Hybridization Ethyne

In the third type of hybridisation of the valence electrons of carbon, two linear 2sp orbitals are formed leaving two unhybridised 2p orbitals. Linear a bonds are formed by overlap of the sp hybrid orbitals with orbitals of neighbouring atoms, as in the molecule ethyne (acetylene) C2H2, Fig. 1, A3. The unhybridised p orbitals of the carbon atoms overlap to form two n bonds the bonds formed between two C atoms in this way are represented as Csp Csp, or simply as C C. 

Self-Test 3.7B Suggest a structure in terms of hybrid orbitals for each carbon atom in ethyne, C2H2. 

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 lower signal is more complicated, and before we can interpret it exactly we need some background information. The magnitude of one-bond C-C coupling constants depends on bond hybridization (ethane 35, ethene 68, benzene 56, ethyne 172 Hz), while two- and three-bond C-C couplings are very small, often around 2-5 Hz. The second thing we have to remember, and this is a new concept, is that the lines in the multiplets from INADEQUATE spectra often come from different spin systems ... 

Similar, but different, redeployment is envisaged when a carbon atom combines with three other atoms, e.g. in ethene (ethylene) (p. 8) three sp2 hybrid atomic orbitals disposed at 120° to each other in the same plane (plane trigonal hybridisation) are then employed. Finally, when carbon combines with two other atoms, e.g. in ethyne (acetylene) (p. 9) two sp1 hybrid atomic orbitals disposed at 180° to each other (idigonal hybridisation) are employed. In each case the s orbital is always involved as it is the one of lowest energy level. 

Figure 1.28 Formation of the bonding molecular orbitals of ethyne from two sp-hybridized carbon atoms and two hydrogen atoms. (Antibonding orbitals are formed as well but these have been omitted for simplicity.)...

These interactions (dd, di, ii) are a function of dipole moment and polarizability. It has been shown that the dipole moment cannot be replaced entirely by the use of electrical effect substituent constants as parameters52. This is because the dipole moment has no sign. Either an overall electron donor group or an overall electron acceptor group may have the same value of /x. It has also been shown that the bond moment rather than the molecular dipole moment is the parameter of choice. The dipole moments of MeX and PhX were taken as measures of the bond moments of substituents bonded to sp3- and sp2-hybridized carbon atoms, respectively, of a skeletal group. Application to substituents bonded to sp-hybridized carbon atoms should require a set of dipole moments for substituted ethynes. 

To minimise repulsion, the angle between the two sp hybrid orbitals will be 180°. We can consider ethyne (C H ), in which both carbon atoms are sp hybridised. Each carbon atom uses its two sp hybrid orbitals to form a bonds with a hydrogen atom and with the other carbon atom. The unhybridised 2p orbitals left on the carbon atoms overlap side-on to form two ji bonds. 

The third observation relates to acetylene (ethyne, C2H2), which is linear, i.e. bond angles of 180°, and contains two it bonds. This introduces what we term triple bonds, actually a combination of one a bond and two n bonds. In this molecule, we invoke another type of hybridization for carbon, that of sp hybrid orbitals. These are a mix of the 2s orbital with one 2p orbital, giving two equivalent sp orbitals. Each hybrid orbital takes one electron, whilst the remaining two electrons are accommodated in two different 2p orbitals (Figure 2.17). 

Sokolova et al. (80) reported two absorptions, at 3050 and 2960 cm-1, for species on Pd/Al203 at temperatures between 223 and 373 K. Using inelastic electron tunneling spectroscopy, Bayman et al. (81) obtained complex spectra of species formed from C2H2 and C2D2 adsorbed on Pd/ A1203. These show the presence of a mixture of sp2- and. syL-hybridized hydrocarbons in addition to bands indicative of undissociated 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. 

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