Molecules cyclic

The H3 and molecules are special cases of AH2 molecules in that neither of them has the linear or bent shape already discussed. They are both cyclic molecules although H3 is known only in excited electronic states since, in its ground state, it is unstable with respect to H + H2. 

The following applications include the removal of straight-chain from branched-chain or cyclic molecules. For example, type 5A sieves will adsorb n-butyl alcohol but not its branched-chain isomers. Similarly, it separates n-tetradecane from benzene, or n-heptane from methylcyclohexane. 

The meaning of the word aromaticity has evolved as understanding of the special properties of benzene and other aromatic molecules has deepened. Originally, aromaticity was associated with a special chemical reactivity. The aromatic hydrocarbons were considered to be those unsaturated systems that underwent substitution reactions in preference to addition. Later, the idea of special stability became more important. Benzene can be shown to be much lower in enthalpy than predicted by summation of the normal bond energies for the C=C, C—C, and C—H bonds in the Kekule representation of benzene. Aromaticity is now generally associated with this property of special stability of certain completely conjugated cyclic molecules. A major contribution to the stability of aromatic systems results from the delocalization of electrons in these molecules. 

Polymerization of raw feedstock. Aliphatic hydrocarbon resins. Raw feedstock contains straight-chain and cyclic molecules and mono- and diolefins. The most common initiator in the polymerization reaction is AICI3/HCI in xylene. The resinification consists of a two-stage polymerization in a reactor at 45°C and high pressure (10 MPa) for several hours. The resulting solution is treated with water and passed to distillation to obtain the aliphatic hydrocarbon resins. Several aliphatic hydrocarbon resins with different softening points can be adjusted. 

The accessibility of the +4 and +6 oxidation states for sulfur and, to a lesser extent, selenium gives rise to both acyclic and cyclic molecules that have no parallels in N-O chemistry. Thus there is an extensive chemistry of chalcogen diimides RN=E=NR (E = S, Se, Te) (Section 10.4). In the case of Te these unsaturated molecules form dimeric structures reflecting the increasing reluctance for the heavier chalcogens to form multiple bonds to nitrogen. The acyclic molecule N=Sp3,... 

One of the most powerful tools for the formation of cyclic molecules is the Diels-Alder reaction (/). The reaction generally involves the combination of a diene with a dienophile according to the diagram. There are surprisingly few limitations on the... 

We ll see numerous instances in future chapters where the chemistry of a given functional group is strongly affected by being in a ring rather than an open chain. Because cyclic molecules are so commonly encountered in all classes of bioniolecules, including proteins, lipids, carbohydrates, and nucleic acids, it s important that the effects of their cyclic structures be understood. 

Chemists in the late 1800s knew that cyclic molecules existed, but the limitations on ring size were unclear. Although numerous compounds containing five- and six-me inhered rings were known, smaller and larger ring sizes had not been prepared, despite many efforts. 

This process, named the Diels-Alder cycloaddition reaction after its discoverers, is extremely useful in organic synthesis because it forms two carbon-carbon bonds in a single step and is one of the few genera) methods available for making cyclic molecules. (As the name implies, a cycloaddition reaction is one in which two reactants add together to give a cyclic product.) The... 

Heterocycle (Sections 15.5, 24.9) A cyclic molecule w hose ring contains more than one kind of atom. l or example, pyridine is a heterocycle that contains five carbon atoms and one nitrogen atom in its ring. 

The next key step, the second dihydroxylation, was deferred until the lactone 82 had been formed from compound 80 (Scheme 20). This tactic would alleviate some of the steric hindrance around the C3-C4 double bond, and would create a cyclic molecule which was predicted to have a greater diastereofacial bias. The lactone can be made by first protecting the diol 80 as the acetonide 81 (88 % yield), followed by oxidative cleavage of the two PMB groups with DDQ (86% yield).43 Dihydroxylation of 82 with the standard Upjohn conditions17 furnishes, not unexpectedly, a quantitative yield of the triol 84 as a single diastereoisomer. The triol 84 is presumably fashioned from the initially formed triol 83 by a spontaneous translactonization (see Scheme 20), an event which proved to be a substantial piece of luck, as it simultaneously freed the C-8 hydroxyl from the lactone and protected the C-3 hydroxyl in the alcohol oxidation state. 

Cyclazines are conjugated cyclic molecules in which planarity is secured by three covalent bonds to a central nitrogen atom. They differ in the size of the individual ring units. In the nomenclature of cyclazines, the name is preceded by the number of jt-centers between the points of attachment to the nitrogen which are arranged by increasing number and put in brackets, e.g. [3.3.3]cyclazine (1) and [2.3.4]cyclazine (2). 

The Ag-C bonds tend to be asymmetric study of silver cycloalkene complexes shows their stability to decrease in the order C5 > C6 > C7 > C8, corresponding to relief of strain in the cyclic molecules consequent upon the lengthening of the double bond on coordination. 

Table 9 shows the PP MO results for this interesting series of highly strained three-membered cyclic molecules. Here a detailed comparison is possible with the best results of an all-electron study, including d functions10 (also reported in Table 9). An analysis of this table reveals how all trends in population analysis, both in charges and overlap populations, are the same in the AE + d and in the simple PP calculations, with very few and very minor exceptions. PP predicts a charge donation to the aliphatic groups, while AE predicts a withdrawal, mainly due to the availability of d orbitals on sulphur, which can allocate extra electronic charge. As outlined in the general notes on population analysis (Section III.D) comparisons should be carried out on a relative basis and,...

With concern to the high internal mobility of the molecules in the high temperature solid state phase, some parallelism to n-alkanes can be stated. In the pseudohexagonal (rotator) phase the latter are also characterized by fast molecular motions. For discrimination and according to Pfitzer 14) and Dale 13) in the following the term pseudorotator phase is used for the mobil crystalline state of cyclic molecules. 

An acyclic molecule has more entropy than a similar cyclic molecule because there are more conformations (cf. hexane and cyclohexane). Ring opening therefore means a gain in entropy and ring closing a loss. 

Twofold extrusion from certain cyclic molecules... 

Orbitals interact in cyclic manners in cyclic molecules and at cyclic transition structures of chemical reactions. The orbital phase theory is readily seen to contain the Hueckel 4n h- 2 ti electron rule for aromaticity and the Woodward-Hof nann mle for the pericyclic reactions. Both rules have been well documented. Here we review the advances in the cyclic conjugation, which cannot be made either by the Hueckel rule or by the Woodward-Hoffmann rule but only by the orbital phase theory. 

The orbital phase theory (Chapter An Orbital Phase Theory by Inagaki in this volume) shows that some saturated cyclic molecules with lone pairs on the ring... 

Hoflmann in 1965. Ten years later, Fukni and Inagaki proposed an orbital phase theory for cyclic molecules and transition states, which includes the Woodward-Hofftnann rule and the Hueckel rule for aromaticity (Scheme 4). In 1982 Inagaki... 

The Flory distribution gives a polydispersity of 2 in the limit of high conversion. Yet, a thought experiment suggests that a small batch of self-condensing molecules would eventually condense to form a single, cyclic molecule. Reconcile this apparent inconsistency. 

Data from f.a.b.-m.s., and also f.d.-m.s., revealed the existence of naturally occurring, large cyclic polysaccharides. The first indication that a molecule may be cyclic comes from its precise molecular-weight determination. Cyclic molecules are 18 mass units less than their linear counterparts. Loss of water may, of course, occur in a number of ways, for example, by dehydration or lactonization, and conclusive evidence for the presence of a cyclic molecule can only be obtained from f.a.b.-m.s. of suitable derivatives, such as the permethyl derivative. Cyclic and dehydrated linear polymers are distinguishable after permethylation, as the cyclic polymer will incorporate one methyl group less than the linear molecule. 

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