CH447 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) have these bands at lower frequencies: 1780-1830 cm-1; 17101770 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:
..
NHR
+
NHR
_:
etc
See spectrum of N-methylaniline, below.
IR Spectra of sp-Hybridized Groups, Nitro Groups, Carboxylate Groups and
Amino Acids
Nitriles (R-CN), isocyanates (R-N=C=O), isothiocyanates (RN=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 16001530 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.
..
:O +
N
.. _
:
O
..
_ ..
O +
: ..
N
.. _
:
O
..
+
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:
R
C
O
_
O
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.
Compound, showing
functional group
Characteristic vibrations (cm-1)
R S H
Thiols (mercaptans)
SH str (weak) ~2550
O
R
S
R
S=O str (strong) ~1050
Sulfoxides
O
Asymm S=O str (strong) 1300
R
S
R
Symm S=O str (strong) 1150
O
Sulfones
O
R
S
O
OH
O
Sulfonic acids
R
S Cl
O
Sulfonyl chlorides
OR
O
O
R
S
Asymm S=O str (strong) 1325 - 1380
Symm S=O str (strong) 1140 - 1190
Sulfonates
O
R
S
NH(R or H)
O
Plus S-O str (sulfonic acids) (strong)
~650
(sulfonates) several bands at 1000 750
Plus N-H str ~3350, 3250 (1o); 3250 (2o)
N-H bend at ~1550 (sulfonamides)
Sulfonamides
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, CHCCH2Cl).
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), 11001035 cm-1 (aryl)
Bromides
C-br str (strong) 650-510 cm-1 (alkyl), 10751030 cm-1
Iodides
C-I str (strong) 600-485 cm-1 (alkyl)