Beta carbon atom 3 eliminations

Athene formation requires that X and Y be substituents on adjacent carbon atoms By mak mg X the reference atom and identifying the carbon attached to it as the a carbon we see that atom Y is a substituent on the p carbon Carbons succeedmgly more remote from the reference atom are designated 7 8 and so on Only p elimination reactions will be dis cussed m this chapter [Beta (p) elimination reactions are also known as i 2 eliminations ] You are already familiar with one type of p elimination having seen m Section 5 1 that ethylene and propene are prepared on an industrial scale by the high temperature dehydrogenation of ethane and propane Both reactions involve (3 elimination of H2... 

This dehydrohalogenation is called an alpha elimination (a elimination) because the hydrogen and the halogen are lost from the same carbon atom. The more common dehy-drohalogenations (to form alkenes) are called beta eliminations because the hydrogen and the halogen are lost from adjacent carbon atoms. 

Synthetic esters like polyol esters or neopentyl polyol esters are made from monobasic fatty acids and polyhedric alcohols with a neopentyl structure (see Figure 7.6). On these molecules, there are no hydrogen atoms on the beta carbon. This carbon is where thermal attack occurs on diesters and eliminating it improves the thermal stability of the molecule. Polyol esters have an increased number of ester groups versus diesters. This feature increases polarity, which will affect the lubricity of the oil at elevated temperatures and give it an advantage over PAOs. 

In the previous section, three general mechanisms of olefin formation by beta-elimination of the elements HX from adjacent carbon atoms, were outlined. Whereas the El and ElcB mechanisms involve preliminary breaking of one bond, the E2 process is concerted, both the C -H and C -X bonds being partially broken at the transition state. When initially designating... 

The above conclusion must be remembered when considering carbon isotope effects for elimination reactions. Certainly the alpha- and beta-carbon isotope effects for elimination from propyl trimethylammonium ion indicate a different extent of rehybridisation at the two carbon atoms in the transition state, but a more definite conclusion requires additional kinetic evidence. 

Elimination reactions which give rise to multiple bonds between carbon and a heteroatom occur with particular facility and show many of the characteristics of olefin-forming processes. Most often one of the eliminating fragments is a hydrogen atom, which is easily removed from a heteroatom and the choice of mechanism, between an anion or a concerted process, will depend on the lability of the beta carbon-X bond, e.g. 

Chemists often call the carbon atom that bears the leaving group (e.g., the halogen atom in the previous reaction) the alpha (a) carbon atom and any carbon atom adjacent to it a beta (j8) carbon atom. A hydrogen atom attached to the /3 carbon atom is called a j8 hydrogen atom. Since the hydrogen atom that is eliminated in dehydrohalogenation is from the j8 carbon atom, these reactions are often called fi eliminations. They are also often referred to as 1,2 eliminations. 

Beta-elimination reactions have been observed in a number of proteins. This reaction occurs primarily at alkaline pH conditions. Abstraction of the hydrogen atom from the alpha-carbon of a cysteine, serine, threonine, phenylalanine, or lysine residue leads to racemization or loss of part of the side chain and the formation of dehydroalanine (26). 

The activating effects of the halogen atoms on the beta-hydrogen coupled with the reluctance of fluorine to depart as an anion from saturated carbon, especially in the presence of other alpha-fluorine atoms - , make the intermediacy of carbanions in these eliminations highly probable. Unfortunately the olefinic product adds alcohol too rapidly to be isolated and this necessitated careful considerations of alternative mechanisms. A minor fraction of the decomposition may follow an alpha-elimination, viz. 

Hydride elimination reactions are characterized by hydrogen atom transfer from a ligand to a metal. The most common type of hydride elimination is /3 elimination, with a proton in a j8 position on an aUcyl ligand transferred to the metal by way of an intermediate in which the metal, the a and /3 carbons, and the hydride are coplanar. An example of j8 elimination—the reverse of 1,2-insertion—is in Figure 14.15. Beta eliminations are important in many catalytic processes. 

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