Carbocation Radicals

Reactive Intermediate Chemistry is an attempt to provide an updated survey and analysis of the field. We have adopted a three-dimensional approach. Reactive Intermediates are considered by type (e.g., carbocations, radicals, carbanions, car-benes, nitrenes, arynes, etc.) they are examined according to the kinetic realms that... 

The relative stabilities of radicals follow the same trend as for carhoca-tions. Like carbocations, radicals are electron deficient, and are stabilized by hyperconjugation. Therefore, the most substituted radical is most stable. For example, a 3° alkyl radical is more stable than a 2° alkyl radical, which in turn is more stable than a 1° alkyl radical. Allyl and benzyl radicals are more stable than alkyl radicals, because their unpaired electrons are delocalized. Electron delocalization increases the stability of a molecule. The more stable a radical, the faster it can be formed. Therefore, a hydrogen atom, bonded to either an allylic carbon or a benzylic carbon, is substituted more selectively in the halogenation reaction. The percentage substitution at allylic and benzyhc carbons is greater in the case of bromination than in the case of chlorination, because a bromine radical is more selective. 

The stabilizing effects of the 1,3-sulfur atoms on the carbocations, radicals, and carbanions generated from 2-aryl-l,3-dithianes and -dithiolanes has permitted the measurement of a variety of bond-making and bond-breaking energies (in DMSO and sulfolane) and their correlation with electron-transfer energies.148... 

Both radicals and carbocations are electron deficient because they lack an octet around the carbon atom. Like carbocations, radicals are stabilized by the electron-donating effect of alkyl groups, making more highly substituted radicals more stable. This effect is confirmed by the bond-dissociation enthalpies shown in Figure 4-7 Less energy is required to break a C—H bond to form a more highly substituted radical. 

Like carbocations, radicals can be stabilized by resonance. Overlap with the p orbitals of a tt bond allows the odd electron to be delocalized over two carbon atoms. Resonance delocalization is particularly effective in stabilizing a radical. 

Electrochemistry is the combination of a heterogeneous electron transfer at an electrode with a chemical reaction. The electron transfer leads to the reactive intermediates carbo-cations [1], carbanions [2], radicals [3], and radical ions [4]. The differences in comparison to chemical reactions are due to the way in which these reactive intermediates are generated. In electrochemistry, the electrode transfers the electron in chemical reactions, chemical reductants or oxidants are used. Furthermore, carbocations, radicals, and carbanions are chemically generated by dissociation, homolysis, or deprotonation. The reaction conditions, the solvents, and the reactants encountered by the electrochemically or chemically formed intermediates can differ substantially and thus can lead to diverse reaction pathways and products. 

Products which could be derived from the carbocation radical VII are obtained in 5-16% yields. It follows that the rate constant for conversion of VII to epoxide must be > (1-2) X 10 1 s". A rate constant of 10 s" for the recombination of a solvent-caged pair involving VII to provide epoxide is not unreasonable. Recall our conclusion that the formation of VII cannot be rate determining. The nonpolar products could have arisen from a reaction that is parallel to epoxidation. 

With alkyl-substituted terminal alkenes various degrees of porphyrin N-alkylation accompany epoxidation. The N-alkylation take place exclusively at the unsubstituted terminus of the double bond and could be a concerted or non-concerted reaction. Formation of an intermediate carbocation on the path to N-alkylation can be excluded because it would require the preferential formation of a primary carbocation. Radicals, on the other hand, show much lower preference for substituted unsubstituted carbon, suggesting that an initial formation of a carbon radical, followed by its collapse to the N-alkyl porphyrin is possible. While carbon radicals cannot be discrete intermediates in the epoxidation reaction vide supra), they can be intermediates in N-alkylation, if N-alkylation is a side reaction to epoxidation. 

Another possibility is the intermediacy of an alkene-derived 7C-cation radical IV. However, the formation of IV from primary alkenes is energetically unfavorable (i.e., the potentials for le oxidation are quite positive for aliphatic primary alkenes), although it is these alkenes which exhibit N-alkylation. Styrene, which has a much lower oxidation potential that would favor the formation of IV, does not exhibit N-alkylation. This dichotomy would tend to eliminate carbocation radicals as plausible intermediates in porphyrin N-alkylation. 

Unhke carbocations, radicals on the whole are less prone to rearrangements. Unhke carbanions, alkoxy and sulfonyloxy groups p-to a radical center do not undergo elimination. However, halogen, sulfur, and selenium groups P-to radicals are ehminated. Thus, radical elimination chemistry can be complementary to that of anions. 

Allylic carbocations, radicals, and anions are conjugated systems involved as reactive intermediates in chemical reactions. The next type of conjugated system that we will examine, conjugated dienes, consists of stable molecules. 

Of the limiting cases, it is easy to distinguish DISP2 (Reaction 14 is unimportant and Reaction 15 the rate-determining step (RDS)) as it has a second-order dependence on B. But it is difficult to distinguish ECE (Reaction 15 unimportant) from DISPl (Reaction 14 unimportant and Reaction 13 the RDS). However, the spectroelectro-chemical experiment shows a rapid falloff in absorbance for the ECE reaction after the cell is switched to open circuit (Fig. 3). Species D, the carbocation radical in this case, decays rapidly when the electrode is unable to oxidize C [21, 40]. 

Use the wave functions shown in equations 4.39 through 4.41 to calculate the n bond orders and the free valence indices for the allyl carbocation, radical, and... 

The familiar octet rule, which states that atoms are most stable when their valence shell is full, suggests that carbon in a molecule will take on four more electrons from other atoms so as to possess an octet of electrons and thereby attain a noble gas configuration. The number of bonds that an atom can make is called its valence number. If each bond that carbon makes is created by the donation of a single electron from an adjacent atom s atomic orbitals, carbon will make four bonds. Carbon is said to have a valence of four. This valence is by far the most common bonding arrangement for C. When carbon has fewer than four bonds it is in a reactive form, namely a carbocation, radical, carbanion, or carbene. When a similar analysis is done for N, O, and F, it is found that these atoms prefer three, two, and one bond(s), respectively. 

Many organic, organometallic, and bioorganic reactions involve intermediates that are indeed reactive and transient. Reactive intermediates such as carbocations, radicals, carban-ions, and carbenes are common to organic and bioorganic transformations, whereas coordi-natively unsaturated transition metals, and low and high oxidation state metals are common to organometallic reactions (see Chapter 12). 

The reactions we are studying are also concerted. A concerted reaction is one that occurs in a single step without any intermediate, while a stepwise process has one or more intermediates. Most of the reactions discussed in Chapters f 0 and 11 are stepwise with intermediates (carbocations, radicals, carbenes, or carbanions), and are thus distinct from pericyclic reactions. Pericyclic and concerted, however, are not inextricably linked. Not all concerted reactions are pericyclic. For example, the Sn2 reaction is concerted, but it is not pericyclic. Pericyclic and concerted really pertain to two very different aspects of a reaction. Pericyclic refers to the geometry of the transition state a cyclic array is required. Concerted refers to a particular kind of reaction coordinate diagram—one without intermediates. The reactions of this chapter are indeed both concerted and pericyclic, but keep clear in your mind the differing implications of these two terms. 

FIGURE 11.19 A comparison of the stabilizations of carbocations, radicals, and Hc2. Notice the electron in the... 

Scheme Ic). This type of mechanism is common in the synthesis of biaryls by the coupling of two electron-rich aryl moieties (see Sect. 4.2.1) [28]. Kita and coworkers discovered that the fluorinated alcohols 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) are excellent solvents in many reactions with At2IX and other hypervalent iodine reactions because of their ability to stabilize carbocation radicals in SET reactiOTis [29, 30]. 

As described in Sections 10.1-10.4, allylic carbocations, radicals, and anions are conjugated TT-electron systems involved as intermediates in chemical reactions. The remaining sections of this chapter focus on stable molecules, especially hydrocarbons called conjugated dienes, which contain two C=C units joined by a single bond asinC=C—C=C. It begins by comparing their structure and stability to isolated dienes, in which the two... 

Benzylic carbocations, radicals, and anions resemble their allylic counterparts in being conjugated systems stabilized by electron delocalization. This delocalization is describable in resonance, valence bond, and molecular orbital terms. 

The relative stability of benzylic carbocations, radicals, and carbanions makes it possible to manipulate the side chains of aromatic rings. Functionalization at the benzylic position, for example, is readily accomplished by free-radical halogenation and provides access to the usual reactions (substitution, elimination) that we associate with alkyl halides. 

Chemical reactions of arenes can take place involving either the ring itself or a side chain. The most characteristic reactions proceed via benzylic carbocations, radicals, or anions. 

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