Triple bonds in ethyne

Ethene, H2C = CH2, serves as the starting material for the synthesis of polyethylene, from which plastic bags and milk jugs are made. Ethyne, H—C=C—H, is used as a fuel for welding torches. The double bond in ethene and the triple bond in ethyne have the same effect on molecular shape as single bonds. Predict the shapes and bond angles of ethene and ethyne. 

Carbon atoms can also share more than one electron pair with another atom to form a multiple covalent bond. Consider the examples of a carbon-carbon double bond in ethene (ethylene) and a carbon-carbon triple bond in ethyne (acetylene). 

There is no need to use a number to locate the position of the triple bond in ethyne and propyne there is only one possible location for it in each compound. For larger molecules, number the longest carbon chain that contains the triple bond from the end that gives the triply bonded carbons the lower numbers. Show the location of the triple bond by the number of its first carbon. If a hydrocarbon chain contains more than one triple bond, we use the infixes -adiyn-, -atriyn-, and so forth. 

Figure 1-21 The double bond in ethene (ethylene) and the triple bond in ethyne (acetylene).

Figure 13-1 (A) Orbital picture of sp-hybridized carbon, showing the two perpendicular p orbitals. (B) The triple bond in ethyne The orbit s of two sp-hybridized CH fragments overiap to create a (T bond and two tt bonds. (C) The two tt bonds produce a cyiindricai eiectron cioud around the molecular axis of ethyne. (D) The electrostatic potential map reveals the (red) belt of high electron density around the central part of the molecular axis.

The 7i electron density in the triple bond of ethyne is cylindrically symmetric. 

There are also carbon-carbon triple bonds. Acetylene (or, more rigorously, ethyne—yne indicating the presence of a triple bond in the molecule), C2H2, is a common hydrocarbon that contains a triple bond HC=CH. Note that here too each carbon atom makes four chemical bonds the triple bond counts as three. Acetylene finds significant commercial use in oxyacetylene torches. 

Similarly, in ethyne (acetylene), C2H2, three electrons from each carbon atom are mutually shared, producing three two-electron bonds, called a triple bond, in which each carbon is attached to only two other atoms ... 

A simple compound with a triple bond is ethyne (acetylene), HC=CH. The Lewis structure for ethyne is shown in Figure 3.14a. It is a linear molecule. One of the CC bonds is a sigma bond. The other two are pi bonds. 

On this basis, formally at least, the series 1-3 contain metal-metal triple bonds. As we shall see, the nature of the metal-metal bonding is quite different from that of carbon-carbon bonding in ethyne. Similarly, it is useful to regard the metal-metal bond order in the series 4-6 as double. However, the nature of the M=M bond is quite different from the C=C bond in ethene. The carbonyls are semi-bridging or bridging in 1-6 and are extensively involved in the metal-metal interactions. This has resulted in some differences of opinion as to whether one should really regard the metal-metal bonds as multiple. 

Alkynes are unsaturated hydrocarbons containing at least one triple carbon-carbon bond. The simplest alkyne is C2H2 (commonly called acetylene), which has the systematic name ethyne. As discussed in Section 14.1, the triple bond in acetylene can be described as one cr bond between two sp hybrid orbitals on the two carbon atoms and two v bonds involving two 2p orbitals on each carbon atom (Fig. 22.10). 

This completes the Lewis structure of ethyne C2H2 with a triple bond. In C2H2 both C atoms have complete octets involving four shared electron pairs, one with the H atom and three with the second C atom. Each H atom has a single shared electron pair. 

You learned that in ethene, the two carbon atoms share two pairs of electrons in a double bond. In ethyne, the carbon atoms share three pairs of electrons to obtain a stable octet. A bond formed by sharing three pairs of electrons between two atoms is called a triple bond. The electron dot diagram for ethyne is shown in Figure 9.20. 

Figure 2.20 Bonding in ethyne (acetylene), C2H2. Overlap of the Is orbitals of H with the sp hybrid orbitals on C (part a) results in a

Analysis of the ELF has also been used to understand the bonding in the uranium methylidyne complexes XjUCH (X=F, Cl, Br) [9]. The ELF of each of these complexes was characterized by a ring-shaped disynaptic basin lying perpendicular to the U-C bond, a defining feature of the ELF in the prototypical C-C triple bond of ethyne, leading the authors to assert the existence of an unprecedented U-C triple bond. 

The carbon—carbon triple bond of ethyne is shorter than the carbon—carbon double bond of ethene, which in turn is shorter than the carbon—carbon single bond of ethane. The reason is that bond lengths are affected by the hybridization states of the carbon atoms involved. 

Benzyne is an extremely reactive species due to the nature of its triple bond. In normal acetylenic species, such as ethyne, the unhybridized p orbitals are parallel to each other above and below the molecular axis, and this facilitates maximum orbital overlap. In benzyne, however, the p orbitals are distorted to accommodate the triple bond within the ring system, thus reducing their effective overlap. Benzyne can also be drawn as a diradical, where the triple bond is drawn as a double bond with a single electron on each carbon. Benzyne can exist as either an ortho-, a meta- or a poro-benzyne, where the diradical can be a 1,2-, a 1,3- or a 1,4-diradical species, respectively. The 1,4-diradical species has been identified in the Bergman cychzation [3]. In this chapter, we will focus solely on the 1,2-dehydro-benzene (o-benzyne species). The term aryne here will be used to refer specifically to 1,2-dehydrobenzenes and derivatives. 

According to the orbital overlap model (Section 1.6F), a triple bond is described in terms of the overlap of sp hybrid orhitals of adjacent carbons to form a sigma bond, the overlap of parallel 2py orbitals to form one pi bond, and the overlap of parallel 2p orbitals to form the second pi bond. In ethyne, each carbon forms a bond to a hydrogen by the overlap of an sp hybrid orbital of carbon with a 15 atomic orbital of hydrogen. 

Thus, as shown in the first and second examples in Table 6.8, the addition of ethanethiol (thioethane, ethyl mercaptan, CH3CH2SH) as well as ethanol (CH3CH2OH) across the triple bond of ethyne (acetylene, HOCH) is catalyzed by the corresponding metal salts and requires high temperature (at which high pressures will develop). These reactions are not generally observed in the presence of acid. 

The four atoms of ethyne are collinear. Therefore, each H—C=C—H bond angle is 180°. In all alkynes, the two triple-bonded carbon atoms and the two atoms direcdy attached to them are colinear. We described the sp hybridization of the carbon atoms of ethyne in Section 1.17. Figure 7.1 shows the bonding in ethyne. 

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