Alkenes—The Products of Elimination Reactions

Because elimination reactions of alkyl halides form alkenes, let s review earlier material on alkene structure and leam some additional facts as well. 

Recall from Section 1.9B that alkenes are hydrocarbons containing a carbon-carbon double bond. Each carbon of the double bond is sp hybridized and trigonal planar, and all bond angles 

Ethylene, the simplest alkene, is a hormone that regulates plant growth and fruit ripening. A ripe banana speeds up the ripening of green tomatoes because the banana gives off ethylene. 

The double bond of an alkene consists of a o bond and a n bond. 2p orbitals 

Overlap of the two sp hybrid orbitals forms the C—C a bond. Overlap of the two 2p orbitals forms the C—C it bond. 

Problem 8.1 Label the a and P carbons in each alkyl halide, and draw all possible elimination products in each reaction. 

Alkenes are classified according to the number of carbon atoms bonded to the carbons of the double bond. A monosubstituted alkene has one carbon atom bonded to the carbons of the double bond. A disubstituted alkene has two carbon atoms bonded to the carbons of the double bond, and so forth. 

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Name the following amine, including R,S stereochemistry, and draw the product of its reaction with excess iodomethane followed by heating with Ag20 (Hofmann elimination). Is the stereochemistry of the alkene product Z or E Explain. 

In order to strengthen evidence in favour of the proposition that concerted inplane 5n2 displacement reactions can occur at vinylic carbon the kinetics of reactions of some /3-alkyl-substituted vinyliodonium salts (17) with chloride ion have been studied. Substitution and elimination reactions with formation of (21) and (22), respectively, compete following initial formation of a chloro-A, -iodane reaction intermediate (18). Both (17) and (18) undergo bimolecular substitution by chloride ion while (18) also undergoes a unimolecular (intramolecular) jS-elimination of iodoben-zene and HCl. The [21]/[22] ratios for reactions of (18a-b) increase with halide ion concentration, and there is no evidence for formation of the -isomer of (Z)-alkene (21) iodonium ion (17d) forms only the products of elimination, (22d) and (23). 

On the other hand the stability of 57 causes the reaction leading to a reversible oxaphosphetane where the isomers 63 and 65 can interconvert via the starting material. The stereoselectivity in this step is thermodynamically controlled. The more stable four-membered ring is anti 65, with the bulky groups on opposite sides of the ring. The product of this reaction after elimination of triphenylphosphine oxide is only the E-alkene 66. 

When 1-bromo-l-methylcyclohexane is heated in ethanol for an extended period of time, three products result one ether and two alkenes. Predict the products of this reaction, and propose a mechanism for their formation. Predict which of the two alkenes is the major elimination product. 

Predict and explain the stereochemistry of E2 eliminations to form alkenes. Predict the products of E2 reactions on cyclohexane systems. 

Figure 8.1 shows several alkenes and how they are classified. You must be able to classify alkenes in this way to determine the major and minor products of elimination reactions, when a mixture of alkenes is formed. 

The ease of reversal of alkene insertion is evident from the numerous syntheses of transition metal-hydride complexes using main group metal alkyls as the source of hydride. The hydride in the products of such reactions usually arises from -hydride abstraction or elimination from intermediate unstable transition metal alkyls. This idea is reinforced by the greater effectiveness of secondary alkyls such as isopropyl or cyclohexyl compounds. However, it has been shown that in at least one case the hydride results from hydrolysis of a Pt-Mg bond, not from the alkyl formed from reaction of a Pt-Cl bond with a Grignard reagent. Several of the reactions listed in Table 1 are spontaneously reversible. Reactions where -hydride elimination has been used in the synthesis of hydrides are listed in Table... 

The carbenium ion intermediate then eliminates the alkene by charge-induced cleavage of a C-C bond. The striking argument for a carbenium ion intermediate is presented by the influence of the y-substituent R on the competition of onium reaction and McL. If R = H, i.e., for propyl-substituted iminum ions, the products of both reactions exhibit similar abundance. If R = Me or larger or if even two alkyls are present, McL becomes extremely dominant, because then its intermediate is a secondary or tertiary carbenium ion, respectively, in contrast to a primary carbenium ion intermediate in case of R = H. The importance of relative carbenium ion stability for onium ion fragmentations (Chap. 6.11.2) will become more apparent when dealing with the mechanism of the onium reaction. 

When 2-bromo-2-methylpropane is dissolved in methanol, it disappears rapidly. As expected, the major product, 2-methoxy-2-methylpropane, arises by solvolysis. However, there is also a signihcant amount of another compound, 2-methylpropene, the product of elimination of HBr from the original substrate. Thus, in competition with the SnI process, which leads to displacement of the leaving group, another mechanism transforms the tertiary halide, giving rise to the alkene. What is this mechanism Is it related to the SnI reaction ... 

An inversion of alkene geometry is made possible by the reaction of their derived epoxides with lithium halides and TFAA (eq 4) the products of this reaction are the corresponding halo-hydrin trifluoroacetates, which undergo a syn elimination upon reaction with Lithium Iodide. 

Pd(II) compounds coordinate to alkenes to form rr-complexes. Roughly, a decrease in the electron density of alkenes by coordination to electrophilic Pd(II) permits attack by various nucleophiles on the coordinated alkenes. In contrast, electrophilic attack is commonly observed with uncomplexed alkenes. The attack of nucleophiles with concomitant formation of a carbon-palladium r-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. However, unlike the mercuration products, which are stable and isolable, the product 1 of the palladation is usually unstable and undergoes rapid decomposition. The palladation reaction is followed by two reactions. The elimination of H—Pd—Cl from 1 to form vinyl compounds 2 is one reaction path, resulting in nucleophilic substitution of the olefinic proton. When the displacement of the Pd in 1 with another nucleophile takes place, the nucleophilic addition of alkenes occurs to give 3. Depending on the reactants and conditions, either nucleophilic substitution of alkenes or nucleophilic addition to alkenes takes place. 

When applied to the synthesis of ethers the reaction is effective only with primary alcohols Elimination to form alkenes predominates with secondary and tertiary alcohols Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid at 140°C At higher temperatures elimination predominates and ethylene is the major product A mechanism for the formation of diethyl ether is outlined m Figure 15 3 The individual steps of this mechanism are analogous to those seen earlier Nucleophilic attack on a protonated alcohol was encountered m the reaction of primary alcohols with hydrogen halides (Section 4 12) and the nucleophilic properties of alcohols were dis cussed m the context of solvolysis reactions (Section 8 7) Both the first and the last steps are proton transfer reactions between oxygens... 

Mild acid converts it to the product and ethanol. With the higher temperatures required of the cyano compound [1003-52-7] (15), the intermediate cycloadduct is converted direcdy to the product by elimination of waste hydrogen cyanide. Often the reactions are mn with neat Hquid reagents having an excess of alkene as solvent. Polar solvents such as sulfolane and /V-m ethyl -pyrrol i don e are claimed to be superior for reactions of the ethoxy compound with butenediol (53). Organic acids, phenols, maleic acid derivatives, and inorganic bases are suggested as catalysts (51,52,54,59,61,62) (Fig. 6). 

I Elimination reactions are, in a sense, the opposite of addition reactions. They occur when a single reactant splits into two products, often with formation of a small molecule such as wateT or HBr. An example is the acid-catalyzed reaction of an alcohol to yield water and an alkene. 

A final piece of evidence involves the stereochemistry of elimination. (Jnlike the E2 reaction, where anti periplanar geometry is required, there is no geometric requirement on the El reaction because the halide and the hydrogen are lost in separate steps. We might therefore expect to obtain the more stable (Zaitsev s rule) product from El reaction, which is just what w e find. To return to a familiar example, menthyl chloride loses HC1 under El conditions in a polar solvent to give a mixture of alkenes in w hich the Zaitsev product, 3-menthene, predominates (Figure 11.22). 

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