NAlkanes

1
N-Alkanes interpreted in terms of 4 vibrati
ons - stretching and bending of CH and CC bon
ds CC bends ca. 500 cm1 (out of spectral w
indow) CC stretches 1200800 cm1, weak ban
ds not of value for interpretation (fingerp
rint) more characteristic CH stretche
s occurs from 3000 - 2840 cm1
CH3 2962 cm1, asymmetrical stretch
2872 cm1, symmetrical stretch
CH2 2926 cm1, asymmetrical stretch 285
3 cm1, symmetrical stretch Note precision!

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n-alkanes
CH2 (s) 720 cm1 Rock Alkanes of 7 or more carb
ons
CH2 (s) 1467 cm1 scissor
CH3 (s) 1375 cm1
C-H Stretches
CH3 (as) 1450 cm1
CH3 (as) 2964 cm1
CH3 (s) 2872 cm1
C-H Bends
CH2 (s) 2853 cm1
CH2 (as) 2924 cm1
C12H25
3
Fingerprinting
Similar But Not Identical
C10H22
C12H26
4
Unconjugated Alkenes
linear alkenes - CC stretch moderate to we
ak absorption at 1667-1640 cm1

CCH - stretch 3000 cm1
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Unconjugated Alkenes
Overtone of 910 cm-1 Absorption (1820 cm1)

CH2 rock 720 cm1
C-H stretch 3049 cm1
CC stretch 1642 cm1
Out of plane CCH bends 991, 910 cm1
1-decene
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cyclic alkenes - CC stretch sensitive to r
ing strain
cumulated alkenes - CCC stretch (asymmetri
c) 20001900 cm1
conjugated alkenes -the alkene bond stretch
ing vibrations in alkenes w/o a center of
symmetry give rise to two CC stretches
-for symmetrical molecules, e.g. butadiene,
only the asymmetric stretch is observed
8
Conjugated Double Bonds
Symmetrical CC stretch 1640 cm1 (weak)
Out of plane CCH bends 990, 892 cm1
C-H stretch 3090 cm1
Asymmetrical CC stretch 1598 cm1 (strong)
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Alkynes
stretch weak absorption at
2260-2100 cm1 - not observed for symme
trical alkynes (v. weak for pseudo
symmetric alkynes - terminal alkynes (R-C
C-H) absorptions are stronger than
internal (R-C C-R) absorptions
C CH stretch - 33333267 cm1 - stro
ng, narrow (as compared to OH or NH)

C CH bend - 700-610 cm1 broad, strong
absorption
- 1400-1220 cm1, overtone of above
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Terminal Alkynes
Alkyne CC stretch 2119 cm1
Alkyne C-H bend overtone 1260 cm1
Alkyne C-H stretch 3310 cm1
Alkyne C-H bend 630 cm1
11
Mononuclear Aromatic Hydrocarbons (benzene)
Out of plane bending of aromatic C-H bonds
most informative - 900675 cm1 - inten
se bands, strongly coupled to adjacent hydrogens
on the ring - position and number of bands giv
es information about the substitution patter
n (particularly useful for alkyl substituted
aromatics. Substitution of polar groups can
give rise to exceptions)
CCH stretch 31003000 cm1
C C stretch 1600-1585 1500-1400 cm1
C C out of plane ring bending 600-420 cm1


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Mononuclear Aromatics
Overtone bands 2000-1650 cm1
Aromatic C-H Stretches 3087, 3062, 3026 cm1
out of plane ring bending 428 cm1
Aromatic C-H in plane bends 1300-1000 cm1
Aromatic C-H out of Plane bends 728 cm1

Aromatic C-C Stretches 1600-1585 1500-1400 cm1
694 cm-1
13
795 cm-1
691 cm-1
768 cm-1
742 cm-
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Alcohols and Phenols Strongly dependent on hydr
ogen-bonding - non-hydrogen bonded OH groups a
bsorb strongly in the 3700-3584
cm1 range. vapor phase dilute, n
on polar solvent very hindered
Intermolecular hydrogen bonding occurs as
conc. increases - accompanied by a shift to
lower freq. (3550-3200 cm1), at the
expense of the free OH band
- Pronounced for intramolecular H-bonding
15
CO stretching Vibrations Alcohols (1260-1000 c
m1) Phenols (1800-1260 cm1)
primary alcohol 1050-1085 cm-1 secon
dary alcohol 1085-1125-1 tertiary alcohol 11
25-1200 cm-1
1073 cm-1
1110 cm1
1202 cm1
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Ethers COC stretching bands are most characte
ristic bands - strong because of strong dipole
moment aliphatic ethers strong band due to as
ymmetrical stretching, 1150-1085
cm1 (usually 1125 cm1) weak band due to sy
mmetrical stretching (lower freq)
Alkyl aryl ethers asymmetrical stretch at 1275-
1200 cm-1 symmetrical stretch at 1075-
1020 cm1 Vinyl alkyl ethers asymmetrical stre
tch at 1225-1200 cm-1 symme
trical stretch at 1075-1020 cm1
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Carbonyls
CO stretch easily recognized, intense band
Ketones, aldehydes, acids, esters, lactones, a
cid halides, anhydrides, amides and lactams all
show CO stretching in the region 1870-1540 cm1
. Position is determined by (1) physical s
tate (2) electronic and mass of neighboring group
s (3) conjugation (4) hydrogen bonding
(5) ring strain
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Ketones
aliphatic normal position of a neat
aliphatic ketone is 1715 cm1

competing effects
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Ketones
conjugation shifts position to lower frequency
alkene or phenyl group causes absorption in t
he 1685-1666 cm1 region. For a,b-unsaturated
carbonyls, 2 absorptions may be
observed
3350 cm-1 Overtone of CO stretch
1685 cm-1
For a,b-unsaturated carbonyls, 2 absorptions
may be observed

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1,3-Diketones
OH stretching
1630 cm-1
1725 cm-1
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Cyclic Ketones
The bond angle influences the absorption
frequency of the CO
In strained rings, interaction with the adjacent
C-C bonds increases the frequency of CO
stretching
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Aldehydes
CO stretch
Aliphatic aldehydes CO stretch at 1740-1720
cm1 Electron withdrawing groups shift to highe
r frequency Conjugative groups shift to lower f
requency (1710-1685 cm1)
CH stretch
28302695 cm1 Often, two bands are observed
(the other is a result of an overtone of the CH
bend of the aldehyde)
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Carboxylic Acids
OH stretch free OH (ca. 3520 cm-1) is observe
d only in the vapor phase or in very dilute (0.01 M) solution in a non-polar solvent
Otherwise, acids exist as dimers
Intense, OH stretching in the 33002500 cm1
region
centered near 3000 cm1
CO stretch monomer 1760 cm1 for aliphatic ac
ids dimer Hydrogen bonding reduces the frequen
cy of the asymmetric CO stretch, especially w
hen intramolecular
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Esters
CO stretch saturated aliphatic esters C
O 17501735 cm1 formates, a,b-unsaturat
ed, and benzoate esters 1730-1715 cm1
phenyl or vinyl esters 1770-1780 cm1
CO stretches (strong absorptions asymmetrical
coupled vibrations) saturated aliphatic es
ters (except acetates) CO 12101163 cm1
acetates 1240 cm1 a,b-unsaturated
esters 13001160 cm1 benzoate esters 13
101250 cm1
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1749 cm1 CO
1243 cm1 CO
1763 cm1 CO
1199, 1164, 1145 cm1 CO
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1730 cm1 CO
1770 cm1 CO
1823 cm1 CO
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Acid Halides
Anhydrides Two carbonyl stretches (symmetrical
and asymmetrical)
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Amides
NH stretching
Two bands
3520 (as), 3400 (s) cm1 (dilute)
3350 (as), 3180 (s) cm1 (conc)
35003400 cm1 (dilute)
3330-3060 (s) cm1, multiple bands due to
H-bonding (conc)
CO stretching (Amide I band)
1690 cm1 (dilute)
1650 cm1 (conc) (except CH3CONH2, which absorbs
at 1694 cm1 )
1680-1700 cm1 (dilute)
1640 cm1 (conc)
1680-1630 cm1 (H-bonding not possible, unless in
protic solvent)
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Amides
NH Bending (Amide II band)
1620-1590 cm1 (dilute)- separate from amide I
1655-1620 cm1 (conc)- overlap with the amide I
band
1550-1510 cm1 (dilute)
1570-1515 (s) cm1 (conc)
Lactams
1750-1700 cm1
1760-1730 cm1
1650 cm1
Secondary lactams do not have an NH bending band
(Amide band II) near 1550 cm-1. This is a
Characteristic of s-trans lactams.
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(II)
1662
1662 cm-1 (I)
1565 cm-1 (II)
1655 cm-1 (I)
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Amines
NH stretching - in dilute solution, primary
amines (RNH2) display 2 bands, near 3500 an
d 3400 cm1. These represent
free asymmetrical and symmetrical
stretches. - in dilute soln, secondary amines
(R2NH) display one band near 3350-3310 cm1.
- weaker and sharper than OH - neat pri
mary aliphatic amines (alkylNH2) absorb
at 3400-3300 and 3330-3250 cm-1. ArylNH2
absorb at slightly higher frequencies.

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Nitriles
2260-2240 cm1
2240-2222 cm1
Nitro compounds 2 bands from the asymmetrical a
nd symmetrical stretching of the bo
nd
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1523 (as)
1347 (s)
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35
Vinyl alkyl ethers CC stretch at 1660-1610
cm-1
often a doublet
trans 1620 cm1
cis 1640 cm1
vinyl ethers- wagging shifted to lower frequency
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