Electron-poor sites/species

Nucleophile (Section 8.1) An electron-rich species that seeks an electron-poor site similar to a Lewis base. 

A full description of how a reaction occurs is called its mechanism. There are two general kinds of mechanisms by which reactions take place radical mechanisms and polar mechanisms. Polar reactions, the most common type, occur because of an attractive interaction between a nucleophilic (electron-rich) site in one molecule and an electrophilic (electron-poor) site in another molecule. A bond is formed in a polar reaction when the nucleophile donates an electron pair to the electrophile. This movement of electrons is indicated by a curved arrow showing the direction of electron travel from the nucleophile to the electrophile. Radical reactions involve species that have an odd number of electrons. A bond is formed when each reactant donates one electron. 

Nucleophiles are electron-rich species that can form a covalent bond by donating two electrons to an electron-poor site. Nucleophiles are negatively charged (anions) or neutral molecules that contain a lone pair of electrons. 

A nucleophile is a species with a negative or partial negative charge. Literally, a nucleophile loves a nucleus. A nucleophile is attracted to a region of positive or partial positive charge (i.e., an electrophile). A nucleophile may be an anion, such as Cl , or the negative portion of a polar molecule, such as the Cl atom in HCl. Nucleophiles are electron-rich. Electron-rich sites and electron-poor sites are attracted to one another. 

Similarly, carboxylic acid and ester groups tend to direct chlorination to the / and v positions, because attack at the a position is electronically disfavored. The polar effect is attributed to the fact that the chlorine atom is an electrophilic species, and the relatively electron-poor carbon atom adjacent to an electron-withdrawing group is avoided. The effect of an electron-withdrawing substituent is to decrease the electron density at the potential radical site. Because the chlorine atom is highly reactive, the reaction would be expected to have a very early transition state, and this electrostatic effect predominates over the stabilizing substituent effect on the intermediate. The substituent effect dominates the kinetic selectivity of the reaction, and the relative stability of the radical intermediate has relatively little influence. 

Hydrogen binds to As sites at the surface when the GaAs electrode is electron rich when the GaAs electrode is electron poor, the hydrogen adsorbates are replaced by OH species at the As sites. Changes in potential were determined by interrupting the cyclic potential scans every 100 ms for a lmn period at various... 

Reaction of dihalocarbenes with alkenes, [1 +2] cycloaddition, is the method of choice for the preparation of 1,1-dihalocyclopropanes. The reaction proceeds stereospecifically preserving the configuration of the alkene in the products. These observations allow the conclusion to be made that the dihalocarbene reacts in the singlet state with alkenes. Experimental data (relative activities of alkenes, selectivity indices as well as theoretical calculations indicate that dihalocarbenes are electrophilic species. This means that they react readily with electron-rich (nucleophilic) alkenes. Dihalocarbenes may also react with electron-poor alkenes, but at a much slower rate. In the case of alkenes with a fairly unreactive double bond, dihalocarbene may also attack other sites of the alkene molecule, e.g. insert into a C-H bond. The selectivity (reactivity) of dihalocarbenes depends on the temperature at 20 C typical dihalocarbenes can be arranged in the order given, with respect to their selectivities versus reactivities (Houben-Weyl, Vol. El9b, p 1598). 

Electrophiles are electron-poor species that can form a covalent bond by accepting two electrons from an electron-rich site. Electrophiles are often positively charged (cations) although they can also be neutral. 

CO2 is a poor donor but a good electron acceptor. Owing to its acidic character, it is frequently used to probe the basic properties of solid surfaces. IR evidence concerning the formation of carbonate-like species of different configurations has been reported for metal oxides [31], which accounts for the heterogeneity of the surface revealed by micro-calorimetric measurements. The possibility that CO2 could behave as a base and interact with Lewis acid sites should also be considered. However, these sites would have to be very strong Lewis acid sites and this particular adsorption mode of the CO2 molecule should be very weak and can usually be neglected [32]. 

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