The Effect of Conjugation on Alkenes

Notice that two atomic orbitals were combined to build the molecular orbitals, and as a result, two molecular orbitals were formed. There were also two electrons, one in each of the atomic p orbitals. As a result of combination, the new n system contains two electrons. Since we fill the lower-energy orbitals first, these electrons end up in y/i, the bonding orbital, and they constitute a new 7t bond. Electronic transition in this system is a r— r transition from xpi to y/2 - 

Notice that the transition of lowest energy in 1,3-bntadiene, y/2 is a r transition and 

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FIGURE 10.12 A comparison of the molecular orbital energy levels and the energy of the r- - r transitions in ethylene and 1,3-butadiene. 

In a qualitatively similar fashion, many auxochromes exert their bathochromic shifts by means of an extension of the length of the conjugated system. The strongest auxochromes invariably possess a pair of unshared electrons on the atom attached to the double-bond system. Resonance interaction of this lone pair with the double bond(s) increases the length of the conjugated system. 

In the new system, the transition from the highest occupied orbital, ijfi, to the antibonding orbital, 1//3, always has lower energy than the transition would have in the system with- 

Notice that the transition of lowest energy in 1,3-butadiene, v 2 Ws is a /r — r transition and that it has a lower energy than the corresponding transition in ethylene, yri — y/2. This result is general. As we increase the number of p orbitals making up the conjugated system, the transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) has 

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The properties of a compound with isolated double bonds, such as 1,4-pentadiene, generally are similar to those of simple alkenes because the double bonds are essentially isolated from one another by the intervening CH2 group. However, with a conjugated alkadiene, such as 1,3-pentadiene, or a cumulated alkadiene, such as 2,3-pentadiene, the properties are sufficiently different from those of simple alkenes (and from each other) to warrant separate discussion. Some aspects of the effects of conjugation already have been mentioned, such as the influence on spectroscopic properties (see Section 9-9B). The emphasis here will be on the effects of conjugation on chemical properties. The reactions of greatest interest are addition reactions, and this chapter will include various types of addition reactions electrophilic, radical, cycloaddition, and polymerization. 

The effect of conjugation also is reflected in infrared carbonyl frequencies (Section 16-3A) and nmr spectra. With respect to the latter, it is found that the protons on the (3 carbon of a,/3-unsaturated carbonyl compounds usually come at 0.7 to 1.7 ppm lower fields than ordinary alkenic protons. The effect is smaller for the a protons. 

The effect of the solvent on the product distribution is observed in the conjugate addition of amines to 1-bromo-l-(phenylsulfonyl)alkenes (54) (equation 54)46. When the reaction is conducted in benzene at room temperature for 4 days, the adduct 55 is formed in good yield. On the other hand, the reaction in DMSO at 80-90 °C for 2.5 h affords 2-(phenylsulfonyl)aziridines (56) and no adduct (55) is isolated. 

The effect of a chiral substituent at the alkene terminus on the course of the 1,7-electrocyclization of diene-conjugated diazo compounds has also been examined [88TL6361 94JCS(P1)3149]. Cyclization of diazo com-... 

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