Hybridization and Bonding in Acetylene

Identify the orbital overlaps involved in the indicated bond in the compound showm (propene). Is this a -IT bond or a a bond  

Acetylene is a linear molecule as indicated in (a) the structural formula and (b) a space-filling model. 

Bonding in acetyiene based on sp hybridization of carbon. The carbon-carbon triple bond is viewed as consisting of one r bond and two TT bonds. 

In general, you can expect that carbon will be sp-hybridized when it is directly bonded to two atoms in a neutral molecule. 

The hydrocarbon shown, called vinytacetylene, is used in the synthesis of neoprene, a synthetic rubber, identify the orbital overlaps involved in the indicated bond. How many a bonds are there in vinylacetylene How many tt bonds  

Begin with two p -hybridized carbon atoms and four hydrogen atoms  

The carbon-carbon double bond in ethylene has a a component and a it component. The a component arises from overlap of sp -hybridized orbitals along the internuclear axis. The tt component results from a side-by-side overlap of 2p orbitals. 

Electrons in a bond are called tt electrons. The probability of finding a tt electron is highest in the region above and below the plane of the molecule. The plane of the molecule corresponds to a nodal plane, where the probability of finding a tt electron is zero. 

FIGURE 1.28 Acetyiene is a iinear moiecuie as indicated in the (a) structurai formuia and a (b) space-fiii-ing modei. 

FIGURE 1.29 (a) Electron configuration of carbon in its most stable state, (b) An electron is promoted from the 2s orbital to the vacant 2p orbital, (c) The 2s orbital and one of the three 2p orbitals are combined to give a set of two equal-energy sp-hybridized orbitals. Two of the 2p orbitals remain unchanged. 

FIGURE 1.30 Representation of orbital mixing in sp hybridization. Mixing of the 2s orbital with one of the p orbitals generates two sp hybrid orbitals. Each sp hybrid orbital has 50% s character and 50% p character. The axes of the two sp hybrid orbitals are colinear. Two 2p orbitals remain unhybridized, and their axes are perpendicular to each other and to the long axis of the molecule. 

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Many of the reactions in which acetylene participates, as well as many properties of acetylene, can be understood in terms of the stmcture and bonding of acetylene. Acetylene is a linear molecule in which two of the atomic orbitals on the carbon are sp hybridized and two are involved in 7T bonds. The lengths and energies of the C—H O bonds and C=C<7 + 27t bonds are as follows ... 

An sp hybridization model for the caibon-caibon triple bond was developed in Section 2.21 and is reviewed for acetylene in Figure 9.2. Figure 9.3 compares the electrostatic potential maps of ethylene and acetylene and shows how the second tt bond in acetylene causes a band of high electron density to encircle the molecule. 

The triple bond is relatively short because of the attractive overlap of three bonding pairs of electrons and the high s character of the sp hybrid orbitals. The sp hybrid orbitals are about one-half s character (as opposed to one-third s character of sp2 hybrids and one-fourth of sp3 hybrids), using more of the closer, tightly held s orbital. The sp hybrid orbitals also account for the slightly shorter C — H bonds in acetylene compared with ethylene. 

The triple bond in acetylene is seen to consist of one c bond joining the line-of-centers between the two carbon atoms, and two n bonds whose lobes of electron density are in mutually-perpendicular planes. The acetylene molecule is of course linear, since the angle between the two sp hybrid orbitals that produce the c skeleton of the molecule is... 

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). 

The triple bond in acetylene, like that in nitrogen, is composed of one a and two tt bonds and here also the tt bonds arc formed between electrons in non-hybridized p states. The a electrons, two to each carbon atom, are located in sp hybrid orbitals, formed by the linear combination of one s and one p wave function, as in beryllium, and the molecule is therefore linear. The arrangement of bonds is shown diagrammatically in Figure si. In a similar way the triple bond in the nitrile group G N is composed also of one n and two tt bonds. 

The sp hybrids about the C atom are used to form the four bonds in diamond, methane, and all alkanes. The sp hybrids are used to form the double bond in all alkenes. The sp hybrids are used in the triple bond in acetylene. The shapes of the molecules of the simple gases ethane, ethylene, and acetylene are well described by the hybrid model. (See Table 3.)... 

Historically, the application of molecular orbital theory to the electronic structures of isoelec-tronic 14-electron molecules such as acetylene, HCN, N2, and O2 was an excellent pioneering demonstration of the value of quantum chemistry. Within the framework of molecular orbital theory, the C - C bond in acetylene is a triple bond involving one a-bond, and two orthogonal 7t-bonds. The a-bond is formed by two sp-hybrid orbitals from each carbon, and the two 71-bonds are formed from the perpendicular p-orbitals. Alternatively, the so-called bent or banana bonds have been invoked to describe the multiple C-C bonds in acetylene (Fig. 1-1) [3-5]. This creates a conceptual dilemma, though one bonding model can be transformed to the other by appropriate linear combinations. It is now realized that both... 

Because the triple bond counts as one effective repulsive unit, each carbon has two effective pairs, which requires a linear arrangement. Thus each carbon atom requires5/7 hybridization, leaving two unchanged p orbitals (see Fig. 9.16). One of the oppositely oriented (see Fig. 9.14) sp orbitals is used to form a bond to the hydrogen atom the other sp orbital overlaps with the similar sp orbital on the other carbon to form the sigma bond. The two pi bonds are formed from the overlap of the twop orbitals on each carbon. This accounts for the triple bond (one sigma and two pi bonds) in acetylene. 

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