Engineering Chemistry S Datta

524 ENGINEERING CHEMISTRY
but the shift to 1655 cm –1 that occurs in these compounds is attributed to H–bonding
—O—H----O = C . True alcoholic OH absorption band near 3700 cm –1 in enols are absent, but
there is absorption band near 2700 cm –1 (s) which is attributed to the OH group (s) indicates
strong absorption, (m) indicates medium absorption.
Amines: Primary and secondary amines contain N—H bond, both types absorb in the
region 3500 – 3300 cm –1 (v) but the two are distinguished by the fact that primary amines show
two bands in their region, whereas secondary amines generally show only one. If there is H-
bonding, then the region is 3400 – 3100 cm –1 (s) C—N – str. 1200 – 1020 cm –1 in aliphatic amines
(w – m), C—N – str. + 200 – 1020 cm –1 in aromatic amines (s), N—H – def 1650–1590 cm –1 (s – m) in
primary amines, N—H – def 1650 – 1550 cm –1 (s).
Because of the overlap of these regions, it is not possible to distinguish the types of
amines in this way.
Aromatic Compounds: Aromatic compounds produce a large number of absorption
bands in the IR region. The following regions are particularly useful for recognising the presence
of benzene, polynuclear aromatics and benzene derivatives.
C—H ⇒ 3080 – 3030 cm –1 (w) (str) and the bands for C = C (in plane vibration) are
1625 – 1600 cm –1 (v), 1590 – 1575 cm –1 (v) and 1525 – 1475 cm –1 (v).
Substituent groups show very light effect on the above bands, but new bands are
introduced according to orientation of the groups. However, because of a great deal of
overlapping of the various bands in the region 1225–970 cm –1 ; this region is not very useful.
On the other hand, these isomers in the aromatic system may be readily distinguished by
the bands due to (C—H) (out of plane) deformation 1, 2 isomer shows ν max
770 – 735 cm –1 (s)
1, 3 isomers at ν max
800 – 750 cm –1 and 725 – 680 (m) ; 1, 4 isomer at ν max
840 – 810 cm –1 (s);
mono-substituted benzene ring shows ν max
at 770 – 730 cm –1 (s) and 710 – 690 cm –1 (s) (Two
bands distinguish it from 1, 2 isomer): ν max
for str. and def. of the bonds are shown by the arrows.
1, 2-disubstituted benzene:
def. at 1376 cm –1 H C
defat3024cm –1
C H
–1
1600 cm
–1
and 1493 cm
1462 and 1449 cm –1
CH 8 10
Alkenes or = C—H (Ar.) show
stretching at 2941 cm –1
1, 3-disubstituted benzene:
H
–1
2941 cm (def )
H
C
H
H
str. min Ar or —C = C—H
H
C
H
–1
H 1370 cm (def)
C = C (Ar) str. is at 1613, 15878 and 1490 cm –1