The Structure of Ethyne Acetylene sp Hybridization

Hydrocarbons in which two carbon atoms share three pairs of electrons between them, and are thus bonded by a triple bond, are called alkynes. The two simplest alkynes are ethyne and propyne. 

Ethyne, a compound that is also called acetylene, consists of a linear arrangement of atoms. The H—C=C bond angles of ethyne molecules are 180°  

We can account for the structure of ethyne on the basis of orbital hybridization as we did for ethane and ethene. In our model for ethane (Section 1.12B) we saw that the carbon orbitals are sp hybridized, and in our model for ethene (Section 1.13) we saw that they are sp hybridized. In our model for ethyne we shall see that the carbon atoms are sp hybridized. 

The mathematical process for obtaining the sp hybrid orbitals of ethyne can be visualized in the following way (Fig. 1.27). 

FIgura 1.27 A process for deriving sp-hybridized carbon atoms. 

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FlSi 10.11 The electronic Structure of ethyne (acetylene). The electrons in the two sp hybrids on each atom pair to form a bonds either with the other C atom or with an H atom. The remaining two unhybridized 2p orbitals on each atom are perpendicular to the axis the electrons in corresponding orbitals on each atom pair to form two it bonds. The overall electron distribution is cylindricaL... 

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. 

The C-H and C-C cr-bonds are all trigonal sp hybrids, with 120° bond angles. The two unhybridized p-orbitals form a 7r-bond, which gives the molecule its rigid planar structure. The two carbon atoms are connected by a double bond, consisting of one o and one tt. The third canonical form of 5/ -hybridization occurs in C-C triple bonds, for example, acetylene (ethyne). Here, two of the p AO s on each carbon remain unhybridized and can form two n -bonds, in addition to two (r-bonds. Acetylene H-C=C-H is a linear molecule, as shown below, since the p-hybrids are oriented 180° apart. 

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