Multiple covalent bonds ethylene

Carbon can form multiple covalent bonds by sharing more than two electrons with a neighboring atom (Section 7.5). In ethylene, the two carbon atoms share four electrons in a double bond. In acetylene, the two carbons share six electrons in a triple bond ... 

Carbon atoms can also share more than one electron pair with another atom to form a multiple covalent bond. Consider the examples of a carbon-carbon double bond in ethene (ethylene) and a carbon-carbon triple bond in ethyne (acetylene). 

Carbon likes to form bonds so well with itself that it can form multiple bonds to satisfy its valence of four. When two carbon atoms are linked with a single bond and their other valencies (three each) are satisfied by hydrogens, the compound is ethane. When two carbons are linked by a double bond (two covalent bonds) and their other valencies (two each) are satisfied by hydrogens, the compound is ethylene. When two carbons are linked by a triple bond (three covalent bonds) and their other valencies (one each) are satisfied by hydrogens, the compound is acetylene. 

Atoms can form different types of covalent bonds. In a single bond, two atoms are held together by one electron pair. Many compounds are held together by multiple bonds, that is, bonds formed when two atoms share two or more pairs of electrons. If two atoms share two pairs of electrons, the covalent bond is called a double bond. Double bonds are found in molecules of carbon dioxide (CO2) and ethylene (C2H4) ... 

Several examples of single bonds consisting of a pair of shared electrons have just been seen. Two other types of covalent bonds are the double bond consisting of two shared pairs of electrons (4 electrons total) and the triple bond made up of three shared pairs of electrons (6 electrons total). These are both examples of multiple bonds. Examples of a double bond in ethylene and of a triple bond in acetylene are shown in Figure 4.13. 

The contribution of n bonding interactions in classical multiple bonds to the bond strength becomes obvious from the EDA results of the HB=BH, H2C=CH2, HN=NH, and HC=CH bonds, which are shown in Table 7. The covalent contributions to the attractive interactions are stronger than the electrostatic contributions in the above molecules. The relative contribution of AE r decreases for the double bonds in the order HB=BH > H2C=CH2 > HN=NH. The degenerate n bond in acetylene contributes 44.4% to the orbital interactions, which means that it is not much weaker than the a bond. Note that the absolute values of AEcr in acetylene (—215.5kcalmoH ) and ethylene (—212.2 kcal mol ) are nearly the same while AE r in acetylene (—172.4 kcal mol ) is more than twice as strong as in ethylene (—79.2 kcalmol ). 

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