Acids, acid carbocations

Antimony pentachloride is a reactive Lewis acid that can be used for Friedel-Crafts reactions and some other Lewis-acid-catalyzed reactions. The HF-SbF5 system is known as magic acid, and carbocations are stabilized in this medium.353 By using the HF-SbF5 system, alkylation of acetophenone (a relatively unreactive aromatic compound) has been achieved (Scheme 87). 

Treatment of 482 as the ether or afcohol with magic acid nerates carbocation 483 which rapidly isomerizes to 484 who sp ra are observable. Quenching experiments carried out with sodium bicarbonate suspensions in methanol or methyl mercaptan at —110 °C produced 485a and 485b. Dissolution of 485a in magic acid... 

Ring closure at the ends of the lycopene molecule to form the carotenes can be formulated most readily through an acid-catalyzed carbocation mechanism (Eq. 22-11). Loss of one or the other of two protons adjacent to the positive charge leads to the (3 ring of (3-carotene or to the a ring of a-carotene.110/123a Compounds with c only one ring may also be... 

We mentioned previously that the hydration of alkenes requires a strong acid as a catalyst, because water itself is too weak an acid to initiate the proton-transfer step. However, if a small amount of a strong acid such as sulfuric acid is present, hydronium ions, H30 , are formed in sufficient amount to protonate reasonably reactive alkenes, although by no means as effectively as does concentrated sulfuric acid. The carbocation formed then is attacked rapidly by a... 

In the presence of the Lewis acid, the carbocation generated in situ can serve as an electrophile to replace, intramolecularly, the silyl substituent at the olefinic carbon153-155. The stereochemical requirements at the silyl substituent appear not to be important (equations 85-87). 

There have been few other examples of secondary deuterium IEs on CH acidity. One reason is their low acidity (carbocations excepted), so that their conjugate bases are not so readily accessible. Another reason is that the titration methods are less applicable, because proton-transfer equilibrium is not established instantaneously . Nevertheless, measurement of IEs in acetylacetone, malononitrile, and related acids ought to be feasible, and they might be larger than the secondary IEs reported here because the deprotonation of these acids can involve rehybridization. 

The alcohol is tertiary and benzylic. In the presence of sulfuric acid a carbocation is formed. 

Keywords Electrophilic polyfluoroolefin Lewis acid polyfluorinated carbocations. 

George Olah published a perspective in which he provided an overview of the research of his group over five decades.1 Christopher Reed surveyed the chemistry of carborane acids, the strongest pure Brpnsted acids known, with a conjugate base that engages in little chemistry.2 These acids are employed to prepare highly acidic carbocations, including protonated benzene, protonated C6o, tertiary carbocations, vinyl cations, and silylium ions. The synthesis and properties of unusual... 

Bakelite, the first synthetic polymer, is an example of a thermoset polymer. It is prepared by the polymerization of phenol and formaldehyde in the presence of ail acid. Carbocations produced by protonation of formaldehyde bond to the ortho and para positions of the highly reactive phenol molecules in a Friedel-Crafts alkylation reaction. The benzylic alcohols that are produced in this step react to produce carbocations that then alkylate additional phenol molecules. A mechanism for the first few steps of this polymerization process is shown in Figure 24.4. 

With only six electrons in the positive carbon s valence shell, a carbocation is a powerful electrophile (Lewis acid), and it may react with any nucleophile it encounters. Like other strong acids, carbocations are unlikely to be found in basic solutions. Carbocations are proposed as intermediates in many types of organic reactions, some of which we will encounter in Chapter 6. 

Other electrophiles include acylium ions produced from acid chlorides, carbocations from tertiary halides or secondary benzylic halides, activated enones, and epoxides all in the presence of Lewis acid. In each case the new bond is highlighted in black. 

Cationic polymerizaton of vinyl and cyclic monomers can be initiated by various initiators such as protonic and Lewis acids, carbocations, and trialkyl-oxonium salts [1]. General reactions for the initiation of the polymerization may be represented as shown below (Scheme 1). 

Common superacids in use are the Bronsted acid FSO3H and the Lewis acid SbFj dissolved in SO ClF or mixtures of SOjClF and SO Fj. To be able to study liquid ionic solutions at very low temperatures (ca —160°C), e.g. by nmr spectroscopy, freons like CHCljF may be added to the solution to keep the viscosity at a tolerable level. Superacids can be up to a billion times stronger acids than sulphuric acid. Carbocations are generated in the reactions of e.g. alcohols and olefins with FSO3H and of chlorides with e.g. SbFj or by hydride abstraction. The superacid chemistry has been treated in a number of reviews (e.g. Olah, 1979). A general survey of the chemistry of superacids is given by Olah et al. (1979b). Other reviews have appeared... 

Similar to alcohols, aliphatic ethers [54], thiols [55], and sulfides are also protonated on oxygen or sulfur, respectively, at -60 °C in magic acid carbocations are subsequently formed upon raising the temperature. Promoted sulfides, excluding tertiary alkyl, are resistant to cleavage up to -i-70 °C [56]. 

B = aminium sait induced reaction in the absence of soiubie, hindered base or trifiic acid cataiyzed reaction Bronsted acid cataiyzed, carbocation mediated reaction. 

Addition of electrophiles is a reaction typical of aliphatic tt bonds (see Example 4.3). Such additions involve two major steps (1) addition of the electrophile to the nucleophilic tt bond to give an intermediate carbocation, and (2) reaction of the carbocation with a nucleophile. Typical electrophiles are bromine, chlorine, a proton supplied by HCl, HBr, HI, H2SO4, or H3PO4, Lewis acids, and carbocations. The nucleophile in step 2 is often the anion associated with the electrophile, e.g., bromide, chloride, iodide, etc., or a nucleophilic solvent like water or acetic acid. 

Enamines are among the most powerful neutral nucleophiles and react spontaneously with alkyl halides. Silyl enol ethers are less reactive and so require a more potent electrophile to initiate reaction. Carbocations will do, and they can be generated in situ by abstraction of a halide or other leaving group from a saturated carbon centre by a Lewis acid, carbocation formed... 

Cationic polymerization is one of the most fundamental methods for synthesizing polymers.A general scheme for the cationic polymerization of vinyl monomers having a cation stabilizing group (R) is shown in Scheme 9.4. In general, proton acids or carbocations generated from their precursors by acid-promoted ionization reactions are used as... 

Figure 8.6 illustrates, for example, the complicated equilibria [19] in initiation by alkyl halides which are widely used as initiators in combination with coinitiator such as aluminum alkyl halides or aluminum halide Lewis acids. Each carbocation can initiate polymerization or remove an alkyl (ethyl) group from the counterion to produce a saturated hydrocarbon, REt, and a more acidic Lewis acid. The propagating cation can also terminate by the same process to produce ethyl-capped polymers and new Lewis acids. Thus, even though the coinitiator used is diethylaluminum chloride there may be major contributions to the polymerization from ethylaluminum dichloride or aluminum chloride. 

Carboxylation of alcohols or olefins with HCO2H or with CO (super-saturated solution) in cone, sulfuric acid via carbocations, usually with rearrangement. 

The acylation of alkanes has also been known for a long time, but for synthetic purposes is limited to simple substrates. The initial step is hydride abstraction by an acylium ion, a process well established in the presence of a powerful Lewis acid, most commonly an aluminum halide, or strong protic acid. The carbocation so formed can then undergo elimination, possibly after hydride or alkyl migration, to give an alkene which is then acylated. In the presence of excess alkane, saturated ketones are formed by a further intermolecular hydride transfer, whereas with an excess of acyl halide, the product is the (conjugated) unsaturated ketone. -" The synthetic potential is obviously likely to be limited to simple substrates. 

F I T T I G Pinacolone Rearrangement Acid catalyzed carbocation rearrangement ot 1,2-diols to ketones... 

Ring closure at the ends of the lycopene molecule to form the carotenes can be formulated most readily through an acid-catalyzed carbocation mechanism (Eq. 

There are three methods for generating carbocations ionization of a C—X" " a bond, reaction of a lone pair on the heteroatom in a C=X bond with a Lewis acid, and reaction of a C=C tt bond with a Lewis acid. Carbocations and H+ are included in the term Lewis acid. 

Oxymercuration-demercuration gives the same products as aqueous sulfuric acid. The carbocations formed from these substrates and H+ are not particularly prone to rearrangement. All chiral products are formed as racemic mixtures. 

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