inorganic-reactions

"When the halogenation reaction is carried out on a cycloalkene, such as cyclopentene, only the trans stereoisomer of the dihalide addition product is formed rather than the mixture of cis and trans isomers that might have been expected if a planar carbocation intermediate were involved. We say that the reaction occurs with anti stereochemistry ..."

"Hydrohalogenation is the addition of hydrogen halides HX (X = Cl, Br, and I) to alkenes to form alkyl halides. Two bonds are broken in this reaction—the weak π bond of the alkene and the HX bond—bond two new σ bonds are formed—one to H and one to X. Because X is more electronegative than H, the H – X bond is polarized, with a partial positive charge on H. Because the electrophilic (H) end of HX is attracted to the electron-rich double bond, these reactions are called electrophilic additions."

"An example of electrophilic aromatic substitution is halogenation. Benzene is normally unreactive in the presence of halogens, because halogens are not electrophilic enough to disrupt its aromaticity. However, the halogen may be activated by Lewis acidic catalysts, such as ferric halides (FeX3) or aluminum halides (AlX3), to become a much more powerful electrophile. (...) The halogenation of benzene becomes more exothermic as we proceed from I2(endothermic) to F2 (exothermic and explosive). Chlorinations and brominations are achieved with the help of Lewis acid catalysts that polarize the X – X bond and activate the halogen by increasing its electrophilic power."

"Alkyl halides are encountered less frequently than their oxygen-containing relatives and are not often involved in the biochemical pathways of terrestrial organisms, but some of the kindsof reactions they undergo— nucleophilic substitutions and eliminations—are encountered frequently. Thus, alkyl halide chemistry acts as a relatively simple model for many mechanistically similar but structurally more complex reactions found in biomolecules."

"Ingold, Hughes, and their collaborators in England, starting in the late 1920s, carried out detailed kinetic and stereochemical investiga tions on what became known as nucleophilic substitution at saturated carbon and polar elimination reactions. Their work relating to uni molecular nucleophilic substitution and elimination, called SN1 and E1 reactions, in which formation of carbocations is the slow rate determining step, laid the foundation for the role of electron-deficient carbocationic intermediates in organic reactions."

"Because MLn consists of an electrophilic metal center and nucleophilic ligands, nucleophilic substitution processes play an important part in the reactivity of complexes."

"Nucleophilic substitution is a fairly general reaction for primary and secondary haloalkanes. The halide functions as the leaving group, and several types of nucleophilic atoms enter into the process."

"Nucleophilic substitution at tetravalent (sp3) carbon is a fundamental reaction of broad synthetic utility and has been the subject of detailed mechanistic study. An interpretation that laid the basis for current understanding was developed in England by C. K. Ingold and E. D. Hughes in the 1930s. Organic chemists have continued to study substitution reactions; much detailed information about these reactions is available and a broad mechanistic interpretation of nucleophilic substitution has been developed from the accumulated data. At the same time, the area of nucleophilic substitution also illustrates the fact that while a broad conceptual framework can outline the general features to be expected for a given system, finer details reveal distinctive aspects that are characteristic of specific systems."

"Secondary alkyl systems undergo, depending on conditions, both eliminations and substitutions by either possible pathway: uni- or bimolecular. Good nucleophiles favor SN2, strong bases result in E2, and weakly nucleophilic polar media give mainly SN1 and E1."

"Tertiary systems eliminate (E2) with concentrated strong base and are substituted in nonbasic media (SN1). Bimolecular substitution is not observed, but elimination by E1 accompanies SN1."

"Another mode of reactivity of carbocations, in addition to regular SN1 and E1 processes, is rearrangement by hydride or alkyl shifts. In such rearrangements, the migrating group delivers its bonding electron pair to a positively charged carbon neighbor, exchanging places with the charge. Rearrangement may lead to a more stable cation — as in the conversion of a secondary cation into a tertiary one. Primary alcohols also can undergo rearrangement, but they do so by concerted pathways and not through the intermediacy of primary cations."

"Nucleophilic substitution at an allylic substrate under SN2 conditions may proceed via nucleophilic attack at the y-carbon, especially when substitution at the a-carbon sterically impedes the normal SN2 reaction. These SN2' reactions with cyclohexenyl systems generally proceed via an anti addition of the nucleophile to the double bond, as depicted below (best overlap of participating orbitals)."

"Unhindered primary alkyl substrates always react in a bimolecular way and almost always give predominantly substitution products, except when sterically hindered strong bases, such as potassium tert-butoxide, are employed. In these cases, the SN2 pathway is slowed down sufficiently for steric reasons to allow the E2 mechanism to take over. Another way of reducing substitution is to introduce branching. However, even in these cases, good nucleophiles still furnish predominantly substitution products. Only strong bases, such as alkoxides, RO-, or amides, R2N-, tend to react by elimination."

"Aliphatic azides are readily prepared by nucleophilic substitution of alkyl halides or sulfonates with sodium azide; the resulting alkyl azides are readily reduced to primary amines, while 1,3 - dipolar cycloadditions of azide derivatives with dipolarophiles such as alkenes, alkynes, and nitriles give various kinds of azaheterocyclic compounds."

"In recent years there has been a proliferation of new reactions and reagents that have been so useful in organic synthesis that often people refer to them by name. Many of these are stereo selective or regioselective methods. While the expert may know exactly what the Makosza vicarious nucleophilic substitution, or the Meyers asymmetric synthesis refers to, many students as well as researchers would appreciate guidance regarding such “Name Reactions”."