Alkenes in elimination reactions

Stereoselectivity was defined and introduced in connec tion with the formation of stereoisomeric alkenes in elimination reactions (Sec tion 5 11)... 

Except for terpene chemistry, the Wagner-Meerwein rearrangement is of limited synthetic importance. It is rather found as an undesired side-reaction with other reactions, for example in the synthesis of alkenes by elimination reactions. 

Practically everything we ve said in previous chapters has been stated without any proof. We said in Section 6.8, for instance, that Markovnikov s rule is followed in alkene electrophilic addition reactions and that treatment of 1-butene with HC1 yields 2-chJorobutane rather than 1-chlorobutane. Similarly, we said in Section 11.7 that Zaitsev s rule is followed in elimination reactions and that treatment of 2-chlorobutane with NaOH yields 2-butene rather than 1-butene. But how do we know that these statements are correct The answer to these and many thousands of similar questions is that the structures of the reaction products have been determined experimentally. 

Similar qualitative relationships between reaction mechanism and the stability of the putative reactive intermediates have been observed for a variety of organic reactions, including alkene-forming elimination reactions, and nucleophilic substitution at vinylic" and at carbonyl carbon. The nomenclature for reaction mechanisms has evolved through the years and we will adopt the International Union of Pure and Applied Chemistry (lUPAC) nomenclature and refer to stepwise substitution (SnI) as Dn + An (Scheme 2.1 A) and concerted bimolecular substitution (Sn2) as AnDn (Scheme 2.IB), except when we want to emphasize that the distinction in reaction mechanism is based solely upon the experimentally determined kinetic order of the reaction with respect to the nucleophile. 

Zaitsev s rule states that the major product in the formation of alkenes by elimination reactions will be the more highly substituted alkene, or the alkene with more substituents on the carbon atoms of the double bond. 

The key butenolide needed by Buszek, for his synthesis of (—)-octalactin A, had already been prepared by Godefroi and Chittenden and coworkers some years earlier (Scheme 13.4).9 Their pathway to 10 provides it in excellent overall yield, in three straightforward steps from l-ascorbic acid. The first step entails stereospecific hydrogenation of the double bond to obtain L-gulono-1,4-lactone 13. Reduction occurs exclusively from the sterically less-encumbered ot face of the alkene in this reaction. Tetraol 13 was then converted to the 2,6-dibromide 14 with HBr and acetic anhydride in acetic acid. Selective dehalogenation of 14 with sodium bisulfite finally procured 10. It is likely that the electron-withdrawing effect of the carbonyl in 14 preferentially weakens the adjacent C—Br bond, making this halide more susceptible to reductive elimination under these reaction conditions. 

Much work has gone into the optimization of results with functionalized alkenes. In the reaction of cyclic alkenes, ds-decomposition of organopalladium halide-alkene complex gives -palladium complex 41. This subsequently undergoes syn-p-hydride elimination since only one such hydrogen is available, deconjugation to 3-substituted alkenes should... 

ZAITSEV S RULE In elimination reactions, the most substituted alkene usually predominates. 

Each step of this E, elimination reaction is reversible and thus the reaction is driven to completion by removing one of the products, the alkene. In these reactions several alkenes can be produced. The Saytzeff rule states that the more substituted alkene is the more stable and thus the one formed in larger amount. And the trans isomer is more stable than the cis isomer. With this information it should be possible to deduce which peaks on the gas chromatogram correspond to a given alkene and to predict the ratios of the products. 

Further exploration of the regioselectivity of alkene formation in elimination reactions (ElcB anc E2). 

The two most common alkene-forming elimination reactions are dehy-drohalogenation—the loss of HX from an alkyl halide—and dehydration—the loss of water from an alcohol. Dehydrohalogenation usually occurs by reaction of an alkyl halide with strong base, such as potassium hydroxide. For example, bromocyclohexane yields cyclohexene when treated with KOH in ethanol solution ... 

Sulfonyl carbanions undergo aldol-type reactions with aldehydes and ketones these are very important owing to the ability of the resultant p-hydroxysulfones to participate in elimination reactions to form alkenes (the Julia reaction, see p. 197). Early experiments used Grignard reagents to form the magnesium sulfonyl carbanions. By this procedure, methyl phenyl sulfone (104) is transformed into the p-hydroxysulfone (105) which subsequently can be either oxidised to the ketone (106) or dehydrated to the alkene (107) (Scheme 45). 

Hofmann rule The principal alkene formed in the decomposition of quaternary ammonium hydroxides that contain different primary alkyl groups is always ethylene, if an ethyl group is present. Originally given in this limited form by A.W. Hofmann, the rule has since been extended and modified as follows When two or more alkenes can be produced in a P-elimination reaction, the alkene having the smallest number of alkyl groups attached to the double bond carbon atoms will be the predominant product. This orientation described by the Hofmann rule is observed in elimination reactions of quaternary ammonium salts and tertiary sulfonium salts, and in certain other cases. 

Saytzeff s rule In elimination reactions, the major reaction product is the alkene with the more highly substituted double bond. This most-substituted alkene is also the most stable. 

Alkenes and alkynes are prepared by elimination reactions in which a carbon-carbon single bond is converted to a double or triple bond. In elimination reactions, atoms or groups are eliminated from adjacent carbons. Elimination once produces double bonds twice produces triple bonds. 

Chapter 5 continues the chemistry of alcohols and alkyl halides by showing how they can be nsed to prepare alkenes by elimination reactions. Here, the students see a second example of the formation of carbocation intermediates from alcohols, but in this case, the carbocation travels a different pathway to a different destination. 

You will see later that alkynes, as well as alkenes, can be formed in elimination reactions. 

In elimination reactions, the halogen and an adjacent hydrogen are removed to form an alkene. 

Chapter 10 introduces the acid-base chemistry of molecules that contain the C=C and C C functional groups. Related reactions that do not fall under the acid-base category are also presented. Chapter 11 uses nucleophiles, which are loosely categorized as specialized Lewis bases, in reactions with alkyl halides. These are substitution reactions. Chapter 12 shows the acid-base reaction of alkyl halides that leads to alkenes (an elimination reaction), and Chapter 13 ties Chapters 11 and 12 together with a series of simplifying assumptions that allows one to make predictions concerning the major product. 

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