19th Ave New York, NY 95822, USA

化学代写|有机化学代写organic chemistry代考|Electrophilic Aromatic Substitutions via Wheland Complexes (“Ar-SE Reactions”)

如果你也在 怎样代写有机化学organic chemistry这个学科遇到相关的难题,请随时右上角联系我们的24/7代写客服。有机化学organic chemistry是化学的一个分支,研究含有碳-碳共价键的有机化合物的结构、性质和反应。对性质的研究包括物理和化学性质,以及对化学反应性的评估,以了解其行为。有机反应的研究包括天然产品、药物和聚合物的化学合成,以及在实验室和通过理论(in silico)研究单个有机分子。

有机化学organic chemistry研究的化学品范围包括碳氢化合物(只含碳和氢的化合物)以及以碳为基础但也含有其他元素的化合物,特别是氧、氮、硫、磷(包括在许多生化制品中)和卤素。有机金属化学是研究含有碳-金属键的化合物。此外,当代研究的重点是涉及其他有机金属的有机化学,包括镧系元素,但特别是过渡金属锌、铜、钯、镍、钴、钛和铬。

my-assignmentexpert™ 有机化学organic chemistry作业代写,免费提交作业要求, 满意后付款,成绩80\%以下全额退款,安全省心无顾虑。专业硕 博写手团队,所有订单可靠准时,保证 100% 原创。my-assignmentexpert™, 最高质量的有机化学organic chemistry作业代写,服务覆盖北美、欧洲、澳洲等 国家。 在代写价格方面,考虑到同学们的经济条件,在保障代写质量的前提下,我们为客户提供最合理的价格。 由于统计Statistics作业种类很多,同时其中的大部分作业在字数上都没有具体要求,因此有机化学organic chemistry作业代写的价格不固定。通常在经济学专家查看完作业要求之后会给出报价。作业难度和截止日期对价格也有很大的影响。

想知道您作业确定的价格吗? 免费下单以相关学科的专家能了解具体的要求之后在1-3个小时就提出价格。专家的 报价比上列的价格能便宜好几倍。

my-assignmentexpert™ 为您的留学生涯保驾护航 在化学Chemical作业代写方面已经树立了自己的口碑, 保证靠谱, 高质且原创的化学Chemical代写服务。我们的专家在有机化学organic chemistry代写方面经验极为丰富,各种有机化学organic chemistry相关的作业也就用不着 说。

我们提供的有机化学organic chemistry及其相关学科的代写,服务范围广, 其中包括但不限于:

化学代写|有机化学代写organic chemistry代考|Electrophilic Aromatic Substitutions via Wheland Complexes (“Ar-SE Reactions”)

化学代写|有机化学代写organic chemistry代考|Electrophilic Aromatic Substitutions via Wheland Complexes (“Ar-SE Reactions”)

For an $A \mathrm{r}-\mathrm{S}_{\mathrm{E}}$ reaction to be able to occur, first the actual electrophile must be produced from the reagent (mixture) used. Then this electrophile initiates the aromatic substitution. It takes place, independently of the chemical nature of the electrophile, essentially according to a two-step mechanism (Figure 5.1). A third step, namely, the initial formation of a $\pi$ complex from the electrophile and the substrate, is generally of minor importance for understanding the reaction event.

In the first step of the actual Ar- $S_{E}$ reaction, a substituted cyclohexadienyl cation is formed from the electrophile and the aromatic compound. This cation and its derivatives are generally referred to as a $\sigma$ or Wheland complex. Wheland complexes are described in the language of the VB method by superpositioning mentally at least three carbenium ion resonance forms (Figure 5.1). In the following, these resonance forms are referred to briefly as “sextet formulas.” There is an additional resonance form for each substituent, which can stabilize the positive charge of the Wheland complex by a +M effect (see Section 5.1.3). This resonance form is an all-octet formula.

Wheland complexes are high-energy intermediates because they do not contain the conjugated aromatic electron sextet present in the product and in the starting material. Consequently, the formation of these complexes is the rate-determining step of Ar-S $\mathrm{S}{\mathrm{E}}$ reactions (cf. Figure 5.1). This, in turn, means that Wheland complexes are also a good-even the best-model for the transition state of $\mathrm{Ar}-\mathrm{S}{\mathrm{E}}$ reactions.

In the second step of the Ar-S $\mathrm{E}_{\mathrm{E}}$ reaction, an aromatic compound is regenerated by cleaving off a cation from the $\mathrm{C}$ atom which was attacked by the electrophile. Most often the eliminated cation is a proton (Figure $5.1, \mathrm{X}=\mathrm{H}$ or $\mathrm{X}^{+}=\mathrm{H}^{+}$).

In a few cases, cations other than the proton are eliminated from the Wheland complex to reconstitute the aromatic system. The tert-butyl cation (Figure $5.1, \mathrm{X}=$ tert-Bu) and protonated $\mathrm{SO}{3}$ (Figure $5.1, \mathrm{X}=\mathrm{SO}{3} \mathrm{H}$ ) are suitable for such an elimination. When the latter groups are replaced in an $\mathrm{Ar}-\mathrm{S}{\mathrm{E}}$ reaction, we have the special case of an ipso substitution. Among other things, ipso substitutions play a role in the few Ar- $\mathrm{S}{\mathrm{E}}$ reactions that are reversible (Section 5.1.2).

化学代写|有机化学代写organic chemistry代考|Thermodynamic Aspects of Ar-SE Reactions

If one compares the heats of reaction for these potentially competing reactions (Figure $5.2$ ), one arrives at the following:
1) The substitution reaction $\mathrm{C}{s p^{2}}-\mathrm{H}+\mathrm{Br}-\mathrm{Br} \rightarrow \mathrm{C}{s p^{2}}-\mathrm{Br}+\mathrm{H}-\mathrm{Br}$ is exothermic by approximately $-11 \mathrm{kcal} / \mathrm{mol}^{2}$. It is irrelevant whether the attacked $s p^{2}$-hybridized carbon atom is part of an olefin or an aromatic compound.
2) In an addition reaction $\mathrm{C}=\mathrm{C}+\mathrm{Br}-\mathrm{Br} \rightarrow \mathrm{Br}-\mathrm{C}-\mathrm{C}-\mathrm{Br}$ there is a decrease in enthalpy of $27 \mathrm{kcal} / \mathrm{mol}$ in the substructure shown, that is, a reaction enthalpy of $-27 \mathrm{kcal} / \mathrm{mol}$. This enthalpy decrease equals the heat of reaction liberated when $\mathrm{Br}{2}$ is added to cyclohexene. However, it does not equal the heat of reaction for the addition of $\mathrm{Br}{2}$ to benzene.

3) When $\mathrm{Br}{2}$ is added to benzene, the above-mentioned $-27 \mathrm{kcal} / \mathrm{mol} \mathrm{must} \mathrm{be} \mathrm{bal-} \mathrm{}$ anced with the simultaneous loss of the benzene conjugation, which is $+36 \mathrm{kcal} / \mathrm{mol}$. All in all, this makes the addition of $\mathrm{Br}{2}$ to benzene by approximately $+9 \mathrm{kcal} / \mathrm{mol}$ endothermic. Moreover, when $\mathrm{Br}{2}$ is added to benzene, the entropy decreases. Consequently, the addition of bromine to benzene would not only be endothermic but also endergonic. The latter means that such an addition is thermodynamically impossible. 4) The exothermic substitution reaction (see above) on benzene is also exergonic because no significant entropy change occurs. This substitution is therefore thermodynamically possible and actually takes place under suitable reaction conditions (Section 5.2.1). 5) Finally, because the addition of $\mathrm{Br}{2}$ to cyclohexene is $27 \mathrm{kcal} / \mathrm{mol}-11 \mathrm{kcal} / \mathrm{mol}=$ $16 \mathrm{kcal} / \mathrm{mol}$ more exothermic than the substitution of $\mathrm{Br}{2}$ on cyclohexene can we conclude that the first reaction also takes place more rapidly? Not necessarily! The (fictitious) substitution reaction of $\mathrm{Br}{2}$ on cyclohexene should be a multi-step reaction and proceed via a bromonium ion formed in the first and also rate-determining reaction step. This bromonium ion has been demonstrated to be the intermediate in the known addition reaction of $\mathrm{Br}_{2}$ to cyclohexene (Section $3.5 .1$ ). Thus, one would expect that the outcome of the competition of substitution vs addition depends on whether the bromonium ion is converted-in each case in an elementary reaction-to the substitution or to the addition product. The Hammond postulate suggests that the bromonium ion undergoes the more exothermic (exergonic) reaction more rapidly. In other words, the addition reaction is expected to win not only thermodynamically but also kinetically.

化学代写|有机化学代写organic chemistry代考|Electrophilic Aromatic Substitutions via Wheland Complexes (“Ar-SE Reactions”)

有机化学代写

化学代写|有机化学代写ORGANIC CHEMISTRY代考|ELECTROPHILIC AROMATIC SUBSTITUTIONS VIA WHELAND COMPLEXES (“AR-SE REACTIONS”)

为一种r−小号和为了能够发生反应,首先必须从试剂中产生实际的亲电试剂米一世X吨在r和用过的。然后这个亲电子试剂开始芳香取代。它的发生与亲电试剂的化学性质无关,基本上是根据两步机制F一世G在r和5.1. 第三步,即初步形成圆周率来自亲电试剂和底物的复合物通常对于理解反应事件并不重要。

在实际 Ar- 的第一步小号和反应,由亲电试剂和芳族化合物形成取代的环己二烯基阳离子。这种阳离子及其衍生物通常被称为σ或惠兰情结。Wheland 配合物是用 VB 方法的语言描述的,通过在心理上叠加至少三种碳离子共振形式F一世G在r和5.1. 在下文中,这些共振形式简称为“六重奏公式”。每个取代基都有一个额外的共振形式,可以通过 +M 效应稳定 Wheland 配合物的正电荷s和和小号和C吨一世这n5.1.3. 这种共振形式是一个全八位组公式。

Wheland 配合物是高能中间体,因为它们不包含存在于产物和起始材料中的共轭芳族电子六重态。因此,这些配合物的形成是 Ar-S $\mathrm{S} {\mathrm{E}}的速率决定步骤r和一种C吨一世这ns(CF.F一世G在r和5.1).吨H一世s,一世n吨在rn,米和一种ns吨H一种吨在H和l一种ndC这米pl和X和s一种r和一种ls这一种G这这d−和在和n吨H和b和s吨−米这d和lF这r吨H和吨r一种ns一世吨一世这ns吨一种吨和这F\ mathrm {Ar} – \ mathrm {S} {\ mathrm {E}} $ 反应。

在 Ar-S 的第二步和和反应中,芳香族化合物通过从化合物中分离出阳离子而再生。C被亲电试剂攻击的原子。大多数情况下,消除的阳离子是质子F一世G在r和$5.1,X=H$这r$X+=H+$.

在少数情况下,除质子外的阳离子会从 Wheland 络合物中消除,以重建芳烃体系。叔丁基阳离子F一世G在r和$, \mathrm{X}=$ tert-Bu) and protonated $\mathrm{SO}{3}$ (Figure $5.1, \mathrm{X}=\mathrm{SO}{3} \mathrm{H}$ ) are suitable for such an elimination. When the latter groups are replaced in an $\mathrm{Ar}-\mathrm{S}{\mathrm{E}}$ reaction, we have the special case of an ipso substitution. Among other things, ipso substitutions play a role in the few Ar- $\mathrm{S}{\mathrm{E}}$可逆反应小号和C吨一世这n5.1.2.

化学代写|有机化学代写ORGANIC CHEMISTRY代考|THERMODYNAMIC ASPECTS OF AR-SE REACTIONS

如果比较这些潜在竞争反应的反应热F一世G在r和$5.2$, 得到以下结果:
1) The substitution reaction $\mathrm{C}{s p^{2}}-\mathrm{H}+\mathrm{Br}-\mathrm{Br} \rightarrow \mathrm{C}{s p^{2}}-\mathrm{Br}+\mathrm{H}-\mathrm{Br}$ is exothermic by approximately $-11 \mathrm{kcal} / \mathrm{mol}^{2}$. It is irrelevant whether the attacked $s p^{2}$-hybridized carbon atom is part of an olefin or an aromatic compound.
2) In an addition reaction $\mathrm{C}=\mathrm{C}+\mathrm{Br}-\mathrm{Br} \rightarrow \mathrm{Br}-\mathrm{C}-\mathrm{C}-\mathrm{Br}$ there is a decrease in enthalpy of $27 \mathrm{kcal} / \mathrm{mol}$ in the substructure shown, that is, a reaction enthalpy of $-27 \mathrm{kcal} / \mathrm{mol}$. This enthalpy decrease equals the heat of reaction liberated when $\mathrm{Br}{2}$ is added to cyclohexene. However, it does not equal the heat of reaction for the addition of $\mathrm{Br}{2}$ to benzene.。

3) 当 $\mathrm{Br}{2}$ is added to benzene, the above-mentioned $-27 \mathrm{kcal} / \mathrm{mol} \mathrm{must} \mathrm{be} \mathrm{bal-} \mathrm{}$ anced with the simultaneous loss of the benzene conjugation, which is $+36 \mathrm{kcal} / \mathrm{mol}$. All in all, this makes the addition of $\mathrm{Br}{2}$ to benzene by approximately $+9 \mathrm{kcal} / \mathrm{mol}$ endothermic. Moreover, when $\mathrm{Br}{2}$ is added to benzene, the entropy decreases. Consequently, the addition of bromine to benzene would not only be endothermic but also endergonic. The latter means that such an addition is thermodynamically impossible. 4) The exothermic substitution reaction (see above) on benzene is also exergonic because no significant entropy change occurs. This substitution is therefore thermodynamically possible and actually takes place under suitable reaction conditions (Section 5.2.1). 5) Finally, because the addition of $\mathrm{Br}{2}$ to cyclohexene is $27 \mathrm{kcal} / \mathrm{mol}-11 \mathrm{kcal} / \mathrm{mol}=$ $16 \mathrm{kcal} / \mathrm{mol}$ more exothermic than the substitution of $\mathrm{Br}{2}$ on cyclohexene can we conclude that the first reaction also takes place more rapidly? Not necessarily! The (fictitious) substitution reaction of $\mathrm{Br}{2}$ on cyclohexene should be a multi-step reaction and proceed via a bromonium ion formed in the first and also rate-determining reaction step. This bromonium ion has been demonstrated to be the intermediate in the known addition reaction of $\mathrm{Br}_{2}$ to cyclohexene (Section $3.5 .1$ )。因此,人们会期望取代与加成的竞争结果取决于溴离子是否被转化——在每种情况下,在基本反应中——转化为取代或加成产物。哈蒙德假设表明溴离子经历更多的放热和X和rG这n一世C反应更迅速。换句话说,加成反应不仅在热力学上而且在动力学上都有望获胜。

化学代写|有机化学代写organic chemistry代考

化学代写|有机化学代写organic chemistry代考 请认准UprivateTA™. UprivateTA™为您的留学生涯保驾护航。

电磁学代考

物理代考服务:
物理Physics考试代考、留学生物理online exam代考、电磁学代考、热力学代考、相对论代考、电动力学代考、电磁学代考、分析力学代考、澳洲物理代考、北美物理考试代考、美国留学生物理final exam代考、加拿大物理midterm代考、澳洲物理online exam代考、英国物理online quiz代考等。

光学代考

光学(Optics),是物理学的分支,主要是研究光的现象、性质与应用,包括光与物质之间的相互作用、光学仪器的制作。光学通常研究红外线、紫外线及可见光的物理行为。因为光是电磁波,其它形式的电磁辐射,例如X射线、微波、电磁辐射及无线电波等等也具有类似光的特性。

大多数常见的光学现象都可以用经典电动力学理论来说明。但是,通常这全套理论很难实际应用,必需先假定简单模型。几何光学的模型最为容易使用。

相对论代考

上至高压线,下至发电机,只要用到电的地方就有相对论效应存在!相对论是关于时空和引力的理论,主要由爱因斯坦创立,相对论的提出给物理学带来了革命性的变化,被誉为现代物理性最伟大的基础理论。

流体力学代考

流体力学力学的一个分支。 主要研究在各种力的作用下流体本身的状态,以及流体和固体壁面、流体流体之间、流体与其他运动形态之间的相互作用的力学分支。

随机过程代写

随机过程,是依赖于参数的一组随机变量的全体,参数通常是时间。 随机变量是随机现象的数量表现,其取值随着偶然因素的影响而改变。 例如,某商店在从时间t0到时间tK这段时间内接待顾客的人数,就是依赖于时间t的一组随机变量,即随机过程

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。

Related Posts

Leave a comment