Stereochemical arrangements

Figure 2-72. A selection of stereochemical arrangements in a rotational list.

Tacticity or stereochemical arrangement of atoms in three-dimensional space in relation to each other along the polymer chain cannot really be termed a structural defect. But researchers have shown that tacticity has an important bearing on the reactivity and thermal stability of PVC. For this reason tacticity is being discussed under this section. 

Hydroxyalkyl)porphyrins,84-85 I07b 109 easily accessible from porphyrins by Friedel-Crafts acylation and subsequent reduction, undergo a stereoselective Claisen rearrangement with N,N-dimethylacetamide dimethyl acetal. The substitution pattern and the stereochemical arrangement of the derived chlorins match those of naturally occurring chlorins9 (see Section 1.2.1.2.). 

Some quantities associated with the rates and mechanism of a reaction are determined. They include the reaction rate under given conditions, the rate constant, and the activation enthalpy. Others are deduced reasonably directly from experimental data, such as the transition state composition and the nature of the rate-controlling step. Still others are inferred, on grounds whose soundness depends on the circumstances. Here we find certain features of the transition state, such as its polarity, its stereochemical arrangement of atoms, and the extent to which bond breaking and bond making have progressed. 

When poly(propylene) was first made, it was found to exist in two possible forms. One was similar to poly(ethylene), but had greater rigidity and hardness the other was found to be amorphous and of little strength. The first of these is now known to be isotactic, that is with a regular stereochemistry at each alternating carbon atom. The other is now known to be atactic, that is with a random distribution of different stereochemical arrangements... 

Figure 4 Sketch of two possible stereochemical arrangements for a chiral monomer. P represents the polymer chain, R represents a vinyl substitutent on a carbon, H represents hydrogen, (a) Linear sketch showing one conformation and two configurations (bracketed and unbracketed). The apex of bonds is a tetrahedrally bonded carbon atom (solid and dashed circles), (b) Newman projection of the same monomer showing the free rotation about the C-C bond.

Figure 18. Diagrams of three possible stereochemical arrangements of Cu-aquo complexes in the interlayer of layer silicates ...

The condition of handedness or lack of superimposabil-ity of molecules differing only with respect to the stereochemical arrangement of identical substituents around a tetrahedral center. In 1893, Lord Kelvin succinctly defined chirality I call any geometrical figure, or any group of points, chiral, and say that it has chirality, if its image in a plane mirror, ideally realized, cannot be brought to coincide with itself. ... 

Note 2 Terms defining stereochemical arrangements are to be italicized only when they form part of the name of a polymer the use of such terms as adjectives, even when immediately preceding names, does not require italics. This practice is illustrated in the examples below. 

In inoiganic compounds, stereochemical arrangements other than octahedral or tetrahedral are often observed. These will be discussed in more detail below. Some selected representations of common structures are shown here. Note that often a hybrid stereoview of the structure is used, in which some lines represent bonds and others the edges of the polyhedron that defines the shape. This has been discussed... 

Most of the stereochemical arrangements found in these complexes are based oil the octahedron or on the tetrahedron, and may be derived from these configurations by a suitable distortion. Both basic systems have cubic symmetry and d-orbital energies are split into a triplet and a doublet. The group-theoretical notation will be used for their description. ... 

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