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The trans addition of bromine to $\mathrm{C}=\mathrm{C}$ double bonds follows the onium mechanism generally formulated in Figure 3.32. Cyclohexene reacts stereoselectively to give (racemic!) trans-1,2-dibromocyclohexane:

trans-Selectivity is also exhibited by the additions of bromine to fumaric or maleic acid, which follow the same mechanism:

This is proven by the fact that the reaction with fumaric acid gives mesodibromosuccinic acid, whereas maleic acid gives the chiral dibromosuccinic acid, of course as a racemic mixture. Note that these reactions are stereospecific.

Instead of a bromonium ion, in certain cases an isomeric acyclic and sufficiently stable cation can occur as an intermediate of the bromine addition to olefins. This holds true for bromine-containing benzyl cations. Therefore, the bromine addition to $\beta$ methyl styrene shown in Figures $3.5$ and $3.6$ takes place without stereocontrol.

The addition of $\mathrm{Cl}{2}$ to $\mathrm{C}=\mathrm{C}$ double bonds is trans-selective only when it takes place via three-membered chloronium ions. But it can also take place without stereocontrol, namely, when carbenium ion intermediates appear instead of chloronium ions. This is observed in $\mathrm{Cl}{2}$ additions, which can take place via benzyl or tert-alkyl cations.

In general, an addition of $\mathrm{I}_{2}$ to $\mathrm{C}=\mathrm{C}$ double bonds is thermodynamically impossible, although an iodonium ion can still form.

## 化学代写|有机化学代写organic chemistry代考|The Formation of Halohydrins; Halolactonization and Haloetherification

Halohydrins are $\beta$-halogenated alcohols. They can be obtained in $\mathrm{H}{2} \mathrm{O}$-containing solvents from olefins and reagents, which transfer $\mathrm{Hal}^{+}$ions. $N$-Bromosuccinimide (transfers $\mathrm{Br}^{+}$; Figures $3.33$ and $3.34$ as well as $3.36$ ), chloramine- $\mathrm{T}$ (transfers $\mathrm{Cl}^{+}$; Figure $3.35$ ), and elemental iodine (transfers $\mathrm{I}^{+}$; Figure 3.36) have this ability. Bromonium and chloronium ions are then attacked by $\mathrm{H}{2} \mathrm{O}$ according to an $\mathrm{S}{\mathrm{N}} 2$ mechanism. This furnishes the protonated bromo- or chlorohydrins, which are subsequently deprotonated. Instead of $\mathrm{H}{2} \mathrm{O}, \mathrm{COOH}$ or $\mathrm{OH}$ groups of the substrate located at a suitable distance can also open the halonium ion intermediate through a nucleophilic backside attack. In this way, cyclic halohydrin derivatives are produced (Figure 3.36). They are referred to as halolactones or haloethers.

With NBS in aqueous DMSO, cyclohexene gives racemic trans-2-bromo-1-cyclohexanol. This stereochemical result means that we have a trans addition. In the analogous bromohydrin formation from 3,3-dimethylcyclohexene, the analogous dimethylated 2-bromo-1-cyclohexanol is also produced trans-selectively as well as regioselectively (Figure 3.33). In the bromonium ion intermediate the backside attack by the $\mathrm{H}{2} \mathrm{O} \mathrm{mol-}$ ecule does not take place at the hindered neopentyl center according to the rules for $S{N}$ 2 reactivity (Section 2.4.4).

As shown in Figure 3.34, trisubstituted olefins are also converted to bromohydrins by NBS in an aqueous organic solvent. This reaction takes place via a trans addition and therefore must take place via a bromonium ion. Once again, the reaction is also regioselective. At first glance, the regioselectivity of this reaction might seem surprising: The $\mathrm{H}{2} \mathrm{O}$ molecule attacks the bromonium ion intermediate at the tertiary instead of at the secondary $\mathrm{C}$ atom. One might have expected the backside $\left(\mathrm{S}{\mathrm{N}} 2\right)$ attack of the nucleophile to be directed at the secondary $\mathrm{C}$ atom of the bromonium ion (cf. Section 2.4.4). However, this is not the case. The bromonium ion is very distorted: The $\mathrm{C}{\text {sec }}-\mathrm{Br}^{+}$bond at $1.9 \AA$ is considerably shorter than and consequently considerably stronger than the $\mathrm{C}{\text {ter }}-\mathrm{Br}^{+}$bond. The latter is stretched to $2.6 \AA$ and thereby weakened. This distortion reduces the ring strain of the bromonium ion. The distortion becomes possible because the stretching of the $\mathrm{C}_{\text {tert }}-\mathrm{Br}^{+}$bond produces a partial positive charge on the tertiary $\mathrm{C}$ atom, which is stabilized by the alkyl substituents located there. In this bromonium ion, the bromine atom has almost separated from the tertiary ring $\mathrm{C}$ atom (but only almost, because otherwise the result would not be $100 \%$ trans addition).

## 化学代写|有机化学代写ORGANIC CHEMISTRY代考|Solvomercuration of Olefins: Hydration of C=C Double Bonds through Subsequent Reduction

Mercury(II) salts add to $\mathrm{C}=\mathrm{C}$ double bonds (Figure 3.37) in nucleophilic solvents according to the onium mechanism of Figure 3.32. However, the heterocyclic primary product is not called an onium, but rather a mercurinium ion. Its ring opening in an $\mathrm{H}{2} \mathrm{O}$-containing solvent gives a trans-configured alcohol, which can then be demercurated with $\mathrm{NaBH}{4}$. The hydration product of the original olefin is obtained and is, in our case, cyclohexanol.

The mechanism of this defunctionalization was discussed in connection with Figure 1.12. It took place via approximately planar radical intermediates. This is why in the reduction of alkyl mercury(II) acetates, the $\mathrm{C}-\mathrm{Hg}$ bond changes to the $\mathrm{C}-\mathrm{H}$ bond without stereocontrol. The stereochemical integrity of the mercury-bearing stereocenter is thus lost. When the mercurated alcohol in Figure $3.37$ is reduced with $\mathrm{NaBD}{4}$ rather than $\mathrm{NaBH}{4}$, the deuterated cyclohexanol is therefore produced as a mixture of diastereomers. Asymmetrically substituted $\mathrm{C}=\mathrm{C}$ double bonds are hydrated according to the same mechanism (Figure 3.38). The regioselectivity is high, and the explanation for this is that the mercurinium ion intermediate is distorted in the same way as the bromonium ion in Figure 3.34. The $\mathrm{H}{2} \mathrm{O}$ preferentially breaks the stretched and therefore weakened $\mathrm{C}{\text {sec }}-\mathrm{Hg}^{+}$bond by a backside attack and does not affect the shorter and therefore more stable $\mathrm{C}_{\text {prim }}-\mathrm{Hg}^{+}$bond.

From the $\mathrm{Hg}$-containing alcohol in Figure $3.38$ and $\mathrm{NaBH}{4}$ one can obtain the $\mathrm{Hg}$ free alcohol. The overall result is a hydration of the $\mathrm{C}=\mathrm{C}$ double bond. According to the nomenclature of Section 3.3.3, its regioselectivity corresponds to a Markovnikov addition. It is complementary to the regioselectivity of the reaction sequence hydroboration/oxidation/hydrolysis (Table 3.1). The latter sequence would have converted the same olefin regioselectively into the primary instead of the secondary alcohol. Besides $\mathrm{H}{2} \mathrm{O}$, simple alcohols or acetic acid can also be added to olefins by solvomercuration/reduction. Figure $3.39$ shows MeOH addition as an example. The regioselectivities of this reaction and of the $\mathrm{H}_{2} \mathrm{O}$ addition in Figure $3.38$ are identical.

## 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。