CH447 CLASS 15INFRARED 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 AnhydridesOpen 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) have these bands at lower frequencies: 1780-1830 cm -1; 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 ChloridesSaturated 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:

NHR..

NHR

etc:

+

_

See spectrum of N-methylaniline, below.

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

Nitriles (R-CN), 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, and alkynes (often weak), are 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 strengthening caused by resonance.

NO O+: :

..

....

_ _..

..

..:: +

NOO

etc

_

+

..

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:

CO

OR _

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.

R S HThiols (mercaptans)

Compound, showingfunctional group Characteristic vibrations (cm-1)

SH str (weak) ~2550

R S R

O

Sulfoxides

S=O str (strong) ~1050

R S R

O

O

Sulfones

Asymm S=O str (strong) 1300

Symm S=O str (strong) 1150

R S

O

O

OH R S

O

O

OR

R S

O

O

NH(R or H)R S

O

O

Cl

Sulfonic acids Sulfonates

Sulfonyl chlorides Sulfonamides

Asymm S=O str (strong) 1325 - 1380

Symm S=O str (strong) 1140 - 1190

Plus S-O str (sulfonic acids) (strong)~650 (sulfonates) several bands at 1000 -750Plus N-H str ~3350, 3250 (1o); 3250 (2o)N-H bend at ~1550 (sulfonamides)

Examples are given below for benzenethiol and benzenesulfonamide.

IR Spectra of some Organophosphorus CompoundsSpectral 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 CompoundsThe 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, CHCCH2Cl).

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)