E2 Reactions and Alkyne Synthesis

Problem 8.21 which mechanism, E1 or E2, will occur in each reaction  

Recall from Section 1.9C that the carbon-carbon triple bond of alkynes consists of one G and two n bonds. 

A single elimination reaction produces the n bond of an alkene. Two consecutive elimination reactions produce the two n bonds of an alkyne. 

The word geminal comes from the Latin geminus, meaning twin. 

Two elimination reactions are needed to remove two moles of HX from a dihalide as substrate. 

The relative strength of C-H bonds depends on the hybridization of the carbon atom sp sp sp . For more information, review Section 1.1 OB. 

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The E2 reaction can also be applied to the synthesis of alkynes. When a vinyl halide is treated with a strong base, loss of HX by what is essentially an E2 process leads to formation of a triple bond. In a simple example, 12-methyltridec-l-ene was treated with bromine in dichloromethane to give dibromide 193. When this was treated with potassium tert-butoxide in petroleum ether, in the presence of 18-crown-6, initial elimination gave vinyl bromide 194 but this reacted again with the base, and an E2 reaction gave alkyne 195 in 72% yield for both chemical steps. Alkyne 195 was used in Mori s synthesis of the sphingosine derivative sulfobacin... 

The study aids for this chapter include the list of methods below that we have mentioned for synthesizing alkenes, as well as key terms and concepts (which are hyperlinked to the Glossary from the bold, blue terms in the WileyPLUS version of the book at wileyplus.com). Following the end of chapter problems you will find graphical overviews of the mechanisms for E2 and El reactions, a Synthetic Connections scheme for alkynes, alkenes, alkyl halides, and alcohols, and a Concept Map regarding organic synthesis involving alkenes. 

In the synthesis of propargylic alcohols, we saw the reaction of an alkynyl nucleophile (either the anion RC=CNa or the Grignard RC CMgBr, both prepared from the alkyne RC CH) with a carbonyl electrophile to give an alcohol product. Such acetylide-type nucleophiles will undergo Sn2 reactions with alkyl halides to give more substituted alkyne products. With this two-step sequence (deprotonation followed by alkylation), acetylene can be converted to a terminal alkyne, and a terminal alkyne can be converted to an internal alkyne. Because acetylide anions are strong bases, the alkyl halide used must be methyl or 1° otherwise, the E2 elimination is favored over the Sn2 substitution mechanism. 

Purpose. The product formed in this multiple elimination reaction is the third intermediate in the b series of Sequence A, and is one of fhe immediate precursors to our target molecule, hexaphenylbenzene.You will investigate the synthesis and properties of alkynes and become familiar with E2 elimination reactions. 

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