1,4-Pentadiyne Explained

1,4-Pentadiyne (penta-1,4-diyne) is a chemical compound belonging to the alkynes. The compound is the structural isomer to 1,3-pentadiyne.

Preparation

Until the late 1960s, no successful synthesis of this seemingly simply preparable molecule was described. Although long-chain and more complex 1,4-diynes had been synthesized successfully before,^[1] synthesis approaches starting from sodium acetylide and propargyl bromide or from the acetylene Grignard reagent with propargyl bromide and copper(I) chloride failed^[2] and mostly 1,3-pentadiyne was obtained as rearrangement product.^[3]

The successful isolation in small amounts succeeded by reacting propargyl bromide and to the respective Normant cuprate converted acetylene grignard (usage of copper(I) chloride) in THF and an, compared to previous attempts additional, flash distillation and GLC of the distillate.^[4]

An improved synthesis method was published by Verkruijsse and Hasselaar in 1979. Copper chloride was substituted by copper(I) bromide as well as propargyl bromide by propargyl tosylate. At lower reaction temperatures and fewer by-products, the alkiyne was obtained after multistep extraction. According to the publication’s authors, this circumvented the problem that the solvent THF and the main compound share similar boiling points.

Moreover, a flash vacuum pyrolysis starting from 3-ethynylcycloprop-1-ene at 550 °C yields the compound and penta-1,2-dien-4-yne as sideproduct.^[5] Alternatively, a photolytic decomposition of cyclopentadienylidene via UV radiation is possible.^[6]

The compounds forms also during the exothermic reaction of allene and the ethynyl radical. This reaction is mainly of interest for astrochemistry.^{[7] [8] [9]}

Properties

At room temperature the substance discolors from a colorless to a yellowish liquid, however, storage in diluted solutions at 0 °C is possible for multiple weeks.

While for 1,4-pentadiene the sp²-hybridization leads to a bond angle of 120° between the single and double bond, in 1,4-pentadiyne it is a 180° angle due to the sp-hybrid orbital. Both triple bonds in 1,4-position destabilize each other according to another study by 3.9 kcal · mol⁻¹, a repulsion between the p orbital lobes close to the sp³-hybridized carbon has been postulated.^[10] According to a QCSID(T) calculation, the alkiyne is destabilized relative to 1,3-pentadiyne by 25 kcal · mol⁻¹.^[11]

Although microwave spectroscopy revealed besides a dipole moment of 0.516 D no significant distortions compared to an ideal tetrahedron, three different ionization energies are reported for the π-system.^[12]

Usage

1,4-pentadiyne is a common starting material for the synthesis of heterobenzenes such as stiba-, arsa- and phosphabenzene and their substituted derivates.^{[13] [14]}

References

1. H. Taniguchi. I. M. Mathai. S. I. Miller. Synthesis and spectral properties of 1,4- and 1,3-pentadiynes. Tetrahedron. 22. 3. 1966. 867–878. 10.1016/0040-4020(66)80058-3.
2. J. M. Todd. Attempted preparation of 1, 4-pentadiyne. 1961. MA. Boston University. 2144/18627 .
3. D. A. Ben-Efraim. F. Sondheimer. The synthesis and some reactions of a series of "skipped" polyacetylenes containing terminal acetylene groups. Tetrahedron. 25. 14. 1969. 2823–2835. 10.1016/0040-4020(69)80026-8.
4. H. D. Verkruijsse. M. Hasselaar. An Improved Synthesis of 1,4-Diynes. Synthesis. 4. 1974. 4 . 292–293. 10.1055/s-1979-28653. 95166709 .
5. Synthesis of Alkenyl- and Alkynylcyclopropenes. Michael M. Haley. Bluegrass Biggs. Will A. Looney. Robert D. Gilbertson. Tetrahedron Lett.. 36. 20. 3457–3460. 1995. 10.1016/0040-4039(95)00634-O.
6. Photochemistry of matrix-isolated cyclopentadienylidene revisited. G. Maier. J. Endres. J. Mol. Struct.. 556. 2000. 1–3 . 179–187. 10.1016/S0022-2860(00)00631-1. 2000JMoSt.556..179M .
7. A crossed molecular beams study of the reaction of the ethynyl radical (C₂H(X²Σ⁺)) with allene (H₂CCCH₂(X¹A₁)). Fangtong Zhang. Seol Kim. Ralf I. Kaiser. Phys. Chem. Chem. Phys.. 10.1039/B822366A. 11. 2009. 23 . 4707–4714. 19492123 . 2009PCCP...11.4707Z .
8. Reactions of ethynyl radicals as a source of C₄ and C₅ hydrocarbons in Titan's atmosphere. F. Stahl. P. v. R. Schleyer. H. F. Schaefer III. R. I. Kaiser. Planet. Space Sci.. 50. 2002. 7–8 . 685–692. 10.1016/S0032-0633(02)00014-4. 2002P&SS...50..685S .
9. Detection of pentatetraene by reaction of the ethynyl radical (C₂H) with allene (CH₂=C=CH₂) at room temperature. Fabien Goulay. Satchin Soorkia. Giovanni Meloni. David L. Osborn. Craig A. Taatjes. Stephen R. Leone. Phys. Chem. Chem. Phys.. 2011. 13. 46 . 20820–20827. 10.1039/C1CP22609F. 22002654 .
10. Donald W. Rogers. Nikita Matsunaga. Frank J. McLafferty. Andreas A. Zavitsas. Joel F. Liebman. On the Lack of Conjugation Stabilization in Polyynes (Polyacetylenes). J. Org. Chem.. 2004. 69. 21. 7143–7147. 10.1021/jo049390o. 15471463 .
11. Identification of C₅H_x Isomers in Fuel-Rich Flames by Photoionization Mass Spectrometry and Electronic Structure Calculations. Nils Hansen. Stephen J. Klippenstein. James A. Miller. Juan Wang. Terrill A. Cool. Matthew E. Law. Phillip R. Westmoreland. Tina Kasper. Katharina Kohse-Höinghaus. 10.1021/jp0569685. J. Phys. Chem. A. 2006. 110. 13. 4276–4388. 16571041 . 2006JPCA..110.4376H .
12. Robert L. Kuczkowski. Frank J. Lovas. R. D. Suenram. The microwave spectrum, structure and dipole moment of 1,4-pentadyine. J. Mol. Struct.. 1981. 72. 143–152. 10.1016/0022-2860(81)85014-4. 1981JMoSt..72..143K . 2027.42/24440 . free.
13. The group 5 heterobenzenes. Arthur J. Ashe III. Acc. Chem. Res.. 1978. 11. 4. 153–157. 10.1021/ar50124a005.
14. Preparation of 2-substituted arsabenzes. Arthur J. Ashe III. Woon-Tung Chan. J. Org. Chem.. 44. 9. 1979. 1409–1413. 10.1021/jo01323a010.