Common Path Combinations

Several pathway combinations occur so frequently that they have been given a name. Most were introduced and discussed in Chapter 4 and are grouped here as a summary. These phrases in your mechanistic vocabulary are the next step toward your eventual construction of a grammatically correct mechanistic sentence.  

The Adg2 is the major addition pathway of alkenes and dienes. Electrophile addition to a pi bond or protonation of a pi bond, path Ag, produces the most stable cation, which is then trapped by a nucleophile, path An. (Section 4.4.2 discusses the various addition types.) If the electrophile is a proton, this reaction is the reverse of the El reaction. The reaction commonly produces a mixture of syn and anti addition. 

This reaction is a minor variant of the Ac1e2 in which the electrophile, usually a proton, attacks the lone pair rather than the less available heteroatom-carbon pi bond. The lone pair is protonated, path p.t., to give a highly polarized multiple bond that can be viewed as a stabilized carbocation. The nucleophilic attack on this carbocation could also be viewed as path An, trapping of a cation by a nucleophile, instead of path AdN- 

After rate-determining ionization of a good leaving group, path Dn, the cation is deprotonated by path Dg to produce a pi bond. This elimination is the reverse of the AdgZ addition. As might be expected, the El process competes with the SnI process. If A acts as a base, El occurs if it acts as a nucleophile, SnI occurs. Section 9.5 will discuss such decisions in detail. 

This reaction is the reverse of the hetero Ade2 reaction. The lone-pair-assisted El uses a properly aligned lone pair to expel the leaving group, path Ep. The resultant cation is then deprotonated, path p.t. 

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Chapter 3 covered the proton transfer electron flow path and reviewed the factors that contribute to acidity. Chapter 4 introduced all the rest of the major electron flow paths along with the four reaction archetypes, substitution, elimination, addition, and rearrangement. This chapter gathers together all the major electron flow paths, introduces a few minor paths, and reviews common path combinations. Section 7.4, Variations on a Theme, shows how the 18 electron flow paths might be reasonably extended and modified. 

Draw a general example of each of these common path combinations. 

Classify the reaction below into one of the four archetypes, and list the possible paths. Using the path restrictions, pick the only path or common path combination that fits for this reaction. Notice how this analysis process turns what appears to be an open-ended problem into a multiple-choice problem. Write out all the steps of the mechanism. 

These reactions are very common (see Figs. 7.16 and 7.17 for the interrelationship of the available paths in acidic and in basic media). In basic media the attack of the nucleophile on a polarized multiple bond forms an anion that in the workup of the reaction is usually protonated (path combination Ad 2). In protic solvents if the nucleophilic attack forms a stronger base, a following irreversible proton transfer step may make the overall reaction favorable (see Figs. 4.43 and 4.44). 

Rearrangement commonly is found with paths Dn and Ae, which make up path combinations, SnL El, Ad 2. Figure 9.9 shows an energy surface map of the spectrum between the diagonal one-step and two-step rearrangement processes. 

Because radicals are in minute concentration, the usual radical mechanisms involve a radical colliding with an even electron molecule in a chain process. A common error is to have a termination step instead of creating a regenerating loop. The three common radical path combinations, Sh2, Adn2, and radical polymerization, all have propagation steps in which radicals collide with an even electron species, creating a new radical. 

A tee junction, shown in Fig. 13.73, is a coaxial component that provides for two RF signals to flow into a common path a star junction is a tee with more than two input paths. This basic coaxial component is one of the building blocks of a branched combiner. 

This chapter examines the overaii performance characteristics of compressors and turbines. This materiai is presented here to famiiiarize the reader with the behavior of these machines, ciassified under the broad term tur-bomaciiinery. Pumps and compressors are used to produce pressure turbines produce power. These machines have some common characteristics. The main eiement is a rotor with biades or vanes, and the path of the fluid in the rotor may be axiai, radiai, or a combination of both. 

According to these equations, the effect of selectively perturbing the spin states of spins i and j is to isolate the cross-relaxation paths common to these two spins. Combining Eqs. 15 and 19, the individual cross-relaxation terms are readily determined from single-selective and double-selective relaxation-rate measurements, that is. 

We can represent this function in the single space, provided we use a common cut line for all three components. This is shown schematically in Fig. 17. Use of the common cut line is equivalent to taking the linear combinations in the double space, then cutting a 27i-wide section out of the entire (4)). The winding numbers n of the Feynman paths that enter the three equivalent reagent channels must all be dehned with respect to the common cut line, since they are analogous to paths starting at different points in the initial state of a unimolecular reaction (Section 11.D). 

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