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Comparison of bond lengths in simple hydrocarbons [5] Molecule Ethane: Ethylene: Acetylene: Formula C 2 H 6: C 2 H 4: C 2 H 2: Class alkane: alkene: alkyne: Structure Hybridisation of carbon sp 3: sp 2: sp C-C bond length 153.5 pm: 133.9 pm: 120.3 pm: Proportion of C-C single bond 100% 87% 78% Structure determination method microwave ...
The bond lengths of these so-called "pancake bonds" [6] are up to 305 pm. Shorter than average C–C bond distances are also possible: alkenes and alkynes have bond lengths of respectively 133 and 120 pm due to increased s-character of the sigma bond.
With 133 pm, the ethylene C=C bond length is shorter than the C−C length in ethane with 154 pm. The double bond is also stronger, 636 kJ mol −1 versus 368 kJ mol −1 but not twice as much as the pi-bond is weaker than the sigma bond due to less effective pi-overlap.
An alkane has only C–H and C–C single bonds. The former result from the overlap of an sp 3 orbital of carbon with the 1s orbital of a hydrogen; the latter by the overlap of two sp 3 orbitals on adjacent carbon atoms. The bond lengths amount to 1.09 × 10 −10 m for a C–H bond and 1.54 × 10 −10 m for a C–C bond.
The average length of a C–C single bond is 154 pm; that of a C=C double bond is 133 pm. In localized cyclohexatriene, the carbon–carbon bonds should be alternating 154 and 133 pm. Instead, all carbon–carbon bonds in benzene are found to be about 139 pm, a bond length intermediate between single and double bond.
Diamond's C-C bond has a distance of away from each carbon since , while graphite's C-C bond has a distance of away from each carbon since . Although both bonds are between the same pair of elements they can have different bond lengths.
Both of these effects tend to reduce the carbon-carbon bond order, leading to an elongated C−C distance and a lowering of its vibrational frequency. In Zeise's salt K[PtCl 3 (C 2 H 4)]. H 2 O the C−C bond length has increased to 134 picometres from 133 pm for ethylene. In the nickel compound Ni(C 2 H 4)(PPh 3) 2 the value is 143 pm.
This is because the bond angle for an alkene, C-C=C, is 122°, while the bond angle for an alkane, C-C-C, is 112°. When these carbons form a small ring, the alkene which has a larger bond angle will have to compress more than the alkane causing more bond angle strain. [4] Cycloalkenes have a lower melting point than cycloalkanes of the same size.