Carbocations stable generation

Carbocations, stable or unstable, are usually generated in one of two general ways ... 

The Focus On box in Chapter 8 on page 298 showed that when carbocations are generated in superacid solution, they undergo extensive rearrangements, usually forming a relatively stable tertiary carbocation. As an example, when 1-butanol is dissolved in superacid at — 60°C, the protonated alcohol is formed. Water does not leave at this temperature because the carbocation that would be formed is primary. When the temperature is raised to 0°C, water leaves but the carbocation rearranges rapidly to the more stable tert-butyl carbocation ... 

A number of methods are available to generate carbocations, stable or unstable. 

With cations ranked according to their relative stability, we then assume that given a choice, the more stable cation will be formed in a reaction because it is lower in energy. The addition of HCl to methylenecyclobutane (100) illustrates this point, where the final product is 1-chloro-l-methylcyclobutane (103), used by Fitjer in a synthesis of cuparene. 29 7 0 n bond reacts as a base, donating a pair of electrons to the acid (H+ = HCl). When the new C—H bond is formed, a carbocation is generated at the other carbon of the jt bond, leading to... 

If 21 is the m or product observed when 3-methyl-l-pentene reacts with HBr, but 20 is the initially formed carbocation, the rearrangement to 22 must be faster than the reaction of bromide ion and 20 (see Chapter 7, Section 7.11, for an introduction to reaction kinetics). Based on this observation, which is known to be the normal reactivity for such alkenes, a simple assumption can be made. If a carbocation is generated next to a carbon atom that can potentially bear a more stable cation, assume that the rearrangement occurs faster than any other process. This is not always true, but it is true most of the time and is a good working assumption for problems encountered in this book. 

However, if either of the methyl groups migrates, a tertiary carbocation is generated. Therefore, we expect a methyl shift to take place, generating a more stable tertiary carbocation. 

Figure 6 6 focuses on the orbitals involved and shows how the rr electrons of the double bond flow in the direction that generates the more stable of the two possible carbocations... 

The triarylmethyl cations are particularly stable because of the conjugation with the aryl groups, which delocalizes the positive charge. Because of their stability and ease of generation, the triarylmethyl cations have been the subject of studies aimed at determining the effect of substituents on carbocation stability. Many of these studies used the characteristic UV absorption spectra of the cations to determine their concentration. In acidic solution, equilibrium is established between triarylearbinols and the corresponding carbocations. 

Mechanistic Transform. A transform involving a sequence of reactive intermediates such as carbocations or carbon radicals which are generated in a stepwise mechanistic manner and which lead finally to stable predecessor structure(s). 

Strong acids or superacid systems generate stable fluorinated carbocations [40, 42] Treatment of tetrafluorobenzbarrelene with arenesulfonyl chlorides in nitro-methane-lithium perchlorate yields a crystalline salt with a rearranged benzo barrelene skeleton [43] Ionization of polycyclic adducts of difluorocarbene and derivatives of bornadiene with antimony pentafluonde in fluorosulfonyl chloride yields stable cations [44, 45]... 

NMR spectroscopy is ideal for detecting charged fluorinated intermediates and has been applied to the study of increasingly stable carbocation and carbanion species. Olah [164, 165] has generated stable fluorocarbocations m SbFj/SOjClF at low temperatures The relatively long-lived perfluoro-rerr-butyl anion has been prepared as both the cesium and tris(dimethylamino)sulfonium (TAS) salts by several groups [166, 167, 168], Chemical shifts of fluonnated carbocations and carbanions are listed m Table 23. 

Protonation and subsequent loss of water should generate a earbocation. Examine all of the carbocations derived from protonation of (3-D-glueose. Identify the most stable carboeation (this is the one that will form most readily), and draw whatever resonance eontributors are needed to describe the geometry, energy, and atomie charges in this cation. Can you explain why substitution oeeurs selectively at Ci ... 

Note that in the S l reaction, which is often carried out under acidic conditions, neutral water can act as a leaving group. This occurs, for example, when an alkyl halide is prepared from a tertiary alcohol by reaction with HBr or HC1 (Section 10.6). The alcohol is first protonated and then spontaneously loses H2O to generate a carbocation, which reacts with halide ion to give the alkyl halide (Figure 11.13). Knowing that an SN1 reaction is involved in the conversion of alcohols to alkyl halides explains why the reaction works well only for tertiary alcohols. Tertiary alcohols react fastest because they give the most stable carbocation intermediates. 

A free radical (often simply called a radical) may be defined as a species that contains one or more unpaired electrons. Note that this definition includes certain stable inorganic molecules such as NO and NO2, as well as many individual atoms, such as Na and Cl. As with carbocations and carbanions, simple alkyl radicals are very reactive. Their lifetimes are extremely short in solution, but they can be kept for relatively long periods frozen within the crystal lattices of other molecules. Many spectral measurements have been made on radicals trapped in this manner. Even under these conditions, the methyl radical decomposes with a half-life of 10-15 min in a methanol lattice at 77 K. Since the lifetime of a radical depends not only on its inherent stabihty, but also on the conditions under which it is generated, the terms persistent and stable are usually used for the different senses. A stable radical is inherently stable a persistent radical has a relatively long lifetime under the conditions at which it is generated, though it may not be very stable. 

Recall that tertiary carbocations are more stable than secondary carbocations. When given the choice, we expect the aUcene to accept the proton in such a way as to form the more stable carbocation intermediate. In order to accomplish this, the proton must add to the less substituted carbon, generating the more substituted... 

It was previously observed that with a catalytic amount of FeCls, benzylic alcohols were rapidly converted to dimeric ethers by eliminating water (Scheme 14). In the presence of an alkyne this ether is polarized by FeCls and generates an incipient benzylic carbocation. The nucleophilic attack of the alkyne moiety onto the resulting benzyl carbocation generated a stable alkenyl cation, which suffer the nucleophilic attack of water (generated in the process and/or from the hydrated... 

The first widely used intermediates for nucleophilic aromatic substitution were the aryl diazonium salts. Aryl diazonium ions are usually prepared by reaction of an aniline with nitrous acid, which is generated in situ from a nitrite salt.81 Unlike aliphatic diazonium ions, which decompose very rapidly to molecular nitrogen and a carbocation (see Part A, Section 4.1.5), aryl diazonium ions are stable enough to exist in solution at room temperature and below. They can also be isolated as salts with nonnucleophilic anions, such as tetrafluoroborate or trifluoroacetate.82 Salts prepared with 0-benzenedisulfonimidate also appear to have potential for synthetic application.83... 

Substituted allenyl cations 47 have been generated from propargyl alcohols 48 under stable carbocation conditions (Sbf s/f SOsII in SO2CIF) (equation 17). On the basis of 13C-NMR chemical shifts, the positive charge has been found to be extensively delocalized with the mesomeric allenyl cations contributing highly to the total ion structure36,37. 

C-NMR spectroscopic studies on a-substituted tris(ethynyl)methyl cations 49 prepared from alcohols 50 (equation 18) provided evidence for the participation of resonance structures with allenyl cationic character38. The parent tris(ethenyl)methyl cation (49, R = H) cannot be generated under stable carbocation conditions (SbFs/FSOsH) presumably due to the highly reactive unsubstituted termini of the three ethynyl groups and the resulting low kinetic stability. The chemical shift data (Table 1) give evidence that in all cases Ca and CY are deshielded more than Cg (relative to their precursor alcohols). 

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