Mechanisms El and

You need to start by drawing the mechanisms of the El and E2 reactions. Look in your class notes and in your textbook and then draw the mechanisms here  

PROBLEM 10.2 Explain why the number 2 is used in the E2 mechanism. You will find that the reason is the same as the reason given for the 2 in Sn2. 

We need to do the same analysis that we did for substitution reactions. So before we go any further, I recommend that you review Section 9.1 in the previous chapter. Do that now and then come back to here. 

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Four types of mechanisms are inherent to Organic Chemistry 1. These are substitution reaction mechanisms (S l and Sf 2) and elimination reaction mechanisms (El and E2). The principles of these four types apply to Organic Chemistry 11, and no review would be complete without a few reminders about these processes. 

Example dehydrogenation, dehydrohalogenation, dehydration Two possible mechanisms El and E2 Slow Step Fast Step... 

As with substitution, we have two mechanisms that are relatively common and one that is much rarer. The two common mechanisms, El and E2, have strong similarities to the S l and 5 2 processes described in the previous chapter and are generally favored by similar conditions to these analogues. The third mechanism, ElcB, is rather different, with no direct analogy in substitution chemistry. We will initially exemplify all the reactions by removal of hydrogen halides, HX, in the presence of base and then explore the scope of the reactions. 

One of the first successful techniques for selectively removing solvent from a solution without losing the dissolved solute was to add the solution dropwise to a moving continuous belt. The drops of solution on the belt were heated sufficiently to evaporate the solvent, and the residual solute on the belt was carried into a normal El (electron ionization) or Cl (chemical ionization) ion source, where it was heated more strongly so that it in turn volatilized and could be ionized. However, the moving-belt system had some mechanical problems and could be temperamental. The more recent, less-mechanical inlets such as electrospray have displaced it. The electrospray inlet should be compared with the atmospheric-pressure chemical ionization (APCI) inlet, which is described in Chapter 9. 

We will discuss shortly the most important structure-reactivity features of the E2, El, and Elcb mechanisms. The variable transition state theoiy allows discussion of reactions proceeding through transition states of intermediate character in terms of the limiting mechanistic types. The most important structural features to be considered in such a discussion are (1) the nature of the leaving group, (2) the nature of the base, (3) electronic and steric effects of substituents in the reactant molecule, and (4) solvent effects. 

These reactions are often promoted by a strong base, which assists the departure of the proton. X is the leaving group. Both El and E2 mechanisms are known, as is a variant designated Elcb, for unimolecular elimination from the conjugate base of the substrate. ... 

All three elimination reactions--E2, El, and ElcB—occur in biological pathways, but the ElcB mechanism is particularly common. The substrate is usually an alcohol, and the H atom removed is usually adjacent to a carbonyl group, just as in laboratory reactions. Thus, 3-hydroxy carbonyl compounds are frequently converted to unsaturated carbonyl compounds by elimination reactions. A typical example occurs during the biosynthesis of fats when a 3-hydroxybutyryl thioester is dehydrated to the corresponding unsaturated (crotonyl) thioester. The base in this reaction is a histidine amino acid in the enzyme, and loss of the OH group is assisted by simultaneous protonation. 

The El reactions can involve ion pairs, just as is true for S l reactions (p. 398), This effect is naturally greatest for nondissociating solvents it is least in water, greater in ethanol, and greater still in acetic acid. It has been proposed that the ion-pair mechanism (p. 400) extends to elimination reactions too, and that the S l, Sn2, El, and E2 mechanisms possess in common an ion-pair intermediate, at least occasionally. ... 

Effect of Solvent on El versus E2 versus ElcB. With any reaction a more polar environment enhances the rate of mechanisms that involve ionic intermediates. For neutral leaving groups, it is expected that El and ElcB... 

In a substitution reaction, the leaving group is replaced with a nucleophile. In an elimination reaction, a beta ((3) proton is removed together with the leaving group, forming a double bond. In the previous chapter, we saw two mechanisms for substitution reactions (SnI and Sn2). In a similar way, we will now explore two mechanisms for elimination reactions, called El and E2. Let s begin with the E2 mechanism. 

In a case like this, E2 wins the competition, and no other mechanisms can snccessfnlly compete. Why not An Sn2 process cannot occnr at a reasonable rate becanse the snbstrate is tertiary (steric hindrance prevents an Sn2 from occnrring). And nnimolecnlar processes (El and SnI) cannot compete becanse they are too slow. Recall that the rate-determining step for an El or SnI process is the loss of a leaving gronp to form a carbocation, which is a slow step. Therefore, El and SnI conld only win the competition if the competing E2 process is extremely slow (when a weak base is nsed). However, when a strong base is nsed, E2 occnrs rapidly, so El and SnI cannot compete. 

In the case of 1,2,4-trithiolanes, both els and trans isomers are obtained with a different reaction mechanism involving a molecule of thioketone and a molecule of thiosulfine.50... 

Figure 6. Uptake of various sugars via the phosphoenolpyruvate group translocator (PTS) mechanism. Please note that all systems shown share the common components El and HPr. For details see the text...

Other terms that he invented include the system of classification for mechanisms of aromatic and aliphatic substitution and elimination reactions, designated SN1, SN2, El, and E2. "S" and "E" refer to substitution and elimination, respectively, "N" to nucleophilic, and "1" and "2" to "molecularity," or the number of molecules involved in a reaction step (not kinetic order, having to do with the equation for reaction rate and the concentration of reactants). Ingold first introduced some of these ideas in 1928 in a... 

Three different mechanisms for the kinetics of Ei can be used to construct three different logic gates AND, OR, and XOR. The degree of cooperativity in the binding of El and R or h determines the steepness of the transition from low to high steady-state concentrations of B. 

To be added in cases of indications for relevance and bioavailability In vivo bioassays with relevant exposure conditions, routes and endpoints Prolonged ELS and metamorphosis In vivo bioassays with sediment extracts In vitro bioassays specific mechanisms Internal effect levels Specific chemicals (e.g. PFACs, HBCDs, PBDEs) ELS not longer than free feeding Chemicals that only are toxic in high concentration (narcotics, nanoparticles) Chemical analysis of lipophillic POPs in water... 

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