Structure and Bonding in Alkynes sp Hybridization

Acetylene is linear, with a carbon-carbon bond distance of 120 pm and carbon-hydrogen bond distances of 106 pm. 

In spite of the fact that few cycloalkynes occur naturally, they gained recent attention when it was discovered that some of them hold promise as anticancer drugs. (See the boxed essay Natural and Designed Enediyne Antibiotics following this section.) 

FIGURE 9.2 The carbon atoms of acetylene are connected by a cr + it + tt triple bond. Both carbon atoms are sp-hybridized, and each is bonded to a hydrogen by an sp-ls t bond. The o-component of the triple bond arises by sp-sp overlap. Each carbon has two p orbitals, the axes of which are perpendicular to each other. One tt bond is formed by overlap of the p orbitals shown in (f ), the other by overlap of the p orbitals shown in (c). Each tt bond contains two electrons. 

FIGURE 9.3 Electrostatic potential maps of ethylene and acetylene. The region of highest negative charge (red) is associated with the TT bonds and lies between the two carbons in both. This electron-rich region is above and below the plane of the molecule in ethylene. Because acetylene has two TT bonds, its band of high electron density encircles the molecule. 

At this point, it s useful to compare some stractural features of alkanes, alkenes, and alkynes. Table 9.1 gives some of the most fundamental ones. To summarize, as we progress through the series in the order ethane ethylene acetylene  

Molecular model of cyclononyne showing bending of the bond angles associated with the triply bonded carbons. This model closely matches the structure determined experimentally. Notice how the staggering of bonds on adjacent atoms governs the overall shape of the ring. 

Angle strain destabilizes cycloalkynes to the extent that cyclononyne is the smallest one that is stable enough to be stored for long periods. The next smaller one, cyclooc-tyne, has been isolated, but is relatively reactive and polymerizes on standing. 

The geometry at carbon changes from tetrahedral trigonal planar 

All of these trends can be accommodated by the orbital hybridization model. The bond angles are characteristic for the sp, sp, and sp hybridization states of carbon and don t require additional comment. The bond distances, bond strengths, and acidities are related to the s character in the orbitals used for bonding. Character is the fraction of the hybrid orbital contributed by an s orbital. Thus, an sp orbital has one quarter s character and three quarters p, an sp orbital has one third and two thirds p, and an sp orbital one half and one half p. We then use this information to analyze how various qualities of the hybrid orbital reflect those of its 5 and p contributors. 

Alkynes resemble alkanes and alkenes in their physical properties. They share with these other hydrocarbons the properties of low density and low water-solubility. They are slightly more polar and generally have slightly higher boiling points than the corresponding alkanes and alkenes. 

Examples of physical properties of alkynes are given in Appendix 1. 

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Now consider the alkynes, hydrocarbons with carbon-carbon triple bonds. The Lewis structure of the linear molecule ethyne (acetylene) is H—O C- H. To describe the bonding in a linear molecule, we need a hybridization scheme that produces two equivalent orbitals at 180° from each other this is sp hybridization. Each C atom has one electron in each of its two sp hybrid orbitals and one electron in each of its two perpendicular unhybridized 2p-orbitals (43). The electrons in the sp hybrid orbitals on the two carbon atoms pair and form a carbon—carbon tr-bond. The electrons in the remaining sp hybrid orbitals pair with hydrogen Ls-elec-trons to form two carbon—hydrogen o-bonds. The electrons in the two perpendicular sets of 2/z-orbitals pair with a side-by-side overlap, forming two ir-honds at 90° to each other. As in the N2 molecule, the electron density in the o-bonds forms a cylinder about the C—C bond axis. The resulting bonding pattern is shown in Fig. 3.23. 

This chapter deals mainly with the structural consequences of and sp hybridizations. The doubly bonded carbons in alkenes are hybridized and the triply bonded carbons in alkynes are hybridized sp. In these molecules, atoms are bound not only by the O bonds we saw in Chapter 2 but by 7t bonds as well. 

The electronic structure of benzyne, shown in Figure 16.19, is that of a highly distorted alkyne. Although a typical alkyne triple bond uses sp-hybridized carbon atoms, the benzyne triple bond uses sp2-hybridized carbons. Furthermore, a typical alkyne triple bond has two mutually perpendicular it bonds formed bv p-p overlap, but the benzyne triple bond has one tt bond formed by p-p overlap and one tt bond formed by sp2 sp2 overlap. The latter tt bond is in the plane of the ring and is very weak. 

Arynes present structural features of some interest. They clearly cannot be acetylenic in the usual sense as this would require enormous deformation of the benzene ring in order to accommodate the 180 bond angle required by the sp hybridised carbons in an alkyne (p. 9). It seems more likely that the delocalised n orbitals of the aromatic system are left largely untouched (aromatic stability thereby being conserved), and that the two available electrons are accommodated in the original sp hybrid orbitals (101) ... 

Both the carbon atom and the nitrogen atom of the cyano group are sp hybridized, and the R—C = N bond angle is 180° (linear). The structure of a nitrile is similar to that of a terminal alkyne, except that the nitrogen atom of the nitrile has a lone pair of electrons in place of the acetylenic hydrogen of the alkyne. Figure 21-1 compares the structures of acetonitrile and propyne. 

Once again, orbital hybridization provides an explanation for the bonding of the carbon atoms. Structurally, the hydrogen and carbon atoms of acetylene molecules lie in a straight line. This same linearity of the triple bond and the two atoms attached to the triple-bonded carbons is found in all alkynes. These characteristics are explained by mixing a 2s and a single 2p orbital of each carbon to form a pair of sp hybrid orbitals. Two of the 2p orbitals of each carbon are unhybridized (see > Figures 2.9 and 2.10). 

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