Methanol hybrid orbitals

Like the carbon atom in methane and the nitrogen atom in methylamine, the oxygen atom in methanol (methyl alcohol) and many other organic mole-cnles can he described as sp -hybridized. The C-O-H bond angle in methanol is 108.5°, very close to the 109.5° tetrahedral angle. Two of the fonr sp hybrid orbitals on oxygen are occnpied by nonhonding electron lone pairs, and two are used to form bonds. 

Figure 1.23 shows the dipole moments of water and methanol. The six valence electrons of oxygen can also be distributed in three sp hybrid orbitals (Figure 1.24). The three sp orbitals ate coplanar and are separated by 120° in a trigonal planar arrangement. Two of the sp orbitals contain a pair of electrons. The other sp orbital contains a single electron that can form a O bond to an atom such as carbon. The single electron in the remaining 2p orbital forms a ir bond with the 2p orbital of another atom such as carbon.

The oxygen atom of methanol is sp hybridized. The C —O—C bond angle, 112°, is close to the tetrahedral bond angle (109.5). The two sets of lone pair electrons are in sp hybrid orbitals that are directed to two of the corners of a tetrahedron. 

The carbon that bears the functional group is sp hybridized m alcohols and alkyl halides Figure 4 1 illustrates bonding m methanol The bond angles at carbon are approximately tetrahedral as is the C—O—H angle A similar orbital hybridization model applies to alkyl halides with the halogen connected to sp hybridized carbon by a ct bond Carbon-halogen bond distances m alkyl halides increase m the order C—F (140 pm) < C—Cl (179 pm) < C—Br (197 pm) < C—I (216 pm)... 

Hirshfeld (1964) pointed out that bond bending not only occurs in ring systems, but also results from steric repulsions between two atoms two bonds apart, referred to as 1-3 interactions. The effect is illustrated in Fig. 12.3. The atoms labeled A and A are displaced from the orbital axes, indicated by the broken lines, because of 1-3 repulsion. As a result, the bonds defined by the orbital axes are bent inwards relative to the internuclear vectors. When one of the substituents is a methyl group, as in methanol [Fig. 12.3(b)], the methyl-carbon-atom hybrid reorients such as to maximize overlap in the X—C bond. This results in noncolinearity of the X—C internuclear vector and the three-fold symmetry axis of the methyl group. Structural evidence for such bond bending in acyclic molecules is abundant. Similarly, in phenols such as p-nitrophenol (Hirshfeld... 

Step I. The reactants (ACh or ASCh, methanol, imidazol(s) and/or imidazolium(s) form a "planar" intermediate complex between the carbonyl carbon of the substrate (which is in an sp "planar" hybridization state) and the oxygen of the attacking nucleophile methanol. The nucleophile is assumed to attack perpendicular to the plane of the substrate acetate (or thio-acetate) moity with the electrons from the methanol oxygen filling the lowest empty molecular orbital which is centered on the carbonyl carbon of the substrate. 

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