Predictions elimination reactions

J.P. Guthrie, Concertedness and E2 Elimination Reactions Prediction of Transition State Position using Two-Dimensional Reaction Surfaces based on Quadratic and Quartic Approximations, Can. J. Chem., 1990, 68, 1643. 

The least sterically hindered p hydrogen is removed by the base m Hofmann elim matron reactions Methyl groups are deprotonated m preference to methylene groups and methylene groups are deprotonated m preference to methmes The regioselectivity of Hofmann elimination is opposite to that predicted by the Zaitsev rule (Section 5 10) Elimination reactions of alkyltrimethylammonmm hydroxides are said to obey the Hofmann rule, they yield the less substituted alkene... 

The amino functional group is not commonly encountered in steroid synthesis except perhaps in steroidal alkaloids. However, certain elimination reactions have been shown to have theoretical and limited preparative importance, largely due to the efforts of McKenna and co-workers. The Hofmann rule for 2 elimination predicts that alkaline elimination of quaternary ammonium salts will occur towards the carbon carrying the most hydrogen atoms cf. the converse Saytzeff orientation, above). In cyclohexyl systems, the requirement for diaxial elimination appears to be important, as in other 2 eliminations, and the Hofmann rule frequently is not obeyed [e.g., (116) (117)]. 

A complication of such reactions is competition from elimination reactions rather than substitution (see Section 18.5). (a) Predict the possible products from the reaction of 2-bromopentane with sodium hydroxide, (b) What can be done to favor the... 

In the previous chapter, we saw that a substitution reaction can occur when a compound possesses a leaving group. In this chapter, we will explore another type of reaction, called elimination, which can also occur for compounds with leaving groups. In fact, substitution and elimination reactions frequently compete with each other, giving a mixture of products. At the end of this chapter, we will learn how to predict the products of these competing reactions. For now, let s consider the different outcomes for substitution and elimination reactions ... 

Substitution and elimination reactions are almost always in competition with each other. In order to predict the products of a reaction, you must determine which mechanism(s) win the competition. In some cases, there is one clear winner. For example, consider a case in which a tertiary alkyl halide is treated with a strong base, such as hydroxide ... 

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. 

We mentioned that predicting the products of substitution and elimination reactions requires three discrete steps ... 

Theoretical predictions, based on AMI MO theory, for gas-phase elimination reactions of 3-chloropropanoic and 2-chlorobutanoic acids are consistent with experimental results four-, five-, and six-membered transition states have been discussed. ... 

Density functional theory and a high-level cib initio procedure (G2+) have been used to explore the potential energy surface for the base-induced elimination reaction of fluoride ion with ethyl fluoride.11 The DFT barriers are smaller and looser than those predicted by the ab initio method but the nature of the transition state cannot be defined with confidence since the predictions are unusually sensitive to the choice of functional and basis set. The results suggest that improvement in density functional methods will require fundamental change in the functionals themselves. 

Predicting products can be challenging when you have to consider substitution and elimination reactions simultaneously. So far, we have seen substitution and elimination reactions separately. But now the truth comes out—substitution and elimination reactions are generally in competition with each other. To predict the products properly, you need to compare all factors for substitution and elimination reactions, and you then need to decide which of the four mechanisms predominates (S l or Sn2 or El or E2). 

Is this conclusion predictable Yes. The thionoester is RC=S perturbed by OR whereas the dithioester is RC=S perturbed by SR. The CO bond being stronger than the CS bond (85.5 versus 65.5 kcal mol ), the jt cs orbital is raised more by OR than by SR. Moreover, reactions with esters being addition-elimination reactions, the weakness of the CS bond facilitates the elimination stage. Always verify that your calculated results make sense chemically. 

The first step is to convert the Fischer projection formulae into sawhorse projections. Note that for a base-induced elimination reaction of the E2 type, the tosyl group must be antiperiplanar to the proton or deutron being abstracted from the neighbouring carbon atom. As a consequence of this reaction mechanism, the configuration of the resulting double bond can be predicted. Because of the higher steric hindrance in the transition states leading to the Z isomers, in both cases the E isomer should be expected as the major product. 

Metastable loss of H2 is only observed for the chloro and bromo compounds. For X = F, the exothermicity of the protonation-dissociation sequence (7 kcalmol-1) is probably not sufficient to exceed the energy barrier for H2 loss, which for the 1,2-H2 elimination is predicted by theoretical calculations to be 18 kcalmol1 (thermochemical data for a 1,1 -H2 elimination are not available)106. For X = I, the protonation-dissociation reaction is exothermic by 41 kcal mol-1 and the absence of a metastable peak for this process is probably due to a very high activation barrier. The existence of an energy barrier for the H2 (HD) elimination from protonated chloro- and bromohalomethane, and their deuterated isotopomers was indeed revealed by the large KER values (140 eV) associated with their metastable decomposition. An intensity ratio of about 50 was measured for metastable peaks due to H2 and HX losses in accordance with the known thermochemical data which establish that H2 loss is less endothermic than HX loss by 10 kcalmol-1 and 28 kcalmol-1 for X = Cl and Br, respectively109. 

These results, obtained on a FT-ICR mass spectrometer, led to the proposal that these reactions proceed through long-lived ion-molecule collision complexes which can undergo secondary reactions within the complex. The mechanism, sketched in Scheme 41, predicts the formation of products originating from attack of F on the neutral by proton transfer, SN2 or elimination reactions. 

Predicting the Major Reaction Pathway of Substitution and Elimination Reactions... 

As can be seen in Figure 9.5, neomenthyl chloride has two hydrogens in an anti-periplanar geometry with the chlorine. Either of these hydrogens can be lost in the elimination reaction, resulting in the formation of two alkenes. (All of the examples presented up to this point were chosen so that only one alkene could be formed.) Let s now address this issue of the direction of the elimination and learn how to predict which will be the major product when more than one alkene can be formed. 

It is possible to predict which product will be formed in larger amounts in these reactions. Most E2 elimination reactions follow Zaitsev s rule ... 

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