Molecular orbitals in ethylene

What is wrong with the following sentence "The it bonding molecular orbital in ethylene results from sideways overlap of two p atomic orbitals."... 

The 7t bonding molecular orbital in ethylene results from the combination of twop atomic orbitals with the same algebraic sign. (A second molecular orbital can form by the combination of two p orbitals with opposite algebraic signs, but it is an antibonding orbital.)... 

Fig. 1.26 Energies of n molecular orbitals in ethylene and the allyl system...

Unlike the pi bonding molecular orbitals in ethylene, those in benzene form delocalized molecular orbitals, which are not confined between two adjacent bonding atoms, but actually extend over three or more atoms. Therefore, electrons residing in any of these orbitals are free to move around the benzene ring. For this reason, the structure of benzene is sometimes represented as... 

These absorptions are ascribed to n-n transitions, that is, transitions of an electron from the highest occupied n molecular orbital (HOMO) to the lowest unoccupied n molecular orbital (LUMO). One can decide which orbitals are the HOMO and LUMO by filling electrons into the molecular energy level diagram from the bottom up, two electrons to each molecular orbital. The number of electrons is the number of sp carbon atoms contributing to the n system of a neuhal polyalkene, two for each double bond. In ethylene, there is only one occupied MO and one unoccupied MO. The occupied orbital in ethylene is p below the energy level represented by ot, and the unoccupied orbital is p above it. The separation between the only possibilities for the HOMO and LUMO is 2.00p. 

Which molecular orbital of ethylene (tt or tt ) is the most impor 1 tant one to look at in a reaction in which ethylene is attacked by an electrophile J... 

In ethylene, both the HOMO and LUMO are formed primarily from p orbitals from the two carbons. The carbons lie in the YZ-plane, and so the p,j orbitals lie above and below the C-C bond. In the HOMO, the orbitals have like signs, and so they combine to form a bonding n molecular orbital. In contrast, in the LUMO, they have opposite signs, indicating that they combine to form an antibonding Tt molecular orbital. 

FIGURE 5.15 Molecular orbitals for ethylene. Promotion of an electron from the ground state to the excited state is known as a n - n transition and is usually accompanied by an absorption of radiation in the ultraviolet region of the spectrum. 

In phenyl substituted ethylene, stilbene CH=CH, the central -system can conjugate with the phenyl -orbitals. The ground state (I) and the lowest excited state (II) MO s in transtilbene are indicated as HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) in Figure 7.6. 

Let us now apply this method to a specific example. Consider the ethylene molecule with D2h symmetry. As can be seen from the character table of the L 2h point group (Table 6.4.2), this group has eight symmetry species. Hence the molecular orbitals of ethylene must have the symmetry of one of these eight representations. In fact, the ground electronic configuration for ethylene is... 

Some molecular orbitals of ethylene (point group Z>2h)- All are completely filled except l 2g in the ground state. The x axis points into the paper. 

So the eight pairs of electrons of this molecule occupy delocalized molecular orbitals lag to 1 3U, while the first vacant orbital is l g- Note that the names of these orbitals are simply the symmetry species of theZ)2h point group. In other words, molecular orbitals are labeled by the irreducible representations of the point group to which the molecule belongs. So for ethylene there are three filled orbitals with Ag symmetry the one with the lowest energy is called lag, the next one is 2ag, etc. Similarly, there are two orbitals with Z iu symmetry and they are called lb u and 2bi . All the molecular orbitals listed above, except the first two, are illustrated pictorially in Fig. 6.4.2. By checking the >2h character table with reference to the chosen coordinate system shown in Fig. 6.4.2, it can be readily confirmed that these orbitals do have the labeled symmetry. In passing, it is noted that the two filled molecular orbitals of ethylene not displayed in Fig. 6.4.2, lag and l iu, are simply the sum and difference, respectively, of the two carbon Is orbitals. 

In a Lewis structure, the double bond of an alkene is represented by two pairs of electrons between the carbon atoms. The Pauli exclusion principle tells us that two pairs of electrons can go into the region of space between the carbon nuclei only if each pair has its own molecular orbital. Using ethylene as an example, let s consider how the electrons are distributed in the double bond. 

In the pi bonding molecular orbital of ethylene, the lobes that overlap in the bonding region between the nuclei are in phase that is, they have the same sign (+ overlaps with +, and — overlaps with —). We call this reinforcement constructive overlap. Constructive overlap is an important feature of all bonding molecular orbitals. 

The orbital in ethylene that receives these electrons is the lowest-energy orbital available, the Lowest Unoccupied Molecular Orbital (LUMO). In ethylene, the LUMO is the tt antibonding orbital. If the electrons in the HOMO of butadiene can flow smoothly into the LUMO of ethylene, a concerted reaction can take place. 

C-Substituents. A double bond, lowers the % energy when it is conjugated to another double bond (see pages 28-30). The phenyl group is similar— the filled n molecular orbitals in styrene come at 2.14/3, 1.41/3, 1.00/3, 0.66/3 below the a level, and the LUMO at 0.66/3 above the a level. The total n stabilisation is 2 x 5.21/3, whereas the total n stabilisation for the separate components benzene and ethylene is 2 x 5.0/3. A C-substituent raises the energy... 

---