Identifying the Mechanism s

We mentioned that there are three main steps for predicting the products of substitution and elimination reactions. In the previous section, we explored the first step (determining the function of the reagent). In this section, we now explore the second step of the process in which we analyze the substrate and identify which mechanism(s) operates. 

As described in the previous section, there are four categories of reagents. For each category, we must explore the expected outcome with a primary, secondary, or tertiary substrate. All of the relevant information is summarized in the following flow chart. It is important to know this flow chart extremely well, but be careful not to memorize it. It is more important to understand the reasons for all of these outcomes. A proper understanding will prove to be far more useful on an exam than simply memorizing a set of rules. 

When the reageht fuhotiohs exclusively as a nucleophile (ahd hot as a base), ohiy substitutioh reactions occur (not elimination). The substrate determines which mechahism operates. 3 2 predominates for primary substrates, and 3 1 predominates for tertiary substrates. For secondary substrates, both 3 2 ahd 3 1 cah occur, although 3 2 is generally favored (especially when a polar aprotic solvent is used). 

When the reagent is both a strong nucleophile and a strong base bimolecular reactions (Sn2 and E2) are favored. 

For primary substrates, Sn2 predominates over E2, unless f-BuOK is used as the reagent, in which case E2 predominates. 

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EXERCISE 10.20 Identify the mechanism(s) expected to occur when 3-bromopentane is treated with sodium hydroxide ... 

PROBLEMS Identify the mechanism(s) expected to occur in each of the following cases. Do not worry about drawing the products yet. We wiU do that in the next section. For now, just identify which mechanisms are operating ... 

MOLECULAR STRUCTURE. In their attempts to identify the mechanism(s) of action of tablet disintegrants, researchers recently have turned their... 

Understanding flux motions and especially identifying the mechanism(s) for pinning at present remain a challenging opportunity. Measurements of a dissipation peak at near 45 K (well below Tc) in Bi-Sr-Ca-Cu-O samples oscillating in a magnetic field and interpreted... 

Many studies have been made to identify the mechanism(s) by which HALS provide such effective UV stabilisation, and these have been reviewed [149-151]. Most authors agree that the >NH moiety is not, in fact, the stabilising species, but that >NO stable radicals and >NOR hydroxylamine ethers formed during photo-oxidation are the true antioxidant species. These are constantly regenerated the nitroxyl scavenging alkyl radicals and the hydroxylamine ethers intercepting hydroperoxy radicals and hydroperoxides. Hydroxylamines may also be formed, which are efficient photostabilisers in their own right. 

Reactions with m-CPBA and f-BuOOH. In an attempt to characterize and identify the mechanism(s) by which these various sulfur-containing heterocylic compounds behave as antioxidants, 2-butylthiophene and 1,3-dithiolane were reacted with the electrophilic... 

As always, a meticulously recorded and studied history is desirable, but additional clinical and laboratory tests can contribute greatly to an accurate diagnosis and help to identify the mechanism(s) of the reaction. Skin tests, detection, and quantitation of released mediators and serum IgE antibodies may all contribute to establishing a more precise diagnosis of an adverse response to an iodinated contrast medium. 

To identify the mechanism(s) responsible for loss of surface-tethered radicals in SI-PMP, several irreversible termination reactions that can result into the permanent loss of surface-tethered radicals, including (a) bimolecular termination, (b) chain transfer to monomer, (c) chain transfer to dithiocarbamyl radical, (d) chain transfer to an adjacent polymer chain, and (e) chain transfer to solvent, were considered by Rahane et al. in their development of a kinetic model to describe SI-IMP [81]. The decrease in the concentration of surface-tethered radicals by chain transfer to monomer and by bimolecular termination reactions is captured by Equation 12.2 ... 

IDENTIFY THE MECHANISM(S) THAT OPERA TE IN EACH OF THE CASES BELOW A 1... 

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