Acetylene hybridization

H. L. Anderson, R. Faust, Y. Rubin, F. Diederich, Fullerene-Acetylene Hybrids On the Way to Synthetic Molecular Carbon Allotropes , Angew. Chem. Int. Ed. Engl. 1994, 33,1366-1368. 

The hypothetical fullerene -acetylene hybrid at right symbolizes the exciting prospects that have arisen from the discovery of fuller-enes, and the profound changes they have induced in our view of acetylene and carbon allotrope chemistry. The preparation of molecular and polymeric acetylenic carbon allotropes, as well as carbon-rich nanometer-sized structures, has opened up new avenues in fundamental and technological research at the interface of chemistry with materials science. 

Ethynylated dihydrofullerenes serve as precursors for buckydumbbells (Scheme 3.3). Coupling of the desilylated compound 10 with CuCl leads to the dimer 11 [24]. Reaction of Cjq with the acetylide Li-C=C-Li leads to the dimer with a bridge consisting of one acetylene unit only. Electronic interaction between the fullerene-units in these two buckydumbbells is negligible. Further examples of CgQ-acetylene-hybrids synthesized by using cyclopropanation reactions are shown in Chapter 4. 

A linear carborane-siloxane-acetylenic hybrid polymer has been synthesized and characterized. The poly (carborane-siloxane-acetylene) is a viscous liquid at room temperature and can be easily processed to a high temperature polymeric thermoset under either thermal or photochemical conditions. Thermal curing of the linear polymer to the thermoset is accomplished above 150 C. This novel thermosetting polymer is readily converted into a ceramic material by heating to 1000 C under inert or oxidative conditions. The tfaermoset and ceramic material are stable in air at elevated temperatures. 

We conclude this introduction to hydrocarbons by describing the orbital hybridization model of bonding m ethylene and acetylene parents of the alkene and alkyne families respectively... 

One more hybridization scheme is important m organic chemistry It is called sp hybridization and applies when carbon is directly bonded to two atoms as m acetylene The structure of acetylene is shown m Figure 2 18 along with its bond distances and bond angles Its most prominent feature is its linear geometry... 

Because each carbon m acetylene is bonded to two other atoms the orbital hybridization model requires each carbon to have two equivalent orbitals available for CT bonds as outlined m 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 orbitals share a common axis but their major lobes are oriented at an angle of 180° to each other Two of the original 2p orbitals remain unhybridized... 

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... 

Section 2 21 Carbon is sp hybridized m acetylene and the triple bond is of the ct + Tt + Tt type The 2s orbital and one of the 2p orbitals combine to give two equivalent sp orbitals that have their axes m a straight line A ct bond between the two carbons is supplemented by two tr bonds formed by overlap of the remaining half filled p orbitals... 

FIGURE 9 2 The carbon atoms of acetylene are con nected by a cr + tt + tt triple bond (a) Both carbon atoms are sp hybridized and each IS bonded to a hydrogen by a (T bond The two tt bonds are perpendicular to each other and are shown sepa rately in (b) and (c)... 

An sp hybridization model for the carbon-carbon 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 tr bond m acetylene causes a band of high electron density to encircle the molecule... 

The property that most separates acetylene from ethane and ethylene is its acidity too can be explained on the basis of the greater electronegativity of sp hybridized... 

Acetylene is linear and alkynes have a linear geometry of their X—C=C—Y units The carbon-carbon triple bond m alkynes is com posed of a CT and two tt components The triply bonded carbons are sp hybridized The ct component of the triple bond contains two electrons m an orbital generated by the overlap of sp hybndized orbitals on adja cent carbons Each of these carbons also has two 2p orbitals which over lap m parrs so as to give two tt orbitals each of which contains two electrons... 

Acetylenic hydrogens are unusual in that they are more shielded than we would expect for protons bonded to sp hybridized carbon This is because the rr electrons circulate around the triple bond not along it (Figure 13 9a) Therefore the induced magnetic field is parallel to the long axis of the triple bond and shields the acetylenic proton (Figure 13 9b) Acetylenic protons typically have chemical shifts near 8 2 5... 

Acetylenes are anomalous m as m H NMR sp Hybridized carbons are less shielded than sp hybridized ones but more shielded than sp hybridized ones... 

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 ... 

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. 

Acetylenes are anomalous in C, as in H NMR. 5/r-Hybridized carbons are less shielded than 5/r -hybr-idized ones, but more shielded than 5/r -hybridized ones. 

When two sp-hybridized carbon atoms approach each other, sp hybrid orbitals on each carbon overlap head-on to form a strong sp-sp a bond. In addition, the pz orbitals from each carbon form a pz-pz it bond by sideways overlap and the py orbitals overlap similarly to form a py-py tt bond. The net effect is the sharing of six electrons and formation of a carbon-carbon triple bond. The two remaining sp hybrid orbitals each form a bond with hydrogen to complete the acetylene molecule (Figure 1.16). 

Figure 1.16 The structure of acetylene. The two sp-hybridized carbon atoms are joined by one sp-sp rr bond and two p-p 7t bonds.

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,... 

The most important alkyne by far is the first member of the series, commonly called acetylene. Recall from Chapter 7 that the C2H2 molecule is linear, with 180° bond angles. The triple bond consists of a sigma bond and two pi bonds each carbon atom is sp-hybridized. The geometries of acetylene and the next member of the series, C3H4, are shown in Figure 22.7. 

Potassium or lithium derivatives of ethyl acetate, dimethyl acetamide, acetonitrile, acetophenone, pinacolone and (trimethylsilyl)acetylene are known to undergo conjugate addition to 3-(t-butyldimethylsiloxy)-1 -cyclohexenyl t-butyl sulfone 328. The resulting a-sulfonyl carbanions 329 can be trapped stereospecifically by electrophiles such as water and methyl iodide417. When the nucleophile was an sp3-hybridized primary anion (Nu = CH2Y), the resulting product was mainly 330, while in the reaction with (trimethylsilyl)acetylide anion the main product was 331. 

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