Bond length terms Links

Ethyne, ethene and ethane contain 10. 12 and 14 valence electrons, respectively. They are isoelectronic with N2, 02 and F., respectively. Along both series of molecules the central link becomes progressively weaker and the bond lengths increase. The carbon-carbon bond length increases from 121 pm in ethyne to 133 pm in ethene and to 155 pm in ethane. The corresponding bond enthalpy terms are 837, 612 and 348 kJ mol Data for the diatomic molecules is contained in the text. Discuss these data in terms of the VSEPR and MO treatments of the molecules. 

The structure of ethene (ethylene) is well known. It has been determined by electron diffraction and is planar (all atoms are in the same plane), with the bond lengths and angles shown on the left. The carbon atoms are roughly trigonal and the C=C bond distance is shorter than that of a typical C-C single bond. The electronic structure of ethene, you will recall from Chapter 4, can be considered in terms of two sp hybridized C atoms with a c bond between them and four a bonds linking them each to two H atoms. The tc bond is formed by overlap of a p orbital on each carbon atom. 

We consider a freely-rotating chain consisting of N bonds (segments) of length a linked linearly at a supplementary angle ip. We assume that no bead-bead interaction is operating, i.e., the chain is not perturbed by volume exclusion. Until Section 4 below our considerations are limited to the case in which this assumption is obeyed. Thus, for simplicity, the term unperturbed is omitted when we refer to wormlike chains. The wormlike chain is a continuous version... 

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