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# 化学代写|有机化学代写organic chemistry代考|Vocabulary of Stereochemistry and Stereoselective Synthesis I

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## 化学代写|有机化学代写organic chemistry代考|Isomerism, Diastereomers/Enantiomers, Chirality

Two molecules that have the same empirical formula can exhibit four different degrees of relationship, which are shown in Figure 3.2.

According to Figure 3.2, molecules of the same empirical formula are either identical or isomers. Isomers either differ in the connectivity of their constituent atoms-this then involves constitutional isomers (structural isomers)-or they do not differ in this way; then they are stereoisomers. Stereoisomers that are related to each other as an image and a mirror image are enantiomers; otherwise they are diastereomers. Enantiomers are enantiomorphic to each other; diastereomers are diastereomorphic. Diastereomers that contain several stereocenters but differ in the configuration of only one stereocenter are called epimers. Finally, epimeric glycosides that are configured differently at the anomeric (i.e., the gem-dioxygenated) $\mathrm{C}$ atom are designated anomers. According to the foregoing analysis, conformers such as gauche- and anti-butane or chair and twist-boat cyclohexane would be considered to be diastereomers of each other. However, under most conditions these conformers interconvert so rapidly that butane and cyclohexane are considered to be single species and not mixtures of stereoisomers.

Let us also visualize the important concept of chirality. Only molecules that differ from their mirror image have enantiomers. Molecules of this type are called chiral. For the occurrence of enantiomers or for the presence of the “property of differing from its mirror image”-that is, chirality-there is a necessary and sufficient condition: it is the absence of an intramolecular rotation/reflection axis.

The most frequently occurring rotation/reflection axis in organic chemistry is $S_{1}$, the intramolecular mirror plane. Its presence makes cis-1,2-dibromocyclohexane (structure $\mathbf{A}$ in Figure 3.3) as well as meso-2,3-dibromosuccinic acid (structure B) achiral. This is true even though there are two stereocenters in each of these compounds. But why is the dibromocyclohexane dicarboxylic acid $\mathbf{C}$ (Figure $3.3$ ) achiral? The answer is that it contains an $S_{2}$ axis, an inversion center, which occurs in organic chemistry rarely. This compound is thus achiral although it contains four stereocenters.

Note that according to the foregoing definition, chirality occurs only in molecules that do not have a rotation/reflection axis. However, if the molecule has only (!) an axis of rotation, it is chiral. For example, both trans-1, 2-dibromocyclohexane (D in Figure 3.3) and the dibromosuccinic acid $\mathbf{E}$ have a two-fold axis of rotation $\left(C_{2}\right)$ as the only symmetry element. In spite of that, these compounds are chiral because the presence of an axis of rotation, in contrast to the presence of a rotation/reflection axis, is not a criterion for achirality.

## 化学代写|有机化学代写organic chemistry代考|Chemoselectivity, Diastereoselectivity/Enantioselectivity,Stereospecificity/Stereoconvergence

Preparatively useful reactions should deliver a single product or at least a preferred product and not a product mixture. If they give one product exclusively or preferentially, they are called selective reactions. One usually specifies the type of selectivity involved, depending on which of the conceivable side products do not occur at all (highly selective reaction) or occur only to a minor degree (moderately selective reaction) (Figure 3.4).
A reaction that preferentially or exclusively gives one of several conceivable reaction products with different empirical formulas takes place with chemoselectivity. A reaction that preferentially or exclusively gives one of several conceivable constitutional isomers is also chemoselective. A reaction that preferentially or exclusively gives one of several conceivable stereoisomers is referred to as moderately or highly stereoselective. When these conceivable stereoisomers are diastereomers, one also talks about a diastereoselective reaction or about the occurrence of diastereoselectivity. When the conceivable stereoisomers are enantiomers, we have an enantioselective reaction, or enantioselectivity.

The objective of stereoselective synthesis is to produce compounds as pure diastereomers (diastereomerically pure) and/or pure enantiomers (enantiomerically pure). To be able to evaluate the success of efforts of this type, one needs quantitative measures for diastereoselectivity and enantioselectivity.

## Matlab代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。