Impurity conjugated double bonds

Batchwise adsorption is usually used for a final purification prior to the isolation of the desired compound and is most commonly used for decolorizing compounds. Colored compounds usually have many conjugated double bonds and thus are adsorbed in preference to many colorless compounds. Charcoal is commonly used as an adsorbent, since it is particularly effective in adsorbing the aromatic compounds which are often the colored impurity. Batchwise adsorption is also used to remove impurities which may inhibit crystallization or produce foaming in a distillation. 

As described at the beginning of the chapter, the types of compounds that absorb UV radiation are those with nonbonded electrons (n electrons) and conjugated double bond systems (tt electrons) such as aromatic compounds and conjugated olefins. Unfortunately, such compounds absorb over similar wavelength ranges, and the absorption spectra overlap considerably. As a first step in qualitative analysis, it is necessary to purify the sample to eliminate absorption bands due to impurities. Even when pure, however, the spectra are often broad and frequently without fine strucmre. For these reasons, UV absorption is much less useful for the quahtative identification of functional groups or particular molecules than anal) tical methods such as MS, IR, and NMR. UV absorption is rarely used for organic stmctural elucidation today in modern laboratories because of the ease of use and power of NMR (Chapter 3), IR (Chapter 4) and MS (Chapters 9 and 10). 

If it is carried out intelligently, the preliminary examination may provide more information with less effort than any other part of the procedure. The simple observation that the unknown is a crystalline solid, for example, eliminates a large fraction of all organic compounds from consideration because many are liquids at room temperature. The color is also informative Most pure organic compounds are white or colorless. A brown color is often characteristic of small amounts of impurities for example, aromatic amines and phenols quickly become discolored by the formation of trace amounts of highly colored air-oxidation products. Color in a pure organic compound is usually attributable to conjugated double bonds (Sec. 8.4). 

Long sequences of conjugated double bonds are created, leading to light absorption and a yellow discoloration of the polymer. The process is prompted by imperfections in the chain and by impurities such as solvents or metal ions. Discoloration can occur early in the life of a polymer, long before physical properties are affected. Pendant groups on a polymer chain can react without affecting the main chain structure. For example, poly(vinyl alcohol) can dehydrate to form ether cross-links ... 

Fig. 1.3. Absorption spectra of small impurities which can exist in commercial polymers (a) conjugated double bonds and (b) conjugated carbonyl groups.

Microcrystalline white powder when pure gray when aluminum impurity present. Monoclinic crystals, d 0.92. Stable in dry air at room temperature, decomp above 125, slowly loses hydrogen at 120. decomp in moist air. may ignite on grinding in air. Solv (parts/100 parts solvent) 30 (ether) 13 (tetrahydrofuran) 10 (dimethylcellosolve) 2 (dibutyl ether) 0.1 (dioxane). Reacts rapidly with water and alcohols reduces aldehydes, ketones, acid chlorides and esters to alcohols nitriles to amines aromatic nitro compounds to azo compounds. Does noi at lack olefinic double bonds unless they are conjugated with a phenyl group and a carbonyl or nitrile group. 

The UV spectrum recorded by the liquid chromatograph s diode array UV detector shows an intense maximum at 236 nm. This indicates that the impurity contains the 4-ene-3-oxo moiety. However, the slightly hypsochromic shift (see Table 1), and especially the broadening of the peak as compared to that of norgestrel, indicate the presence of a double bond in the skeleton, presumably in the 8(14) position, which is not in conjugation with the... 

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