And the neighboring group

The local chemical environment of the polymer-metal complex can be considered to be fairly different from ihat of the monomeric analog due to the effect of the polymer chain and the neighboring groups, even if part of the complex is the same in both. 

Summarizing, the separation of enantiomeric pairs is achieved by the close selective interaction of one enantiomer with a stationary phase chiral center, resulting in stronger molecular interactions between the enantiomer and the neighboring groups or atoms round the stationary phase chiral center. 

Perhaps the best of these chemical (enzymes are chemicals, too) cleavages uses cyanogen bromide (BrCN) and takes advantage of the nucleophilicity of sulfur and the neighboring group effect to induce cleavage at the carboxy side of each methionine in a polypeptide (Fig. 23.38). 

If 1 is a H(f>0 group and the neighboring groups at are also H< 0, then both the force ViC/(Ri, X ) and the local density p(Xh/R 0 are expected to be normal. (By normal we mean here that the forces and the local densities are as in an a polar solvent.) In this case we cannot expect large solvent-induced forces. 

If 1 is H0O and the neighboring groups at R are H< I, then the force is still normal but the density of water molecules might be large (see Appendix D). 

The pify of the leaving group and the hydrophobe chain length can dramatically affect the efficiency of the perhydrolysis reaction. Additionally, the stmcture of the acid portion of the precursor can affect the yield and sensitivity of the reaction to pH. The mono-4-hydroxybenzenesulfonic acid ester of a-decylsuccinic acid (13) undergoes extremely efficient perhydrolysis at much lower pHs than other peracid precursors, eg, decanoyloxybenzene sulfonate (14). This may be because of the neighboring group participation of the adjacent carboxylate as shown in Table 2 (115). 

The activation energy of thermolysis of the azo group was measured by DSC [14]. Type II MAIs, which are composed of various prepolymers such as aliphatic polyester, poly(caprolactone), and aliphatic poly (carbonate), showed almost the same activation energy irrespective of difference in prepolymer structure, suggesting that the neighboring group only affects the active site. 

Q Base (B ) attacks a neighboring hydrogen and begins lo remove the H at the same time as the alkene double bond starts to form and the X group starts to leave. 

An aldehyde or ketone reacts with a primary amine, RNH.2, to yield an imine, in which the carbonyl oxygen atom has been replaced by the =N-R group of the amine. Reaction of the same aldehyde or ketone with a secondary amine, R2NH, yields an enamine, in which the oxygen atom has been replaced by the -NR2 group of the amine and the double bond has moved to a position between the former carbonyl carbon and the neighboring carbon. 

This indicated that retention had taken place. Note that both products are optically inactive and so caimot be told apart by differences in rotation. The meso and d/ dibromides have different boiling points and indexes of refraction and were identified by these properties. Even more convincing evidence was that either of the two threo isomers alone gave not just one of the enantiomeric dibromides, but the dl pair. The reason for this is that the intermediate present after the attack by the neighboring group (17) is symmetrical, so the external nucleophile Br can attack... 

The bromine atom is much more selective than the chlorine atom. As indicated on page 906, it is often possible to brominate tertiary and benzylic positions selectively. High regioselectivity can also be obtained where the neighboring-group mechanism (p. 899) can operate. 

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