Practice Problems on Chromophores

Transcription

1 hromophores.1 Practice Problems on hromophores 1. Which one of the following compounds is visibly colored? 2. Which one of the following molecules is most likely to be colored? a 3 S H H H (H 3 ) 2 H H H 2 H 2

2 hromophores.2 3. Which one of the following compounds is not coloured? H r H H r H H H 3 H 3 H H H 4. s of May 23, 2007, it is a criminal offense in the US to aim laser pointers at aircraft. If aircraft windows could be tinted, what window colour can protect the pilots eyes from a green laser pointer? ) Green ) lear ) Yellow ) Red ) lue 5. The wavelength of maximum fluorescence is typically longer than the wavelength of maximum absorption. Which statement best explains this phenomenon? ) Just prior to fluorescence, the molecule is in an excited vibrational state. ) Fluorescence is a process that is quantum mechanically forbidden. ) Fluorescence is a very slow process. ) portion of the absorbed energy is lost as heat prior to fluorescence. ) The Pauli xclusion Principle is violated in fluorescence.

3 hromophores.3 6. Which one of the following absorbs the longest wavelength of light? H 3 S H H 3 S H 2 H 2 H 2 H 2 H S 3 H S 3 H H 3 S H 2 H 3 H 3 H 2 S 3 H H 3 H 3 7. Which one of the following molecules is highly coloured? H 3 H 2 2 H H H H H 2 H H H

4 hromophores.4 8. nthracene is colourless, while naphthacene is yellow-orange. Which one of the following statements is most likely to be correct? anthracene naphthacene ) nthracene is colourless because it absorbs all visible light ) nthracene absorbs in the UV region; naphthacene in the blue region ) nthracene absorbs no visible light, while naphthacene absorbs in the yelloworange region ) aphthacene absorbs at longer wavelengths because it has a less extended conjugated system of electrons ) aphthacene absorbs at shorter wavelengths because it has a more extended conjugated system of electrons 9. Which one of the following statements about chlorophyll and heme is correct? ) The physiological function of chlorophyll is to carry oxygen, while the function of heme is to absorb light. ) hlorophyll has a hydrophobic tail that tethers it to chloroplast membranes. ) hlorophyll contains a = bond that is reduced to in heme. ) oth heme and chlorophyll contain diarylazo chromophores. ) Two forms of heme exist, one of which has an aldehyde auxochrome.

5 hromophores Which one of the following is least likely to be coloured? H H H H Mg H 2 S 3 H H 3 S H H 3 S H 10. In photodynamic therapy, why is it important to use a photosensitizer that absorbs at a long wavelength? ) Molecular oxygen has the highest absorption at long wavelengths. ) Photosensitizers that absorb at a short wavelength are too expensive. ) Long wavelengths in the IR region are required to vibrationally excite oxygen. ) The high energy of long-wavelength light creates reactive oxygen species that are more cytotoxic than those generated with short-wavelength light. ) Tissue contains very few natural chromophores that absorb at long wavelengths, allowing the light to travel deeper into the tissue. 11. In the lab involving the extraction of spinach, which one of the following statements best describes why thin layer chromatography was used? ) To identify the plant pigments present in spinach ) To quantify the plant pigments present in spinach ) To separate the plant pigments present in spinach ) To structurally modify the plant pigments present in spinach ) To isomerize the plant pigments present in spinach

6 hromophores Which one of the following compounds would adsorb most strongly to silica gel? SH 3 r 2 H 13. mixture consisting of all five compounds in the previous question was spotted on a silica-gel TL plate. The plate was then developed in hexane. Which compound has the lowest R f value? 14. Referring to previous question, which compound would have the lowest R f if the TL plate were developed in acetone instead? 15. TL plate is spotted with a mixture of a, b, and c. fter developing with acetone as the eluant, three spots appeared with R f values of 0.1, 0.5, and 0.9. Which R f values most likely correspond to the given compounds? H 2 H 2 S a b c ) a (Rf 0.1), b (Rf 0.5), c (Rf 0.9) ) c (Rf 0.1), b (Rf 0.5), a (Rf 0.9) ) c (Rf 0.1), a (Rf 0.5), b (Rf 0.9) ) b (Rf 0.1), c (Rf 0.5), a (Rf 0.9) ) b (Rf 0.1), a (Rf 0.5), c (Rf 0.9)