene are heated to 130—160°C ia the presence of alkaH metals on activated AI2O3 (33). 

Some chlorine and chlorate also form through competing reactions. Chlorine dioxide is also evolved from mixtures of powdered sodium chlorite and acidic clays or alumina. 

A competing reaction that consumes ethylene oxide is hydrolysis to ethylene glycol and oligomeric glycol by-products. 

The generated CIO2 has a competing reaction -with excess ozone to form chlorate. Mechanistically, ... 

A secondary competing reaction can occur where chlorine dioxide disproportionates in the alkaline solution, producing sodium chlorite and chlorate ... 

Abstraction of other hydrogens occurs to a very small degree and leads to small amounts of 2-chlotopropene [557-98-2J and 1-chlotopropene [590-21 -6]. Significant competing reactions include the addition reaction forming 1,2-dichloropropane [78-87-5] (eq. 1,... 

An understanding of competing reactions in the manufacturing process is important if by-products are to be minimized. Three competing reactions are possible under conditions of the reaction. 

Reversible inhibition is characterized by an equiUbrium between enzyme and inhibitor. Many reversible inhibitors are substrate analogues, and bear a close relationship to the normal substrate. When the inhibitor and the substrate compete for the same site on the enzyme, the inhibition is called competitive inhibition. In addition to the reaction described in equation 1, the competing reaction described in equation 3 proceeds when a competitive inhibitor I is added to the reaction solution. 

The closely related N- arylazoaziridine system (278) decomposes in refluxing benzene to give aryl azides and alkenes, again stereospecifically (70T3245). However, biaryls, arenes and other products typical of homolytic processes are also formed in a competing reaction, although this pathway can be suppressed by the use of a polar solvent and electron withdrawing aryl substituents. 

Oxaziridines are powerful oxidizing agents. Free halogen is formed from hydrobromic acid (B-67MI50800). Reduction by iodide in acidic media generally yields a carbonyl compound, an amine and two equivalents of iodine from an oxaziridine (1). With 2-alkyl-, 2-acyl and with N-unsubstituted oxaziridines the reaction proceeds practically quantitatively and has been used in characterization. Owing to fast competing reactions, iodide reduction of 2-aryloxaziridines does not proceed quantitatively but may serve as a hint to their presence. 

Scales for bases that are too weak to study in aqueous solution employ other solvents but are related to the equilibrium in aqueous solution. These equilibrium constants provide a measure of thermodynamic basicity, but we also need to have some concept of kinetic basicity. For the reactions in Scheme 5.4, for example, it is important to be able to make generalizations about the rates of competing reactions. 

There is usually a competing reaction with solvent when lydrogen halide additions to alkenes are carried out in nucleophilic solvents ... 

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