This document summarizes the infrared spectral features of various organic compounds. It discusses the absorption regions and functional groups of hydrocarbons such as alkanes, alkenes, alkynes and aromatics. It also covers alcohols, ethers, aldehydes, ketones, carboxylic acids, esters, amides, and amines. For each functional group, it provides examples and describes the characteristic absorption bands that can be used for identification in infrared spectroscopy. In conclusion, it notes that infrared spectroscopy provides detailed information about functional groups and molecular structure.

1. IR SPECTRAL FEATURES OF
SOME ORGANIC COMPOUNDS

2. Features of an IR Spectrum
Two regions of absorption :
GFR (4000-1500 cm-1): Identification of groups
FPR (1500-400 cm-1): Molecular fingerprint (Spectral matching)
Nature (Intensity and Shape) of spectral bands : Strong (s),
medium (m), weak (w); Broad and Narrow

3. HYDROCARBONS:
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

4. #

5. 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 bending occurs in ranges of 1000-650cm-1.
Example:1-hexene, cyclohexene, cis-2-pentene.

6. 3.ALKYNES:
Terminal 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-hexyne,1-octyne,4-octyne

7. 4.AROMATIC HYDROCOMPOUND:
The C-H stretching for SP2 carbon appears 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.

8. *For aromatic compounds, the most characteristic C=C stretching
bands are at 1600cm-1, 1580cm-1, 1500cm-1 and 1450cm-1.
* If there is no absorption in this region, it is a fair proof that the
compound is not aromatic.

9. 5.ALCOHOL AND PHENOL:
Alcohol and phenol show strong and broad hydrogen bonded O-H
stretching bands 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.

11. 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

12. 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.

13. 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.

14. 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

15. 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.

16. 11.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

17. 12.AMINES
•Primary amine-N-H stretch occurs in the range of 3500-3300cm-1.
•Secondary amine-N-N stretch occurs in the range of 3500-3300cm-1
Ex: butylamine, dibutylamine, tributylamine, n-methylaniline.

18. CONCLUSION
• Infra-red spectrum of a compound provides more information than
is normally available from the electronic spectra.
• The position of the peek or the band not only tells the presence of a
particular group but also reveals a good deal about the environments
affecting the group.
• Further study of the spectrum reveals whether it is aldehydic,
ketonic, esters, amide etc.
• Due to the different positions of absorptions, it is also possible to
know the axial and the equatorial positions of certain groups in a
cyclic structure.

19. REFERENCE:
1.Pharmaceutical drug analysis Ashutoshkar , New age international
publishers page 330-335.
2.Introduction to spectroscopy .Donald L. Pavia, Gray
M.Lampman, George S. Kritz. 3rd edition ,page 13-82.
3. Spectroscopy .B.K Sharma, Goel Publishing house, page 193-
333.
4. Elementary organic spectroscopy .Y.R Sharma.S.Chand and
company Ltd,page 90.
5.Spectrometric identification of organic compound.Robert
M.Silverstein ,Francis X.Webster,David J.Kiemle. 7th edition page
72-125

20. Thank you