Methyl carbocation, electronic structure

Before we move on from the hybrid orbitals of carbon, we should take a look at the electronic structure of important reactive species that will figure prominently in our consideration of chemical reactions. First, let us consider carbanions and carbocations. We shall consider the simplest examples, the methyl anion CHs and the methyl cation CH3+, though these are not going to be typical of the carbanions and carbocations we shall be meeting, in that they lack features to enhance their stability and utility. 

Carbocations are electron-deficient species that are the most important intermediates in several kinds of reactions. A common model for carbocation stmcture is a planar species exhibiting sp hybridization, as shown in Figure 2.4 for methyl cation. The p-orbital that is not utilized in the hybrids is empty and is often shown bearing the positive charge since it represents the orbital available to accept electrons. There is a vacant p orbital perpendicular to the plane of the molecule this is the LUMO (lowest unoccupied molecular orbital). In all reactions of carbocations there is an interaction between this LUMO and the HOMO (highest occupied molecular orbital) of another molecule. A structure with an empty p orbital should be more stable than a structure in which an orbital with s character is empty. In general, a carbocation is a purely ionic species. 

It is possible to describe the stabilization of a carbocation by an adjacent methyl group in a more familiar way with PMO theory by using a localized molecular orbital to represent the electron density associated with C-H a bonding. The structure on the left in Figure 4.59 represents the nondelocalized carbocation, while the structure on the right shows the effect of the interaction described in Figure 4.55(b). Exactly as was the case in Figure 4.56, the... 

C, indicate that the mesomeric effect e.g. MejC- -F Me2C=F ) of the halogens involved decreases in the order F > Cl > Br. All this and related work by Olah s group on fluorinated carbocations has been reviewed, and the results of MO studies of fluorinated methyl, allyl, or trans-2 vinylcyclopropylmethyP cations and radicals have been presented in detail. The electronic structure of difluoromethane and the partial double-bond character of C— F bonds in CHjF, CHjFj, and CHF, have received attention at the hands of MO pundits. 

The structures, geometries, and relative stabilities of simple alkyl radicals are similar to those of alkyl carbocations. Methyl radical is planar, and all other radicals are nearly so, with bond angles near 120° about the carbon with the unpaired electron. This geometry indicates that carbon is sp hybridized and that the unpaired electron occupies the unhybridized 2p orbital. As mentioned, the order of stability of allyl radicals, like alkyl carbocations, is 3° > 2° > 1° methyl. 

An important tool for the investigation of carbocation structure is measurement of the C nmr chemical shift of the carbon atom bearing the positive charge.66 This shift approximately correlates with electron density on the carbon. 13C chemical shifts for a number of ions are given in Table 5.2.67 As shown in the table, the substitution of an ethyl for a methyl or a methyl for a hydrogen causes a downfield shift, indicating that the central carbon... 

The pinacol rearrangement is a dehydration of an alcohol that results in an unexpected product. When hot sulfuric acid is added to an alcohol, the expected product of dehydration is an alkene. However, if the alcohol is a vicinal diol, the product will be a ketone or aldehyde. The reaction follows the mechanism shown, below. The first hydroxyl group is protonated and removed by the acid to form a carboca-tion in an expected dehydration step. Now, a methyl group may move to fonn an even more stable carbocation. This new carbocation exhibits resonance as shown. Resonance Structure 2 is favored because all tire atoms have an octet of electrons. The water deprotonates Resonance Structure 2, forming pinacolone and regenerating the acid catalyst. 

A carbocation (also called a carbonium ion or a carbenium ion) is a species that contains a carbon atom bearing a positive charge. The positively charged carbon atom is bonded to three other atoms, and it has no nonbonding electrons, so it has only six electrons in its valence shell. It is sp2 hybridized, with a planar structure and bond angles of about 120°. For example, the methyl cation (+CH3) is planar, with bond angles of exactly 120°. The unhybridized p orbital is vacant and lies perpendicular to the plane of the C—H bonds (Figure 4-13). The structure of +CH3 is similar to the structure of BH3, discussed in Chapter 2. 

Like carbocations, free radicals are sp2 hybridized and planar (or nearly planar). Unlike carbocations, however, the p orbital perpendicular to the plane of the C—H bonds of the radical is not empty it contains the odd electron. Figure 4-15 shows the structure of the methyl radical. 

Mercuration exhibits a carbocation-like pattern, but with the superposition of a large steric effect. For unsubstituted terminal carbons, the rate increases from ethene to propene to 2-methylpropene. This trend also holds for internal alkenes, as 2-methyl-2-butene is more reactive than 2-butene. However, steric effects become dominant for 2,3-dimethylbutene. This incursion of steric effects in oxymercuration has long been recognized and is exemplified by the results of Nelson and co-workers, who found separate correlation lines for mono- and disubstituted alkenes. Hydroboration by 9-BBN (structures) shows a different trend steric effects are dominant and reactivity decreases with substitution. Similar trends apply to rates of addition of dibromob-orane and disiamylborane. The importance of steric factors is no doubt due in part to the relatively bulky nature of these boranes. However, it also reflects a decreased electron demand in the hydroboration TS. 

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