Commercial Importance of Alkynes

Acetylene is by far the most important commercial alkyne. Acetylene is an important industrial feedstock, but its largest use is as the fuel for the oxyacetylene welding torch. Acetylene is a colorless, foul-smelling gas that burns in air with a yellow, sooty flame. When the flame is supplied with pure oxygen, however, the color turns to light blue, and flame temperature increases dramatically. A comparison of the heat of 

When acetylene was first used for welding, it was considered a dangerous, explosive gas. Acetylene is thermodynamically unstable. When the compressed gas is subjected to thermal or mechanical shock, it decomposes to its elements, releasing 234 kJ (56 kcal) of energy per mole. This initial decomposition often splits the container, allowing the products (hydrogen and finely divided carbon) to burn in the air. 

Acetylene is safely stored and handled in cylinders that are filled with crushed firebrick wet with acetone. Acetylene dissolves freely in acetone, and the dissolved gas is not so prone to decomposition. Firebrick helps to control the decomposition by minimizing the free volume of the cylinder, cooling and controlling any decomposition before it gets out of control. 

Methylacetylene also is used in welding torches. Methylacetylene does not decompose as easily as acetylene, and it burns better in air (as opposed to pure oxygen). Methylacetylene is well suited for household soldering and brazing that requires higher temperatures than propane torches can reach. The industrial synthesis of methylacetylene gives a mixture with its isomer, propadiene (allene). This mixture is sold commercially under the name MAPP gas (MethylAcetylene-ProPadiene). 

An oxygen-acetylene flame is hot enough to melt steel for welding. A cutting torch uses an extra jet of oxygen to burn away hot steel. 

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Introduction 392 9-2 Nomenclature of Alkynes 393 9-3 Physical Properties of Alkynes 394 9-4 Commercial Importance of Alkynes 395 9-5 Electronic Structure of Alkynes 396 9-6 Acidity of Alkynes Formation of Acetylide Ions 397 9-7 Synthesis of Alkynes from Acetylides 399 9-8 Synthesis of Alkynes by Elimination Reactions 403 Summary Syntheses of Alkynes 404 9-9 Addition Reactions of Alkynes 405... 

CO. Alkynes will react with carbon monoxide in the presence of a metal carbonyl (e.g. Ni(CO)4) and water to give prop>enoic acids (R-CH = CH-C02H), with alcohols (R OH) to give propenoic esters, RCH CHC02R and with amines (R NH2) to give propenoic amides RCHrCHCONHR. Using alternative catalysts, e.g. Fe(CO)5, alkynes and carbon monoxide will produce cyclopentadienones or hydroquinols. A commercially important variation of this reaction is hydroformyiation (the 0x0 reaction ). 

There are no other alkynes that are of commercial importance, and so acetylene will be the only member of this series that is considered in fire discussions. There are... 

Several linear cooligomers of butadiene are prepared with alkenes and alkynes. Commercially important 1,4-hexadiene (103) is prepared by the reaction of ethylene and butadiene catalysed by Ni [40], Fe [41] and Rh [42], The experiment carried out using deuterated ethylene (100) supports the mechanism that the insertion of butadiene to M—H forms the 7i-allyl complex 99. Insertion of ethylene (100) to 99 gives 101, and its -elimination affords the cooligomer 102, tetradeuterated at C-1,1,2,6 of 103. 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

Despite the industrial importance of amines and imines, hydroamination, i.e. the direct reaction of alkenes or alkynes with primary or secondary amines, is only used in one commercial process where isobutene and ammonia are converted in the presence of a zeolite catalyst to /-butylaminc. Turnover frequencies are generally very low and consequently, high catalyst loadings are necessary, which in turn demands efficient recycling. 

An early example having potential commercial importance comes from tlie Trost laboratory s synthesis of vitamin D analogs (Scheme 6-23) [51], Their combination of vinyl bromide 129 and alkyne 130 to form triene 131 led to a concise and efficient synthesis of (-i-)-alphacalcidiol (134). In this reaction, vinyl bromide 129 participates in a bimolecular Heck reaction with alkyne 130 and the resulting alkenylpalladium intermediate 133 undergoes subsequent intramolecular Heck reaction with the pendant terminal alkene to provide 131. Under the reaction conditions, some of the desired product undergoes a [1,7]-hydrogen shift to yield 132. After thermal recycling of the minor component, a remarkable 76% yield of 131 was obtained. 

The partial reduction of substrates containing triple bonds is of considerable importance not only in research, but also commercially for stereoselectively introducing (Z)-double bonds into molecular frameworks of perfumes, carotenoids, and many natural products. As with catalytic hydrogenation of alkenes, the two hydrogen atoms add syn from the catalyst to the triple bond. The high selectivity for alkene formation is due to the strong absorption of the alkyne on the surface of the catalyst, which displaces the alkene and blocks its re-adsorption. The two principal metals used as catalysts to accomplish semireduction of alkynes are palladium and nickel. 

The alkyne series is characterized by molecules with one triple bond each. They are quite reactive. Ethyne is commonly known as acetylene, a commercially important fuel and raw material in the manufacture of rubber and other industrial chemicals. 

Alkanes and alkane-like substances have captured the interest of researchers in isomer enumeration for a long time because of their commercial importance. For example, Henze and Blair published the first isomer enumeration of alkanes in 1931. Here we provide, for reference, tables that list the number of isomers of alkanes, alkenes, alkynes, and stereoalkanes (Table 4), ketones and esters (Table 5), and primary, secondary, and tertiary alcohols (Table 6) up to 25 carbon atoms. 

Some related reactions of commercial importance, alkyne carbonylation to acrylic acid and acrylic esters, are discussed in Chapter 12. 

Formaldehyde, acetaldehyde, and acetone are important commercial chemicals, synthesized by special methods. In the laboratory, aldehydes and ketones are most commonly prepared by oxidizing alcohols, but they can also be prepared by hydrating alkynes and by Friedel-Crafts acylation of arenes. Aldehydes and ketones occur widely in nature (see Figure 9.1). 

We can only note that alkene, alkyne, allyl, and related compounds of many types are known. Important ruthenium starting materials are RuC12(COD) and Ru(COD)(2-methylallyl)2, both of which are commercially available. 

The alkyne series of hydrocarbons is characterized by having molecules with one triple bond each. They have the general formula C H2 2 and the name ending -yne. Like other unsaturated hydrocarbons, the alkynes are quite reactive. Ethyne is commonly known as acetylene. It is the most important member of the series commercially, being widely used as a fuel in acetylene torches and also as a raw material in the manufacture of synthetic rubber and other industrial chemicals. 

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