The Structure of Ethane

The structure of ethane. The carbon-carbon bond is formed by a- overlap of two carbon sp hybrid orbitals. (For clarity, the smaller lobes of the sp hybrid orbitals are not shown.) 

Problem 1.8 Draw a line-bond structure for propane, CH3CH2CH3. Predict the value of each bond angle, and indicate the overall shape of the molecule. 

Although sp hybridization is the most common electronic state of carbon, it s not the only possibility. Look at ethylene, C2H4, for example. It was recognized well over 100 years ago that ethylene carbons can be tetravalent only if the two carbon atoms share four electrons and are linked by a double bond. Furthermore, ethylene is planar (flat) and has bond angles of approximately 120°. 

An 5p2.hybridized carbon. Three equivalent sp hybrid orbitals (green) lie in a plane at angles of 120 to one another, and a single unhybridized p orbital (blue) is perpendicular to the sp plane. 

To complete the structure of ethylene, four hydrogen atoms form cr bonds with the remaining four sp orbitals. Ethylene has a planar structure with H-C-H and H-C=C bond angles of approximately 120° (the. H-C-H bond angles are 116.6°, and the H-C=C bond angles are 121.7°). Each C-H bond has a length of 107,6 pm and a strength of 444 kj/mol (106 kcal/mol). 

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One more reaction of the ethylene halides must be mentioned, as, in it, we have a direct proof that the structure of ethane is as represented, viz., the symmetrical structure. Our evidence of this structure, thus far is simply indirect, i.e., from the proof that the other isomeric di-halogen ethane has the unsymmetrical structure. 

The third member of the alkane series, propane, has three carbon atoms and eight hydrogen atoms, giving it the molecular formula C3Hg. The next member, butane, has four carbon atoms in a continuous chain and the formula C4H10. Compare the structures of ethane, propane, and butane in Figure 22-3. 

The view of the structure of ethane which has been reached in this way leads to the conclusion that all the six hydrogen atoms are alike in their relation to the carbon atoms. This has been found to be true, for although chloroethane, C2H5CI, and other similar substitution-products have been made in a number of ways, no case of isomerism among mono-substitution-products has ever been discovered. The structure assigned to ethane does suggest, however, the possibility of the existence of two dichloroethanes, namely. 

Alkanes can have different conformations. By analyzing the structure of ethane, we can define certain aspects regarding its conformations. Conformations are different arrangements of the atoms in a molecule, as a result of rotation around a single bond. 

FIGURE 12.6 The structure of ethane, as represented by (a) its structural formula, (b) a ball-and-stick model, (c) a space-filling model, and (d) a wedge-dash projection. 

PROBLEM 1.9 Using the structure of ethane as a guide, draw the structure for propane, CjHg. 

The principles used to describe the bonding for methane can be extended to larger molecules that have additional carbon atoms and carbon-carbon bonds rather than only the C-H bonds found in methane. Ethane (5) has a covalent bond between two carbon atoms, for example, and both carbons have four bonds directed to the corners of a tetrahedron. The structure of ethane is shown in Figure 3.12, first with the Lewis dot formula for ethane (5a) and then as a structure made by overlapping two tetrahedrons of carbon (5b), each carbon with three hydrogen atoms and the fourth bond between carbon and carbon. The carbon-carbon bond is represented by overlap of the tetrahedrons in 5b. 

Figure 11.2 shows the stmctures of the first four alkanes ( = 1 to n = 4). Nat-mal gas is a mixture of methane, ethane, and a small amount of propane. We discussed the bonding scheme of methane in Chapter 10. The carbon atoms in all the alkanes can be assumed to be -hybridized. The structures of ethane and propane are straightforward, for there is oidy one way to join the carbon atoms in these molecules. Butane, however, has two possible bonding schemes resirlting in different com-poimds called n-butane ( stands for normal) and isobirtane. n-Butane is a straight-chain alkane because the carbon atoms are joined in a continuoirs chain. In a branched-chain alkane like isobutane, one or more carbon atoms are bonded to a nonterminal carbon atom. Isomers that differ in the order in which atoms are connected are called structural isomers. 

The structure of ethane with D3d symmetry is shown on the right. In this... 

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