Trigonal planar carbon atoms

Trigonal planar geometry Is common In carbon compounds that have double bonds. For example, ethylene has two trigonal planar carbon atoms. Each carbon atom has three ligands two hydrogen atoms and one CH2 group. 

The central event in an epimerization reaction is the removal of a proton from the chiral a-carbon of an amino acid residue. This event generates an enolate with a trigonal planar carbon atom. Replacement of the proton on the face opposite from which it was abstracted results in the inversion of the configuration of the a-carbon. In theory, this event can occur at any stage of peptide synthesis. In practice, however, epimerization is observed almost exclusively during the amide-bond-formation step. This discussion will be confined to that type of epimerization. 

Butanal has a trigonal planar carbon with a polar C=0 bond, so it exhibits dipole-dipole interactions in addition to van der Waals forces. There is no H atom bonded to O, so two butanal molecules cannot hydrogen bond to each other. 

We ll start with the most straightforward aromatic compound benzene. Benzene is a planar symmetrical hexagon with six trigonal (sp ) carbon atoms, each having one hydrogen atom in the plane of the ring. All the bond lengths are 1.39 A (compare C-C 1.47 A and C=C 1.33 A). All the shifts are the same (Bq 128.5). 

The most prominent feature of the host framework in 3 is the formation of the [COj" 6(NH2)2CS] aggregate, which may be described as two concave, three-leaved propellers attached to the same core. All three independent thiourea molecules C(l), C(2) and C(3) are hydrogen-bonded on the same side to the trigonal planar carbonate ion, which has a twofold axis passing through atoms C(5) and 0(2), whereas their symmetry-related partners are bound to the opposite face (Figure 8.9a). The stability of the host... 

Because an sp carbon is bonded to three atoms that define a plane, it is called a trigonal planar carbon. The unhybridized p orbital is perpendicular to the plane defined by the axes of the sp orbitals (Figure 1.12b). 

In ethene the situation is rather different here, each carbon atom has one 2s and two 2p orbitals hybridised to form three sp single-pear orbitals which are trigonal planar (shown shaded in each half of Figure 2. JO). The remaining 2p orbital is not hybridised,... 

The element before carbon in Period 2, boron, has one electron less than carbon, and forms many covalent compounds of type BX3 where X is a monovalent atom or group. In these, the boron uses three sp hybrid orbitals to form three trigonal planar bonds, like carbon in ethene, but the unhybridised 2p orbital is vacant, i.e. it contains no electrons. In the nitrogen atom (one more electron than carbon) one orbital must contain two electrons—the lone pair hence sp hybridisation will give four tetrahedral orbitals, one containing this lone pair. Oxygen similarly hybridised will have two orbitals occupied by lone pairs, and fluorine, three. Hence the hydrides of the elements from carbon to fluorine have the structures... 

The carbon atoms of the double bond have a trigonal planar configuration and free rotation about the C—C bond is prevented by the n bond. The inability to rotate means that geometrical isomers can be produced, with substituents a and b, thus ... 

To ensure that the arrangement of four atoms in a trigonal planar environment (e.g., a sp -hybridized carbon atom) remains essentially planar, a quadratic term like V(0) = (fe/2) is used to achieve the desired geometry. By calculating the angle 9 between a bond from the central atom and the plane defined by the central... 

Multiple bonds are treated as a single unit m the VSEPR model Formaldehyde is a trigonal planar molecule m which the electrons of the double bond and those of the two single bonds are maximally separated A linear arrangement of atoms m carbon diox ide allows the electrons m one double bond to be as far away as possible from the elec Irons m the other double bond... 

The chemistry of propylene is characterized both by the double bond and by the aHyUc hydrogen atoms. Propylene is the smallest stable unsaturated hydrocarbon molecule that exhibits low order symmetry, ie, only reflection along the main plane. This loss of symmetry, which implies the possibiUty of different types of chemical reactions, is also responsible for the existence of the propylene dipole moment of 0.35 D. Carbon atoms 1 and 2 have trigonal planar geometry identical to that of ethylene. Generally, these carbons are not free to rotate, because of the double bond. Carbon atom 3 is tetrahedral, like methane, and is free to rotate. The hydrogen atoms attached to this carbon are aUyflc. 

We use different hybridization schemes to describe other arrangements of electron pairs (Fig. 3.16). For example, to explain a trigonal planar electron arrangement, like that in BF, and each carbon atom in ethene, we mix one s-orbital with two /7-orbitals and so produce three sp2 hybrid orbitals ... 

The vinylic carbon atoms, highhghted above, are both sp hybridized, and therefore trigonal planar. As a result, aU four groups (connected to the vinylic positions) are... 

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