Bonding in ethyne

Go to the Chemistry 12 Electronic Learning Partner for more information about the bonding in ethyne. 

FIGURE 3.27 The pattern of bonding in ethyne (acetylene). The carbon atoms are sp hybridized, and the two remaining p-orbitals on each C atom form two Tt-bonds. The resulting pattern is very similar to that for nitrogen (Fig. 3.15), but two C—H groups replace the N atoms. 

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. 

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. 

The electron density can also be characterized by its ellipticity, the extent to which it deviates from cylindrical symmetry, reflecting the contribution of rr orbitals. While the C=C bond in ethyne is cylindrically symmetrical, the C-C bonds in ethene and benzene have greater extension in the direction of the rr component. Ellipticity is defined by... 

O Reading Check Infer, by looking at the bonds in ethyne, why it is highly reactive with oxygen. 

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. 

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

A discussion of bonding in ethyne in terms of 2c, 2c bonds is possible it the carbon atoms are assumed to be sp hybridized. These hybrids are used to form three bonding 2c a orbitals, one between the two C atoms and one from each C atom to its appended H. These orbitals each accommodate an electron pair, and four electrons are unaccounted for. 

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). 

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. 

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 1.30 (a) The structure of ethyne (acetylene) showing the sigma-bond framework and a schematic depiction of the two pairs of p orbitals that overlap to form the two tt bonds in ethyne. 

Describe the bonding in ethyne (acetylene), C2H2, which has the Lewis dot structure ... 

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.

We next consider the bonding in ethyne, a molecule in which each carbon atom is bonded to two... 

Bonding in ethyne the a bonds are colinear the n bonds lie above and below, and in front and behind the carbon-carbon sigma bond. 

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. 

The sp hybrid orbital has 50% s character, which is greater than the 33% and 25% s characters of the sp hybrid orbitals of alkenes and the sp and hybrid orbitals of alkanes. We recall that as the percent s character of hybrid orbitals increases the electrons in the hybrid orbitals are closer to the nucleus. Therefore, the bonding electrons in an sp hybrid orbital of a C—H bond in ethyne are closer to the nucleus than the electrons in the hybrid orbital of the C—H bonds of ethylene or ethane. The greater s character of the sigma bonds of acetylene and alkynes affects their physical properties. 

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