Elimination, of alkyl halides

Elimination bimolecular (E2) mechanism (Section 5.15) Mechanism for elimination of alkyl halides characterized by a transition state in which the attacking base removes a proton at the same time that the bond to the halide leaving group is broken. 

AJkynes can be made by dehydrohalogenation of vinylic halides in a reaction that is essentially an E2 process. In studying the stereochemistry of this elimination, it was found that (Z)-2-chloro-2-butenedioic acid reacts 50 times as fast as the corresponding isomer. What conclusion can you draw about the stereochemistry of eliminations in vinylic halides How does this result compare with eliminations of alkyl halides ... 

These A -acyl phosphazenes also decompose further on heating to PhaMePO and RCN. The elimination of methyl iodide probably occurs by a course closely related to the elimination of alkyl halides in the Arbusov reaction, where the alkoxy-group undergoes nucleophilic attack by halide ion. 

It is important to emphasize that in this reaction a spontaneous elimination of alkyl-halide takes place. Detection of the eliminated alkylhahde is an additional evidence for the mechanism of the formation of 2,5-dihydro-l,2-oxaphosphole-2-oxide derivatives. 

This means that if a reaction is carried out on a compound that has no stereoisomers, it cannot be stereospecific but at most stereoselective. The concerted reactions, including SN2 displacements, E2 elimination of alkyl halides, anti and Syn addition to alkenes are all stereoselective. In the case of chiral or geometric substrates the nature of the product depends on the unique stereoelectronic requirement of the reaction. These are examples of stereospecific reactions. 

In contrast to the inner salts, the tertiary salts are sufficiently stable that they can be prepared and purified quite easily by the usual methods for handling somewhat hygroscopic materials. They are readily soluble in a number of solvents so, with the exception of the ferric salt which seems to give some difficulty, can be purified by crystallization. They decompose at their melting points with elimination of alkyl halide... 

Partially fluorinated vinyl ethers of fluoroolefins are quite susceptible to the action of Lewis acids. Reaction usually proceeds with ionization of the allylic C-F bond and results in formation of C=0 group and elimination of alkyl halide. Indeed, 3-chloro-2-methoxyhexafluoro-2-butene 82 reacts with A1C13 with formation of trichlorovinyl ketone 83, and cyclic alkoxyfluoroalkenes demonstrate similar behavior in reaction with aluminum or tin(IV) halides [170] ... 

The elimination of alkyl halides is done under basic conditions, the elimination of alcohols is done under acid conditions. Under basic conditions, an E2 elimination would require the loss of a hydroxide ion as a leaving group. Since the hydroxide ion is a strong base, it is not a good leaving group and so the elimination of alcohols under basic conditions is difficult to achieve. 

Show how substitutions and eliminations of alkyl halides might be used to synthesize other types of compounds. 

El eliminations of alkyl halides are rarely useful for synthetic purposes because they give mixtures of substitution and elimination products. Explain why the sulfuric acid-catalyzed dehydration of cyclohexanol gives a good yield of cyclohexene even though the reaction goes by an El mechanism. (Hint What are the nucleophiles in the reaction mixture What products are formed if these nucleophiles attack the carbocation What further reactions can these substitution products undergo )... 

In Chapter 7, we saw that eliminations of alkyl halides usually follow Zaitsev s rule that is, the most substituted product predominates. This rule applies because the most-substituted alkene is usually the most stable. In the Hofmann elimination, however, the product is commonly the /east-substituted alkene. We often classify an elimination as giving mostly the Zaitsev product (the most-substituted alkene) or the Hofmann product (the least-substituted alkene). 

Hydrogen and chlorine are anti to each other in the Z isomer and are syn in the E isomer. Since the Z isomer reacts fifty times faster than the E isomer, elimination must proceed more favorably when the substituents to be eliminated are anti to one another. This is the same stereochemical result that occurs in E2 eliminations of alkyl halides. 

If the monomers contain reactive groups that could be attacked by carbanions they will not be suitable for anionic polymerizations. Halogen-containing vinyl monomers are difficult to polymerize in these systems because of the elimination of alkyl halides. Interactions between many initiators and the carbonyl groups of methacrylates or acrylates necessitate the use of special reaction conditions, like very low temperatures, for the anionic polymerization of these monomers. 

The synthesis of stereodefined acyclic alkenes via 3-elimination reactions—such as (1) dehydration of alcohols, (2) base-induced eliminations of alkyl halides or sulfonates (tosyl or mesyl esters), and (3) Hofmann eliminations of quaternary ammonium salts—often suffers from a lack of regio- and stereoselectivity, producing mixtures of isomeric alkenes. 

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