Secondary side reactions

Byproducts are also generated by secondary reactions, occurring in parallel with the desired reaction. These may be represented as  

In this case it may be possible to adjust the reaction conditions to maximise conversion to C. 

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In the Pidgeon process discussed above, a secondary side reaction occurs between the CaO and Si02 forming dicalcium silicate ... 

Step VI in Scheme 1 is an irreversible secondary side reaction—oligomeriza-... 

The major chemical changes in wood caused by fiberboard manufacture are secondary side reactions which are both beneficial and detrimental to the final properties achieved. Defibering is accomplished by hydrolytic breakdown of lignin and hemicelluloses under wet acidic conditions combined with high process temperatures. Board conversion and consolidation is attended by pyrolytic reactions which... 

Mg-Al mixed oxides obtained by thermal decomposition of anionic clays of hydrotalcite structure, present acidic or basic surface properties depending on their chemical composition [1]. These materials contain the metal components in close interaction thereby promoting bifunctional reactions that are catalyzed by Bronsted base-Lewis acid pairs. Among others, hydrotalcite-derived mixed oxides promote aldol condensations [2], alkylations [3] and alcohol eliminations reactions [1]. In particular, we have reported that Mg-Al mixed oxides efficiently catalyze the gas-phase self-condensation of acetone to a,P-unsaturated ketones such as mesityl oxides and isophorone [4]. Unfortunately, in coupling reactions like aldol condensations, basic catalysts are often deactivated either by the presence of byproducts such as water in the gas phase or by coke build up through secondary side reactions. Deactivation has traditionally limited the potential of solid basic catalysts to replace environmentally problematic and corrosive liquid bases. However, few works in the literature deal with the deactivation of solid bases under reaction conditions. Studies relating the concerted and sequential pathways required in the deactivation mechanism with the acid-base properties of the catalyst surface are specially lacking. 

Another coulometric approach is based on controIled-current technique. In this case, a constant current (iapp < iL(t = 0)) is performed, with corresponding potential varied with time span of the electrolysis in Fig. 6a. While the iapp is higher than iL(t > 0), a secondary (side) reaction occurs during the electrolysis (Fig. 6b [2]). As a result, the current efficiency of the analyte falls below 100 %, due to the potential drift with increasing time. 

BEC An abbreviation for background equivalent concentration, by-product ions Ionic species formed as a result of secondary reactions that take place in a reaction/collision cell. Also refer to secondary (side) reactions. 

One characteristic of this reaction that can cause problems is that secondary and tertiary amines are produced in addition to the primary amine. It has been proposed that these side reactions occur through reaction of the imine iatermediate with the product amine, followed by the loss of ammonia and further hydrogenation (10). 

The tertiary amine is formed in a similar manner from the imine and a secondary amine. This side reaction can be minimized by carrying out the hydrogenation in the presence of ammonia, which tends to shift the equiHbrium back towards the imine. When a compound with two or more nitrile groups is hydrogenated, the formation of both cycHc and acycHc secondary and tertiary amines is possible, depending on whether the side reaction is intramolecular or intermolecular. For example, for the hydrogenation of adiponitfile ... 

Alcohols undergo dehydration in supercritical and hot water (41). Tertiary alcohols require no catalyst, but secondary and primary alcohols require an acid catalyst. With 0.01 MH2SO4 as a catalyst, ethanol eliminates water at 385°C and 34.5 MPa to form ethene. Reaction occurs in tens of seconds. Only a small amount of diethyl ether forms as a side reaction. 

Generally, isolated olefinic bonds will not escape attack by these reagents. However, in certain cases where the rate of hydroxyl oxidation is relatively fast, as with allylic alcohols, an isolated double bond will survive. Thepresence of other nucleophilic centers in the molecule, such as primary and secondary amines, sulfides, enol ethers and activated aromatic systems, will generate undesirable side reactions, but aldehydes, esters, ethers, ketals and acetals are generally stable under neutral or basic conditions. Halogenation of the product ketone can become but is not always a problem when base is not included in the reaction mixture. The generated acid can promote formation of an enol which in turn may compete favorably with the alcohol for the oxidant. 

If homolytic reaction conditions (heat and nonpolar solvents) can be avoided and if the reaction is conducted in the presence of a weak base, lead tetraacetate is an efficient oxidant for the conversion of primary and secondary alcohols to aldehydes and ketones. The yield of product is in many cases better than that obtained by oxidation with chromium trioxide. The reaction in pyridine is moderately slow the intial red pyridine complex turns to a yellow solution as the reaction progresses, the color change thus serving as an indicator. The method is surprisingly mild and free of side reactions. Thus 17a-ethinyl-17jS-hydroxy steroids are not attacked and 5a-hydroxy-3-ket-ones are not dehydrated. 

The elimination from sulfonates of secondary alcohols is frequently easier than more direct methods applied to the free alcohols. As with the latter, there are the possibilities of isomeric olefin formation and rearrangement reactions. In addition, displacement and hydrolysis may occur, but these side reactions can usually be suppressed. 

It is a reaction of wide scope both the phosphite 1 and the alkyl halide 2 can be varied. Most often used are primary alkyl halides iodides react better than chlorides or bromides. With secondary alkyl halides side reactions such as elimination of HX can be observed. Aryl halides are unreactive. 

With secondary and tertiary alkyl halides an Ea-elimination is often observed as a side-reaction. As the alkyl halide reactant an iodide is most often employed, since alkyl iodides are more reactive than the corresponding bromides or chlorides. With phenoxides as nucleophiles a C-alkylation can take place as a competing reaction. The ratio of 0-alkylation versus C-alkylation strongly depends on the solvent used. For example reaction of benzylbromide 4 with /3-naphth-oxide 5 in yV,A-dimethylformamide (DMF) as solvent yields almost exclusively the /3-naphthyl benzylether 6, while the reaction in water as solvent leads via intermediate 7 to formation of the C-benzylated product—l-benzyl-2-naphthol 8—as the major product ... 

Neben-quantenzahl, /. secondary (or subsidiary) quantum number, -raum, m. additional space anteroom, -reaktlon, /. by-reaction, side reaction, secondary reaction, -rechnung, /. auxiliary calculation, -reihe,/. = Neben-serie. -rohr, n. side tube (or pipe), branch tube (or pipe). -roUe, /. subordinate r61e. -sache, /. secondary matter. 

Alkylation lo yield a leriiary amine may occur easily if the formation involves cyclization (ii). Catalysts may have a marked influence. In reductive alkylation of ammonia wilh cyclohexanones, more primary amine was formed over Ru and Rh and more secondary amine over Pd and Pt. Reduction of the ketone to an alcohol is an important side reaction over ruthenium. 

The mechanisms of these acid-catalyzed epoxide openings are more complex than they at first appear. They seem to be neither purely SN1 nor SN2 but instead to be midway between the two extremes and to have characteristics of both. Take the reaction of 1,2-epoxy-l-methylcyclohexane with HBr shown in Figure 18.2, for instance. The reaction yields only a single stereoisomer of 2-bromo-2-methyl-cyclohexanol in which the —Br and —OH groups are trans, an S 2-li.ke result caused by backside displacement of the epoxide oxygen. But the fact that Br attacks the more hindered tertiary side of the epoxide rather than the less hindered secondary side is an SN1 -like result in which the more stable, tertiary carbocation is involved. 

Recent investigations have been concerned with the reactivities observed with secondary silanes R2SiH2. In these cases, a dehydrogenative coupling of silanes to disilanes is observed as a side reaction of the hydrosilation. However, the hydrosilation can be totally suppressed if the olefins are omitted. The key intermediate in the coupling reaction has been identified as a silylene complex (sect. 2.5.4). 

Small amts of H chloride, N monoxide, N, and other prods which have been found in the decompn of AP are accounted for by secondary reactions and/or higher temp decompns (Ref 42, p 5). Kinetic work on the decompn of AP has been hampered by side reactions and the profound effect which small amts of impurities, especially metal salts, have on the decompn (see below)... 

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