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# 化学代写|有机化学代写organic chemistry代考| Nucleophilic Substitution Reactions at the Saturated C Atom

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## 化学代写|有机化学代写organic chemistry代考|Nucleophiles and Electrophiles; Leaving Groups

Stated with some exaggeration, organic chemistry is comparatively simple to learn because most organic chemical reactions follow a single pattern. This pattern is
$$\text { nucleophile }+\text { electrophile } \underset{\text { pair shift(s) }}{\text { valence electron }} \operatorname{product(s)}$$
A nucleophile is a species that attacks the reaction partner by making a pair of electrons available to it; it is thus an electron pair donor. An electrophile is a species that reacts by accepting a pair of electrons from the reaction partner so that it can be shared between them. An electrophile is thus an electron pair acceptor.

## 化学代写|有机化学代写organic chemistry代考|Good and Poor Nucleophiles

Which nucleophiles can be alkylated rapidly and are thus called “good nucleophiles”? Or, in other words, which nucleophiles have “high nucleophilicity”? And which nucleophiles can be alkylated only slowly and are thus called “poor nucleophiles”? Or, in other words, which nucleophiles have “low nucleophilicity”? Or let us ask from the point of view of the alkylating agent: Which alkylating agents react rapidly in $S_{N}$ reactions and thus are “good alkylating agents” (good electrophiles)? Which alkylating agents react slowly in $\mathrm{S}_{\mathrm{N}}$ reactions and thus are “poor alkylating agents” (poor electrophiles)? As emerges from these definitions, good and poor nucleophiles, high and low nucleophilicity, good and poor alkylating agents, good and poor electrophiles, and high and low electrophilicity are kinetically determined concepts.

Answers to all these questions are obtained via pairs of $\mathrm{S}_{\mathrm{N}}$ reactions, which are carried out as competition experiments. In a competition experiment two reagents react simultaneously with one substrate (or two substrates react simultaneously with one reagent). Two reaction products can then be produced. The main product is the compound that results from the more reactive (synonymous with “faster reacting”) reaction partner.

## 化学代写|有机化学代写ORGANIC CHEMISTRY代考|Leaving Groups and the Quality of Leaving Groups

In Figure $2.3$ substructures have been listed in the order of their suitability as leaving groups in $\mathrm{S}{\mathrm{N}}$ reactions. Substrates with good leaving groups are listed on top and substrates with increasingly poor leaving groups follow. At the bottom of Figure $2.3$ are substrates whose functional group is an extremely poor leaving group. In part the effect of nucleophiles on substrates of the latter type does result in a reaction, but it is not an $\mathrm{S}{\mathrm{N}}$ reaction. This is, for example, the case when the nucleophile abstracts an acidic proton in the $\alpha$ position to the functional group instead of replacing the functional group. For example, $\mathrm{S}{\mathrm{N}}$ reactions with ammonium salts, nitro compounds, sulfoxides, sulfones, sulfonium salts, phosphonic acid esters, phosphine oxides, and phosphonium salts usually fail as a result of such a deprotonation. Another reaction competing with the substitution of a functional group by a nucleophile is an attack on the functional group by the nucleophile. For example, $\mathrm{S}{\mathrm{N}}$ reactions of nitriles, phosphonic acid esters, and phosphonium salts often fail because of this problem.

Alcohols, ethers, and carboxylic acid esters occupy an intermediate position. These compounds as such-except for the special cases shown in Figure 2.3-do not enter into any $\mathrm{S}{\mathrm{N}}$ reactions with nucleophiles. The reason for this is that poor leaving groups would have to be released $\left({ }^{-} \mathrm{OH},{ }^{-} \mathrm{OR},{ }^{-} \mathrm{O}{2} \mathrm{CR}\right.$; see below for details). However, these compounds can enter into $\mathrm{S}{\mathrm{N}}$ reactions with nucleophiles when they are activated as oxonium ions, for example via a reversible protonation, via bonding of a Lewis acid (LA in Figure 2.3), or via a phosphorylation. Thus, upon attack by the nucleophile, better leaving groups (e.g., $\mathrm{HOH}, \mathrm{HOR}, \mathrm{HO}{2} \mathrm{CR}, \mathrm{O}=\mathrm{PPh}_{3}$ ) can be released.

## 化学代写|有机化学代写ORGANIC CHEMISTRY代考|NUCLEOPHILES AND ELECTROPHILES; LEAVING GROUPS

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