Infrared spectrophotometer application in organic analysis

Applications in organic analysis 1. The combination of the atomic groups in a compound is determined by the characteristic functional groups present in the infrared spectrum.

(1) The structure of para-cresol of brominated tetrachlorinated chlorinated compound. In the past, it was thought that it had three possible structures, but it could not be identified and confirmed. Only one structure was confirmed by infrared spectroscopy.

(2) The dialdehyde acetal condensation ketone should be of formula (I). If formula (I) is replaced by pyridyl, the chemical properties and (I) are not the same. It has an enediol type reaction such as (II). However, in the polar olefin solution, the 3700 cm(-1)-spectrum band of the free hydroxyl group was not seen, but the hydrogen bond at the 2750 cm(-1) appeared. It is known that it has formed intramolecular hydrogen bonds.

(I) Hydroxyketols (II) Alkenediols Formula 2. Determination of Isomers - Determination of Stereoisomers and Isomers (1) Determination of cis-transomers - cis-trans isomer radicals Since there is no center of symmetry in the arrangement order, the C=C double bond is at 1630 cm(-1), 724 cm(-1), and the trans C=C is at a higher frequency.

(2) Identification of isomers—In the infrared spectrum 900-660 cm(-1), homologs with different benzene ring substitution positions can be seen. For example, the absorption bands of the three isomers of xylene are very different. Ortho-position at 742cm(-1), meta-position at 770cm(-1), alignment at 800cm(-1), and para-xylene symmetry, its C=C double bond (benzene skeleton) is 1500cm ( -1) becomes smaller and the 600 cm(-1) band disappears.

Again, n-propyl, isopropyl, and t-butyl can be seen from the bending vibration of the methyl group in the infrared spectrum. Only one absorption band appears at 1375 cm (-1), which is expressed as n-propyl; if there is an equal intensity double peak at 1375 cm (-1), it is isopropyl; if it is at 1390 cm (-1) and 1365 cm ( -1) If there is a strong and weak band, it is tert-butyl.

Ethanol and methyl ether have the same molecular formula C2H6O, ethanol has a hydroxyl absorption band at 3500 cm(-1), C-0 stretching vibration is at 1050 to 1250 cm(-1), and hydroxyl bending vibration is at 950 cm(-1). Methyl ether absorbs no hydroxyl groups at 3500 cm (-1). Its first strong 1150 ~ 1250cm (-1), the two isomers are easily distinguishable.

3. Inspection of Chemical Reactions—Whether a chemical reaction has been carried out completely, infrared spectroscopy may be used, as both the raw material and the intended product have characteristic absorption bands.

For example, when the secondary alcohol is a ketone, the absorption of the hydroxy group of the secondary alcohol of the raw material should disappear, and the carbonyl group 171 cm(-1) of the ketone should appear in the product until the reaction is complete. 4. Analysis of unknowns - The unknowns can be separated and purified first, elemental analysis can be made, formulas can be written, and the degree of unsaturation can be calculated. The information of the main functional group of this unknown can be obtained from the infrared spectrum to determine which compound it belongs to. Combining UV, NMR, etc. can identify the structure of this compound.