Side-chain substituents, effect

Kish MM, Ohanessian G, Wesdemiotis C. The Na affinities of alpha-amino acids side-chain substituent effects. Inter Mass Spectrom. 2003 227 509-24. 

Kish, M.M., Ohanessian, G. and Wesdemiotis, C. (2003) The Na+ affinities of a-amino acids side-chain substituent effects. Int. J. Mass Spectrom., 227, 509-524. 

Figure 5. Effect of side chain substituents on polymer crystallinity. Polymers with two para substituents (second row) are more crystalline than polymers with mixed para and meta substituents.

Most optically active polysilanes owe their optical activity to induced main-chain chirality, as outlined above. However, backbone silicon atoms with two different side-chain substituents are chiral. Long-chain catenates, however, are effectively internally racemized by the random stereochemistry at silicon, and inherent main-chain chirality is not observed. For oligosilanes, however, inherent main-chain chirality has been demonstrated. A series of 2,3-disubstituted tetrasilanes, H3Si[Si(H)X]2SiH3 (where X = Ph, Cl, or Br), were obtained from octaphenylcyclote-trasilane and contain two chiral main-chain silicon atoms, 6.16 These give rise to four diastereoisomers the optically active S,S and R,R forms, the activity of which is equal but opposite, resulting in a racemic (and consequently optically inactive) mixture and the two meso-forms, S,R and R,S, which are optically inactive by internal compensation. It is reported that the diastereoisomers could be distinguished in NMR and GC/MS experiments. For the case of 2-phenyltetrasilane, a racemic mixture of (R)- and (A)-enantiomers was obtained. 

In a series of papers on the interaction of several purified /3-lactamase preparations with various substrates (75,77,101,112,116,117-120), evidence was presented which indicates a possibly meaningful correlation between the structure of the substrate, its effect on the conformation of the enzyme, and the catalytic activity. Much of the evidence (partly reviewed in reference 2) is based on interactions with penicillins differing in the structure of the side chain (substituent R in Fig. 1). 

Estimating the reactivity of various aromatic nuclei in lignin involves studies to determine the protodedeuteration rates of certain selectively deuterated model compounds. The combined effects of hydroxyl, methoxyl, and side-chain substituents are approximately additive in guaiacyl models while syringyl models show significant deviation from the predicted behavior. The reactivity of the position meta to methoxyl groups is higher while those of the ortho and para positions are lower than anticipated on the basis of combined substituent effects. 

A convenient route to the preparation of substituted cylohexanes is the reduction of appropriately substituted benzenes. Total reduction is effected by heterogeneous catalytic hydrogenation at temperatures in the region of 100-200 °C and usually under pressure. Under these conditions functional side-chain substituents may be variously affected (as a result of reduction or hydrogen-olysis), and specialised texts should be consulted for coverage of this vast field.15... 

In some cases, several of these processes occur simultaneously, depending on the sample size, the heating rate, the pyrolysis temperature, the environment, and the presence of any additives. Although polymer degradation schemes can be greatly altered by the presence of comonomers, side-chain substituents, and other chemical constituent factors, the ultimate thermal stability is determined by the relative strengths of the main-chain bonds. Many additives and comonomers employed as flame retardants are thermally labile and as a result the thermal stability of the polymer system is reduced. In order to reduce the observed effects of the flame-retardant additives on the thermal stability of the polymeric materials, more thermally stable and hence inherently flame-resistant polymers are of increasing interest. 

Goff, D.A., Koolpe, G.A., Kelson, A.B., Vu, H.M., Taylor, D.L., Bedford, C.D., Musallam, H.A., Koplovitz, I., Harris, R.N. (1991). Quaternary salts of 2-[(hydroxyimino)methyl]imidazole. 4. Effect of various side-chain substituents on therapeutic activity against anticholinesterase intoxication. J. Med. Chem. 34 1363-8. 

The external effects of the environment on polymer blends are chemical in nature, and normally lead to degradation of the polymers. Chain scission, depolymerization and reactions on the side-chain substituents all contribute to overall deterioration of blend properties. These are described for blends containing polyvinyl chloride, polystyrene, acrylics and polyolefins mixed with a variety of other polymers. The general feamres of radiation damage and the detrimental effects of processing are reviewed. 

The C = C double bond causes dramatic changes in the corrosion-inhibiting property only in the acyl moiety. With the amino acid side chain, the effectiveness is enhanced by the presence of a C = C group only if the vicinal phenyl ring is substituted by hydroxy/methoxy groups. The nitrogen substituent reduces the inhibition capacity. 

The impact of alkyl side chain substituents on conjugated pol5uners on the photovoltaic properties of bulk heterojimction solar cells has been studied extensively (128). However, their impact on small molecules has not received adequate attention. In order to assess the effect of side chains, a series of star-shaped molecules based on a triphenylamine core, bithiophene, and dicyanovinyl units deriva-tized with various alkyl end-capping groups of methyl, ethyl, hexyl and dodecyl have been S5mthesized. These molecules were studied to get structure-property relationships. 

The following rules may be inferred from the data (Table IV, page 354) relating the effect of side chain substituents to the pK values of the nitrogen atoms in the tetrapyrrole rings ... 

There were two schools of thought concerning attempts to extend Hammett s treatment of substituent effects to electrophilic substitutions. It was felt by some that the effects of substituents in electrophilic aromatic substitutions were particularly susceptible to the specific demands of the reagent, and that the variability of the polarizibility effects, or direct resonance interactions, would render impossible any attempted correlation using a two-parameter equation. - o This view was not universally accepted, for Pearson, Baxter and Martin suggested that, by choosing a different model reaction, in which the direct resonance effects of substituents participated, an equation, formally similar to Hammett s equation, might be devised to correlate the rates of electrophilic aromatic and electrophilic side chain reactions. We shall now consider attempts which have been made to do this. 

Chapters 9, 10 and 11 describe methods for substitution directly on the ring with successive attention to Nl, C2 and C3. Chapters 12 and 13 are devoted to substituent modification as C3. Chapter 12 is a general discussion of these methods, while Chapter 13 covers the important special cases of the synthesis of 2-aminoethyl (tryptaminc) and 2-aminopropanoic acid (tryptophan) side-chains. Chapter 14 deals with methods for effecting carbo cyclic substitution. Chapter 15 describes synthetically important oxidation and reduction reactions which are characteristic of indoles. Chapter 16 illustrates methods for elaboration of indoles via cycloaddition reactions. 

In the foregoing discussions of theoretical models and experimental results, we have focused on linear polymers. We have seen the effect of chain substituents on viscosity. All other things being equal, bulky substituents tend to decrease f and thereby lower 17. The effect is primarily due to the opening up of the liquid because of the steric interference with efficient packing arising from the substituents. With side chains of truly polymeric character, the picture is quite different. 

In the methacrylate homologous series, the effect of side-chain bulkiness is just the opposite. In this case, however, the pendant groups are flexible and offer less of an obstacle to free rotation than the phenyl group in polystyrene. As chain bulk increases, molecules are wedged apart by these substituents, free volume increases, and Tg decreases. 

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