Atomic orbital model

6 Bonding in Organic Molecules. Atomic-Orbital Models 

Many important molecules such as ammonia, water, and hydrogen fluoride have atoms with unshared pairs of electrons  

If we formulate each of these molecules in such a way to minimize repulsions between like charges, a basically tetrahedral arrangement will be expected because this will place the nuclei (and electron pairs) as widely separated as possible. The water molecule could be formulated this way, as in 6, with the 

This simple picture predicts that the H-O-H bond angle should be tetrahedral, 109.5°. But actually it is 104.5°. 

A similar, but smaller, effect is expected for ammonia because now the repulsion is only between the one unshared pair and bonding pairs. The ammonia H—N—H angle is 107.3°, which is only slightly smaller than the tetrahedral value of 109.5°. 

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Carbon has six electrons around the atomic core as shown in Fig. 2. Among them two electrons are in the K-shell being the closest position from the centre of atom, and the residual four electrons in the L-shell. TTie former is the Is state and the latter are divided into two states, 2s and 2p. The chemical bonding between neighbouring carbon atoms is undertaken by the L-shell electrons. Three types of chemical bonds in carbon are single bond contributed from one 2s electron and three 2p electrons to be cited as sp bonding, double bond as sp and triple bond as sp from the hybridised atomic-orbital model. 

Starting with Bohr s version of 1913, the evolution of this model was examined in an attempt to highlight the assumptions and approximations that were made at each stage. As in the case of many other papers in this volume, there is an educational motivation for raising these questions, especially in view of the central role of the atomic orbital model at all levels of chemical education. My suspicion is that many chemical educators do not appreciate the extent to which this model is an approximation and the conditions under which it ceases to be applicable. 

Figure 2. [TCNE] [Bu,N].+ spin density obtained by MaxEnt reconstruction using an atomic orbital model, and subsequent projection onto the molecular plane of [TONE]-. Positive contour steps are 50 mpE/A- and negative contours are dashed (step 10mpB/A2). A significant off-centring is present.

BONDING IN ORGANIC MOLECULES. ATOMIC-ORBITAL MODELS... 

Saturated compounds such as the alkanes and their derivatives, which have normal tetrahedral angles for the bonds to carbon, can be formulated readily in terms of atomic orbitals with sp3 o- bonds to carbon. An example is shown in Figure 6-11, which also shows how an atomic-orbital model can be drawn in abbreviated style. The lines in this drawing correspond to bonds and are labeled as sp3 with sp3 (the overlapping orbitals of the C-C bond) or as sp3 with s (the overlapping orbitals of the C-H bonds). 

Figure 6-11 Abbreviated atomic-orbital model of ethane showing only the orbitals of the outer-shell electrons...

The results are hardly clearcut, but the bonding orbitals are considerably closer to sp3 (25% s and 75% p) than they are to 100% p. We recommend that the bonding orbitals of nitrogen and oxygen be considered to be sp3 and the unshared pairs designated simply as (n)2. An abbreviated atomic orbital model of methanol, CH3OH, made on this basis is shown in Figure 6-13. 

Figure 6-19 Atomic-orbital model of benzene showing the arrangements of the p2 orbitals on each of the carbons...

Atomic-orbital models, like that shown for benzene, are useful descriptions of bonding from which to evaluate the potential for electron delocalization. But they are cumbersome to draw routinely. We need a simpler representation of electron delocalization. 

Figure 6-21 Atomic-orbital model, valence-bond structures, and resonance-hybrid formula for the 2-propenyl radical...

Exercise 6-9 Set up atomic-orbital models to represent the hybrid structures of N03 , C032 , and N20. 

Exercise 6-10 Set up an atomic-orbital model of each of the following structures with normal values for the bond angles. Evaluate each model for potential resonance (electron delocalization). If resonance appears to you to be possible, draw a set of reasonable valence-bond structures for each hybrid. ... 

Draw atomic-orbital models for each of the following substances. Each drawing should be large and clear with all bonds labeled as either values expected for the bond angles and whether the molecule or ion should be planar or nonplanar. 

Draw an atomic-orbital model for each of the compounds listed in Exercise 6-15 that is consistent with the geometry deduced for each. 

Draw atomic-orbital models for thiophene and imidazole that are consistent with their being planar compounds with six 77-electron systems associated with five atomic nuclei. 

Exercise 10-17 Make atomic-orbital models of the Y and 2-fluoroethyl carbocations (CH3CHFe and FCH2CH2 ). Predict which should be formed more rapidly by the addition of H to fluoroethene. Give your reasoning. 

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