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# 化学代写|有机化学代写organic chemistry代考|Radical 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|Bonding and Preferred Geometries in C Radicals, Carbenium Ions and Carbanions

At the so-called radical center an organic radical R. has an electron septet, which is an electron deficiency in comparison to the electron octet of valence-saturated compounds. Carbon atoms are the most frequently found radical centers and most often have three neighbors (see below). Carbon-centered radicals with their electron septet occupy an intermediate position between the carbenium ions, which have one electron less (electron sextet at the valence-unsaturated $\mathrm{C}$ atom), and the carbanions, which have one electron more (electron octet at the valence-unsaturated $\mathrm{C}$ atom). Since there is an electron deficiency present both in $\mathrm{C}$ radicals and in carbenium ions, the latter are more closely related to each other than $\mathrm{C}$ radicals are related to carbanions. Because of this, $\mathrm{C}$ radicals and carbenium ions are also stabilized or destabilized by the same substituents.

Nitrogen-centered radicals $\left(\mathrm{R}{s p^{3}}\right){2} \mathrm{~N}$ · or oxygen-centered radicals $\left(\mathrm{R}{s p^{3}}\right) \mathrm{O} \cdot$ are less stable than $\mathrm{C}$-centered radicals $\left(\mathrm{R}{s p^{3}}\right)_{3} \mathrm{C}$. They are higher in energy because of the higher electronegativity of these elements relative to carbon. Nitrogen- or oxgencentered radicals of the cited substitution pattern consequently have only a limited chance to exist. Which geometries are preferred at the valence-unsaturated $\mathrm{C}$ atom of $\mathrm{C}$ radicals, and how do they differ from those of carbenium ions or carbanions? And what types of bonding are found at the valence-unsaturated $\mathrm{C}$ atoms of these three species? It is simplest to clarify the preferred geometries first (Section 1.1.1). As soon as these geometries are known, molecular orbital (MO) theory will be used to provide a description of the bonding (Section 1.1.2).

We will discuss the preferred geometries and the MO descriptions of C radicals and the corresponding carbenium ions or carbanions in two parts. In the first part we will examine $\mathrm{C}$ radicals, carbenium ions, and carbanions with a trivalent central $\mathrm{C}$ atom. The second part treats the analogous species with a divalent central $\mathrm{C}$ atom. A third part (species with a monovalent central C atom) can be dispensed with because the only species of this type that is important in organic chemistry is the alkynyl anion, which, however, is of no interest here.

## 化学代写|有机化学代写organic chemistry代考|Stability of Radicals

Stability in chemistry is not an absolute but a relative concept. It always refers to a stability difference with respect to a reference compound. Let us consider the standard heats of reaction $\Delta H^{0}$ of the dissociation reaction $\mathrm{R}-\mathrm{H} \rightarrow \mathrm{R} \cdot+\mathrm{H} \cdot$, that is, the dissociation enthalpy (DE) of the broken C-H bond. It reflects, on the one hand, the strength of this $\mathrm{C}-\mathrm{H}$ bond and, on the other hand, the stability of the radical R-produced. As you see immediately, the dissociation enthalpy of the $\mathrm{R}-\mathrm{H}$ bond depends in many ways on the structure of $R$. But it is not possible to tell clearly whether this is due to an effect on the bond energy of the broken $\mathrm{R}-\mathrm{H}$ bond and/or an effect on the stability of the radical $R \cdot$ that is formed.

To what must one ascribe, for example, the fact that the dissociation enthalpy of $\mathrm{a}^{s p} \mathrm{C}^{n}-\mathrm{H}$ bond depends essentially on $n$ alone and increases in the order $n=3,2$, and 1 ?
To help answer this question it is worthwhile considering the following: the dissociation enthalpies of bonds such as $\mathrm{C}{s p^{n}}-\mathrm{C}, \mathrm{C}{s p^{n}}-\mathrm{O}, \mathrm{C}{s p^{n}}-\mathrm{Cl}$, and $\mathrm{C}{s p^{n}}-\mathrm{Br}$ also depend heavily on $n$ and increase in the same order, $n=3,2$, and 1. The extent of the $n$-dependence of the dissocation energies depends on the element which is cleaved off. This is only possible if the $n$-dependence reflects, at least in part, an $n$-dependence of the respective $\mathrm{C}{s p^{n}}$-element bond. (Bond enthalpy tables in all textbooks ignore element but not on the value of $n$ !) Hence, the bond enthalpy of every $\mathrm{C}{s p^{n}-\mathrm{element}}$ bond increases in the order $n=3,2$, and 1. This is so because all $\mathrm{C}{s p^{n}}-$ element bonds become shorter in the same order. This in turn is due to the $s$ character of the $\mathrm{C}{s p^{n}}-\mathrm{el}-$ ement bond, which increases in the same direction.

An immediate consequence of the different ease with which $\mathrm{C}_{s p^{n}}$-element bonds dissociate is that in radical substitution reactions, alkyl radicals are preferentially formed. Only in exceptional cases are vinyl or aryl radicals formed. Alkynyl radicals do not appear at all in radical substitution reactions. In the following we therefore limit ourselves to a discussion of substitution reactions that take place via radicals of the general structure $\mathrm{R}^{1} \mathrm{R}^{2} \mathrm{R}^{3} \mathrm{C}$.

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