26 CHARACTERISTIC INFRARED ABSORPTION BANDS OF FUNCTIONAL GROUPS Class of Compounds -Absorption, cm 1 Intensity -Assignment Class of Compounds Absorption, cm 1 Intensity Assignment Alkanes and 2850-3000 s C-H stretch Carboxylic Acids 2500-3500 s, broad O-H stretch Alkyls 1450-1470 s C-H bend R-C(O)-OH 1710-1715 s, broad C=O stretch skills required to be able to "interpret" an "unknown" IR spectra. A symmetric 'doublet' with medium intensity around 1370 cm-1 is characteristic for an isopropyl group, while an asymmetric 'doublet' between 1365 and 1390 cm-1 is often due to a t-Bu-group. A medium or strong peak in this area corresponds to aromatic ring. In order to determine the nature of an unknown compound correctly, other techniques such as NMR (1H,13C, HETCOR, DEPT), Mass spectrometry, the chemical reactivity (solubility tests, sometimes derivatives) and physical properties (melting point, boiling point, refractive index) are required as well. In some instances, it is useful to remember that aromatics in general show a lot more bands than compounds that do not contain an aromatic ring. Peaks that are due to N-H-stretching modes are sharper than O-H-peaks (3300-3500 cm-1). The number of basic stretching and bending modes expected for a molecule increases with the number of atoms in the molecule. View desktop site, Label the following stretches on the IR for An aromatic compound will typically show IR bands in the following regions : i) a C-H stretch at 3080-3030 cm-1 (often rather weak); ii) a variable number of two, three or four C=C stretches between 1600 and 1450 cm-1 whose intensities can vary from case to case; They show many small bands of no diagnostic value, but some are useful. functional groups are provided below. The following has been observed for simple benzene derivatives (bending modes of tri-, tetra- and pentasubstituted
There are no peaks in this area for tertiary amines. previous case which are weaker and sharper. Carboxylic acids contain both C=O and OH groups. 3. iv) weak overtone and combination bands are useful indicators of an aromatic compound
With the help of quantum mechanical calculations (Schroedinger equation) you can find the frequencies of basic stretching and bending modes: masses of involved atoms The pattern of overtone bands in the region 2000-1665 cm-1 reflect the substitution pattern on the ring. If the masses of the involved atoms increase, the peak will shift to lower wavenumbers e.g. The C-N-triple bond of nitriles (example 10) (2100-2160 cm-1). Aromatic; C-H: stretch: 3000-3100 medium: C=C: stretch: 1400-1600 medium-weak, multiple … Acetone (2-propanone) is the "classic" carbonyl containing compound with the obvious C=O stretch in the middle of the spectra. The two strong bands arise from the symmetric and antisymmetric "C-O" stretches of the ester "COO" group, and the weaker band is the ether "CO" stretch of the "OCH"_3 group. Primary amines (example 7) have two peaks (sym./asym. The characteristic absorption of the alcohol, 2-propanol,
IR: aromatics. "C-C" Stretching The benzene ring has characteristic medium-strength "C-C" stretching bands in the "1600-1585 cm"^(-1) and "1500-1400 cm"^(-1) regions. The differences in wavenumbers are mainly due to different hybridization (=bond strength, the s-character in the bond, the stronger the bond is) and number of ligands on the carbon atom. C-O, C-Cl, O-H, etc. CO2 one expect to find 3N-5 (3*3-5=4) modes. If you are presented with two spectra and told that one is aromatic and one is not, a quick glance at the sheer multitude of bands in one of the spectra can tell you that it is the aromatic compound. IR Spectrum of Propylbenzene (Typical Absorptions of Aromatic Compounds) aryl C−H stretch 3100-3050 cm-1 aromatic overtones 1941, 1870, 1801, 1604 cm-1 (monosubstituted shown here) ←1538 cm←1584 cm-1 -1 1496 & 1453 cm-1 C=C in-plane vibration 743 cm-1 698 cm-1 monosubstituted C−H bend Region I Region II Region III Region IV This model assumes that the two masses (with known weight) are connected with a spring (with known strength). Unfortunately, the change in vibration modes is always accompanied with change in rotational mode (Stokes and Anti-Stokes). Help for aromatic C-H and C=C stretches. ... Aromatic C-C Stretches 1600-1585; 1500-1400 cm–1 Aromatic C-H in plane bends 1300-1000 cm–1 out of plane ring bending 428 cm–1. Identification of Organic Compounds. Although IR spectroscopy can reveal a lot of information about a molecule, but it is not enough to identify an unknown compound for sure. benzenes have been omitted for clarity). The actual number of your vibration changes quite at bit, especially for highly polar compounds (Why ?). Nobody is expected to memorize all those numbers. The C-C-triple bond of alkynes (2130-2150 cm-1) is usually fairly weak, if observed at all. For all 'anhydride type systems' (O=C-X-C=O, X=O, NR, S, CH2) two peaks are observed in this region due to a symmetric and an asymmetric stretching mode. If the force constant F (= bond strength) increases, the stretching frequency will increase as well (in cm-1). From 2700-4000 cm-1 (E-H-stretching: E=B, C, N, O) In this range typically E-H-stretching modes are observed. Note that the peak is a very strong absorption. The C N stretching frequency in the infrared spectra of Schiff's base complexes—I. Vibrational modes within a molecule can be described using the anharmonic oscillator model. If there is no symmetry in the molecule most of them will be observed the IR spectrum; the remaining modes will be observed in a Raman spectrum. Note that this is at slightly higher frequency than is the CH stretch in alkanes. The C-H-stretching modes can be found between 2850 and 3300 cm-1, depending on the hydrization. Based on all these data, a structural suggestion for an unknown compound can be made. Distinguishing aromatic subtypes:
Example 1: Alkane - Undecane, Example 4: Substituted Arene - Ethylbenzene, Example 6: Carboxylic Acid - Hexanoic Acid, Example 8: Secondary Amine - Diethylamine, Example 9: Tertiary Amine - Triethylamine, Example 11: Carboxylic Acid Chloride - Valeryl chloride, Example 12: Carboxylic Acid Anhydrides - Propionic anhydride, Example 17: Nitro compounds - 1-Nitropropane. Sometimes it is only observed as a shoulder. Alkyl halides are compounds that have a C–X bond, where X is a halogen: bromine, chlorine, fluorene, or iodine (usually Br or Cl in the organic chemistry teaching labs). e. Below 1000 cm-1 (out of plane modes, C-X: X=Cl, Br, I, heavier atoms) QD272.S6 S55, A more complete listing of functional group absorption frequencies Latter ones are usually weak or medium in intensity. John Wiley and Sons, 1981. The spectrum of toluene is shown below. containing compound with the obvious C=O stretch in the middle of the spectra. The = C–H stretch in aromatics is observed at 3100-3000 cm -1.