Substitutions classification

A more detailed classification of chemical reactions will give specifications on the mechanism of a reaction electrophilic aromatic substitution, nucleophilic aliphatic substitution, etc. Details on this mechanism can be included to various degrees thus, nucleophilic aliphatic substitutions can further be classified into Sf l and reactions. However, as reaction conditions such as a change in solvent can shift a mechanism from one type to another, such details are of interest in the discussion of reaction mechanism but less so in reaction classification. 

The imides, primaiy and secondary nitro compounds, oximes and sulphon amides of Solubility Group III are weakly acidic nitrogen compounds they cannot be titrated satisfactorily with a standard alkaU nor do they exhibit the reactions characteristic of phenols. The neutral nitrogen compounds of Solubility Group VII include tertiary nitro compounds amides (simple and substituted) derivatives of aldehydes and ketones (hydrazones, semlcarb-azones, ete.) nitriles nitroso, azo, hydrazo and other Intermediate reduction products of aromatic nitro compounds. All the above nitrogen compounds, and also the sulphonamides of Solubility Group VII, respond, with few exceptions, to the same classification reactions (reduction and hydrolysis) and hence will be considered together. 

Classification of Substituents in Electrophilic Aromatic Substitution Reactions... 

The triaryknethane dyes are broadly classified into the triphenyknethanes (Cl 42000—43875), diphenylnaphthyknethanes (Cl 44000—44100), and miscellaneous triphenylmethane derivatives (Cl 44500—44535). The triphenyknethanes are classified further on the basis of substitution in the aromatic nuclei, as follows (/) diamino derivatives of triphenylmethane, ie, dyes of the malachite green series (Cl 42000—42175) (2) triamino derivatives of triphenylmethane, ie, dyes of the fuchsine, rosaniline, or magenta series (Cl 42500—42800) (J) aminohydroxy derivatives of triphenylmethane (Cl 43500—43570) and (4) hydroxy derivatives of triphenylmethane, ie, dyes of the rosoHc acid series (Cl 43800—43875). Monoaminotriphenyknethanes are known but they are not included in the classification because they have Httie value as dyes. 

The richness and complexity of the present section is considerable. Almost any heterocycle conveniently substituted can be transformed into another chosen ring system, and this is shown in the two excellent volumes of van der Plas ( Ring Transformations of Heterocycles ) (B-73MI4Q4Q2). The arrangement of the aforementioned book (from the starting heterocycle point of view) does not suit this section and for the purposes of this chapter an alternative classification has been selected. When no explicit references are given, the material has been taken from (B-73MI4Q4Q2). 

Kinds of Catalyzed Organic Reactions A fundamental classification of organic reactions is possible on the basis of the lands of bonds that are formed or destroyed and the natures of eliminations, substitutions, and additions of groups. Here a more pragmatic hst of 20 commercially important lands or classes of reactions will be discussed. In all instances of sohd-catalyzed reactions, chemisorption is a primary step. Often molecules are dissociated on chemisorption into... 

The classification can be illustrated with a few examples. All of the nucleophilic substitutions shown in Scheme 3.4 are of the exo-tet classification. The reacting atom is of sp hybridization (tetrahedral = tet), and the reacting bond, that is, the bond to the leaving grov, is exocyclic to the forming ring ... 

The classification of hydrocar bons as aliphatic or ar omatic took place in the 1860s when it was aheady apparent that there was something special about benzene, toluene, and their- derivatives. Their molecular- for-mulas (benzene is CgHg, toluene is CyKj ) indicate that, like alkenes and alkynes, they are unsaturated and should undergo addition reactions. Under conditions in which bromine, for example, reacts rapidly with alkenes and alkynes, however, benzene proved to be inert. Benzene does react with Br-2 in the presence of iron(III) bromide as a catalyst, but even then addition isn t observed. Substitution occurs instead ... 

A further classification is into the classes of substitutions (symbolized S), additions (Ad), and eliminations (E). 

In this reaction I" is the nucleophile, and Br" is called the leaving group (or nucleofuge). Beyond this, the classification symbolism may include a designation of the molecularity of the reaction. Molecularity is the number of reactant molecules included in the transition state. The above reaction is an 8 2 reaction, because both reactants are present in the transition state. On the other hand, this substitution... 

Al, as shown in structure 3), the molecularity (1 or 2), and the ionic form of the substrate [A for conjugate acid RC(OH)OR and B for conjugate base RCOOR]. Note that alkyl-oxygen fission constitutes nucleophilic substitution and is therefore equivalent to the classification ... 

A further factor which must also be taken into consideration from the point of view of the analytical applications of complexes and of complex-formation reactions is the rate of reaction to be analytically useful it is usually required that the reaction be rapid. An important classification of complexes is based upon the rate at which they undergo substitution reactions, and leads to the two groups of labile and inert complexes. The term labile complex is applied to those cases where nucleophilic substitution is complete within the time required for mixing the reagents. Thus, for example, when excess of aqueous ammonia is added to an aqueous solution of copper(II) sulphate, the change in colour from pale to deep blue is instantaneous the rapid replacement of water molecules by ammonia indicates that the Cu(II) ion forms kinetically labile complexes. The term inert is applied to those complexes which undergo slow substitution reactions, i.e. reactions with half-times of the order of hours or even days at room temperature. Thus the Cr(III) ion forms kinetically inert complexes, so that the replacement of water molecules coordinated to Cr(III) by other ligands is a very slow process at room temperature. 

In Brown s classification a diazonium ion is a reagent of very low reactivity and correspondingly high substrate selectivity and regioselectivity. This follows from the fact that benzenediazonium salts do not normally react with weakly nucleophilic benzene derivatives such as toluene. More reactive heteroaromatic diazonium ions such as substituted imidazole-2-diazonium ions will even react with benzene (see Sec. 12.5). 

In discussing base catalysis it will prove convenient to adopt, at the outset, a distinction first proposed by Bunnett and Garst22, who noted that the observed cases of catalysis in nucleophilic aromatic substitution could be broadly divided into two categories. The classification was in terms of the relative rates of the catalyzed and uncatalyzed reactions. Since all of the systems could be accommodated empirically by eqn. (4),... 

In any heterolytic reaction in which a new carbon-carbon bond is formed one carbon atoms attacks as a nucleophile and the other as an electrophile. The classification of a given reaction as nucleophilic or electrophilic is a matter of convention and is usually based on analogy. Although not discussed in this chapter, 11-12-11-28 and 12-14-12-19 are nucleophilic substitutions with respect to one reactant, though, following convention, we classify them with respect to the other. Similarly, all the reactions in this section (10-93-10-123) would be called electrophilic substitution (aromatic or aliphatic) if we were to consider the reagent as the substrate. 

A frequently used classification is based on the degree of substitution and types of side groups for characterization (Ebringerova, 2005 Sedlmeyer, 2011) ... 

Fig. 2.33 Classification of y-peptides according to their substitution patterns (Seebach s nomenclature) and folding propensity...

Phosphinidenes [1] are low-valent organophosphorus compounds that have attracted attention since the early 1980s when they were first discovered [2]. They are known in two classifications, one being the six-electron singly substituted phosphorus species (A) and the other in which the phosphorus atom carries an additional ri -stabifizing group, typically, but not necessarily, a transition metal group (B). Much has been learned about the reactivities of the complexed phos-... 

In a ligand substitution reaction, two groups must always receive attention. There is a bond to the leaving group to be broken and a bond to the entering group to be formed. The relative importance of these two processes provides a basic dichotomy for the classification of substitutions. If a reaction rate is sensitive to... 

Casanola-Martin GM et al. (2006) New tyrosinase inhibitors selected by atomic linear indices-based classification models. Bioorg Med Chem Lett 16(2) 324-330 Tan CY, Wainman D, Weaver DF (2003) N-, alpha-, and beta-substituted 3-amino-propionic acids design, syntheses and antiseizure activities. Bioorg Med Chem 11(1) 113-121... 

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