Proton removal weak base

In the El mechanism, the leaving group has completely ionized before C—H bond breaking occurs. The direction of the elimination therefore depends on the structure of the carbocation and the identity of the base involved in the proton transfer that follows C—X heterolysis. Because of the relatively high energy of the carbocation intermediate, quite weak bases can effect proton removal. The solvent m often serve this function. The counterion formed in the ionization step may also act as the proton acceptor ... 

Protonation, if forced upon pyrrole, is found to take place not on nitrogen but on the a-carbon atom (19). This occurs because incorporation of the nitrogen atom s lone pair of electrons into the aromatic 6jre system leaves the N atom positively polarised protons tend to be repelled by it, and are thus taken up by the adjacent a-carbon atom. The basicity situation rather resembles that already encountered with aniline (p. 70) in that the cation (19) is destabilised with respect to the neutral molecule (18a). The effect is much more pronounced with pyrrole, however, for to function as a base it has to lose all aromatic character, and consequent stabilisation this is reflected in its related pKa (-0-27) compared with aniline s of 4-62, i.e. pyrrole is a very weak base indeed. It can in fact function as an acid, albeit a very weak one, in that the H atom of the NH group may be removed by strong bases, e.g. eNH2 the resultant anion (20) then retains the aromatic character of pyrrole, unlike the cation (19) ... 

As a result, inert and aprotic solvent toluene is suitable for the titration of weak bases in non-aqueous media as solvent, although benzene which is more carcino-genic aromatic hydrocarbon used widely in literature for non-aqueous titrations. The major advantage of toluene is tliat it does not compete for protons with the reactant in the titrations because of its autoprotolysis constant approaching zero. The major disadvantages of solubility can be removed by using small amount of amphiprotic solvents. 

The proton in the thiazolium ring is relatively acidic (p Ta about 18) and can be removed by even weak bases to generate the carbanion or ylid an ylid is a species with positive and negative charges on adjacent atoms. This ylid is an ammonium ylid with extra stabilization provided by the sulfur atom. 

Perchloric acid is the strongest of the common acids in acetic acid solution and the titration medium usually used for non-aqueous titration of bases is perchloric acid in acetic acid. Addition of acetic anhydride, which hydrolyses to acetic acid, is used to remove water from aqueous perchloric acid. Weak bases compete very effectively with acetic acid for protons. Oracet blue, quinalidine red and crystal violet (very weak bases) are used as indicators in this type of titration. A typical analysis is shown in Figure 3.8 for LDOPA. 

Palladium-catalyzed allylic oxidations, in contrast, are synthetically useful reactions. Palladium compounds are known to give rise to carbonyl compounds or products of vinylic oxidation via nucleophilic attack on a palladium alkene complex followed by p-hydride elimination (Scheme 9.16, path a see also Section 9.2.4). Allylic oxidation, however, can be expected if C—H bond cleavage precedes nucleophilic attack 694 A poorly coordinating weak base, for instance, may remove a proton, allowing the formation of a palladium rr-allyl complex intermediate (89, path by694-696 Under such conditions, oxidative allylic substitution can compete... 

The difference between the most kinetically favoured cyclisations is easily seen in the type of base needed to cyclise chloro-alcohols to three- and five-membered cyclic ethers (epoxides and THFs). Chloroethanol 19 cyclises only as the oxyanion specific base is needed, i.e. a strong enough base to remove the OH proton completely. By contrast, 4-chlorobutanol cyclises by general base catalysis the proton is removed during the cyclisation 22 and weak bases will do.1, 2... 

Ethoxide ion, a strong base, removes a proton from acetic acid. The formation of the weak base, acetate ion, in this step drives the equilibrium to the final products, the alcohol and the carboxylate anion. 

If to a mixture of a weak acid and its conjugate base one adds protons (e.g., HC1), the conjugate base (the acetate ion) will combine with the protons to form undissociated acetic acid to maintain the Kd at the appropriate level [Equation (3.5)]. The proton concentration and the pH will remain relatively stable. Likewise, if to the mixture of a weak acid and its conjugate base one added OH- ions (e.g., NaOH), the OH" will combine with the protons present to form the inert H20. To maintain the Kd [Equation (3.5)], acetic acid will dissociate to replenish the protons removed, again resulting in reestablishment of nearly the original pH. 

Hints. First draw good diagrams of the reagents. NaHC03 is a salt and a weak base—strong enough only to remove which proton Then work out which bonds are formed and which broken, decide whether to push or pull, and draw the arrows. What are the other products ... 

Nitroalkanes form enolate-like anions in quite weak base. As in base-catalysed enolization, a proton is removed from a carbon atom and a stable oxyanion is formed. 

We have not usually bothered with the base that removes the proton from the intermediate. Here it is chloride ion as the by-product is HCl, so you can see that even a very weak base will do. Anything, such as water, chloride, or other counterions of strong acids, will do this job well enough and you need not in general be concerned with the exact agent. 

So how do we find GAC or GBC Normally, general species catalysis is a weak addition to specific catalysis. We must remove that more powerful style of catalysis by working at a specific pH because SAC or SBC depends on pH alone. If we find that the rate of the reaction changes with the concentration of a weak base at constant pH, we have GBC. Note that, if the proton transfer is between heteroatoms, as in this example, some other bond-making or bond-breaking steps must be happening too as proton transfer between heteroatoms is always a fast process. Proton transfer to or from carbon can be slow. 

Silyl enol ether formation again results from silylation of carbonyl oxygen but this time no alcohol is added and a weak base, usually a tertiary amine, helps to remove the proton after silylation. 

The key part of the molecule for reactivity is the thiazolium salt in the middle. The proton between the N and S atoms can be removed by quite weak bases to form an ylid. You saw sulfonium ylids in Chapter 46, and there is some resemblance here, but this ylid is an ammonium ylid with extra stabilization from the sulfur atom. The anion is in an sp2 orbital, and it adds to the reactive carhonyl group ofpyruvate. 

Ammonia does not contain hydroxide ions, so it is not an Arrhenius base. As you can see, however, an ammonia molecule can remove a proton from water, leaving a hydroxide ion behind. Thus, ammonia is a Bronsted-Lowry weak base. 

In the extreme case of an acid-catalysed dehydration, the polar solvent fulfils the role of a very weak base in removing a proton from the carbonium ion intermediate (Ei process [z o]). The sensitivity of the product-forming transition state to stabihsation by alkyl groups, destabilisation by electronegative substituents, and developing jr-overlap of orbitals, is at a maximum, so that the product is the thermodynamically stable olefin. In the case of a mixture of olefins, this will be close to the equilibrium composition. Clear instances of thermodynamic control are seen in the formation under acidic conditions of 3-methyl-5a-cholest-2-ene (16) [8ya] and the 6-methyl-AS-olefin (18) [122] from the corresponding C<3)-... 

Table 19.3 lists common bases that can be used to deprotonate carboxylic acids. It is noteworthy that even a weak base like NaHC03 is strong enough to remove a proton from RCOOH. 

In the first step, the weak base water acts as a nucleophile. In the second step, the weak base chloride ion is shown removing a proton. This second step also could have been written with water acting as the base. Notice that in this example most of the lone pairs have been omitted from the Lewis structures. Reactions in the chemical literature often are written in this way. 

When writing mechanisms in acid and base, keep in mind that protons are removed by bases. Even very weak bases like HS04 , the conjugate base of sulfuric acid, can remove protons. Protons do not just leave a substrate as FT because the bare proton is very unstable Nonetheless the designation —is often used when a proton is removed from a molecule. (Whether this designation... 

Chloride ion generally is a moderate nucleophile and a weak base. However, in the following dehydrohalogenation reaction, chloride ion functions as a base, removing a proton to bring about elimination of the elements of HCl. 

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