Pentyne isomers

One of the papers initiating this field deals with a pentyne isomer, as early as 1888 Faworski [25] reported the isomerization of 3-methylbutyne 7 to 8 (Scheme 1.4)... 

Because of the almost complete absence of the 1,2,3-isomer in the product mixture when propyne, 1-butyne, or 1-pentyne were passed over the W08-Si02 catalyst, it was concluded that cyclotrimerization over this catalyst does not occur via a cyclobutadiene complex (39). 

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. 

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... 

Deuterium labeling corroborates the migration of a j5-H atom. If no j5-hydrogen is available after propargylic insertion (as, e.g., in 3-hexyne or 2-pentyne), the Fe+ ion either catalyzes interconversion to an isomer which possesses a j5-H atom (3-hexyne 2-hexyne),... 

Problem 12.46. Which of the following exist as a pair of cis and Irons isomers Show structures of the cis and trans isomers, (a) 4-Methyl-2-pentene, ib) 2,4-dimethyl-2-pentene, (c) 4-methyl-2-pentyne, (dichloro-cyclopropane, (e) 1,2-dichlorocyclopropane. 

The name of the preceding compound is 4-methyl-2-pentyne. As with alkenes, changing the location of the multiple bond produces an isomer. For example, 4-methyl-l-pentyne is a different compound from 4-methyl-2-pentyne. 

Any of several isomers of dibromopentane give mostly 2-pentyne on dehydrohalogenation with fused KOH at 200°C. In each case, the alkyne initially formed rearranges to the most stable isomer, 2-pentyne. 

Write structural formulas for all of the isomers of pentyne (CsHs). 

Draw structural formulas for two isomers of pentyne that differ only in the position of the triple bond. 

Which isomer, 2,2-dibromopentane or 3,3-dibromopentane, would give the better yield of 2-pentyne using sodium amide as the base ... 

Is 2-methylcyclobutene an isomer of 1-pentyne Explain your answer. 

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