Hybridization in acetylene

Section 2.21 Carbon is sp-hybridized in acetylene, and the triple bond is of the a + tt + tt type. The 2s orbital and one of the 2p orbitals combine to give two equivalent sp orbitals that have their axes in a straight line. A cr bond between the two carbons is supplemented by two tt bonds formed by overlap of the remaining half-filled p orbitals. 

We assume here that since two geometrically equivalent bonds are formed by each carbon, this atom must be sp-hybridized in acetylene. On each carbon, one sp hybrid bonds to a hydrogen and the other bonds to the other carbon atom, forming the c bond skeleton of the molecule. 

It is also common to attribute the fact that the CH bond is shorter in C2H2 than in C2H6 to sp hybridization in the former case, as more s-character in the hybrid orbital implies a larger overlap integral. However, it should be noted that the smaller CH bond length and the consideration of sp carbon hybrids in acetylene in order to define quasi-localized molecular orbitals are two manifestations (one physical, the other mathematical) of the same real feature the number of atoms bonded to each C atom and their geometric arrangement. 

FIGURE 2 20 Bonding in acetylene based on sp hybridization of carbon The carbon-carbon triple bond is viewed as consisting of one cr bond and two tt bonds... 

One more hybridization scheme is important in organic chemistry. It is called sp hybridization and applies when carbon is directly bonded to two atoms, as in acetylene. The structure of acetylene is shown in Figure 2.18 along with its bond distances and bond angles. Its most prominent feature is its linear geometr-y. 

Because each carbon in acetylene is bonded to two other atoms, the orbital hybridization model requires each carbon to have two equivalent orbitals available for a bonds as outlined in Figure 2.19. According to this model the carbon 2s orbital and one of its 2p orbitals combine to generate two sp hybrid orbitals, each of which has 50% s character and 50% p character. These two sp or bitals share a common axis, but their major lobes ar e or iented at an angle of 180° to each other. Two of the or iginal 2p or bitals remain unhybridized. 

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. 

To illustrate this rule, consider the ethylene (C2H4) and acetylene (C2H2) molecules. You will recall that the bond angles in these molecules are 120° for ethylene and 180° for acetylene. This implies sp2 hybridization in C2H4 and sp hybridization in C2H2 (see Table 7.4). Using blue lines to represent hybridized electron pairs,... 

One of the simplest examples of sp hybridization is found in acetylene, H-C = C-H, a colorless gas used in welding. Both carbon atoms in the acetylene molecule have linear geometry and are sp-hybridized. When the two sp-hybridized carbon atoms approach each other with their sp orbitals aligned... 

Types of hybridization exhibited by the C atoms in acetylene, C2H2, include which of the following ... 

The carbon atoms in acetylene are sp hybridized, with linear (180°) bond angles. The triple bond contains one sigma bond and two perpendicular pi bonds. 

Predict the hybridization, geometry, and bond angles for the carbon atoms in acetylene, C2H2. 

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... 

New parameters of cyclopropene (155) have been calculated from existing MW data. A near-equilibrium structure has also been derived from scaled moments of iner-tia (Table 16). The lengths of the C—C single bond and the methylene C—H bond and H—C—H angle are similar to those in 1 (Table 1). The C=C bond is, however, considerably shorter than in ethene 1.337 (2) A, and (=)C—H is between C—H in ethene (Section II. A) and in acetylene, 1.0586 and 1.0547 A. Bond-length relations indicate that the methylene carbon in 155 uses approximately the same hybrid orbitals as 1, sp" and sp (Section II.A), to form bonds within the ring and to substituents, while the —CH= carbon in 155 is characterized by sp and sp hybrids, respectively ... 

Furthermore, compounds containing triple bonds can be obtained from sp hybrid orbitals, sp hybridisation produces two orbitals oriented at 180° to each other, as shown by the filled lobes in Fig. 1.4(a), which can form co-linear cr-bonds. This leaves two p-electrons, shown by the shaded lobes in Fig. 1.4(a), which are free to form rc-bonds aligned with the cr-bond, as in acetylene, C2H2, shown in Fig. 1.4(b). 

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). 

A further way of making four bonds from the carbon is to hybridize the 2s and one 2p orbital to give two hybrids in which the orbitals are at 180° to each other. The remaining two 2p orbitals are used to form two 7i-bonds at 90° each other (see 1.5). In this case there is a triple bond between the carbon and another atom as, for example, in ethyne (acetylene, 1.6). 

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