Download Infrared Spectroscopy: Identifying Functional Groups in Organic Compounds - Prof. Peter B. and more Study notes Organic Chemistry in PDF only on Docsity! Analysis of Infrared Spectra I. Molecular Vibrations and Absorption of Infrared Radiation A. Molecular vibrations Every molecule executes a unique set of vibrations. Absorption of infrared(IR) radiation can occur only at these vibrational frequencies. Some of these vibrations, called fundamentals, involve mostly vibration of one bond even in a large molecule. Because most bonds have characteristic vibrational frequency ranges, e.g. C=O from 1800 to 1650 cm-1, a compound's IR absorption frequencies tell us what kinds of bonds it contains. This makes IR spectroscopy a valuable tool in structure determination, because it provides information on a compound's functional groups allowing us to distinguish alkanes, from alcohols, from aldehydes etc. B. Relative intensity of IR absorptions For IR radiation to be absorbed, the dipole moment of the molecule must change as the vibration occurs. (Such a vibration is said to be "infrared active".) For our purposes, this means that the bond which is vibrating must be polarized. Absorption intensity is proportional to bond polarity; highly polarized bonds give rise to strong IR absorptions. A second factor that affects the relative intensity of IR absorption bands is the relative number of bonds in the molecule that absorb at the same frequency. The C-H stretching absorption of alkanes has a weak to medium intensity. But most organic molecules have many C-H bonds and exhibit medium to strong C-H stretching absorption. C. Regions of the infrared spectrum It is useful to divide the IR spectrum into regions that are treated differently when spectra are analyzed. The high frequency end, 4000-1500 cm-1, is where most absorption bands characteristic of specific functional groups occur and is often called the functional group region. tag-IR spectrum with functional group and fingerprint regions labelled. The low frequency end, 1500-400 cm-1 contains many absorptions that are characteristic of large portions of the molecule. Identical fingerprint spectra is strong evidence that two samples are identical. II. Functional Groups and Compound Classes Most functional groups exhibit unique "group frequencies". Observation of infrared absorption at its characteristic frequency with the appropriate intensity is definitive evidence for the presence of a functional group. In some cases group frequencies overlap and more than one IR absorption must be used . The table below contains useful group frequencies that you can apply to identify the functional groups that define different classes of organic compounds. Characteristic IR Absorptions of Functional Groups a Class, functional group Group frequency (cm- 1) Relative absorption intensity Alkanes, alkyl groups C-H stretch C-H bend 2980-2850 1470-1450, 1400- 1360 medium to strong medium Alkenes =C-H stretch C=C stretch 3090-3010 1680-1620 medium very weak(trans) to medium(cis,term) Alkynes #C-H stretchb C#C stretch b 3300-3250 2250-2200 medium, (terminal alkynes only) weak(internal) to medium (terminal) Aromatics C-H stretch arom. C-C stretch 3100-3000 1630-1590, 1520-1480 medium medium Alcohols O-H stretch C-O stretch 3650-3300 1150-1030 strong, broad strong Amines 3450-3280 weak to medium, broad
