Ethyne orbital hybridization

Ethyne sp hybrid orbitals farm two cf bonds two remaining p orbitals on each carbon form orthogonal rr bonds... 

The remaining p-orbitals (one on each carbon) form the pi orbital. In ethyne, sp hybridization occurs to give two hybrid orbitals on each atom with lobes pointing along the axis. The two remaining p-orbitals on each carbon form two pi orbitals. Hybrid atomic orbitals can also involve d-orbitals. For instance, square-planar complexes use sp d hybrids octahedral complexes use sp if. 

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. 

In the ethyne molecule, C2H2, the two carbon atoms form sp hybrid orbitals. An electron from the 2s sub-shell is again promoted into the 2p sub-shell. The 2s orbital hybridizes with one of the 2p orbitals to give two sp hybridized orbitals. The 2p and 2p orbitals do not participate in the hybridization process (Figure 14-40). 

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. 

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.)...

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). 

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. 

For this system, we find that the spin-coupled orbitals do not remain associated with the same first-row atom throughout the reaction. Instead, orbital /2 from the ethyne moiety becomes a linear combination of an sp -like hybrid from the ethyne and another such hybrid from the HCNO, as is shown for the transition state in the middle column of Figure 5. After the transition state, this orbital becomes almost entirely associated with the HCNO carbon atom. 

Ethyne (acetylene) has a C-C triple bond. Each carbon bonds to only two other atoms to form a linear CH skeleton. Only the carbon 2s and 2px have the right symmetry to bind to only two atoms at once so we can hybridize these to form two sp hybrids on each carbon atom leaving the 2py and 2pz to form Jt MOs with the 2p orbitals on the other carbon atom. These sp hybrids have 50% each s and p character and form a linear carbon skeleton. 

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). 

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. 

Hybridization of the carbon to which a proton is attached also influences electron density. As the proportion of s character increases from sp to sp to sp orbitals, bonding electrons move closer to carbon and away from the protons, which then become deshielded. For this reason, methane and ethane resonate at 8 0.23 and 0.86, respectively, but ethene resonates at 8 5.28. Ethyne (acetylene) is an exception in this regard, as we shall see. Hybridization contributes to shifts in strained molecules, such as cyclobutane (8 1.98) and cubane (8 4.00), for which hybridization is intermediate between sp and sp. ... 

In ethyne with its triple bond (HC=CH), two p orbitals on each carbon are left as p orbitals and two linear sp hybrids are formed (Figure 3.24). The electrons in the sp hybrids form the carbon-hydrogen bonds and a carbon-carbon bond. The remaining two electrons on each carbon form carbon-carbon bonds as in ethene. 

---