Acetylene, bond angles structure

X-ray structural study of a platinum(II)-acetylene complex (6J) it seems likely that the acetylenic bond angles in the complexes (XXII) will be found to be closer to 120° than 180°. Both proton NMR and infrared spectra of the complexes (XXII) indicate that the R groups are in different structural environments (31). 

One more hybridization scheme is important m organic chemistry It is called sp hybridization and applies when carbon is directly bonded to two atoms as m acetylene The structure of acetylene is shown m Figure 2 18 along with its bond distances and bond angles Its most prominent feature is its linear geometry... 

Figure 8.1 The structure of acetylene, H —C=C—H. The H--0 C bond angles are 180°, and the C=C bond length is 120 pm. The electrostatic potential map shows that the r bonds create a negative (red) belt around the molecule.

An X-ray structure analysis of 74 (R=C4Hg) revealed that the unsaturated portion of the molecule was planar, with the angles between adjacent acetylenic bonds deviating by 13 -15° from 180°, the value for a strain-free molecule. Since the connection of the alkyne moieties to the aromatic rings was only shifted slightly (2-3°), distortion of the acetylene linkages appears as the major source of instability in these macrocycles. 

Arynes present structural features of some interest. They clearly cannot be acetylenic in the usual sense as this would require enormous deformation of the benzene ring in order to accommodate the 180° bond angle required by the sp1 hybridised carbons in an alkyne (p. 9). It seems more likely that the delocalised 7i orbitals of the aromatic system are left largely untouched (aromatic stability thereby being conserved), and that the two available electrons are accommodated in the original sp2 hybrid orbitals (101) ... 

Both the carbon atom and the nitrogen atom of the cyano group are sp hybridized, and the R—C = N bond angle is 180° (linear). The structure of a nitrile is similar to that of a terminal alkyne, except that the nitrogen atom of the nitrile has a lone pair of electrons in place of the acetylenic hydrogen of the alkyne. Figure 21-1 compares the structures of acetonitrile and propyne. 

The C-H and C-C cr-bonds are all trigonal sp hybrids, with 120° bond angles. The two unhybridized p-orbitals form a 7r-bond, which gives the molecule its rigid planar structure. The two carbon atoms are connected by a double bond, consisting of one o and one tt. The third canonical form of 5/ -hybridization occurs in C-C triple bonds, for example, acetylene (ethyne). Here, two of the p AO s on each carbon remain unhybridized and can form two n -bonds, in addition to two (r-bonds. Acetylene H-C=C-H is a linear molecule, as shown below, since the p-hybrids are oriented 180° apart. 

X-ray crystal structures of both 97 (all-tra/is isomer) and 99 trans is,trans isomer) have been obtained [39], and they confirm the relief of angle strain at the acetylenic carbon atoms that the phosphorus atoms provide in these small-ring pericyclynes (actual geometries shown in Fig. 9-28 average phosphorus atom endocyclic bond angles = 96° in 97 and 91° in 99 average acetylene carbon bond angles = 174° in 97 and 163° in 99). 

Predict the geometric structure of the acetylene molecule, C2H2. What are the bond angles in this molecule ... 

Acetylene prefers a linear molecular arrangement with bond angles of 180 rather than 120 as assumed in the resonance contributing structures. 

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