Charge reagents

The spirit of this kind of calculation is to give a rough and ready visualization to the potential reactivity of a molecule. For example, Figure 16.3 is a contour map for aspirin. These maps look much better in colour, and it is often possible to spot the route that an approaching charged reagent would take. 

C. Methoxide Ion, Arylsulfide Ions, and Other Charged Reagents. 312... 

Special interactions of the charged reagent with the substrate can lead to kinetic complications and to exceptional substrate reactivity. For example, the strongly basic alkoxide ion promotes ionization of... 

Chart 2. Experimental reactivity diagrams for charged reagents. Rates relative to a given position (= 1). Data deduced from refs. 20, 29, and 99. 

In the light of what has been said above, any factors that influence the relative availability of electrons (the electron density) in particular bonds, or at particular atoms, in a compound might be expected to affect very considerably its reactivity towards a particular reagent a position of high electron availability will be attacked with difficulty if at all by, for example, eOH, whereas a position of low electron availability is likely to be attacked with ease, and vice versa with a positively charged reagent. A number of such factors have been recognised. 

Figure 14.3 Principle of atmospheric pressure chemical ionization. The dissolved analyte is sprayed through a capillary. Evaporation of the solvent is supported by a heated gas stream. Within the source, a plasma is formed by a Corona discharge needle, which creates the charged reagent gas (here HgO+j. The ionization of the analyte (M) is performed by the transfer of the charge (proton) via ion-molecule reactions.

The increase in stability with the DMSO content is a general feature for Meisenheimer adduct formation and is observed to similar extents for pyridine as well as benzene adducts. It is mainly caused by the enhanced nucleophilicity of the RO ion, resulting from a decreased specific solvation of the charged reagent as the concentration of the protic solvent is decreased and dispersion interactions between the large, polarizable adduct and the dipolar aprotic solvent.36,83... 

In what follows we will be concerned with the rates of ionic reactions under nonequilibrium conditions. We shall use the term nucleophile repeatedly and we want you to understand that a nucleophile is any neutral or charged reagent that supplies a pair of electrons, either bonding or nonbonding, to form a new covalent bond. In substitution reactions the nucleophile usually is an anion, Y 0 or a neutral molecule, Y or HY . The operation of each of these is illustrated in the following equations for reactions of the general compound RX and some specific examples ... 

An electrophile is any neutral or charged reagent that accepts an electron pair (from a nucleophile) to form a new bond. In the preceding substitution reactions, the electrophile is RX. The electrophile in other reactions may be a carbon cation or a proton donor, as in the following examples ... 

This inorganic style of attraction is rare in organic reactions. A more common cause of organic reactions is attraction between a charged reagent (cation or anion) and an organic compound that has a dipole. An example that we shall explore in this chapter is the reaction between sodium cyanide (a salt, NaCN) and a carbonyl compound such as acetone. Sodium cyanide is made up of sodium cations, Na+, and cyanide anions, CN-, in solution. 

Owing to the presence of acidic groups in melanins (carboxyls, phenolic groups) positively charged reagents react faster than anions or neutral species, especially in basic media. Thus, cationic nitroxides react much faster than anionic ones, and the reaction is twofold faster at pH 10 than pH 5. The slow reaction with Nitro Blue Tetrazolium is dramatically accelerated in the presence of a cationic detergent (92). 

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