Nucleophilic substitutions, aliphatic

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 attack by a reagent of a molecule might be hampered by the presence of other atoms near the reaction site. The larger these atoms and the more are there, the higher is the geometric restriction, the steric hindrance, on reactivity. Figure 3-6e illustrates this for the attack of a nucleophile on the substrate in a nucleophilic aliphatic substitution reaction. 

Thus, to name just a few examples, a nucleophilic aliphatic substitution such as the reaction of the bromide 3.5 with sodium iodide (Figure 3-21a) can lead to a range of stereochemical products, from a l l mbrture of 3.6 and 3.7 (racemization) to only 3.7 (inversion) depending on the groups a, b, and c that are bonded to the central carbon atom. The ring closure of the 1,3-butadiene, 3.8, to cyclobutene... 

Figure 3-22 shows a nucleophilic aliphatic substitution with cyanide ion as a nucleophile, i his reaction is assumed to proceed according to the S f2 mechanism with an inversion in the stereochemistry at the carbon atom of the reaction center. We have to assign a stereochemical mechanistic factor to this reaction, and, clearly, it is desirable to assign a mechanistic factor of (-i-1) to a reaction with retention of configuration and (-1) to a reaction with inversion of configuration. Thus, we want to calculate the parity of the product, of 3 reaction from the parity of the... 

Nucleophilic aliphatic substitution (Chapter 8) Reaction m which a nucleophile replaces a leaving group usually a halide ion from sp hybridized carbon Nucleophilic aliphatic substitution may proceed by either an S l or an Sfj2 mechanism... 

The thenyl chlorides appear to be more reactive in nucleophilic aliphatic substitution than the benzyl analogs. Thus, 2-thenyh chloride gives, in the reaction with sodium cyanide in ethanol, a mixture of ethyl 2-thenyl ether (25% yield) and 2-thenyl cyanide (32% yield), whereas benzyl chloride gives a high 3deld of benzyl cyanide uncontaminated with benzyl ether. When 2-thenyl chloride and benzyl chloride were allowed to compete for a deficiency of sodium amyloxide, 2-thenyl chloride reacted three times faster. In acetone solution 2-thenyl cyanide is obtained smoothl. ... 

S909). First, an addition of the cyano group takes place to give intermediate 127, in which nucleophilic substitution of aliphatic nitro group provides the final product 128 (Scheme 20). 

In TFE the secondary isotope effect for solvolysis ( h/ d3)s is found to have the value 1.46 and that for Na,Np-rearrangement (kH/kD3)r has the value 1.42 (Szele and Zollinger, 1981). The two isotope effects are similar to the largest values observed in nucleophilic aliphatic substitutions following the DN + AN mechanism (Shiner,... 

There is a special interest in the role of neighboring group participation by sulfinyl groups in nucleophilic aliphatic substitution. Thus Martin and Uebel218 found that trans-4-chlorothiane-S-oxide 36 is solvolyzed (50% v/v aqueous ethanol, 140 °C) 630 times faster than the cis isomer 37. This was attributed to the intervention of 38 for the former. 

In Part 2 of this book, we shall be directly concerned with organic reactions and their mechanisms. The reactions have been classified into 10 chapters, based primarily on reaction type substitutions, additions to multiple bonds, eliminations, rearrangements, and oxidation-reduction reactions. Five chapters are devoted to substitutions these are classified on the basis of mechanism as well as substrate. Chapters 10 and 13 include nucleophilic substitutions at aliphatic and aromatic substrates, respectively, Chapters 12 and 11 deal with electrophilic substitutions at aliphatic and aromatic substrates, respectively. All free-radical substitutions are discussed in Chapter 14. Additions to multiple bonds are classified not according to mechanism, but according to the type of multiple bond. Additions to carbon-carbon multiple bonds are dealt with in Chapter 15 additions to other multiple bonds in Chapter 16. One chapter is devoted to each of the three remaining reaction types Chapter 17, eliminations Chapter 18, rearrangements Chapter 19, oxidation-reduction reactions. This last chapter covers only those oxidation-reduction reactions that could not be conveniently treated in any of the other categories (except for oxidative eliminations). 

Mechanisms for nucleophilic aliphatic substitution at glycosides, 41, 277 Mechanisms of hydrolysis and rearrangements of epoxides, 40, 247 Mechanisms of oxygenations in zeolites, 42, 225 Mechanisms, nitrosation, 19, 381... 

In contrast with aliphatic nucleophilic substitution, nucleophilic displacement reactions on aromatic rings are relatively slow and require activation at the point of attack by electron-withdrawing substituents or heteroatoms, in the case of heteroaromatic systems. With non-activated aromatic systems, the reaction generally involves an elimination-addition mechanism. The addition of phase-transfer catalysts generally enhances the rate of these reactions. 

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