1. I.R INTERPRETATION
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2. INFRARED SPECTRA INTERPRETATION
When analysing the spectra of unknown compound , concentrate
first on determining the presence or absence of few major
functional group .
The C=O , O-H , C-O , C=C , C≡C , C≡N, and NO2 peaks are
the most conspicuous and give immediate structural information
if they are present.
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3. 1.Look if carbonyl group (C = O) is present?
The C = O group gives strong absorption in the region 1820-
1600cm-1 , the peak is strong and medium width.
2. If C=O is present , check for the presence of the following groups:
(I) Acid
(ii) Amides
(iii) Ester
(iv) Anhydrides
(v) Aldehydes
(vi) ketones
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4. 3. If C=O is absent
i. Alcohol
ii. Phenols
iii. Amines
iv. Ether
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5. 4. Double bond and / or aromatic ring :
(i) C=C is a weak absorption near 1650 cm-1 .
(ii) Medium to strong absorption in the region of 1600 - 1550cm-1
often imply an aromatic ring.
(iii) Confirm the double bond or aromatic ring by consulting the C-H
region , if C-H stretch occurs to the left of 3000cm-1 ,then it is
aromatic or vinyl . If C-H stretch occurs to right of 3000cm-1 then it
is aliphatic. 5

6. 5. Triple bond :
(i) C ≡ N is a medium sharp peak near 2250cm-1
(ii) C ≡ C is a weak sharp peak near 2150 cm-1
(iii) Check also for acetylinic C-H near 3300cm-1 , which gives an
idea if the triple bond is placed terminally.
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7. 6. Nitrogroup :
Two strong absorption at 1600 - 1500cm-1 and 1390 - 1300cm-1
7. Hydrocarbon
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8. HYDROCARBONES:
1.ALKANES:
-They yield four stretching and bending vibration of C-H and C-C
bonds.
1 .C - C BENDING VIBRATION :
2. C - H STRETCHING VIBRATION
3. CH3 BENDING VIBRATIONS:
4. CH2 BENDING VIBRATION :
Example
Decane , cyclohexane 8

9. #
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10. 2.ALKENE :
Alkenes show many more peaks than alkanes
(a) = C - H stretch for sp2 C-H occurs at region slightly greater
than 3000cm-1
(b) = C - H out of plane (oop) bending occurs in ranges of 1000-
650cm-1.
Example:1-hexene, cyclohexene, cis-2-pentene.
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12. 3.ALKYNES:
Terminali alkyne show prominent peak at about 3300cm-1 for
SP hybridized C-H.
≡C – H stretching vibration occurs at 3300cm-1
C≡C stretch occurs near 2150cm-1 this is only for terminal
alkynes (absent or very weak for non terminal alkynes)
Example:1-octyne,4-octyne
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14. 4.AROMATIC HYDROCOMPOUND:
The C-H stretching for SP2 carbon aapears at value greater
than 3000cm-1. since the C-H stretch for alkenes appears in the
same range ,it may be difficult to use the C - H stretching bands
to differentiate between alkenes and aromatic compounds.
Example: Toluene, o-diethylbenzene ,m-diethylbenzene, p-
diethylbenzene, and styrene.
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16. 5.ALCOHOL AND PHENOL:
Alcohol and phenol show strong and broad hydrogen bonded O-H
stretching bands centering between 3400cm-1 and 3300cm-1. In
solution it will also be possible to observe a free O-H stretching band
at 3600cm-1.
Example: 1-hexanol,2-butanol,para cresol.
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18. 6.ETHERS:
Ether show prominent C-O stretching band at 1300 to 1000cm-1.
Absence of C=O and O-H is required to ensure that C-O stretch is
not due to an ester and phenol .phenyl alkyl ether gives two strong
bands at 1250cm-1 and 1040cm-1.
Example: dibutylether, anisole
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20. 7.ALDEHYDE:
Aldehydes show a very strong band for C=O that appears in the range
of 1740 to 1725cm-1 ,for simple aliphatic aldehydes. This band is
shifted to lower frequency with conjugation to a C=C or phenyl group
.
EXAMPLE : Nonanal , crotonaldehyde , benzaldehyde.
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22. 8.KETONE:
ketone show a very strong band for c=o group that appears in the
range of 1720 to 1708cm-1 for simple aliphatic ketones .this bond is
shifted to lower frequency with conjugation to a c=c or phenyl group
.ring strain moves the absorption to a higher frequency in cyclic
ketones .
An overtones is seen at 3430cm-1
Ex:3-methyl-2-butanone,mesityl
oxides,acetophenone,cyclopentanone,2,4-pentanedione.
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24. 9.Carboxylic acid:
It shows very strong bands that appears in the range of 1730-
1700cm-1 for simple aliphatic carboxylic acids, this band is
shifted to lower frequency conjugation to a C=C or phenyl group.
Example : isobutyric acid
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26. 10.ESTER
Esters show a vey strong band for the C=O group that appears in the
range of 1750-1735cm-1 for simple aliphatic esters. The C=O band is
shifted to lower frequency when it is conjugated to C=C or phenyl
group.
Ex: ethyl butyrate, methyl methacrylate, vinyl acetate, methyl
benzoate, methyl salicylate.
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28. 13.AMIDES
Amides show a very strong band for the C=O group that appears in
the range of 1680 to 1630cm-1.
N-H stretch in primary amides (-NH2) gives two bands near
3350cm-1 (asymmetric), and 3180cm-1 (symmetric) secondary
amides (-NH) have one band at 3300cm-1. Tertiary amides will not
show an N-H stretch because they do not contain N-H.
EX: Propionamide, N-Methylacetamide
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30. 11.ANHYDRIDES
EX: Propionic anhydride.
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31. 12.AMINES
N-H stretch occurs in the range of 3500-3300cm-1.
Ex: butylamine, dibutylamine, tributylamine, n-methylaniline.
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32. Referances
1. Pharmaceutical drug analysis Ashutosh kar , New age international
publishers page 330-335. Spectroscopy
2. B.K Sharma, Goel Publishing house, page 193-333.
3. Elementary organic spectroscopy .Y.R Sharma.S.Chand and
company Ltd, page no 90.
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