Reacted at different

An achiral reagent cannot distinguish between these two faces. In a complex with a chiral reagent, however, the two (phantom ligand) electron pairs are in different (enantiotopic) environments. The two complexes are therefore diastereomeric and are formed and react at different rates. Two reaction systems that have been used successfully for enantioselective formation of sulfoxides are illustrated below. In the first example, the Ti(0-i-Pr)4-f-BuOOH-diethyl tartrate reagent is chiral by virtue of the presence of the chiral tartrate ester in the reactive complex. With simple aryl methyl sulfides, up to 90% enantiomeric purity of the product is obtained. 

Note The reagent can be employed on sihca gel, kieselguhr. Si 50 000 and cellulose layers. At room temperature sugars and sugar derivatives react at different rates depending on the functional groups present [1], e.g. ketoses react more rapidly than aldoses. It is possible to differentiate substance types on this basis [1, 3]. 

Now suppose that the substrate of a chemical reaction is a weak acid, with both the conjugate acid HS and conjugate base S being capable of undergoing reaction. Usually these two species will react at different rates because of the considerable difference in their electronic configurations. The rate equation for this system is... 

In this case study, an enzymatic hydrolysis reaction, the racemic ibuprofen ester, i.e. (R)-and (S)-ibuprofen esters in equimolar mixture, undergoes a kinetic resolution in a biphasic enzymatic membrane reactor (EMR). In kinetic resolution, the two enantiomers react at different rates lipase originated from Candida rugosa shows a greater stereopreference towards the (S)-enantiomer. The membrane module consisted of multiple bundles of polymeric hydrophilic hollow fibre. The membrane separated the two immiscible phases, i.e. organic in the shell side and aqueous in the lumen. Racemic substrate in the organic phase reacted with immobilised enzyme on the membrane where the hydrolysis reaction took place, and the product (S)-ibuprofen acid was extracted into the aqueous phase. 

Further evidence against the formation of a free carbonium ion in the alkylation reaction is obtained from the fact that in the presence of boron trifluoride-phosphoric acid catalyst, but-l-ene, but-2-ene, and i-butene react at different rates with alkylbenzenes, yet they would each give the same carbonium ion. In addition, only the latter alkene gave the usual activation order (in this case the hyper-... 

A kinetic resolution depends on the fact that the two enantiomers of a racemic substrate react at different rates with the enzyme. The process is outlined in Figure 6.1, assuming that the (S) substrate is the fast-reacting enantiomer (ks > ka) and Kic = 0-In ideal cases, only one enantiomer is consumed and the reaction ceases at 50% conversion. In most cases, both enantiomers are transformed and the enantiomeric composition ofthe product and the remaining starting material varies with the extent... 

They react at different rates with other chiral compounds. These rates may be so close together that the distinction is practically useless, or they may be so far apart that one enantiomer undergoes the reaction at a conveni t rate while the other does not react at all. This is the reason that many compounds are biologically active while their enantiomers are not. Enantiomers react at the same rate with achiral compounds. ... 

In general, it may be said that enantiomers have identical properties in a symmetrical environment, but their properties may differ in an unsymmetrical environment. Besides the important differences previously noted, enantiomers may react at different rates with achiral molecules if an optically active catalyst is present they may have different solubilities in an optically active solvent., they may have different indexes of refraction or absorption spectra when examined with circularly polarized light, and so on. In most cases these differences are too small to be useful and are often too small to be measured. 

It is now possible to see why, as mentioned on page 126, enantiomers react at different rates with other chiral molecules but at the same rate with achiral molecules. In the latter case, the activated complex formed from the (/ ) enantiomer and the other molecule is the mirror image of the activated complex formed from the (S) enantiomer and the other molecule. Since the two activated complexes are... 

In a batch reactor, the relative monomer concentrations will change with time because the two monomers react at different rates. For polymerizations with a short chain life, the change in monomer concentration results in a copolymer composition distribution where polymer molecules formed early in the batch will have a different composition from molecules formed late in the batch. For living polymers, the drift in monomer composition causes a corresponding change down the growing chain. This phenomenon can be used advantageously to produce tapered block copolymers. 

Note A -, A A - and d -3-Ketosteroids react at different rates — as a function amongst other things of the acid strength of the reagent — so they can be differentiated [3, 5, 6). 

Another more specific characteristic can allowed the differentiation between homogeneous and colloidal catalysts. In Pd-catalysed allylic alkylation rac-3-acetoxy-1,3-diphenyl-l-propene with dimethyl malonate, homogeneous and colloid species reacts at different rates with both substrate enantiomers, obtaining in the case of Pd colloids an excellent kinetic resolution (see Section 4) [44]. 

There are two possible approaches for the preparation of optically active products by chemical transformation of optically inactive starting materials kinetic resolution and asymmetric synthesis [44,87], For both types of reactions there is one principle in order to make an optically active compound we need another optically active compound. A kinetic resolution depends on the fact that two enantiomers of a racemate react at different rates with a chiral reagent or catalyst. Accordingly, an asymmetric synthesis involves the creation of an asymmetric center that occurs by chiral discrimination of equivalent groups in an achiral starting material. This can be done either by enan-tioselective (which involves the reaction of a prochiral molecule with a chiral substance) or diastereoselective (which involves the preferential formation of a single diastereomer by the creation of a new asymmetric center in a chiral molecule) synthesis. 

Dibenzoylmethane (8b) has been the subject of much interest as regards the possibility that its polymorphism is associated with keto-enol tautomerism. Chemical and spectroscopic studies showed that this is not so (33a). This compound had previously been reported to be trimorphic (33b), but one form appears, in fact, to be a eutectic mixture of the other two. The molecules in these two polymorphs are both in the same state of tautomerism they differ in the torsional angle about the (CH)-(CO) bond and in the type of hydrogen bonding in which they participate. It is noteworthy that solutions prepared from these forms at low temperature have differences in chemical and spectroscopic properties that are maintained for some time. For example, such solutions prepared and held at —35° react at different rates with FeCl3. 

It seems necessary at this time to emphasize that the above generalizations may pertain only as concerns O-H-N-O explosives and that diametrically opposed conclusions may apply to aluminized mixtures. Evidence is available that kinetic factors (which extend far beyond the time of the C-J condition), rather than thermodynamic factors, govern the extent of utilization of aluminum m the detonation. If, as seems reasonable, the rates of the aluminum reactions are highly temperature dependent, and if aluminum reacts at different rates with HaO, CO, and COa, detonation properties of aluminized explosives should depend very strongly on exact equilibrium compositions of these species in the C-J condition and in the early stages of the gas expansion. For such reasons, Eqs. (1) and (7) may be inapplicable for use with aluminized mixtures. 

The presence of stereocenters in sugars causes their C=0 groups to have diastereotopic faces (Section 5.4) that react at different rates, resulting in unequal amounts of diastereomers. 

Both the ally lie alcohol and tert-hutyX hydroperoxide are achiral, but the product epoxide is formed in high optical purity. This is possible because the catalyst, titanium tetraiso-propoxide, forms a chiral (possibly dimeric [36]) complex with resolved diethyl tartrate [(+)-DET] which binds the two achiral reagents together in the reactive complex. The two enantiotopic faces of the allylic double bond become diastereotopic in the chiral complex and react at different rates with the tert-butyl hydroperoxide. Many other examples may be found in recent reviews [31, 37-39]. 

It is now possible to see why, as mentioned on p. 95, enantiomers react at different rates with other chiral molecules but at the same rate with achiral molecules. In the latter case, the activated complex formed from the R enantiomer and the other molecule is the mirror image of the activated complex formed from the S enantiomer and the other molecule. Since the two activated complexes are enantiomeric, their energies are the same and the rates of the reactions in which they are formed must be the same (see Chapter 6). However, when an R enantiomer reacts with a chiral molecule that has, say, the R configuration, the activated complex has two chiral centers with configurations R and R, while the activated complex formed from the S enantiomer has the configurations S and R. The two activated complexes are diastereomeric, do not have the same energies, and consequently are formed at different rates. 

Figure 9.1 Illustration of some processes in which sorbed species behave differently from dissolved molecules of the same substance. (a) Dissolved species may participate directly in air-water exchange while sorbed species may settle with solids. (b) Dissolved species may react at different rates as compared with their sorbed counterparts due to differential access of other dissolved and solid-phase reactants. ...

When we measure the pH of 0.10 M CH,COOH(aq), we find a value close to 3. In contrast, when we make the same measurement on 0.10 M HCl(aq), we find a pH close to 1. We have to conclude that the H,0+ molarity in 0.10 M CH3COOH(aq) is lower than that in 0.10 M HCl(aq). Similarly, we find that, whereas the pH of 0.10 M NH3(aq) is close to 11, that of 0.10 M NaOH(aq) is close to 13. That is, the H.O molarity is higher in 0.10 M NH3(aq) than it is in 0.10 M NaOH(aq). The explanation must be that CH3COOH is not fully deprotonated and NH3 is not fully protonated in water. That is, they are, respectively, a weak acid and a weak base. The incomplete deprotonation of CH3COOH explains why solutions of HC1 and CH3COOH with the same molarity react at different rates (Fig. 10.12). 

Getting these depth numbers is very important, because every person is unique in his reactions while hypnotized. Some people react at different speeds than others some react to a particular hypnotic experience by going deeper into hypnosis, others sometimes find the depth of their hypnotic state decreased by the same experience. Thus by getting these state reports from you every so often I can tell whether to go a little faster or slower, where to put emphasis in the suggestions I use to guide you, etc. These depth reports are not always what I expect, but it s more important for me to know where you really are than just assume you re there because I ve been talking that way ... 

If, on the other hand, primary and secondary amine hydrogens react at different rates, it is necessary to use a kinetic scheme to obtain the evolution of the concentration of different fragments along the reaction. The epoxy -amine reaction may take place both by a noncatalytic path (specific rate constant k ) and by a reaction catalyzed by OH groups (specific rate constant k). The secondary amine hydrogen is usually less reactive than the primary amine hydrogen. Specific rate constants for the secondary amine... 

The third method used in the resolution of racemates is the kinetic resolution. The success of this method is depending on the fact that the two enantiomers react at different rates with a chiral entity. The chiral entity should be present in catalytic amounts it may be a biocatalyst (enzyme or a microorganism) or a chemocatalyst (chiral acid or base or even a chiral metal complex). Kinetic resolution of racemic compounds is by far the most common transformation catalyzed by lipases, in which, the enzyme discriminate between the two enantiomers of racemic mixture, so that one enantiomer is readily transferred to the product faster than the other.1"18 (cf. fig 3)... 

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