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

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## 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”)

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。

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

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。

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