Alkynes from acetylenic acids

Acetylene and terminal alkynes are more acidic than other hydrocarbons They have s of approximately 26 compared with about 45 for alkenes and about 60 for alkanes Sodium amide is a strong enough base to remove a proton from acetylene or a terminal alkyne but sodium hydroxide is not... 

The reaction of alkynes with nitric acid or mixed acid is generally not synthetically useful. An exception is the reaction of acetylene with mixed acid or fuming nitric acid which leads to the formation of tetranitromethane. A modification to this reaction uses a mixture of anhydrous nitric acid and mercuric nitrate to form trinitromethane (nitroform) from acetylene. Nitroform is produced industrially via this method in a continuous process in 74 % yield. " The reaction of ethylene with 95-100 % nitric acid is also reported to yield nitroform (and 2-nitroethanol). The nitration of ketene with fuming nitric acid is reported to yield tetranitromethane. Tetranitromethane is conveniently synthesized in the laboratory by leaving a mixture of fuming nitric acid and acetic anhydride to stand at room temperature for several days. ... 

Alkynes and ynamines have also been developed as useful participants in RCM approaches to pyrrolidine derivatives (Scheme 13). Mori and co-workers reported the formation of a cyclic dienamide via ene-ynamide RCM <020L803>. They also studied substituent effects on the course of RCM of enynes and reported the formation of both pyrrolidines and piperidines <02MI678>. The syntheses of 5- and 6-ring carbocycles and O-containing heterocycles from appropriate precursors were also reported in that work. Synthetically-useful dienylboronic acids have been prepared from acetylenic boronates <00AG(E)3101>. ... 

Alkynes (acetylenes, RCsCR) may be prepared by the elimination of a hydrogen halide from alkenyl halides under vigorous conditions. This is exemplified by the preparation of phenylacetylene from cinnamic acid via the dibromide and (o-bromostyrene (Scheme 3.26). The contrast between the conditions required for the bromodecarboxylation and for the second elimination to form the alkyne reveals the difference in reactivity between an alkyl and an alkenyl halide. Alternative modes of elimination, such as allene formation or rearrangement reactions, restrict the use of this procedure. 

Alkynes of chain length longer than two carbons can be formed from acetylene by using sodamide (NaNH2). Sodamide reacts with acetylene in an acid-base type of reaction. The hydrogen attached to the triply-bonded carbon shows appreciable acidity. Hence,... 

Enol formates, which are formylating agents under neutral conditions, are obtained from 1-alkynes and formic acid by arene-ruthenium(II) catalysis Heating acetylenes (phenylacetylene, diphenylacetylene, 1-octyne or 4-octyne) with formic acid to 100°C produces ketones and carbon monoxide. It was shown by NMR spectroscopy that the process involves the intermediate formation of enol formates (equation 35). Treatment of terminal alkynes with carbon dioxide and secondary amines in the presence of a ruthenium catalyst affords vinyl carbamates, e.g. equation 36 d88. reviews, see References 189 and 190. 

This reaction was first reported by Fittig and Schrohe in 1875 and subsequently extended by Kutscheroff in 1881. It is an acid-catalyzed hydration of alkynes into ketones. In this reaction, dilute sulfuric acid and mercuric salt are used as catalysts, and mercuric chloride can form a complex with acetylene in aqueous solution. This reaction has been used to prepare ketones from higher alkynes, such as propyne, and vinylacetylene as well as in commercial production of acetaldehyde from acetylene. ... 

The carbonylation of alkynes in the presence of methanol or water and carbon monoxide produces a,(3-unsaturated carboxylic acids or esters (Scheme 50).l This reaction is rarely used in large-molecule synthetic applications however, it has been very important in the industrial preparation of acrylic acid from acetylene. 

The removal by a base of a hydrogen from the terminal position of acetylenes to give acetylides is mentioned. Terminal alkynes are moderately acidic molecules. 

Most other alkynes are prepared from acetylene by taking advantage of the acidity of the C — H bond. In the presence of a very strong base, such as sodium amide (NaNH2), the amide anion removes the proton from acetylene to form ammonia and the salt sodium acetylide. The acetylide can then react with an alkyl halide, such as CHsBr ... 

We have already discussed one important chemical property of alkynes the acidity of acetylene and terminal alkynes In the remaining sections of this chapter several other reactions of alkynes will be explored Most of them will be similar to reactions of alkenes Like alkenes alkynes undergo addition reactions We 11 begin with a reaction familiar to us from our study of alkenes namely catalytic hydrogenation... 

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