Infrared spectroscopy aromatic compounds

Analytical and Test Methods. o-Nitrotoluene can be analyzed for purity and isomer content by infrared spectroscopy with an accuracy of about 1%. -Nitrotoluene content can be estimated by the decomposition of the isomeric toluene diazonium chlorides because the ortho and meta isomers decompose more readily than the para isomer. A colorimetric method for determining the content of the various isomers is based on the color which forms when the mononitrotoluenes are dissolved in sulfuric acid (45). From the absorption of the sulfuric acid solution at 436 and 305 nm, the ortho and para isomer content can be deterrnined, and the meta isomer can be obtained by difference. However, this and other colorimetric methods are subject to possible interferences from other aromatic nitro compounds. A titrimetric method, based on the reduction of the nitro group with titanium(III) sulfate or chloride, can be used to determine mononitrotoluenes (32). Chromatographic methods, eg, gas chromatography or high pressure Hquid chromatography, are well suited for the deterrnination of mononitrotoluenes as well as its individual isomers. Freezing points are used commonly as indicators of purity of the various isomers. 

Air Monitoring. The atmosphere in work areas is monitored for worker safety. Volatile amines and related compounds can be detected at low concentrations in the air by a number of methods. Suitable methods include chemical, chromatographic, and spectroscopic techniques. For example, the NIOSH Manual of Analytical Methods has methods based on gas chromatography which are suitable for common aromatic and aHphatic amines as well as ethanolamines (67). Aromatic amines which diazotize readily can also be detected photometrically using a treated paper which changes color (68). Other methods based on infrared spectroscopy (69) and mass spectroscopy (70) have also been reported. 

Infrared radiation, electromagnetic spectrum and, 419, 422 energy of. 422 frequencies of, 422 wavelengths of, 422 Infrared spectroscopy, 422-431 acid anhydrides, 822-823 acid chlorides, 822-823 alcohols. 428, 632-633 aldehydes, 428. 730-731 alkanes, 426-427 alkenes, 427 alkynes, 427 amides. 822-823 amines, 428, 952 ammonium salts, 952-953 aromatic compound, 427-428, 534 bond stretching in, 422... 

Speck, John C., Jr., The Lobry de Bruyn-Alberda van Ekenstein Transformation, 13, 63-103 Spedding, H., Infrared Spectroscopy and Carbohydrate Chemistry, 19, 23-49 Sprinson, D. B., The Biosynthesis of Aromatic Compounds from d-G1u-cose, 16, 235-270... 

Infrared Spectroscopy (Review) Aromatic compounds are readily identified by their infrared spectra because they show a characteristic C—C stretch around 1600 cm-1. This is a lower C—C stretching frequency than for isolated alkenes (1640 to 1680 cm-1) or conjugated dienes (1620 to 1640 cm-1) because the aromatic bond order is only about 1 The aromatic bond is therefore less stiff than a normal double bond, and it vibrates at a lower frequency. 

Trace analysis by IR spectroscopy, involving pre-concentration, separation, and computer techniques has been reported by Hannah et al. (1978). The term trace analysis is used to refer to concentrations in the ppm range up to 1%. In instances where interference is at its minimum, analysis may be performed in a straightforward manner by using difference techniques. However, there are many cases in which analysis may be complicated by the fact that the trace material is structurally similar to the matrix material. Moreover, the presence of other trace compounds is intolerable if their spectra interfere with that of the compound under investigation. In this case it is often necessary to use pre-concentration or separation techniques. This method is illustrated by analyses of aromatic isomers, gasoline additives, drugs, and polymer additives. The different aspects of trace analyses by infrared spectroscopy are discussed by Smith (1986). 

The most important samples for analysis by infrared spectroscopy for crude oil chemists are organic substances. For organic molecules, the infrared spectrum can be divided into three important regions. First is the absorption of infrared radiation within the wave number range of 4000 and 1300 cm 1 which is caused by functional groups and different bond types. Second is the absorption between 1300 and 909 cm 1 that is typical for more complex interactions in the molecules. And last is the absorption between 909 and 650 cm 1, which is usually associated with the presence of aromatic compounds in the sample. 

The use of near-infrared spectroscopy is also an effective approach to the analysis of artificial sweeteners. The near-infrared spectrum of saccharin is shown in Figure 6.8c. As aromatic compounds are not very common in food products, the modes due to these compounds are therefore very useful. Although saccharin is found at low concentrations in foods, it is normally added as a concentrated solution and this can be monitored by near-infrared measurements in the presence of other ingredients. 

Nuclear magnetic resonance spectroscopy ( C CP/MAS Solid State NMR) and Fourier transform infrared spectroscopy (FT-IR) were also performed for the freeze dried NOM sample. The results were both very noisy and paramagnetic compounds such as iron and manganese interfered with the - C NMR analysis. After 20 h of run time the sample showed mostly alkyl and alkyl-oxygen carbon, thus very little aromatic compounds. 

Preparations of azide derivatives from styrene-maleic anhydride copolymers, cellulose, and gelatin by attaching aromatic azide compounds are described in the literature. Most of the resultant polymers crosslink rapidly when exposed to light of 260 wavelength. Also, as much as 90% of the hydroxy groups of poly(vinyl alcohol) can be esterified withp-azido-benzoyl chloride. These reactions must be carried out in mixtures of chloroform and aqueous sodium hydroxide. Based on infrared spectroscopy, the following crosslinking mechanism was proposed ... 

The compounds Fe(CO), g(CNC4H3Me2-l,3X, (n = 1-5) are characterized by IR vibrational spectroscopy (vco and Vnc> 1800-2200-on region) and by NM R spectroscopy (aromatic and methyl protons). Selected spectroscopic data for these complexes are given in the Table I. Infrared spectroscopy may conveniently be used for the monitoring of the progress of the substitution reaction, and NMR spectroscopy for an estimate of product purity. The complexes may also be characterized by mass spectrometry... 

Table 4.2 Characteristic infrared bands of aromatic compounds. From Stuart, B., Modem Infrared Spectroscopy, ACOL Series, Wiley, Chichester, UK, 1996. University of Greenwich, and reproduced by permission of the University of Greenwich...

Essential oils are volatile compounds responsible for the aromas commonly associated with many plants (see essay "Terpenes and Phenylpropanoids")- The chief constituent of the essential oil from cloves is aromatic and volatile with steam. In this experiment, you will isolate the main component derived from this spice by steam distillation. Steam distillation provides a means of isolating natural products, such as essential oils, without the risk of decomposing them thermally. Identification and characterization of this essential oil will be accomplished by infrared spectroscopy. 

Infrared spectroscopy is an excellent technique for determining the structure of a polymer. For example, polyethylene and polypropylene have relatively simple spectra because they are saturated hydrocarbons. Polyesters have stretching frequencies associated with the C=0 and C—O groups in the polymer chain. Polyamides (nylon) show absorptions that are characteristic for the C=0 stretch and N—H stretch. Polystyrene has characteristic features of a monosubstituted aromatic compound (see Technique 25, Figure 25.12). You may determine the infrared spectra of the linear polyester from Experiment 46A and polystyrene from Experiment 46C in this part of the experiment. Your instructor may ask you to analyze a sample that you bring to the laboratory or one supplied to you. 

None of the unknowns to be issued for this experiment will be simple aromatic hydrocarbons. All aromatic compounds will have a principal functional group as a part of their structure. Nevertheless, in many cases it will be useful to be able to recognize the presence of an aromatic ring. Although infrared and nuclear magnetic spectroscopy provide the most reliable methods of determining aromatic compounds, often they can be detected by a simple ignition test. 

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