Heterolysis reactive intermediates

Homolysis and heterolysis require energy. Both processes generate reactive intermediates, but the products are different in each case. 

Figure 6.2 illustrates homolysis and two different heterolysis reactions for a carbon compound using curved arrows. Three different reactive intermediates are formed. 

Three reactive intermediates resulting from homolysis and heterolysis of a C—Z bond... 

Thus, homolysis and heterolysis generate radicals, carbocations. and carbanions, the three most important reactive intermediates in organic chemistry. 

Problem 6.2 By taking into account electronegativity differences, draw the products formed by heterolysis of the carbon-heteroatom bond in each molecule. Classify the organic reactive intermediate as a carbocation or a carbanion. 

One possible stepwise mechanism involves heterolysis of the A-B bond to form two ions A and B% followed by reaction of B with anion C to form product B-C, as outlined in the accompanying equations. Species B is a reactive intermediate. It is formed as a product in Step [1], and then goes on to react with C in Step [2]. 

Polar protic solvents like H2O and ROH solvate both cations and anions well, and this characteristic is important for the SnI mechanism, in which two ions (a carbocation and a leaving group) are formed by heterolysis of the C-X bond, The carbocation is solvated by ion-dipole interactions with the polar solvent, and the leaving group is solvated by hydrogen bonding, in much the same way that Na" and Br are solvated in Section 7.8C. These interactions stabilize the reactive intermediate. In fact, a polar protic solvent is generally needed for an SnI reaction. SnI reactions do not occur in the gas phase, because there is no solvent to stabilize the intermediate ions. 

Direct irradiation of the silyl ketone (9) affords the products shown in scheme 2. The same products can be obtained on sensitised irradiation. An analysis of the reaction suggests that two reaction paths are operative.The asters (10) are photochemically reactive in methanol. The preference for the reaction paths observed is dependent upon the substituent in the naphthyl ring. Thus with unsubstitution or with a 4-methyl substituent the reaction mode is dominated by C-0 bond heterolysis yielding intermediates from which the ether (11) and the acid (12) are formed. With a 4-cyano substituent the reaction is dominated by C-0 homolysis yielding the alkane (13) as the major product. The introduction of a 3-mathoxy substituent produces a system which is intermediate between the two extremes. 

The radical mechanism involves homolysis of the Co—C bond and the ionic mechanism involves heterolysis of this bond. Since coenzyme Bjj has both Co(II) and Co(I) derivatives, either of these mechanisms is chemically reasonable. The radical pathway provides low-spin Co(II) which is EPR active and therefore easily detectable. However, detection alone does not prove that this is the catalytic pathway. It should be noted that an important function of the protein in these systems is to constrain and protect reactive intermediates and inhibit unwanted side reactions. For example, the substrate radical or anion might combine with the cobalt to give an organocobalt complex as the product of the second step in both reactions (8.13) and (8.14). 

Draw the products of homolysis or heterolysis of each indicated bond. Use electronegativity differences to decide on the location of charges in heterolysis reactions. Classify each carbon reactive intermediate as a radical, carbocation, or carbanion. 

Breaking bonds generates reactive intermediates. Homolysis generates radicals with unpaired electrons. Heterolysis generates ions. 

As the understanding of the ionic intermediates has progressed, advantage has been taken of the fact that bromination, like SN1 heterolysis, is a carbocation-forming reaction. Kinetic data on this addition have therefore been used to examine in detail how the basic concepts of physical organic chemistry work as regards transition-state shifts with reactivity (Ruasse et al, 1984). Bromination lends itself particularly well to the quantitative application of the BEMA HAPOTHLE (acronym for Bell, Marcus, Hammond, Polanyi, Thornton and Leffler Jencks, 1985). In particular, it has been possible to evaluate the transition-state dependence on the solvent and substituents. The major disadvantage that bromination shares with many... 

The accepted reaction mechanism for the electrophilic aromatic nitration was postulated by Ingold in 1969[3] and involves several steps (Scheme 5.1). Firstly, the nitric acid is protonated by a stronger acid (sulfuric). The protonated nitric acid gives water and the nitronium ion (N02+) which is the electrophilic active species for nitration of aromatics. Nitric acid heterolysis is considered to be accelerated by the polarity of the solvent, and solvation of nitronium ion in different media affects its reactivity and the selectivity of the reaction. Combination of nitronium ion and an aromatic molecule form an intermediate named the Wheland complex or er-complex. The loss of a proton from the er-complex gives the aromatic nitrocompound (Scheme 5.1). 

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