Unimolecular 1,2-elimination reactions

In the previous section, we looked at the bimolecular 1,2-elimination reaction. We will now look at the unimolecular 1,2-elimination reaction, which is given the label El. This reaction bears many similarities to the SN1 reaction that we studied in an earlier chapter. 

This reaction occurs in two steps, the first of which is the rate limiting loss of the leaving group to give the carbonium ion. This step is unimolecular, and so the concentration of the other reagent is not involved in the rate equation. The second step involves the attack by the incoming nucleophile to result in overall substitution. 

instead of the addition of an incoming nucleophile in the second step, the (5-hydrogen is lost, then the overall result would be a unimolecular 1,2-elimination reaction. Write the first ionisation step, and then involve the elimination step in the second stage. 

This is the mechanism for the El elimination reaction. The similarities with the SN1 reaction are plain. Like the E2 and SN2 reactions that often 

The first step is exactly the same in the SN1 and El mechanisms, and thus those considerations that applied to the first step of the SN1 mechanism apply equally to the first step of the El mechanism. These factors include the stability of the carbonium ion intermediate, the loss of stereochemistry at that centre if no other factors intervene, and the possible rearrangement of the carbonium ion intermediate. 

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El reaction (Section 11.10) A unimolecular elimination reaction in which the substrate spontaneously dissociates to give a carbocation intermediate, tvhich loses a proton in a separate step. 

It is possible to take advantage of the differing characteristics of the periphery and the interior to promote chemical reactions. For example, a dendrimer having a non-polar aliphatic periphery with highly polar inner branches can be used to catalyse unimolecular elimination reactions in tertiary alkyl halides in a non-polar aliphatic solvent. This works because the alkyl halide has some polarity, so become relatively concentrated within the polar branches of the dendrimer. This polar medium favours the formation of polar transition states and intermediates, and allows some free alkene to be formed. This, being nonpolar, is expelled from the polar region, and moves out of the dendrimer and into the non-polar solvent. This is a highly efficient process, and the elimination reaction can be driven to completion with only 0.01 % by mass of a dendrimer in the reaction mixture in the presence of an auxiliary base such as potassium carbonate. 

The first step (loss of the leaving group) is the rate-determining step, much Uke we saw for SnI processes. The base does not participate in this step, and therefore, the concentration of the base does not affect the rate. Because this step involves only one chemical entity, it is said to be uiumolecular. Unimolecular elimination reactions are called El reactions, where the 1 stands for unimolecular. ... 

In competition with these oxidation reactions are two unimolecular elimination-reactions which eventually lead to 64 and 65 (Table V, reactions 128 to 133). Product 65 may also have the radical at C-3, namely, 66, as a precursor (reaction 134). As expected, the precursor radicals 62 and 66 are rapidly oxidized in the presence of Fe3+ ions, and the formation of 65 and 65 is suppressed (see Table V). 

Preliminary results on the enantioselective formation of sulfur and nitrogen mediumsized heterocycles by base-induced ring opening of hetero-oxabicyclic [3.2.1] and [3.3.1] systems have been reported.91 The reaction involves a deprotonation-C—O bond elimination sequence. The kinetics and mechanism of gas-phase unimolecular elimination reactions of some substituted aminoazoles have been studied as an aid to heterocycle synthesis.92... 

An ab initio study on the unimolecular elimination reactions of methacrylonitrile has revealed a direct four-centre elimination of HCN and three-centre elimination of H2 channels.17 A methylcyanoethylidene intermediate has also been identified. 

The gas-phase unimolecular elimination reactions of 2-substituted ethyl N,N-dimethylcarbamates23,24 and several heterocyclic carbamates25 have been studied using the Moller-Plesset MP2/6-31G method. On the basis of these calculations, the mechanism appears to be concerted, asynchronous, through a six-membered cyclic transition-state structure. 

El reaction or unimolecular elimination reaction (Section 9.5) An elimination reaction that occurs in two steps through a carbocation intermediate. 

Recently measurements on chemical lasers [282-285] and of infrared chemiluminescence [286] have provided information about the partitioning of energy in unimolecular elimination reactions, following production of... 

C2H5Br -> C2H4 + HBr (unimolecular elimination reaction) CjHsBr-l-HBr - C2H6-t-Br2 (wall reaction)... 

But if an alkyl halide can form a stable carbocation, the unimolecular elimination reaction is faster. It s referred to as unimolecular because the slow step has only one molecule in the transition state, just like substitution reactions. 

Elimination reactions to produce alkenes may compete in reactions in which nucleophilic aliphatic substitution is the desired process. Unimolecular elimination reactions, El, compete with substitutions, and bimolecular elimination processes, E2 (E stands for elimination and 2 for bimolecular), compete with S[ j2 transformations. These competitions are shown in Equations 14.6 and 14.7. The nature of El reactions is discussed in detail in Section 10.3 and that of E2 processes in Section 10.2. 

El Reaction (Section 7.8) The unimolecular elimination reaction. The ionization of the starting material is followed by the loss of a proton to base. The product has a new it bond. 

There are several thermal elimination reactions that find use in synthesis. Some of these are concerted processes. The transition state energy requirements and stereochemistry of concerted elimination processes can be analyzed in terms of orbital symmetry considerations. We will also consider an important group of unimolecular -elimination reactions in Section 6.8.3. 

Self-Assessment Exercises 79a. Nucleophilic substitution corresponds to a substitution (either Sxjl or S[.j2) for aliphatic compounds. Electrophilic aromatic substitution is typical for aromatic compounds (an atom is replaced by an electrophile) 79b. An addition reaction is the opposite of an elimination reaction. In an addition reaction, two or more atoms (molecules) combine to form a larger one. 79c. S[,jl reaction involves the formation of carbocation. Sf.j2 reaction, on the other hand, is a one-step process in which bond breaking and bond making occur simultaneously at a carbon atom with a suitable leaving group. 79d. El reactions are unimolecular elimination reactions that proceed via carbocation intermediates. E2 reactions are bimolecular, one-step reactions that require an antiperiplanar conformation at the time of rr-bond formation and /3-bond breaking and do not involve carbocations. 

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