Double bonds crystal effects

The presence and the number of double bonds. Crystallization of paraffinic compounds is difficult but occurs easiest if the chains can assume a linear zigzag conformation. A chain with a cis double bond cannot do this it tends to form a bend (see Figure 15.19b, SOS). This makes crystallization more difficult. The effect will be greater for more double bonds. Cf. the series SSS-OOO-LiLiLi-LnLnLn. 

Halogenated Butyl Rubber. Halogenation at the isoprene site ia butyl mbber proceeds by a halonium ion mechanism leading to a double-bond shift and formation of an exomethylene alkyl haUde. Both chlorinated and brominated mbber show the predominate stmcture (1) (>80%), by nmr, as described eadier (33,34). Halogenation of the unsaturation has no apparent effect on the isobutylene backbone chains. Cross-linked samples do not crystallize on extension due to the chain irregularities introduced by the halogenated isoprene units. 

Chlorogenic acid forms a 1 1 complex with caffeine, which can be crystallized from aqueous alcohol and yields very little free caffeine on extraction with chloroform. Other compounds with which caffeine will complex in this way include isoeugenol, coumarin, indole-acetic acid, and anthocyanidin. The basis for this selection was the requirement for a substituted aromatic ring and a conjugated double bond in forming such a complex. This kind of complex does modify the physiological effects of caffeine.14 Complex formation will also increase the apparent aqueous solubility of caffeine in the presence of alkali benzoates, cinnamates, citrates, and salicylates.9... 

In contrast to the allyl system, where the reduction of an isolated double bond is investigated, the reduction of extensively delocalized aromatic systems has been in the focus of interest for some time. Reduction of the systems with alkali metals in aprotic solvents under addition of effective cation-solvation agents affords initially radical anions that have found extensive use as reducing agents in synthetic chemistry. Further reduction is possible under formation of dianions, etc. Like many of the compounds mentioned in this article, the anions are extremely reactive, and their intensive studies were made possible by the advancement of low temperature X-ray crystallographic methods (including crystal mounting techniques) and advanced synthetic capabilities. 

The molecular structure of the parent compound was investigated in the vapor and in the solid phase using X-ray, XN and GED methods. The reported data are shown in Table 16. In both phases a clear bond length separation could be detected with a localized three-membered ring and its three adjacent double bonds. The symmetry-equivalent cyclopropane bonds are rather long in C3v-symmetric BUL (1.533-1.542 A), which can be explained by the common electron-withdrawing effect of the 7r-systems in a. svM-ciinal conformation. For comparison, the unaffected bonds in unsubstituted cyclopropane are 1.499 A in the crystal and 1.510 A in the gas phase. Therefore, the bond lengths in BUL... 

A variety of systems are known in which there are observable crystal-field effects on the configuration about a double bond. Thus, acetaldehyde phenylhy-drazone occurs in solution as an equilibrium mixture of syn and anti forms (in benzene the ratio is 3 2), but in the crystal only the anti isomer is found (60). 

The distances between saturated hydrocarbon molecules in crystals can be calculated by the use of these radii, with consideration also of the possibility of molecular or group rotation. Another factor must be introduced for aromatic molecules.64 The double bonds in these molecules project above and below the plane of the ring in such a way as to give to the ring an effective thickness of about 3.4 A, as observed in anthracene, durene, hexamethylbenzene, benzbisantkrene, and many other aromatic hydrocarbons. The same value is also found between the layer of graphite. 

The experimental crystal structure determination showed that this molecule has bond distances with unexpected values, which were not consistent with classically localized bond-valence forms in particular, the C-9=N-4 and C-12=N-6 bonds, which are both formally double bonds, and not of equal length. In addition, the N-l-N-2, N-3-N-4, and N-5-N-6 bonds were expected to be shorter than the C-N single bonds, C-l-N-3, C-2-N-3, and C-ll-N-5. In fact, none of the former three is shorter than any other of the latter three. They are shorter than the C-12-C-10 bond in the ring system. The bond distances in C-ll-S-l-C-1 fragment are normal for their types. These two C-S bonds have effectively ideal lengths and are in the normal range. 

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