The Effect of Conjugation on

The wavelength at which the Tr— Tr transition occurs increases as the number of conjugated double bonds in the compound increases. For example, the A, ax for methyl vinyl ketone is at a longer wavelength (219 nm) than the for acetone (195 nm), because methyl 

The A ax values of the -tt tt transition for several conjugated polyenes are listed in Table 14.6. Thus, the Ama of a compound can be used to estimate the number of conjugated double bonds in a compound. Notice that both the A ax and the molar absorptivity increase as the number of conjugated double bonds increases. 

The increases as the number of conjugated double bonds increases. 

Conjugation raises the energy of the HOMO and lowers the energy of the LUMO. 

If a compound has enough conjugated double bonds, it will absorb visible light (light with wavelengths 400 nm) and the compound will be colored. For example, S-carotene, a precursor of vitamin A with a A x = 455 nm, is an orange substance found in carrots. 

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The effect of conjugation on bond stability is revealed by comparing 1,3-pentadiene and 1,4-pentadiene. Figure 10-43 shows that both have eight C—bonds, two C—C bonds, and two C C bonds. The only significant... 

The effect of conjugation on the n- and rr -orbitals is analogous to the previous example with dienes but the n orbital remains virtually unchanged as a result of the conjugation. Figure 2.12 shows the effect of conjugation on the lowest-energy n —> n and n —> it transitions. 

Consider the effects of conjugation on electron transfer and of free rotation around a C — C bond modifying the closeness of metal centers. 

The effect of conjugation on the UV-Vis spectrum of a simple chromophore is shown in Table 2.2, in which we can see that the conjugated MVK has an increased Amax and for both absorption bands. 

Identify the C=O absorptions in the spectra of these esters and note the effect of conjugation on the C=O stretch. You should also identify, and compare the wavenumbers of, the C—O absorptions of 2-methy propanal (Figure 3.14) and the conjugated aldehyde benzaldehyde (Figure 3.2). 

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 on the Long Wavelength Absorption Band... 

Figure 21.4 illustrates the effects of conjugation on the location of the carbonyl absorption in some representative compounds. 

The naturally occurring ionone pair 37 and 38 demonstrate again that the effect of conjugation on acyclic carbonyl NMR chemical shift is smaller than that for cyclic a,P-unsaturated ketones. 

In addition, in the visible region, ligand field and charge transfer spectra are also observed. For further information regarding the origin of UV-visible spectra and the effect of conjugation on chromophores, Rao [3], Dodd [4] and Jaffe and Orchin [5] should be consulted. 

To understand the effects of conjugation on molecules, we need now to look at their molecular orbitals. We ll concentrated only on the electrons in n orbitals—you can take it that all the C—C and C—H a bonds are essentially the same as those of all the other molecules you met in Chapter 4. We ll start with the simplest compound that can have two conjugated n bonds butadiene. As you would expect, butadiene prefers to be planar to maximize overlap between its p orbitals. But exactly how does that overlap happen, and how does it give rise to bonding ... 

Nevertheless, the effect of conjugation on the carbon acid compared with methane is enormous (-30 pH units) and brings proton removai from carbon within the range of accessible bases... 

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