Shape and Hybridisation

Recent additions to analyses of drug-like physical properties are simple descriptors of shape and aromaticity. The fraction of aromatic atoms and mol. wt were the two most important parameters in accounting for the solubility of a set of 3563 molecules. " The parameter Fsp, equal to the ratio of tetrahedral (sp hybridised) carbon atoms to total carbon atoms,was found to gradually increase in moving from research compounds (Fsp = 0.36) through phases... 

Atoms that are linked by electron-pair bonds are positioned so that orbital overlap is maximised. The orbitals used are also sensitive to bond overlap and hybridisation, so that atomic orbitals frequently mix to give hybrid orbitals with greater overlapping power. The shapes of atomic orbitals and hybrid orbitals are quite definite and point in fixed directions. This leads to the fact that covalent bonding is directional. From a geometrical point of view, the array of covalent bonds in a solid resembles a net. 

Good introduction to range of bonding, including intermolecular bonding. Includes useful material on atomic orbitals and hybridisation and shapes of molecules. 

When elements in Period 2 form covalent bonds, the 2s and 2p orbitals can be mixed or hybridised to form new, hybrid orbitals each of which has. effectively, a single-pear shape, well suited for overlap with the orbital of another atom. Taking carbon as an example the four orbitals 2s.2p.2p.2p can all be mixed to form four new hybrid orbitals (called sp because they are formed from one s and three p) these new orbitals appear as in Figure 2.9. i.e. they... 

Still better agreement could be expected (in particular in terms of the observed width of the features) if the non-spherical nature of the C84 cage, the off-centre position of the Sc ions and the non-rectangular shape of the C 2p-based hybridisation band were to be taken into account [34]. 

Because amines are tetrahedral so they are chiral if they have three different substituents. However, it is not possible to separate the enantiomers of a chiral amine because amines can easily undergo pyramidal inversion. This process interconverts the enantiomers. The inversion involves a change of hybridisation where the nitrogen becomes sp2 hybridised rather than sp3 hybridised. Because of this, the molecule becomes planar and the lone pair of electrons occupy a p orbital. Once the hybridisation reverts back to sp3, the molecule can either revert back to its original shape or invert. 

The nitrile group (CN) is linear in shape with both the carbon and the nitrogen atoms being sp hybridised. The triple bond linking the two atoms consists of one o bond and two n bonds. Nitriles are strongly polarised. The nitrogen is a nucleophilic centre and the carbon is an electrophilic centre. Nucleophiles react with nitriles... 

The four sp3 orbitals for these three atoms (i.e. N, O and Cl) form a tetrahedral arrangement having one or more of the hybridised orbitals occupied by a lone pair of electrons. For an isolated atom, nitrogen forms a pyramidal shape where the bond angles are slightly less than 109.5 (c. 107 )fig.(a). This compression of the bond angles is because of the orbital containing the lone... 

The unhybridised 2py orbital has the usual dumb-bell shape. Each of the sp2 hybridised orbitals has a deformed dumb-bell shape similar to an sp3 hybridised orbital. The difference between the sizes of the major and minor lobes is larger for the sp2 hybridised orbital than that in case of sp3 hybridised orbitals. 

The hybridised orbitals and the 2py orbital occupy spaces as far apart from each other as possible. The lobes of the 2p orbital occupy the space above and below the plane of the x and z axes. The three hybridised sp2 orbitals (major lobes shown only) will then occupy the remaining space such that they are as far apart from the 2py orbital and from each other as possible. Thus, they are all placed in the x-z plane pointing towards the corner of a triangle (trigonal planar shape). The angle between each of these lobes is 120. We are now ready to look at the bonding of an sp2 hybridised carbon. 

The shape of a molecule and the functional group depends on the type of hybridisation of atoms involved in its formation,... 

Alkanes These are the open chain organic compounds having the general formula Cn H2n + 2. In them all the bonds are o-bonds and so they are also called saturated hydrocarbons. All the carbon atoms in any alkane are sp3 hybridised and so their shape in tetrahedral. Since C-C and C-H o-bonds are strong. So the alkanes are unreactive to most of the chemical reagents. 

The functional group in alkenes is ( > C = C< ). The alkene functional group (R,C = CR,) is planar in shape with bond angles of 120. The two carbon atoms involved in the double bond are both sp2 hybridised. Each carbon has three sp2 hybridised orbitals which are used for a bonds while the p orbital is used for a n bond. Thus, the double bond is made up of one a bond and one n bond ... 

The localised M.O. lead to directed valence and the shape may depend on type of hybridisation used. The following table illustrates the point. 

Table Hybridisation and Structure involving lonepair Hybridi- Bonded Lone Bond Shape Example sation pair pair angle...

In the nitrogen atom, the same hybridisation of the atomic orbitals occurs and so four sp hybrid orbitals are formed. These are distributed around the central atom in the same tetrahedral manner as in carbon. Thus, suggest the shape of the ammonia molecule. 

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