Infrared Spectroscopy 3

CH437 CLASS 15

INFRARED SPECTROSCOPY 3

Synopsis. IR spectra associated with common functional groups continued. Includes acid chlorides, acid anhydrides, groups containing sp-hybridized carbon, amines, carboxylates, nitro groups, aminoacids, various sulfur and phosphorus compounds and finally halogen compounds.

Acid Anhydrides

Open chain saturated anhydrides have bands at: 1800-1850 cm-1; 1740-1790 cm-1 (always two bands, with the higher wavenumber band usually more intense), due to asymmetric and symmetric vibrations (see spectrum of propionic anhydride below and class 13).

Cyclic (5-ring) anhydrides have these bands at lower frequencies: typically 1780-1830 cm-1 and 1710-1770 cm-1 (two bands, the lower wavenumber band is more intense)

Conjugated unsaturation or aromatic character lowers the absorption position by 20-30 cm-1.

Acid Chlorides

Saturated acid chlorides have a strong band at 1790-1815 cm-1 (see spectrum of ethanoyl chloride, below), but when the carbonyl group is conjugated to an aromatic system a second (usually less intense) band appears at ca. 1730 cm-1. This is due to Fermi resonance involving the C=O stretching fundamental (~1790 cm-1) and the first overtone of an intense Ar-C bending mode at ca. 900 cm-1 (see benzoyl chloride below and class 13).

IR Spectra of Amines

The characteristic absorption of amine groups (arising from N-H stretching) occurs at 3300-3500 cm-1, in roughly the same region as hydroxyl absorption. However, N-H absorption bands are usually much sharper and less broad, and for NH2, there are two peaks, as shown for 1-butylamine, below.

Secondary amines give only one band at ca. 3300 cm-1, often weak for dialkylamines, but stronger for mixed or diarylamines. N-H bending (scissoring) occurs around 1600 cm-1 (alkylamines) or 1500 cm-1 (arylamines) and sometimes (as in the example above) its overtone is enhanced in intensity by Fermi resonance interaction with the symmetric N-H stretching mode. The C-N stretching vibration (ca. 1100 cm-1 for alkyl C-N) is often weak, but arylamines have generally a strong band at ca. 1300 cm-1, due to resonance:

See spectrum of N-methylaniline, below.

IR Spectra of sp-Hybridized Groups, Nitro Groups, Carboxylate Groups and Amino Acids

Nitriles (R-CºN), isocyanates (R-N=C=O), isothiocyanates (R-N=C=S), ketenes (R2C=C=O), allenes (R2C=C=CR2) and carbodiimides (RN=C=NR) all contain a central sp-hybridized carbon atom, and they all have characteristic absorptions in the 2300-2100 cm-1 region, often strong. These, alkynes (often weak), diazoalkanes and diazonium salts are the only common fundamental vibrations occurring in this region. Two examples are given below.

Aliphatic nitro compounds show asymmetric and symmetric stretches at 1600-1530 and 1390-1300 cm-1, respectively. The corresponding stretches for aromatic nitro compounds are found at 1550-1490 and 1355-1315 cm-1, respectively, again because of bond weakening caused by resonance.

See examples below.

Asymmetric stretching of the carboxylate group occurs at 1600 cm-1, whereas symmetric stretching is at ca. 1400 cm-1: at such low frequency because of the extensive delocalization in the ion:

Amino acids exist largely as zwitterions (RCH(NH3+)-COO-) and hence their IR spectra are characterized by broad N-H stretching band and low frequency C-O stretching bands (asymmetric and symmetric), that often overlap with N-H bending bands, as shown for leucine, below.

IR Spectra of Sulfur Compounds

The IR spectral characteristics of some organosulfur groups are summarized in the table.

Examples are given below for benzenethiol and benzenesulfonamide.

IR Spectra of some Organophosphorus Compounds

Spectral characteristics are summarized in the table below for selected phosphorus functional groups.

Phosphines RPH2, R2PH / Band position (cm-1)
P-H str / 2320-2270 (strong, sharp)
PH2 bend (RPH2) / 1090-1075 (asymm), 840-810 (sym)
(both med)
P-H bend (R2PH) / 990-885 (med)
Phosphine oxides R3P=O, Ar3P=O
P=O str / 1210-1140 (v. strong)
Phosphate esters (RO)3P=O
P=O str / 1300-1240 (v. strong)
R-O str / 1088-920 (strong, one or two bands)
P-O str / 845-725 (med)

The IR spectra of methyl phenyl phosphine (PhP(Me)H) and diphenyl methyl phosphate ({PhO}2P(O)OMe) are shown below.

IR Spectra of Halogen Compounds

The running of IR spectra of alkyl and aryl halides on a dispersive instrument requires a KBr plate or disc, because of the low frequency of C-Hal str. for the heavier (Br, I) halogens.

The major IR features are summarized in the table below and below that is the IR spectrum of 3-chloropropyne (propargyl chloride, CHºCCH2Cl).

Fluorides / C-F str (strong) at 1400-1000 cm-1. Monoalkyl fluorides absorb at the low frequency end, polyfluorinated alknes and aryl fluorides at the upper end.

Chlorides

/ C-Cl str (strong) 785-540 cm-1 (alkyl), 1100-1035 cm-1 (aryl)
Bromides / C-br str (strong) 650-510 cm-1 (alkyl), 1075-1030 cm-1
Iodides / C-I str (strong) 600-485 cm-1 (alkyl)

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