Pyrolytic elimination reactions

Enby 6 is an example of a stereospecific elimination reaction of an alkyl halide in which the transition state requires die proton and bromide ion that are lost to be in an anti orientation with respect to each odier. The diastereomeric threo- and e/ytAra-l-bromo-1,2-diphenyl-propanes undergo )3-elimination to produce stereoisomeric products. Enby 7 is an example of a pyrolytic elimination requiring a syn orientation of die proton that is removed and the nitrogen atom of the amine oxide group. The elimination proceeds through a cyclic transition state in which the proton is transferred to die oxygen of die amine oxide group. 

Elimination reactions of fluorine compounds are not limited to the removal of simple molecules Frequently, large molecules or combination of smaller ones are formed as by-products, especially in pyrolytic reactions For example perhalo genated acid chlorides lose not only carbon monoxide but also chlorine fluoride [106, 107] (equations 74 and 75)... 

A third category of syn eliminations involves pyrolytic decomposition of esters with elimination of a carboxylic acid. The pyrolysis of acetate esters normally requires temperatures above 400° C and is usually a vapor phase reaction. In the laboratory this is done by using a glass tube in the heating zone of a small furnace. The vapors of the reactant are swept through the hot chamber by an inert gas and into a cold trap. Similar reactions occur with esters derived from long-chain acids. If the boiling point of the ester is above the decomposition temperature, the reaction can be carried out in the liquid phase, with distillation of the pyrolysis product. 

A third category of syn eliminations involves pyrolytic decomposition of esters with elimination of a carboxylic acid. The pyrolysis of acetate esters normally requires temperatures above 400° C. The pyrolysis is usually a vapor-phase reaction. In the... 

Reactivities and activation parameters for pyrolytic unimolecular first-order elimination reactions of A -acetylurea, A -acetylthiourca, /V,7V -diacetylthiourea and N-acetylthiobenzamide have been interpreted with reference to those for other amide derivatives.55 The first-order rate constants for pyrolysis of RCONHCSNHCe R (R = Me, R = H R = Ph, R = H, 4-N02, 3-C1, 4-C1, 4-Me) have also been measured at 423-500 K and correlated with Hammett [Pg.378]

The term stereoselective is often confused with the term stereospecific, and the literature abounds with views as to the most satisfactory definition. To offer some clarification, it is perhaps timely to recall a frequently used term, introduced a decade or so ago, namely the stereoelectronic requirements of a reaction. All concerted reactions (i.e. those taking place in a synchronised process of bond breaking and bond forming) are considered to have precise spatial requirements with regard to the orientation of the reactant and reagent. Common examples are SN2 displacement reactions (e.g. Section 5.10.4, p. 659), E2 anti) elimination reactions of alkyl halides (e.g. Section 5.2.1, p.488), syn (pyrolytic) elimination reactions (Section 5.2.1, p.489), trans and cis additions to alkenes (e.g. Section 5.4.5, p. 547), and many rearrangement reactions. In the case of chiral or geometric reactants, the stereoisomeric nature of the product is entirely dependent on the unique stereoelectronic requirement of the reaction such reactions are stereospecific. 

Many convenient methods for the introduction of carbon-carbon double bonds into a saturated carbon chain involve the removal of two atoms or groups from adjacent carbon atoms. Usually, but not invariably, one of these groups is hydrogen (i.e. the removal of HX). Two main types of elimination reactions are recognised - heterolytic processes in solution and pyrolytic reactions in the gas phase. A detailed discussion of the mechanisms of these reactions may be found in all standard and advanced textbooks in each of the reactions discussed below the probable mechanism is noted in relation to the aim of obtaining good yields of regio- or stereoisomerically pure compounds. 

In this case the Zs/Z-selectivity does not matter because a terminal double bond is generated. In principle the Zs/Z-selectivity depends on the stability of the ylide employed (see chapter 9). The most valuable feature of the Wittig procedure is that, in contrast to elimination and pyrolytic reactions, it gives rise to alkenes with unambiguous position of the double bond. 

Example 3.11. An Ei reaction Pyrolytic elimination from a sulfoxide. 

The chain scission can be seen as a pyrolytic elimination reaction. All mechanisms described in Section 2.2 may take place during chain scission. A reaction of chain scission with a cyclic transition state may take place, for example, during cellulose pyrolysis ... 

In an elimination reaction, fragments of a molecule are removed and form a new molecule. Elimination involving a free radical mechanism operates in many pyrolytic reactions. First, an initiation occurs by pyrolytic cleavage, followed by... 

Banthorpe, D. V., Davies, H. f. S. Elimination reactions. II. Pyrolytic and base-promoted decompositions of thujyl compounds. J. Chem. 

This is an example of a five-membered ring version of the Ei mechanism. Moreover, it is an example of a synthetic reaction that proceeds via two different pathways depending upon the exact conditions. These pyrolytic elimination reactions are valuable synthetically, because there is normally no opportunity for the starting material to rearrange, which is always a possibility when the elimination occurs via the El route, because in that pathway the reaction proceeds via a carbonium ion intermediate. 

The other type of pyrolytic elimination reaction involves free radicals. The steps are similar to those that we studied in the free radical substitution reactions, i.e. there is an initiation step, followed by several propagation steps, and then there are some termination steps. Free radical elimination is found in polyhalides and primary monohalides. For the general primary monohalide, R2CHCH2X,... 

The other type of pyrolytic elimination reaction involves free radicals, for example in polyhalides and primary monohalides. These reactions involve the normal sequence for radical reactions, namely initiation, propagation and termination steps. 

Pyrolytic An elimination reaction that occurs on heating, requiring no other reagent. 

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