Pentyne and 1-hexyne

In acetic acid, both 1-pentyne and 1-hexyne give the syn addition product. With 2-butyne and 3-hexyne, the major products are p-chlorovinyl acetates of //-configuration.154 Some of the dichloro compounds are also formed, with more of the E- than the Z-isomer being observed. 

In the preparation of 1-pentyne and 1-hexyne (exp. 10) complete conversion of the alkyl bromides is effected by using an excess of sodium acetylide. A reasoning based on economics prompts the use of an excess of the alkyl halide if alkali vinylacetylide or alkali diacetylide (generated from alkali amide and dichlorobutene or dichlorobutyne, respectively) are to be alkylated. If slightly mare than the stoichiometiical amount of alkyl bromide is used, no serious separation problems will be encountered during the final distillation. A relatively small amount of DMSO is added to enhance the solubility of the alkyl bromides, thereby facilitating the alkylation reaction. 

Pentyne and 1-hexyne are hydrogenated with [Ti(Cp)2(CO)2] to 1-pentene and 1-hexene, respectively, but phenylacetylene gives ethylbenzene. Reduced compounds of [Ti(Cp)2Cl2] with Na, Mg, Ca, sodium naphthalenide, or butyllithium catalyze the hydrogenation of a variety of alkynes to alkenes. 

The nickel-catalyzed hydrogenation of 1-pentyne and 1-hexyne provided only the 1-alkene and alkane, the selectivity being about 0.75 91, 92). It must therefore be concluded that, generally, reaction sequences analogous to steps (5) and (6) are difficult. 

The metathesis of linear alkynes has also been reported, e.g. the metathesis of propyne, 1-pentyne, 2-pentyne, and 2-hexyne (31-33). This reaction can be visualized as the cleavage and formation of carbon-carbon triple bonds ... 

Beside the aromatic vinylidene complexes also alkyl vinylidene complexes [Ru(bdmpza)Cl(=C=CHPr)(PPh3)] (33c) and [Ru(bdmpza) Cl(=C=CHBu)(PPh3)] (33d) have been synthesized (Scheme 20), following the same procedure as earlier by using 1-pentyne or 1-hexyne. IR and NMR spectroscopic data of 33c and 33d are similar to those of 33a and 33b. Again only one major isomer was isolated in the case of [Ru(bdmpza)Cl(=C=CHPr)(PPh3)] (33c), though the... 

Cobalt metal catalyses the co-trimerization of one molecule of acetonitrile with two of 1 -pentyne or 1 -hexyne to yield mixtures of the pyridines 610 and 611 (R = Pr or Bu). The regioselctivity of the reaction of nitriles with terminal acetylenes has been investigated. It was found that, in general, mixtures of 2,4,6- and 2,3,6-trisubstituted pyridines are produced. The selectivity depends on the size of the substituents attached to the triple bond and the cyanide group. If either of these is large, only 2,4,6-substituted pyridines are obtained. Ethyl propiolate and aliphatic or aromatic nitriles (RCN) yield, in addition to the usual type of product, i.e. 612 and 613, the 2,3,5-isomer 614. Pyridines 617 result from the action of alkynes RC=CH (R = Ac, PrCO or C02Me) on the electron-rich... 

CONDENSATIONS WITH SODAMIDE IN LIQUID AMMONIA Acetylenic compounds are conveniently prepared with the aid of Uquid ammcx as a solvent. The preparation of a simple acetylenic hydrocarbon ( -butylacetylene or 1-hexyne) and also of phenylacetylene is described. Experimental details are also given for two acetylenic carbinols, viz., 1-ethynyl-eyciohoxanul and 4-pentyn-l-ol. It will be noted that the scale is somewhat laige smaller quantities can readily be prepared by obvious modifications of the directions. 

Problem 8.10 Synthesize the following compounds from HC=CH and any other organic and inorganic reagents (do not repeat steps) (a) 1-pentyne, (b) 2-hexyne. M... 

Both CO-S-methyl- 1-pentyne and (5 )-4-methyl-l-hexyne yield optically active products when their triple bonds are reduced to double bonds. 

The disproportionation of acetylenes has also been reported Pennella, Banks, and Bailey69) found that 2-pentyne could be converted to 2-butyne and 3-hexyne when contacted with tungsten oxide-silica. The results were consistent with a four-center reaction scheme the mole ratio of 3-hexyne to 2-butyne was always 1.0 to 1.2 (theoretical 1.0) and no significant amounts of other products with carbon numbers less than 10 were found. 

Ne.xt ask, What is an immediate precursor of 2-bexyne WeVe seen that an internal alkyne can be prepared by alkylation of a terminal alkyne anion. In the present instance, we re told to start with 1-pentyne and an alky] halide. Thus, alkylation of the anion of 1-pentyne with iodomethane should yield 2-hexyne ... 

Addition of HF to acetylene homologs, giving difluoroalkanes, has been described both for small218 and for large batches (5-10 moles).220 In the latter case it is necessary to avoid formation of hot reaction zones (resinification) and decrease in temperature which can lead to dangerous accumulation of unchanged reagents. Propyne and 1-butyne are led as gas into liquid ice-cold anhydrous HF (2.5 moles per mole of alkyne) pentyne and hexyne are dropped into HF at — 50° (copper vessels). 

The reaction mixtures are poured into water and the products collected on a filter. The excess of sulfur dioxide and paraldehyde is removed by triturating with alcohol and ether several times. The polymer from 1-pentyne is further purified by dissolution in dioxane and reprecipitating by the addition of water. The products from 1-hexyne and 1-heptyne are purified similarly with acetone as the solvent. 

In this experiment, we will conduct some modern organic chemistry using a palladium catalyst. It is a rare opportunity for students in undergraduate laboratories to experience this powerful chemistry. We will react the lodosubstituted aromatic compounds, shown below, with 1-pentyne, 1-hexyne, or 1-heptyne in the presence of the catalysts, palladium acetate and cuprous iodide, to yield 4-substituted-l-pentynyl, 4-substituted-l-hexynyl, or 4-substituted-l-heptynylaromatic compounds. This reaction is called the Sonogashira coupling reaction. The reaction will be carried out in refluxing 95% ethanol as the solvent. In addition, piperazine will be employed both as a base and as a hydride donor. 

Add 0.200 mmol of one of the four iodo substrates shown to a 25-mL round-bottom flask. Use a 4-place analytical balance for weighing the substrates and all of the materials listed next. Now add 55 mg of piperazine and a clean magnetic stir bar to the flask. Add 1.25 ml of 95% ethanol to the flask to dissolve the materials. Now add 16.5 mg of palladium (II) acetate and 10 mg of copper (I) iodide to the flask. Finally, use an automatic pipette to dispense 70 pX. of 1-pentyne. 1-hexyne, or 1-heptyne, depending on which alkyne you were assigned, to the round-botom flask. Attach a water-cooled condenser to the flask. Heat the contents at reflux for 30 minutes on a hot plate, with stirring. 

PROBLEM 15.37 The following IR spectra belong to 5-phenyl-1-pentyne and 6-phenyl-2-hexyne. Which is which Explain your reasoning. 

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