Bond length, alkanes alkenes

Aromatic compounds have special characteristics of aromaticity, which include a low hydro-gen carbon atomic ratio, C-C bonds that are quite strong and of intermediate length between such bonds in alkanes and those in alkenes, tendency to undergo substitution reactions rather than the addition reactions characteristic of alkenes, and delocalization of n electrons over several carbon atoms. The last phenomenon adds substantial stability to aromatic compounds and is known as resonance stabilization. 

Experimental and theoretical C-H bond lengths for methyl groups in alkanes or alkyl moieties are given in Table 11, for C-H bonds adjacent to the C=C bonds in alkenes in Table 12, and for C-H adjacent to the C bonds in... 

Two C-H bonds lengths are determined for some alkanes since the conformers studied have a symmetry plane and the C-H bonds on the symmetric plane are indicated as in-plane (ip) and the other pair of C-H bonds above and below the symmetry plane are called out-of-plane (op). For some alkenes considered, three points are given in Fig. 12 as there are three different C-H bonds adjacent to the double bond. For the QC methods employed, the ip and op C-H bonds of methyl groups are shown as almost identical. However, the ro bond lengths derived by Bouchy et al. [35] differ widely (0.025 A). 

The effect of introducing /j -hybridized atoms into acyclic molecules was discussed in Section 2.2.1, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are seen when sp centers are incorporated into six-membered rings. Whereas the energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecy-clohexane ° and to 4.9 kcal/mol in cyclohexanone. The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffraction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. The C(l)—C(2) bond length is 1.335 A, and the C(l)-C(2)-C(3) bond angle is 123°. The substituents at C(3) and C(6) are tilted from the usual axial and equatorial directions and are referred to as pseudoaxial and pseudoequatorial. 

Bond Lengths and Bond Strengths in Alkanes, Alkenes, and Alkynes... 

The Qj p—H bond has the shortest bond length and is therefore the strongest bond. The C —H bond has the longest bond length and is therefore the weakest bond. Compare the spectra of an alkane, an alkene, and an alkyne (Figure 15.10). In each case, we draw a hne at 3000 cm ... 

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