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 group / Band position/cm-1 (type of vibration) / Intensity of absorption and comments
Alkanes and alkyl / 2850-2960 (C-H str),
1370-1470 (C-H def) / m-s
m-s: doublet for -CMe2 and -CMe3
Alkenes / 3010-3020 (C-H str),
1640-1680 (C=C str) / m
m
Aromatic / 3010-3040 (C-H str)
~2000
1500-1600 (ring def) / w
w, combination bands
m-s, usually 2 or 3 bands
Alkynes / 3300 (C-H str),
2100-2260 (CC str) / s
w-m
Halides / 600-800 (C-Cl str), 500-600 (C-Br str), 500 (C-I str) / All s
Alcohols / 3400-3650 (O-H str),
1050-1150 (C-O str) / s, broad
s
Carbonyl compounds / 1670-1780 (C=O str) / s
Aldehydes / 2700-2900 (C-H str) / m, often two (doublet)
Carboxylic acids / 2500-3100 (O-H str) / s, very broad
Amines / 3300-3500 (N-H str)
1490-1650 (N-H def)
1030-1230 (C-N str) / m, often doublet
w-m, often too weak
m
Nitriles (-CN) / 2210-2260 (CN str) / m
Nitro compounds (-NO2) / 1515-1560; 1345-1385 (NO2 str) / s, sym and asym stretch

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 aromatic (out of plane)

terminal (880 – 1000 cm-1) monosubs (700 cm-1 and 750 cm-1)

cis (670 – 730 cm-1) o-disubs (730 – 770 cm-1)

trans (965 – 975 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:

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).

Alkene double bonds at a ring fusion absorb at lower frequencies:

On the other hand, exocyclic double bonds have higher frequencies in highly strained rings because of increased “s-character” of C=C:

Like acyclic alkenes, increasing substitution increases the C=C stretching frequency:

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:

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:

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.

Halogen -substituents cause shifts to higher frequency, as a result of the inductive (-I) effect of the halogen atom:

Also, the carbonyl stretching frequency of cyclic ketones depends on the conformational position (equatorial or axial) of the -halogen:

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.

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)

Hydrogen bonding can lower the stretching frequency of the ester (or other) carbonyl group, as in methyl salicylate, below.

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:

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; 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.

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 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.

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.