CH447 CLASS 14

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CH447 CLASS 14
INFRARED SPECTROSCOPY 2
Synopsis. IR spectra associated with common functional groups. Includes
discussion of electronic effects and influence on bond order (and hence vibration
frequency)
The major uses of IR spectra are:
1. Identification of an unknown compound by comparison of
its whole IR spectrum with those of known compounds,
held in data banks.
2. Identification of functional groups in an unknown or new
compound.
This course emphasizes the latter use.
Particular functional groups have characteristic absorption bands, depending on
the atoms and bonding arrangement of the atoms in the functional group. Also,
the actual strength of a particular bond in a particular functional group depends
on the rest of the molecule. Hence characteristic IR absorption bands are nearly
always given as ranges: sometimes narrow, sometimes broad, as shown in the
table below.
Functional
Band position/cm-1 (type of vibration)
group
Alkanes
alkyl
Intensity of absorption and
comments
and 2850-2960 (C-H str),
1370-1470 (C-H def)
m-s
m-s: doublet for -CMe2
and -CMe3
Alkenes
Aromatic
3010-3020 (C-H str),
m
1640-1680 (C=C str)
m
3010-3040 (C-H str)
w
~2000
w, combination bands
1500-1600 (ring def)
m-s, usually 2 or 3 bands
Alkynes
Halides
3300 (C-H str),
s
2100-2260 (CC str)
w-m
600-800 (C-Cl str), 500-600 (C-Br str), All s
500 (C-I str)
Alcohols
3400-3650 (O-H str),
s, broad
1050-1150 (C-O str)
s
1670-1780 (C=O str)
s
Aldehydes
2700-2900 (C-H str)
m, often two (doublet)
Carboxylic
2500-3100 (O-H str)
s, very broad
3300-3500 (N-H str)
m, often doublet
1490-1650 (N-H def)
w-m, often too weak
1030-1230 (C-N str)
m
Nitriles (-CN)
2210-2260 (CN str)
m
Nitro
1515-1560; 1345-1385 (NO2 str)
s, sym and asym stretch
Carbonyl
compounds
acids
Amines
compounds
(-
NO2)
IR Spectra of Some Common Functional Groups
IR Spectra of Hydrocarbons
These may be summarized as below and the following spectra serve as
examples.
CH stretch (centered on 3000 cm-1)
aromatic, alkene (3010 cm-1)
alkyne (3030 cm-1)
alkane (< 3000 cm-1): 2 – 4 peaks
Saturated CH deformation
1430 – 1470 cm-1 and 1380 cm-1 for CH3, CH2. For other examples of CH
deformations - see
spectra below
C=C stretch (centered on 1600 cm-1)
alkene (1600 – 1650 cm-1) (often weak, one peak) aromatic (1500 – 1600
cm-1) (several)
CC stretch (  2100 cm-1)
C=C-H deformation
alkene
terminal (880 – 1000 cm-1)
cis (670 – 730 cm-1)
trans (965 – 975 cm-1)
aromatic (out of plane)
monosubs (700 cm-1 and 750 cm-1)
o-disubs (730 – 770 cm-1)
m-disubs (700 cm-1 and 780 cm-1)
p-disubs (810 - 840 cm-1)
C=C Stretching Vibrations
These generally occur in the region 1670-1640 cm-1, for simple acyclic alkenes,
and are weak for symmetrically substituted alkenes. Their frequencies roughly
follow the pattern of relative bond strengths, monosubstituted < disubstituted <
trisubstituted < tetrasubstituted: ~1640, 1650, 1670 and 1670 cm -1, respectively.
Also, trans 1,2-disubstituted alkenes absorb at higher frequency (~1670 cm -1)
than corresponding cis isomers (~1658 cm-1).
Conjugation weakens the C=C bond, thus shifting stretching frequencies to lower
values:
C
C
C
+
C
C
C
C
_
..
C
 C=C ~ 1600 cm-1
 C=C = 1630 cm-1
Cyclic alkene C=C stretching frequencies sometimes fall outside the normal
range (1670-1640 cm-1), due to the variation in bond strength caused by the
alkene double bond being composed of higher “p-character” in small rings
(because of ring strain). The higher the “s-character” of a double bond, the higher
its strength. Cyclopropene is an exception (C=C = 1656 cm-1).
 C=C (cm-1)
1650
1646
1611
1566
Ring strain increases
(angle C=C-C decreases)
"p-character" of C=C increases
Bond strength increases
Alkene double bonds at a ring fusion absorb at lower frequencies:
C=C = 1611 cm-1
On the other hand, exocyclic double bonds have higher frequencies in highly
strained rings because of increased “s-character” of C=C:
 C=C (cm-1)
1780
CH2
CH2
CH2
CH2
1657
1678
1651
Like acyclic alkenes, increasing substitution increases the C=C stretching
frequency:
R
C=C (cm-1)
R
R
1611
1679
1650
R
R
R
C=C (cm-1)
1646
1675
1681
IR Spectra of Alcohols and Phenols
The characteristic peaks in the IR spectra of alcohols and phenols are
summarized below and examples are given by the following spectra.
OH stretch
“free”  3600 cm-1 (sometimes not observed) hydrogen bonded
( 3400 cm-1) (intense
and broad!)
CO stretch
1o ( 1050 cm-1), 2o ( 1100 cm-1) 3o ( 1150 cm-1) phenols ( 1230
cm-1)
Ethers show C-O stretches but not O-H stretches.
The broad hydrogen bonded OH stretching band disappears on dilution of the
sample if the hydrogen bonding is intermolecular: this absorption is not affected
by dilution if intramolecular hydrogen bonding is present.
The higher frequency of phenolic C-O stretching bands are due to bond
strengthening caused by resonance:
..
:OH
..
+ OH
etc
.._
When the OH group is attached to a an allylic (or alkyne equivalent, e.g.
CHCCH2-OH), a benzylic carbon, or a ring carbon, the C-O absorptions are
shifted 30-40 cm-1 with respect to the base 1o, 2o and 3o frequencies (see benzyl
alcohol below).
IR Spectra of Ethers
These are summarized below and examples are given in the following spectra.
CO stretch
Alkyl (1060 – 1150 cm-1) aryl and vinyl (1200 – 1275 cm-1) and also 1075 –
1200 cm-1
The C-O stretching frequency of vinyl ethers is higher than that of dialkyl ethers
because of the bond strengthening effect of resonance:
C
C
..
OR
..
_
..
C
C
+
OR
..
See ethoxybenzene, below.
Epoxides (oxiranes) possess two C-O stretching bands, an asymmetric mode at
950-815 cm-1 and a symmetric mode at 1280-1230 cm-1 (both medium-strong).
They also exhibit a strong band at 880-750 cm-1, due to ring C-O bending.
Alcohols (and phenols) show C-O stretches, but also O-H stretches. Carboxylic
acid derivatives show C-O stretches, but also C=O stretches.
IR Spectra of Carbonyl Compounds
Carbonyl functional groups in aldehydes and ketones, as well as in carboxylic
acids and their derivatives, are the easiest to identify by their IR spectra. Most
carbonyl groups give an intense sharp peak in the region 1670 – 1780 cm-1 (and
nearly all within the range 1640 – 1840 cm-1), which arises from the stretching of
the C=O bond. Furthermore, the exact position of the absorption within this range
can often be used to identify the exact type of carbonyl group (aldehyde, ketone,
carboxylic acid, ester, amide, acid chloride and anhydride: open chain or cyclic,
conjugated, aromatic or alkyl).
Aldehydes
The C=O stretching frequency behavior is summarized below.
CH3CH2CH=O (alkyl) CH3CH=CHCH=O (conjugated) PhCH=O (aromatic)
1730 cm-1
1705 cm-1
1705 cm-1
Many aldehydes also have a doublet at ~2800 cm -1 (at the CH stretching
position), due to Fermi resonance (see class 13). See spectra below.
Ketones
The carbonyl stretching frequency rises with increase in ring strain (increase in
“s-character” of C=O) and with donor resonance effect (as in ,-unsaturated
and aromatic ketones) – see the examples below.
1715 cm-1
O
O
1750 cm-1
O
CH3
C
CH3
O
1780 cm-1
six-membered rings and
saturated open chain ketones
smaller cyclic ketones
O
CH3
CH
CH
C
O
CH3
C
conjugated ketones 1690 cm-1
CH3
aromatic ketones 1690 cm-1
Halogen -substituents cause shifts to higher frequency, as a result of the
inductive (-I) effect of the halogen atom:
O
C
C
F
Also, the carbonyl stretching frequency of cyclic ketones depends on the
conformational position (equatorial or axial) of the -halogen:
O
O
Cl
Cl
Equatorial chlorine C=O = 1750 cm-1
Axial chlorine C=O = 1725 cm-1
Esters
In esters (and also acid chlorides and anhydrides) the inductive effect is more
pronounced than the opposite resonance effect, so that the C=O bond is
strengthened and basically stretches at higher frequency than aldehydes or
ketones.
..
:O
O
C
CH3
C
OCH3
-I effect of O
strengthens
C=O bond
CH3
..
OCH
3
..
+R effect of O
weakens C=O
bond
Conjugation at the carbonyl carbon, however, shifts the frequency back to lower
values (see methyl benzoate, below). Cyclic esters (lactones) have C=O
stretching frequencies that reflect the size of the ring and the % s-character of
the C=O bond (see the -, - and -lactones, below)
conjugated and aromatic esters
saturated open chain esters
O
O
CH3 C
O CH3
CH3
CH
1735 cm-1
CH
O
C
OCH3
C OCH3
1715 cm-1
1715 cm-1
cyclic esters (lactones)
O
O
1735 cm-1
O
O
1770 cm-1
O
O
1820 cm-1
Hydrogen bonding can lower the stretching frequency of the ester (or other)
carbonyl group, as in methyl salicylate, below.
OCH3
C
O
H

C=O
= 1680 cm-1
O
Compare the spectra of ethyl benzoate and ethyl salicylate, below.
Amides
In amides, the resonance effect is stronger than the opposite inductive effect,
resulting in much lower values of C=O than ketones:
.. _
:O :
..
:O
C
R
..
N
C
R
O
+
..
N
+R effect weakens C=O bond
C
R
..
N
-I effect strengthens C=O bond
The solid phase IR spectra of primary and secondary amides show the C=O
stretching mode at 1680-1630 cm-1. This low frequency range arises partly
because of the above effect but is also due to hydrogen bonding, which is absent
in very dilute solutions, where C=O is about 1690 cm-1. Cyclic amides (lactams)
give the expected increase in C=O stretching frequency with drop in ring size.
See below of a summary of amide C=O stretching frequencies.
Saturated open chain: 1630-1690 cm-1.
Conjugated and aromatic: 1645-1705 cm-1.
Lactams (cyclic amides): 1670 cm-1 (6-ring and larger); 1700 cm-1 (5-ring); 1745
cm-1 (4-ring).
The N-H bending band (1640-1620 cm-1) often overlaps or almost overlaps the
C=O stretching band and is intense, giving the appearance of either a broad
band a doublet in that region. The NH2 group of primary amides appears as a
doublet at 3350-3180 cm-1, corresponding to symmetric and asymmetric
stretching modes. See the spectra below for illustration of some of these points.
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.
IR Spectra of sp-Hybridized Groups, Nitro Groups, Carboxylate Groups and
Amino Acids
Nitriles (R-CN), 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.
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.
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:
O
R
C
O
O
_
R
_
C
R
O
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.
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