Primary carbocations

The diarylmethyl cations listed in Table 5.1 are 6-7pATr+ units less stable than the corresponding triarylmethyl cations. This indicates that the additional aryl group has a cumulative, although not necessarily additive, effect on stability of the carbocation. Primary benzylic cations (monparylmethyl cations) are generally not sufficiently stable for determination of pATr+ values. A particularly stable benzylic ion, the 2,4,6-trimethyl-phenylmefliyl cation, has a pATr+ of — 17.4. 

Primary carbocations Should you wish to use carbocations in a reaction mechanism, you must consider the relative stability of these entities. Tertiary carbocations are OK, and in many cases so are secondary carbocations. Primary carbocations are just not stable enough, unless there is the added effect of resonance, as in benzylic or ally lie systems. 

Methyl Forms least stable carbocation Primary Secondary Tertiary Forms most stable carbocation... 

The protonated hydroxyl group —OH2 is a good leaving group. A simple ionization to a carbocation would form a primary carbocation. Primary carbocations, however, are very unstable. Thus, a methyl shift occurs as water leaves, so a primary carbocation is never formed. A tertiary carbocation results. (You can visualize this as two steps if you prefer.)... 

CH3)2CH+> H2C==CH—CH+ CH3CH+ > H2C=CH > Ph > CHj. The stabilities of various carbocations can be determined by reference to the order of stability for alkyl carbocations, 3 > 2° > 1 > CH3. The acetyl cation has a stability similar to that of the r-butyl cation. Secondary carbocations, primary benzylic cations, and primary allylic cations are all more stable than primary alkyl cations. Vinyl, phenyl, and methyl carbocations are less stable than primary alkyl cations. 

Unlike tertiary and secondary carbocations, primary carbocations are too high in energy to be intermediates in chemical reactions. Since primary alcohols are converted, albeit rather slowly, to aUcyl halides on treatment with hydrogen halides, they must follow some other mechanism that avoids carbocation intermediates. This alternative mechanism is believed to be one in which the carbon-halogen bond begins to form before the carbon-oxygen bond of the alkyloxonium ion is completely broken. 

When an alkene that does not have the same substituents on its sp carbons undergoes an electrophilic addition reaction, the electrophile can add to two different sp carbons. We have just seen that the major product of the reaction is the one obtained by adding the electrophile to the sp carbon that results in the formation of the more stable carbocation (Section 4.3). For example, when propene reacts with HCl, the proton can add to the number-1 carbon (C-1) to form a secondary carbocation, or it can add to the number-2 carbon (C-2) to form a primary carbocation. The secondary carbocation is formed more rapidly because it is more stable than the primary carbocation. (Primary carbocations are so unstable that they form only with great difficulty.) The product of the reaction, therefore, is 2-chloropropane. 

The rate of an SnI reaction depends only on the concentration of the alkyl halide. The halogen departs in the first step, forming a carbocation that is attacked by a nucleophile in the second step. Therefore, carbocation rearrangements can occur. The rate of an SnI reaction depends on the ease of carbocation formation. Tertiary alkyl halides, therefore, are more reactive than secondary alkyl halides since tertiary carbocations are more stable than secondary carbocations. Primary carbocations are so unstable that primary alkyl halides cannot undergo SnI reactions. An SnI reaction takes place with racemization. Most SnI reactions are solvolysis reactions The solvent is the nucleophile. 

B and C are unreactive because they would form less stable carbocations (primary or secondary rather than tertiary) in the first step of the mechanism. 

Alkyl halides and alcohols, depending on the structure of the alkyl group, react with magic acid to give rise to carbocations. Primary and secondary alcohols are... 

Table 10.2 shows that, just as with allg l carbocations, secondary vinyl cations are more stable than primary vinyl cations. However, it also shows that even a secondary dnyl cation is not even as stable as a primary alkyl carbocation. Primary carbocations serve as a kind of mechanistic stop sign They are generally too unstable to be viable intermediates in most reactions. Presumably, we should treat unstabilized tdnyl cations the same way. 

Figure 6.3 compares potential energy diagrams for these two competing modes of addition. According to Hammond s postulate, the transition state for protonation of the double bond resembles the carbocation more than the alkene, and for formation of the more stable carbocation (tertiary) is less than that for formation of the less stable carbocation (primary). The major product is derived from the carbocation that is formed faster, and the energy difference between a primary and a tertiary carbocation is so great and their rates of formation so different that essentially all of the product is derived from the tertiary carbocation. 

Carbocation rearrangements take place by hydride and alkyl group shifts. They usuaUy result in interconversion of secondary carbocations or conversion of a secondary into a tertiary carbocation. Primary alkyloxonium ions can rearrange by a concerted process consisting of loss of water and simultaneous hydride or alkyl shift to give secondary or tertiary carbocations. 

Compounds that undergo reactions by an SnI path must be capable of forming relatively stable carbocations. Primary halides of the type ROCH2X form carbocations that are stabilized by resonance ... 

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