Elimination of a Proton

For steric reasons, (E)-alkenes (and transition states leading to ( )-alkenes) are usually lower in energy than (Z)-alkenes (and the transition state leading to them) because the substituents can get farther apart from one another. A reaction that can choose what it forms is therefore likely to favor the formation of ( )-alkenes. For alkenes formed by El elimination, this is exacdy what happens the less hindered ( )-alkene is favored. Carbocations can also lead to two other pathways, which do not yield to stable products but, instead, to other carbocations, that is, rearrangement and addition to an unsaturated linkage, for which the whole spectrum of reaction types is stiU open. 

---

This involves the formation of a carbenium ion which is best described as a hybrid of the two structures shown. This then rearranges by migration of a bond, and in so doing forms a more stable tertiary carbenium ion. Elimination of a proton yields camphene. 

The base [FeBr4] facilitates the elimination of a proton from the carbonium ion (I). 

Aromatic ethers and furans undergo alkoxylation by addition upon electrolysis in an alcohol containing a suitable electrolyte.Other compounds such as aromatic hydrocarbons, alkenes, A -alkyl amides, and ethers lead to alkoxylated products by substitution. Two mechanisms for these electrochemical alkoxylations are currently discussed. The first one consists of direct oxidation of the substrate to give the radical cation which reacts with the alcohol, followed by reoxidation of the intermediate radical and either alcoholysis or elimination of a proton to the final product. In the second mechanism the primary step is the oxidation of the alcoholate to give an alkoxyl radical which then reacts with the substrate, the consequent steps then being the same as above. The formation of quinone acetals in particular seems to proceed via the second mechanism. ... 

When the radicals have p hydrogens, alkenes are formed by a process in which carbocations are probably bypassed. Instead, the oxidation and the elimination of a proton probably occur in a single step through an alkylcopper species. The oxidation state of copper in such an intermediate is Cu(III). 

Benzene and substituted benzenes reaet with electrophiles, leading to new functionality. The two-step mechanism involves initial attack by an electrophile to form an intermediate (benzenium ion), followed by elimination of a proton to generate the substituted benzene. 

A meehanistie seheme that was proposed involves the sueeessive or synehronous elimination of a proton and a ehloride ion from the 5-methyl group and from the ehloroethynyl group attaehed to it, followed by addition of the nueleophile (NH3) to the intermediate bipolar earbene ion stabilized by eonjugation (Seheme 103). 

The next step is an attack by the carbocation on the benzene ring, followed by the elimination of a proton and the formation of a benzene alkylate ... 

Here the alcoholic hydroxyl is first protonated and then eliminated as water. The allylcarbenium ion (2) is initially stabilized by elimination of the proton at C-14. Then the ether link is opened after protonation of the ring oxygen with the formation of carbenium ion (3), whereby the neighboring C-C bond of the piperidine ring is cleaved with aromatization of the C ring. The carbenium ion (4) formed is stabilized by elimination of a proton and ring closure to apomorphine (5). 

A striking result of this reinvestigation (128, 129) is the observation that the ratio of the product ketone to the acetylene formed from a-bromo-p-aminostyrene is a function of the pH (Table Vll) but that the rate at which they are formed is not. As the pH increases from 3.9 to 13.1, the relative yield of acetylene increases from 16% to 85%. Therefore, the acetylene formation by elimination of a proton from the vinyl cation (path b in route D in Scheme XI) is more susceptible to an increase in base strength than is ketone formation via the enol (path a). This observation is a rare case of pH control over product composition in a 1-El reaction. 

Chlorination can be accompanied by other reactions that are indicative of carbocation intermediates. Branched alkenes can give products that are the result of elimination of a proton from a cationic intermediate.35... 

Normally, the dominant products are the alkene and acetate ester, which arise from the carbocation intermediate by, respectively, elimination of a proton and capture of an acetate ion.269... 

Thus reaction of the 1-propyl cation (13) with water (reaction type a) will yield propan-l-ol (14), elimination of a proton from (13) will yield propene (15, reaction type b), while rearrangement of (13, reaction type d)—in this case migration of He—will yield the 2-propyl cation... 

In most of the hitherto known cationic domino processes another cationic process follows, representing the category of the so-called homo-domino reactions. In the last step, the final carbocation is stabilized either by the elimination of a proton or by the addition of another nucleophile, furnishing the desired product. Nonetheless, a few intriguing examples have been revealed in which a succession... 

The study of several other model ethers derived from benzylic and allylic alcohols for which -elimination of a proton is possible confirmed that this reaction is general and occurs at low temperatures in the presence of strong acid. 

In this process the elimination of a proton results in the formation of an alkene. Thus the dehydration of alcohols gives rise to alkene in presence of cone. H2S04... 

When oligoisobutenes are formed from gaseous isobutene at ambient temperature by BF3 and H20 the initial group is CH3, formed by addition of a proton to the monomer [8]. The predominant terminal groups are double bonds [8] formed by transfer reactions involving elimination of a proton from the growing carbonium ion ... 

The anion presumably plays only a minor role, if any, especially in aqueous systems. Now the formation of 3-hexene may be explained in either of two ways. The intermediate carbonium ion, written in brackets, can undergo hydride migration to form a new carbonium ion, which can then collapse by proton loss to form the 3-hexene. Such a process does not require the intermediate formation of 2-hexene. The alternate explanation involves the discrete formation of 2-hexene followed by addition and elimination of a proton to give the desired 3-hexene. There is no question but that the hydride migration occurs, and with great... 

Worthy of note in this reaction is that citrate displays prochirality (see Section 3.4.7). The methylene carbons may be considered prochiral, in that enzymic elimination of a proton is likely to be entirely stereospecific. In addition, the apparently equivalent side-chains on the central carbon are also prochiral and going to be positioned quite differently on the enzyme. This means that only one of these side-chains is involved in the dehydration-rehydration... 

---