Question

Based on table 1,

why Lmax shifts towards higher wavelengths when you go from ethylene to b-carotene?

Figure 1. violet blue green yellow orange red purple 500 800 nm The greater the degree of conjugation of chromophores the greater the chance that the absorption of light will occur at higher wavelengths. Table 1 illustrates the effect of conjugation (where max gives the wavelength of maximum absorption): # of conjugated double bonds 175 Table 1. Compound Ethylene 1,3-Butadiene 1.3.5-Hexatricne B-Carotene 258 465 The absorbance (A) of a sample at a particular wavelength is dependent on its concentration (c, in moles/liter), on the path length of the sample (I. in cm), and on a physical constant characteristic of the absorbing sample (molar absorbtivity coefficient, t). This dependence is expressed in the Beer-Lambert law by (eq. 1): A = elc (eq. 1) A substance appears colored if it absorbs some wavelengths (or a band of wavelengths) from white light. The light that is reflected has these wavelengths missing and the color we perceive results from the wavelengths that remain. Figure 2 illustrates a color wheel that can be used to anticipate the colors resulting when selected wavelength regions are absorbed. Figure 2.

Answer 1

The Imax name depends on the no. of coringated double bond in a compound In Ethylene there is only I double bond. &, if you see ß-carotene there are Il conyugated double bonds. with the increase in double bonds, conjugation increases So, absorption increases That's why ethylene (dman -175) werens, &-corrotone (amax - 465).