Ir spectroscopy ii

Dr. BASAVARAJAIAH S. M.
Assistant Professor and Coordinator
P.G. Department of Chemistry
Vijaya College
Bangalore-560 004
INFRARED SPECTROSCOPY-II

Contents:
Applications of Infrared spectroscopy
Identification of organic compounds,
Structure determination
Qualitative analysis of functional group
Quantitative analysis
Distinction between two types of hydrogen bonding
Study of chemical reaction
Study of Keto-Enol tautomerism
Conformational analysis
Geometrical isomerism
Study of complex molecules
Detection of impurity in a compound
Identification of the organic compounds by IR
Hydrocarbons, Aromatic compounds, Alcohol, Phenols, Ethers, Aldehydes,
Ketones, Esters, Acid chlorides, Anhydrides, Amides, Amines, Nitriles,
Isocynates, Isothiocynates, Imines and Nitro compounds.

Applications of IR spectroscopy
Identification of organic compounds
Structure determination
Qualitative analysis of functional group
Quantitative analysis
Distinction between two types of hydrogen bonding
Study of chemical reaction
Study of Keto-Enol tautomerism
Conformational analysis
Geometrical isomerism
Study of complex molecules
Detection of impurity in a compound

Identification of Organic Compounds:
The identify of an organic compound can be established from
its finger print region (1400-670 cm-1.)
The identity of an organic compound is confirmed if its finger
print region exactly matches with the known spectrum of that
compound.
It may be noted that the compounds containing the same
functional groups may have similar absorption above 1500 cm-1
but differ considerably in finger print regions.

Structure Determination:
This technique helps to establish the structure of an unknown
compound.
All major functional groups absorb at their characteristic wave
numbers.
The shift due to environmental effects may also be locked into.
From the data available due to due to absorption frequencies,
the probable structure can be predicted.
If some chemical data is available, it can lead to the
confirmation of the structure.

Qualitative Analysis of Functional Group:
The presence or absence of absorption bands help in
predicting the presence of certain functional groups in the
compound.
 The presence of oxygen reveals that the group may be –
OH, -C=O, -COOR, -COOH, anhydride etc.
But an absorption band between 3600-3200 cm-1 limits
possibilities.
The band in this region may be due to –OH str.

The quantity of the substance can be determined either in pure
form or as a mixture of two or more compounds.
In this, characteristic peak corresponding to the drug substance
is chosen and log I0/It of peaks for standard and test sample is
compared.
This is called base line technique to determine the quantity of
the substance.
Quantitative Analysis:

Distinction Between Two Types of Hydrogen Bonding:
Hydrogen bonding gives rise to downward frequency shifts.
Stronger the bonding, greater the absorption shift towards lower
wavenumber from normal value.
Generally, intermolecular hydrogen bonds are sharp and well
defined
Intermolecular hydrogen bonds are concentration dependent.
On dilution, intensities of such bands decreases and finally
disappears.

Study of Keto-Enol Tautomerism:
Diketone and ketoesters exhibit keto-enol tautomerism provided they
have a-H atom in them.
The infrared spectrum of such a compound contain bands due to
C=O, O-H and C=C bonds. Consider acetyl acetone. It exists in keto-
enol isomers in equilibrium.

Conformational Analysis:
This technique is quite useful in determining the relative stability of
various conformations of cyclic compounds.
Cyclohexane exists in both chair and boat form.
The spectral examination of cyclohexane in the region 1350-700 cm-1
reveals five bands expected for chair form.
This shows the greater stability of chair conformation over boat
conformation.
By Infrared spectroscopy, axial and equatorial substitution in
cyclohexane can also be distinguished.
It has been found that an equatorial substituent usually absorbs at a
higher frequency than does the same substitution at the axial.

Geometrical Isomerism:
It is known that a vibration is infra red active only if it causes a
change in dipole moment of the molecule.
Also the intensity of absorption depends upon the change in the
dipole moment.
This technique clearly makes a distinction between cis and trans
isomers.
Consider 1, 2-dicholro ethene.
C=Cstr absorption at 1580 cm-1 for cis isomer and not observed in case
of trans isomer.

Study of Complex Molecules:
This technique is also useful to establish the structure the of complex
molecules. For example β-lactam ring Penicillin.
Since Penicillin gives a strong band at 1770 cm-1, the β -lactam
structure of Penicillin is confirmed.

Progress of chemical reaction can be determined by examining the
small portion of the reaction mixture withdrawn from time to time.
The rate of disappearance of a characteristic absorption band of the
reactant group and/or the rate of appearance of the characteristic
absorption band of the product group due to formation of product is
observed.
Study of Chemical Reaction:
IR spectrum of the test sample to be determined is compared
with the standard compound.
If any additional peaks are observed in the IR spectrum, then it is
due to impurities present in the compound.
Detection of Impurity in a Compound:

Identification of the Organic Compounds by IR
Hydrocarbons: Alkanes, Alkenes and Alkynes
A. Alkanes:
Examples: Decane, Mineral oil and Cyclohexane

Alkenes:
Examples: 1-Hexene, Cyclohexene, cis-2-Pentene and Trans-2-Pentene

C. Alkynes:
Examples: 1-Octyne, 4-Octyne

D. Aromatic rings:
Examples: Toluene, o, m, and p-Diethyl benzene, and Styrene.

E. Alcohols and phenols:
Alcohols and phenols will show strong and broad hydrogen bonded O-H
stretching bands centering between 3400 and 3300 cm-1. In solution, it
will also be possible to observe a free O-H stretching band at about 3600
cm-1 (sharp and weak) to the left of the hydrogen bonded O-H peak. In
addition, a C-O stretching band will appear in the spectrum at 1260-1000
cm-1.
Examples; 1-Hexanol, 2-Butanol and p-Cresol

C-O and O-H Stretching vibrations in alcohols and phenols

F. Ethers:
Ethers show at least one C-O band in the range 1300-1000 cm-1.
Simple aliphatic ethers can be distinguished from alkanes by the
presence of the C-O band. In all other respects, the spectra of simple
ethers look very similar to those of alkanes. Aromatic ethers, epoxides,
and acetals are involved.
Examples: Dibutyl ether, Anisole

Dialkyl Ethers: The asymmetric C-O-C stretching vibration leads to
a single strong absorption that appears at about 1120 cm-1, as seen in the
spectrum of dibutyl ether. The symmetric stretching band at about 850
cm-1 is usually very weak. The asymmetric C-O-C absorption also
occurs at about 1120 cm-1 for a six-membered ring containing oxygen.

G. Carbonyl compounds:
The carbonyl group is present in aldehydes, ketones, acids, esters, amides, acid
chlorides, and anhydrides. This absorbs strongly in the range from 1850-1650 cm-1
because of its large change in dipole moment.
Since the C=O stretching frequency is sensitive to attached atoms, the common
functional groups already mentioned absorb at characteristic values.
The normal base values for the C=O stretching vibrations of the various functional
groups are as follows.

The range of values for C=O stretching vibrations of the various functional groups
may be explained through the use of inductive effects, resonance effects, and hydrogen
bonding.
A higher frequency absorption results in ester, since oxygen is more electronegative
than the carbon, this –I effect dominates in an ester to raise the C=O frequency above
that of a ketone.
 A resonance effect may be observed when the unpaired electrons on a nitrogen atom
conjugate with the carbonyl group in amides, resulting in increased single bond
character and a lowering of the C=O absorption frequency below that of a ketone.

In acid chlorides, the highly electronegative halogen atom strengthens the C=O bond
through an enhanced inductive effect and shifts the frequency to values even than the
esters.
Anhydrides are likewise shifted to frequencies higher than the esters because of a
concentration of electronegative oxygen atoms.
In addition, anhydrides give two absorption bands that are due to symmetric and
asymmetric stretching vibrations.
A carboxylic acid exists in monomeric form in very dilute solution, and it absorbs at
about 1760 cm-1 because of the electron withdrawing effect.
However, acids in concentrated solution, in the form of neat liquid, or in the solid
state tend to dimerise via hydrogen bonding, this weakens the C=O bond and lowers
the stretching force constant K, resulting in a lowering of the carbonyl frequency of
saturated acids to about 1710 cm-1.

Ketones absorb at a lower frequency than aldehydes because of their additional alkyl
group, which is electron donating (compared to H) and suppliers electrons to the C=O
bond. This electron releasing effect weakens the C=O bond in the ketone and lowers the
force constant and the absorption frequency.
Factors that influence the C=O stretching vibration
Conjugation Effects:
The introduction of a C=C bond adjacent to a carbonyl group results in
delocalization of the π electrons in the C=O and C=C bonds. This conjugation
increases the single bond character of the C=O and C=C bonds in the resonance
hybrid and hence lowers their force constants, resulting in a lowering of the
frequencies of carbonyl and double bond absorption.

α-Substitution effect:
Hydrogen bond effect:

H: Aldehydes:
Aldehydes show a very strong band for the carbonyl group (C=O) that appears in the
range of 1740-1725 cm-1 for simple aliphatic aldehydes. This band is shifted to lower
frequencies with conjugation to a C=C or phenyl group. A very important doublet
can be observed in the C-H stretch region for the aldehyde C-H near 2850 and 2750
cm-1. The presence of this doublet allows aldehydes to be distinguished from other
carbonyl containing compounds.

I. Ketones:
Ketones show a very strong band for the C=O group that appears in the range
of 1720-1708 cm-1 for simple aliphatic ketones. This band is shifted to lower
frequencies with conjugation to a C=C or phenyl group. An α-halogen atom
will shift the C=O frequency to a higher frequency in cyclic ketone.

The C=O stretching vibrations in conjugated ketones.
The C=O stretching vibrations for cyclic ketones and ketene.

J-Carboxylic acid:
Carboxylic acids show a very strong band for the C=O group that appears in the
range of 1730-1700 cm-1 for simple aliphatic carboxylic acids in the dimeric
form. This band is shifted to lower frequencies with conjugation to a C=C or
phenyl group. The O-H stretch appears in the spectrum as a very band
extending from 3400 to 2400 cm-1. This broad band centers on about 3000 cm-1,
and partially obscures the C-H stretching bands. If this very broad O-H stretch
band is seen, along with a C=O peak, it almost certainly indicate the compound
is a carboxylic acid.

K. Esters:
Esters show a very strong band for the C=O group that appears in the range of 1750-
1735 cm-1 for simple aliphatic esters. The C=O band is shifted to lower frequencies
when it is conjugated to a C=C or phenyl group. On the other hand, conjugated of a
C=C or phenyl group with the single bonded oxygen of an ester leads to an increased
frequency from the range given above. Ring strain moves the C=O absorption to a
higher frequency in cyclic esters (lactones).

L. Amides:
Amides show a very strong band for the C=O group that appears in the range of 1680-
1630 cm-1. The N-H stretch is observed in the range of 3475-3150 cm-1. Primary
amides (R-CONH2), show two bands in the N-H region while secondary amides (R-
CONHR), show only one band. The presence of N-H bands plus an unusually low value
for the C=O would suggest the presence of an amide functional group. Tertiary amides,
R-CONR2, will show the C=O in the range of 1680-130 cm-1, but will not show an N-H
stretch.

M. Acid chlorides:
Acid chlorides show a very strong band for the C=O group that
appears in the range of 1810-1775 cm-1 for aliphatic acid chlorides.
Acid chloride and anhydrides are the most common functional
groups that have a C=O appearing at such a high frequency.
Conjugation lowers the frequency.

N. Anhydrides:
Anhydrides show two strong bands for the C=O groups. Simple alkyl substituted
anhydrides generally give bands near 1820 and 1750 cm-1. Anhydrides and acid
chlorides are the most common functional groups that have a C=O peak appearing
at such a high frequency. Conjugation shifts each of the bands to lower frequencies
(about 30 cm-1 each). Simple five membered ring anhydrides have bands at near
1860 and 1780 cm-1.

Nitriles, Isocynates, Isothiocynates and Imines:
Nitriles, Isocynates and Isothiocynates all have sp-hybridized carbon atoms
similar to the C C bond. They absorb in the region 2100-2270 cm-1. On the other
hand C N bond of an imine has an sp2 carbon atom. Imines and similar
compounds absorb near where double bonds appear, 1690-1640 cm-1.

Nitro compounds:
Nitro compounds show two strong bands in the infrared spectrum.
One appears near 1550 cm-1 and the other near 1350 cm-1. Although
these two bands may partially overlap the aromatic ring region, 1600-
1450 cm-1, it is usually easy to see the NO2 peaks.

Ir spectroscopy ii

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