1.4- Pentadiene, molecular orbitals

Draw the structure of 1,3-pentadiene. Use valence-bond and molecular orbital pictures to describe the bonding for the (T-framework and -n-orhitals, respectively. 

In 1,3-pentadiene, however, the p orbitals are all able to overlap in such a way that a lower energy molecular orbital can be formed. We have more physical data available for 1,3-butadiene, so let us consider this slightly simpler conjugated system instead. 

Pentadiene, like 1,3-butadiene, has four tt electrons. However, unlike the delocalized TT electrons in 1,3-butadiene, the tt electrons in 1,4-pentadiene are completely separate from one another. In other words, the electrons are localized. The molecular orbitals of 1,4-pentadiene have the same energy as those of ethene—a compound with one pair of localized tt electrons. Thus, molecular orbital theory and contributing resonance structures are two different ways to show that the tt electrons in 1,3-butadiene are delocalized and that electron delocalization stabilizes a molecule. 

Record the absorption spectrum between 400 and 200 nm of (a) 1-octene, (b) 1,3-butadiene, and (c) a nonconjugated diolefin (e.g., 1,4-pentadiene). What is the effect of a conjugated system on the absorption spectrum What does the spectmm tell you about the relative energy of the molecular orbitals in each compound ... 

This reaction can be analysed by means of a simple extension of the analysis of the [l,3]-shift just discussed. Pentadiene has two carbon and two hydrogen atoms more than propylene the total number of orbitals is thus raised by ten to twenty-eight, as is the number of valence electrons. However, after the four acc and three fixed ctch bonds are combined to form seven localized molecular orbitals and their antibonding counterparts to seven more, we are left with 14 mobile electrons in 7 doubly occupied MOs. There is no need to draw a new correspondence diagram for the reaction to a slight amplification of Fig. 8.6 is all that is required. 

The narrower gap between the HOMO and the LUMO in 1,3-butadiene results from the conjugation of the double bonds. Molecular orbital calculations indicate that a much larger gap should occur in isolated alkadienes. This is borne out experimentally. Isolated alkadienes give absorption spectra similar to those of alkenes. Their /Imax are at shorter wavelengths, usually below 200 nm. As we mentioned, 1,4-pentadiene has its /Imax at 178 nm. 

Give a molecular orbital description for each of the following a. 1,3-pentadiene b. 1,4-pentadiene c. 1,3,5-heptatriene... 

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