Higher alkynes

The products of the radiolysis of butyne-2, propyne, pentyne-2, hexyne-3, and butyne-1, have been determined by Rondeau et Dimers, trimers and tetra-mers, in decreasing importance, are the most significant but many minor products including aromatic compounds, are observed. A free-radical mechanism is suggested. 

The radiolysis of benzene has been the subject of many studies in both the liquid phase and the gas phase. In all cases the radiolysis leads mainly to polymer formation, but hydrogen and acetylene as well as other minor products are formed in much lower yields. 

In the liquid-phase radiolysis benzene is particularly resistant to decomposition with the major product, polymer, having a G-value of 0.75 (refs. 468-470). The polymer has been found to contain biphenyl, phenylcyclohexadiene, phenylcyclo-hexene, Cjs compounds and higher-molecular-weight material A brief and rather uncertain reaction scheme for the radiolysis of liquid benzene is 

CgHj and C6H7— polymer CsHe + CfiHg polymer 

Hydrogen atoms add to benzene rather than abstracting H2 therefore does not come from hydrogen atoms. Burns has shown that there is a dependence of G c,h . andGcjHi on the linear energy transfer of the system studied, with the C -values increasing for increased let. This supports the occurrence of reactions (9) to (11) in the track of the radiation. 

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The reaction is endothermic and the equilibrium favors ethylene at low temperatures but shifts to favor acetylene above 1150°C Indeed at very high temperatures most hydro carbons even methane are converted to acetylene Acetylene has value not only by itself but IS also the starting material from which higher alkynes are prepared... 

The pattern is repeated m higher alkynes as shown when comparing propyne and propene The bonds to the sp hybridized carbons of propyne are shorter than the corre spondmg bonds to the sp hybridized carbons of propene... 

Spectra of Higher Alkynes on Metal Single Crystals... 

Almost no studies of the adsorption of diolefins and higher alkynes... 

As with alkenes, isomers are possible for butyne and higher alkynes, depending on the position of the triple bond in the chain. Unlike the alkenes, however, no cis-trans isomers are possible for alkynes because of their linear geometry. 

If one bases the yield of DMCDeT on reacted butyne, then the system containing tri(o-phenylphenyl)phosphite produces satisfactory results. Although large quantities of butadiene dimers are formed, only a trace of CDT is produced and hardly any higher oligomers, which facilitates the work-up. This combination, therefore, has definite advantages for the synthesis of cyclodecatriene derivatives from expensive higher alkynes. 

An interesting case is the coordination polymerisation of acetylene and higher alkynes. It may proceed by a mechanism quite similar to the metathesis polymerisation of cycloalkenes involving metal carbene and metallacycle (metallacyclobutene) species [45], The initiation and propagation steps in alkyne polymerisation (leading to a polymer of cis structure) in the presence of a catalyst with a diphenylcarbene initiating ligand are as follows ... 

Higher alkynes can be synthesised from alkenes through a two-step process which involves the electrophilic addition of bromine to form a vicinal dibromide then dehydrohalogenation with strong base(Following fig.). The second stage involves the loss of two molecules of hydrogen bromide and so two equivalents of base are required. 

If you look back on the arylations and alkenylations that have so far been discussed in this section, you will find out that the acetylene itself was never used as the nucleophile, but always a higher alkyne. That is no coincidence, since in the presence of an amine, acetylene and Cul form the poorly soluble Cu2C2 most of which precipitates. If at all, the very small portion of this species that remains in solution would couple with the arylating and alkenylating agents on both C atoms. This would at best result in the formation of the respective bis-coupling product of acetylene. 

Sodium acetylides are used in the synthesis of higher alkynes. For example HteC Na+ + CaHjiX -... 

This has been predominantly due to the lack of an efficient catalyst of sufficiently high activity and selectivity for the carbonylation of higher alkynes such as propyne [4]. Besides a high catalyst activity, not only is a high chemos-electivity of the carbonylation to unsaturated esters required, but a high regioselec-tivity of the carbonylation to the desired branched isomer (MMA) is, obviously, also essential. 

In recent years, attention has been focused on alkyne carbonylation catalysts based on the metals nickel, palladium, and platinum, modified with a variety of tertiary (bi)phosphines [5]. TTie main goal has been to develop chemo- and regio-selective carbonylation catalysts for application to higher alkyne substrates for the synthesis of certain fine chemicals. Many of these catalysts do allow the carbonylation to proceed under milder conditions than those applied in the catalytic Reppe process, and some of these catalysts do provide the branched regioisomer product from higher alkynes with good selectivity. However, in all cases reaction rates are very low, i.e., below 100 (and in most cases even below 10) mol/mol metal per h, as are the product yields in mol/mol metal (< 100). These catalyst productivities are far too low for large-scale industrial application in the production of commodity-type products, such as (meth)acrylates. 

A new class of cationic palladium catalysts for the carbonylation of alkynes is described which, under mild conditions, shows unprecedented high activity and selectivity for the carbonylation of (higher) alkynes. As a particularly interesting application, the catalysts allow the development of a commercially attractive and environmentally friendly process for the carbonylation of propyne to methyl methacrylate. 

Alkynes add hydrogen fluoride readily without a catalyst, in accordance with Markovnikov s rule. However, unlike the higher alkynes, acetylene does not react with HF between — 70° and 0° at atmospheric pressure. 

Sodium acetylide can react with alkyl halides in a substitution reaction to produce higher alkynes. The reaction involves substitution of acetylide ion for halide ion as shown ... 

Since sodium methyl acetylide is the salt of the extremely weak acid, methyl acetylene, the acetylide ion is a stronger base, thus this reaction involves substitution of acetylide ion for halide ion. From this it can readily be seen that the metal ion, sodium, bonds to the released halide ion and the acetylide ion bonds to the alkyl group yielding a higher alkyne, plus a metal halide as the byproduct. 

Higher alkynes may form structures analogous to numbers 11 to 16 in Table 4.2 but analogues of 16A to 21 are only formed by terminal alkynes. Molecules containing more than one multiple bond, e.g. 1,2-propadiene, 1,2- and 1,3-butadiene, etc. can have their adsorbed states formulated in many different ways, but in general both C=C bonds are connected to the surface in the same way, i.e. both are either TT or di-a. It will be evident that species containing an odd number of hydrogen... 

The cleanliness of the reactions of propyne, butynes and higher alkynes on certain metals, especially palladium and copper, and of ethyne on silver and... 

Higher alkynes can be synthesized fhom acetylene by reacting with NaNH followed by treatment with the appropriate alkyl halide.Terminal alkynes can be deprotonated using strong bases such as sodium amide. Hydroxides or alkoxides are not strong enough to deprotonate an acetylenic hydrogen. 

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. ... 

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