Color in Organic Compounds

Development of theories regarding the sharing of electrons led to the concept of the covalent bond, a particularly appropriate one for dealing with the carbon-carbon bonds found in almost all organic compounds. Linus Pauling s quantum mechanical calculations based on the idea of resonance led to an approach that came to be known as the valence bond (VB) approach, whereas another approach based upon linear combination of atomic orbitals to form molecular orbitals led to the molecular orbital (MO) approach. The latter model is very helpfid in understanding the reasons why certain types of organic molecules are colored, and can even predict the intensities of absorption bands for simple molecules. 

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Color in Organic Compounds. In organic molecules, particularly those containing conjugated chains of alternating single and double... 

Concerning the various factors responsible for color in organic compounds, it is worthwhile to point out that the chromophore must be part of a conjugated system. This is illustrated through the examples in Fig. 13.77. When the azo group is connected to methyl groups the resultant compound is colorless. When it is attached to aromatic rings, the compound... 

Theory internal H bond proposed as basis of color in organic compounds. 

Now that we understand the origin of color in organic compounds, we are finally able to explore how bleaches function. Most bleaching agents react with conjugated -ir systems, disrupting the extended conjugation ... 

Figure 1.1 The position of carbon in the periodic table. Other elements commonly found in organic compounds are shown in the colors typically used to represent them.

Light-activated shift of double bonds in organic compounds can promote the reaction of oxygen with a molecule. Once oxygen has bonded to a fuel component, oxidized organic compounds such as alcohols, aldehydes, esters, ethers, and acids can form. These compounds can then continue to react with other fuel components to form color bodies, gums, and insoluble deposits. 

By far the greatest number of photochromatic compounds, however, are organic. As in the inorganic compound mercury dithizon-ate, photochromism in organic compounds occurs when one form of a compound is exposed to light, which converts the compound to an isomer of a different color. In most such instances, the two isomers differ in that one is a closed form of the isomer and the other is an open form. In some cases, however, the two isomers differ in that one occurs in a trans form and the other in a cis form. Trans and cis isomers are forms of a compound in which two constituent atoms or groups are located on opposite or the same side of a molecule, respectively. 

Beilstein s test. A test to detect halogens in organic compounds. Copper gauze is heated in a flame until the flame shows no green color if the addition of an organic compound produces a green flame, a halogen is present. 

The pH of a solution can be determined using a pH meter (Figure 18-1) or by the indicator method. Acid-base indicators are intensely colored complex organic compounds that have different colors in solutions of different pH (Section 19-4). Many are weak acids or weak bases that are useful over rather narrow ranges of pH values. Universal indicators are mixtures of several indicators they show several color changes over a wide range of pH values. 

Alkane molecules are rather unreactive (except for being very flammable), but alkenes react with many other substances. When a drop of bromine is added to an alkene, for example, the deep orange color of the bromine immediately disappears as the bromine adds across the double bond to form a dibromo derivative. The double bond is called a functional group because its presence in a molecule causes reactivity at that particular site. There are a dozen or so functional groups that appear frequently in organic compounds. Some of the most common ones are Hsted in Table 2. The same molecule may contain several functional groups. Aspirin, for example, is both a carboxylic acid and an ester, and cholesterol is an alkene as well as an alcohol. 

In 1936 K. A. Jensen (2, 3) measured the moments of a number of platinum (II) dihalido complexes with tertiary phosphines and other such hgands. He showed that the cis complexes had moments of up to about 11.0 Debye units (D) and that atom polarization in complex compounds is much higher than in organic compounds. He also showed that all the complexes of this type had had their cis or trans configurations incorrectly assigned on the basis of color, in analogy with the diammine di-chloro platinum (II) complexes. 

Transitions between moiecuiar orbitais in organic compounds e.g. poly-aromatics, organic dyes, biological colorants such as chlorophylls and carotenes (see Chap. 4). The lowest energy transitions are those between the HOMO and... 

When testing for antimony in organic compounds, a small sample is ashed in a micro crucible and the residue is treated with a freshly prepared 1 % solution of diphenylamine or diphenylbenzidine in concentrated sulfuric acid. A blue color appears at once or within several minutes if antimony is present. The limit of identification of this procedure is 5 y antimony. 

This can be colored, as turbid surface water may contain humus as well as iron and manganese, either in the oxidized form or bound to organic matter. Water of this type is treated by aeration + coagulation + addition of alkali + clarification + sand filtration. The removal of color and organic compounds usually requires a method based on chemical precipitation. Some reduction in organic matter content, of course, can be achieved by simple filtration. The removal of iron and manganese, on the other hand, usually requires ... 

Note that some of the metals frequently encountered in simple organic compounds give characteristic flame colorations Na, yellow K, lilac through blue glass Ca, brick-red Ba, apple-green Cu, bright blue-green. Ag and Pb, no characteristic flame. 

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