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# 化学代写|有机化学代写organic chemistry代考|Additions That Take Place or Can Take Place without Stereocontrol Depending on the Mechanism

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## 化学代写|有机化学代写organic chemistry代考|Additions via Carbenium Ion Intermediates

Carbenium ions become intermediates in two-step additions to olefins when they are more stable than the corresponding cyclic onium intermediates. With many electrophiles (e.g., $\mathrm{Br}{2}$ and $\mathrm{Cl}{2}$ ), this occasionally is the case. With others, such as protons and carbenium ions, it is always the case. (Figure 3.40).

In Section 3.5.1 it was mentioned that $\mathrm{Br}{2}$ and $\mathrm{Cl}{2}$ form resonance-stabilized benzyl cation intermediates with styrene derivatives and that gem-dialkylated olefins react with $\mathrm{Br}{2}$ but not with $\mathrm{Cl}{2}$ via halonium ions. Because $\mathrm{C}-\mathrm{Cl}$ bonds are shorter than $\mathrm{C}-\mathrm{Br}$ bonds, chloronium ions presumably have a higher ring strain than bromonium ions. Accordingly, a $\beta$-chlorinated tertiary carbenium ion is more stable than the isomeric chloronium ion, but a $\beta$-brominated tertiary carbenium ion is less stable than the isomeric bromonium ion.

Protons and carbenium ions always add to $\mathrm{C}=\mathrm{C}$ double bonds via carbenium ion intermediates simply because no energetically favorable onium ions are available from an attack by these electrophiles. An onium intermediate formed by the attack of a proton would contain a divalent, positively charged H atom. An onium intermediate produced by the attack of a carbenium ion would be a carbonium ion and would thus contain a pentavalent, positively charged $\mathrm{C}$ atom. Species of this type are at best detectable under exotic reaction conditions or in the norbornyl cation (Figure 2.25).

Preparatively it is important that mineral acids, carboxylic acids, and tert-carbenium ions can be added to olefins via carbenium ion intermediates. Because of their relatively low stability, primary carbenium ions form more slowly in the course of such reactions than the more stable secondary carbenium ions, and these form more slowly than the even more stable tertiary carbenium ions (Hammond postulate!). Therefore, mineral and carboxylic acids add to asymmetric olefins regioselectively to give Markovnikov products (see Section $3.3 .3$ for an explanation of this term). In addition, these electrophiles add most rapidly to those olefins from which tertiary carbenium ion intermediates can be derived.

An example of an addition of a mineral acid to an olefin that takes place via a tertiary carbenium ion is the formation of a tertiary alkyl bromide from a 1,1dialkylethylene and HBr (Figure 3.41).

Figure $3.42$ shows an addition of a carboxylic acid to isobutene, which takes place via the tert-butyl cation. This reaction is a method for forming tert-butyl esters. Because the acid shown in Figure $3.42$ is a $\beta$-hydroxycarboxylic acid whose alcohol group adds to an additional isobutene molecule, this also shows an addition of a primary alcohol to isobutene, which takes place via the tert-butyl cation. Because neither an ordinary carboxylic acid nor, of course, an alcohol is sufficiently acidic to protonate the olefin to give a carbenium ion, catalytic amounts of a mineral or sulfonic acid are also required here.

Carbenium ion additions to $\mathrm{C}=\mathrm{C}$ double bonds are of greater preparative importance when they take place intramolecularly as ring closure reactions (see Figure 3.45).
In polyenes even tandem additions are possible. The best known and the most impressive example is the biomimetic synthesis of the steroid structure.

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

Nucleophiles can be added to acceptor-substituted olefins. In that case, enolates and other “stabilized carbanions” occur as intermediates. Reactions of this type are discussed in this book only in connection with 1,4-additions of ylids (Section 9.2.2), organometallics (Section 8.6), or enolates (Section 10.6) to $\alpha, \beta$-unsaturated carbonyl and carboxyl compounds.

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