Free Radicals and Carbenes

Thus, nanosecond techniques are usually limited to a few nanoseconds on the short end, presently related to the pulse duration (2-20 ns) of typical nanosecond lasers. Long times are frequently limited by either the stability of electronic-optical components, or by the fact that in the absence of other processes many reaction intermediates (e.g., free radicals and carbenes) can undergo rapid self-reactions. Nanosecond techniques rarely extend beyond a few tens of milliseconds. 

Many reactive intermediates can decay via self-reactions, giving dimers or disproportionation products, as is the case of free radicals and carbenes. When these self-reactions are not the ones under study, it is desirable to keep the transient concentration low enough to minimize this type of interference. For example, for a radical that dimerizes with fet = 3 x 10 M s and a concentration c of lO M, its first half-life (ti/2 = 1/kc) would be 33 ps. Note that excited triplet states also undergo bimolecular decay by triplet-triplet... 

The major carbon centered reaction intermediates in multistep reactions are carboca-tions (carbenium ions), carbanions, free radicals, and carbenes. Formation of most of these from common reactants is an endothermic process and is often rate determining. By the Hammond principle, the transition state for such a process should resemble the reactive intermediate. Thus, although it is usually difficult to assess the bonding in transition states, factors which affect the structure and stability of reactive intermediates will also be operative to a parallel extent in transition states. We examine the effect of substituents of the three kinds discussed above on the four different reactive intermediates, taking as our reference the parent systems [ ]+, [ ]-, [ ], and [ CI I21-... 

C60 chemically behaves as an electron-deficient polyalkene, with rather localized double bonds. Thus, the reactions that mainly take place involve cycloadditions, additions of nucleophiles, free radicals, and carbenes, as well as rp-complexation with various transition metal elements. 

Q Draw and describe the structures of carbocations, carbanions, free radicals, and carbenes and the structural features that stabilize them. Explain which are electrophilic and which are nucleophilic. 

Chemical inertness of alkanes is reflected in one of their old names, paraffins , from the Latin parum affinis (without affinity). However, saturated hydrocarbons can be involved very easily in a total oxidation with air (simply speaking, burning) to produce thermodynamically very stable products water and carbon dioxide. It should be emphasized that at room temperature alkanes are absolutely inert toward air, if a catalyst is absent. At the same time, some active reagents, e.g., atoms, free radicals, and carbenes, can react with saturated hydrocarbons at room and lower temperatures. These compounds are easily transformed into various products under elevated (above 1000 °C) temperatures, in the absence of other reagents. 

The interaction of high-energy irradiation with alkanes leads, at the first stage of the process, to the excitation of the hydrocarbon molecule [3a]. Furthermore, the excited molecule decomposes to generate free radicals and carbenes. Radiolysis of methane produces ethane, ethylene, and higher... 

II.2. Reactions with Atoms, Free radicals and Carbenes... 

A general consideration of reactive intermediates in 1956 by Leffler suggested that assignments of reaction mechanisms very often assume intermediates that have not been isolable for direct study and the physical reahty of such intermediates depends on their relation to similar substances that do happen to be stable enough to study directly. Study of reactive intermediates focuses not only on ions, free radicals, and carbenes but also on other reactive species such as benzynes and ketenes, which were recognized around 1900. With advances in experimental methods of generation and detection of such species, as well as improvements in... 

In formulating the transformations of starting materials to products for all of the functional groups, evidence has accumulated that the pathways by which the former are converted to the latter frequently involve the intermediacy of the same types of transient, highly reactive species. These intermediate species include carbocations, carbanions,free radicals, and carbenes. 

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