Preparing Alkynes

Just as alkenes can be prepared from alkyl halides, alkynes can be prepared from alkyl dihalides  

An alkyl dihalide has two leaving groups, and the transformation is accomplished via two successive elimination (E2) reactions  

In this example, the dihalide used is a geminal dihahde, which means that both halogens are connected to the same carbon atom. Alternatively, alkynes can also be prepared from vicinal dihalides, in which the two halogens are connected to adjacent carbon atoms  

In total, three equivalents of the amide ion are required two equivalents for the two E2 reactions, and one equivalent to deprotonate the terminal alkyne and form the alkynide ion. After the alkynide ion has formed and the reaction is complete, a proton source can be 

Equilibrium favors formation of weaker acid and weaker base 

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Both strategies are applied to the preparation of alkynes In this section we shall see how to prepare alkynes while building longer carbon chains By attaching alkyl groups to acetylene more complex alkynes can be prepared... 

Table 9 2 summarizes the methods for preparing alkynes Section 9 8 Like alkenes alkynes undergo addition reactions... 

For reviews of methods for preparing alkynes, see Friedrich, K. in Patai Rappoport The Ghemistry of Functional Groups, Supplement G, pt. 2 Wiley NY, 1983, p. 1376 Ben-Efraim, D.A. in Patai The Chemistry of the Carbon-Carbon Triple Bond, pt. 2 Wiley NY, 1978, p. 755. For a comparative study of various methods, see Mesnard, D. Bemadou, F Miginiac, L. J. Ghem. Res. (S), 1981, 270, and other papers in this series. 

The transition metal cross-couplings of allenes described here offer practical solutions for the modification of 1,2-dienes and access to the preparation of highly functionalized 1,3-dienes, alkynes and alkenes, which are often not easily accessible in a regio- and stereoselective manner by classical methods. Some of the prepared alkynes or functionalized allenes serve as important intermediates in syntheses of natural products, biologically active compounds, e.g. enynes and enyne-allenes, and new materials. It can be predicted that further synthetic efforts will surely be focused on new applications of allenes in transition metal-catalyzed cross-coupling reactions. 

Metal acetylides or alkynides react with primary alkyl halides or tosylates to prepare alkynes (see Sections 5.5.2 and 5.5.3). 

What procedure is available for preparing alkynes without adding carbon atoms ... 

The elimination of 1,2- or 1,1-dihalogenoalkanes or that of halogenoalkenes by base is effective for preparing alkynes. This route is especially effective owing to a variety of halides that are commercially available for use as raw materials. Usually, caustic alkali, alkali alcoholate, alkali amide, or n-butyl lithium are used as the base. Under certain conditions side reactions occur, leading to isomerization or rearrangement to form more stable internal alkynes or allene derivatives. 

Unlike sulfoxide elimination, selenoxide elimination can be used to prepare alkynes, providing the syn elimination process is sterically feasible, and has been used for the preparation of terminal alkynes e.g. 104 equation 43). ... 

Elimination reactions can also be used to prepare alkynes. Dihalides can be converted into alkynes by treatment with base, provided that the two halogen atoms are on the same, or adjacent, carbon atoms. In general, alkyne formation requires more drastic reaction conditions than alkene formation, and a strong base, such as sodium amide, is usually employed (Reactions 5.4 and 5.5). The product of Reaction 5.5, phenylethyne, is shown in Figure 5.1 as a ball and stick model. 

Elimination reactions involving dihalides can be used to prepare alkynes. A strong base such as sodium amide is usually required. 

How are alkynes prepared Alkynes react as Br0nsted-Lowry bases in acid-base reactions with acids such as HBr or with aqueous acid. Alkynes react as Lewis bases with dihalogens such as Brg and in hydroboration reactions (see Chapter 10). Alkynes often come from reactions involving other alkynes, using the fact that a terminal alkyne is a weak acid, as introduced in Chapter 6 (Section 6.2.6). Treatment of a terminal alkyne with a base such as sodium... 

In the next chapter, we will learn a method for preparing alkynes (compounds containing C C triple bonds). In the following reaction, a dihalide (a compound with two halogen atoms) is treated with an excess of strong base (sodium amide), resulting in two successive E2 reactions. Draw the mechanism for this transformation. 

The two basic methods used to prepare alkynes are double elimination from 1,2-dihaloalkanes and alkylation of alkynyl anions. This section deals with the first method, which provides a synthetic route to alkynes from alkenes Section 13-5 addresses the second, which converts terminal alkynes into more complex, internal ones. 

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