Compounds structural units

Compound Structural unit 8iso(PPm) fl(ppm) Skew Reference... 

In a reaction, bonds are broken and made. In some cases free electrons are shifted also. The rcaciion center contains all the bond.s being broken or made during the reaction as well as all the electron rearrangement processes. The reaction uhstme-ture is the structural subunit of atoms and bonds around the reaction center that has to be present in a compound in order for the reaction to proceed in the foi"ward (synthesis) direction (Figure 10,3-32). Both characteristics of a reaction can be used to. search for reactions with an identical reaction center and reaction substructure but with different structural units beyond the reaction substructure. For example, this can be achieved by searching in a reaction database. 

Conceptually the most simple syntheses of complex molecules involve the joining of structural units in which all functional groups and all asymmetric centres are preformed. This technique can usually only be applied to compounds in which these units are connected by —C—X— bonds rather than C—C. It is illustrated here by the standard syntheses of oligonucleotides, peptides, and polydentate macrocyclic ligands. 

In spite of the diverse nature of alkaloid structures, two structural units, i.e. fused pyrrolidine and piperidine rings in different oxidation states, appear as rather common denominators. We therefore chose to give several examples for four types of synthetic reactions which have frequently been used in alkaloid total synthesis and which provide generally useful routes to polycyclic compounds with five- or six-membered rings containing one nitrogen atom. These are ... 

Furthermore, if the structural unit to be polymerized is a vinyl group, mixed reactions can be carried out with a second compound such as a vinyl derivative or maleic anhydride. 

Section 4 1 Functional groups are the structural units responsible for the character istic reactions of a molecule The hydrocarbon chain to which a func tional group is attached can often be considered as simply a supporting framework The most common functional groups characterize the fami lies of organic compounds listed on the inside front cover of the text... 

Later in this chapter we 11 explore the criteria for aromaticity in more detail to see how they apply to cyclic polyenes of different ring sizes The next several sections intro duce us to the chemistry of compounds that contain a benzene ring as a structural unit We 11 start with how we name them... 

Firefly lucifenn is an example of an azole that contains a benzene ring fused to the five membered ring Such structures are fairly common Another example is benzimidazole present as a structural unit m vitamin B12 Some compounds related to benzimidazole include purine and its ammo substituted derivative adenine one of the so called het erocychc bases found m DNA and RNA (Chapter 28)... 

Analyzing an NMR spectrum m terms of a unique molecular structure begins with the mfor matron contained m Table 13 1 By knowing the chemical shifts characteristic of various proton environments the presence of a particular structural unit m an unknown compound may be inferred An NMR spectrum also provides other useful information including... 

The structural unit associated with an electronic transition m UV VIS spectroscopy IS called a chromophore Chemists often refer to model compounds to help interpret UV VIS spectra An appropriate model is a simple compound of known structure that mcor porates the chromophore suspected of being present m the sample Because remote sub stituents do not affect Xmax of the chromophore a strong similarity between the spectrum of the model compound and that of the unknown can serve to identify the kind of rr electron system present m the sample There is a substantial body of data concerning the UV VIS spectra of a great many chromophores as well as empirical correlations of sub stituent effects on k Such data are helpful when using UV VIS spectroscopy as a tool for structure determination... 

Nucleic acids are acidic substances present m the nuclei of cells and were known long before anyone suspected they were the primary substances involved m the storage transmission and processing of genetic information There are two kinds of nucleic acids ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) Both are complicated biopolymers based on three structural units a carbohydrate a phosphate ester linkage between carbohydrates and a heterocyclic aromatic compound The heterocyclic aro matic compounds are referred to as purine and pyrimidine bases We 11 begin with them and follow the structural thread... 

Two nitrogen containing heterocyclic aromatic compounds—pyrimidine and purine— are the parents of the bases that constitute a key structural unit of nucleic acids... 

Acylation (Section 12 7 and Chapter 20) Reaction in which an acyl group becomes attached to some structural unit in a molecule Examples include the Fnedel-Crafts acylation and the conversion of amines to amides Acyl chloride (Sections 4 1 and 20 1) Compound of the type... 

Aliphatic (Section 2 1) Term applied to compounds that do not contain benzene or benzene like rings as structural units (Historically aliphatic was used to descnbe compounds de rived from fats and oils )... 

If the thermodynamic data for a compound of interest have not been determined and abulated, it may be possible to estimate AHf or AGj from tabulated data pertaining to dividual structural units. Procedures have been developed for estimating thermodynamic characteristics of hydrocarbons and derivatives by summing the contributions of the constituent groups. The group increments are derived from experimental thermochemical iata and therefore depend on the existence of reliable data for the class of compounds of merest. 

In this exfflTipIe, addition to the double bond of an alkene converted an achiral molecule to a chiral one. The general term for a structural feature, the alteration of which introduces a chirality center in a molecule, is prochiral. A chirality center is introduced when the double bond of propene reacts with a peroxy acid. The double bond is a prochiral structural unit, and we speak of the top and bottom faces of the double bond as prochiral faces. Because attack at one prochual face gives the enantiomer of the compound formed by attack at the other face, we classify the relationship between the two faces as enantiotopic. 

Acridine is a heterocyclic aromatic compound obtained from coal tar that is used in the synthesis of dyes. The molecular formula of acridine is C13H9N, and its ring system is analogous to that of anthracene except that one CH group has been replaced by N. The two most stable resonance structures of acridine are equivalent to each other, and both contain a pyridine-like structural unit. Write a structural formula for acridine. 

Carboranes (or more correctly carbaboranes) are compounds having as the basic structural unit a number of C and B atoms arranged on... 

It is also interesting that the hypothetical helicene 255 has been incorporated in a study of the jt -electron spectra of a series of infinite helicenes composed of various structural units (77BAU2532). A compound 256, closely resembling an indolo-[2,3-c]carbazole but possessing an additional six-membered ring between the... 

The intermediate product S3 (see above) will also take part in this polymerization. Infrared spectra of the polysulfuroxide show a strong absorption at 1123 cm which was assigned to the structural unit -S-S(=0)-S- by comparison with model compounds like trisulfane-2-oxides (RS)2S=0 [31] and SsO (see above). In chloroform solution S2O reacts at —60 °C to S5O and SO2 (see above). 

The process proceeds through the reaction of pairs of functional groups which combine to yield the urethane interunit linkage. From the standpoint of both the mechanism and the structure type produced, inclusion of this example with the condensation class clearly is desirable. Later in this chapter other examples will be cited of polymers formed by processes which must be regarded as addition polymerizations, but which possess within the polymer chain recurrent functional groups susceptible to hydrolysis. This situation arises most frequently where a cyclic compound consisting of one or more structural units may be converted to a polymer which is nominally identical with one obtained by intermolecular condensation of a bifunctional monomer e.g., lactide may be converted to a linear polymer... 

In Section 3 of this chapter it was mentioned that polymers obtained by intermolecular condensation of bifunctional monomers may often be prepared alternatively by an addition polymerization of a cyclic compound having the same composition as the structural unit. Typical examples are shown in Table III. The processes indicated are appropriately regarded as addition polymerizations. Each of these polymers may also be prepared through the condensation of suitable bifunctional monomers. The dimethylsiloxane polymer, for example, may be prepared, as indicated in Table I (p. 45), through the condensation of dimethyl dihydroxysilane formed by hydrolysis of the di-chlorosilane... 

The chemical and physical properties of the polymers obtained by these alternate methods are identical, except insofar as they are affected by differences in molecular weight. In order to avoid the confusion which would result if classification of the products were to be based on the method of synthesis actually employed in each case, it has been proposed that the substance be referred to as a condensation polymer in such instances, irrespective of whether a condensation or an addition polymerization process was used in its preparation. The cyclic compound is after all a condensation product of one or more bifunctional compounds, and in this sense the linear polymer obtained from the cyclic intermediate can be regarded as the polymeric derivative of the bifunctional monomer(s). Furthermore, each of the polymers listed in Table III may be degraded to bifunctional monomers differing in composition from the structural unit, although such degradation of polyethylene oxide and the polythioether may be difficult. Apart from the demands of any particular definition, it is clearly desirable to include all of these substances among the condensation... 

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