SURVEY OF SPECTRA

1
CARBONYL COMPOUNDS
( CO BOND STRETCH)
Aldehydes Ketones Esters Amides Acid Chlorides
SURVEY OF SPECTRA
2
CO STRETCHING
3
Typical Infrared AbsorptionRegions
CO
WAVELENGTH (mm)
C-Cl
CO
CN
O-H
C-H
C N
Very few bands
C-O
CC
N-H
C C
C-N
XCY
C-C

NO NO
(C,O,N,S)
FREQUENCY (cm-1)
4
THE CARBONYL STRETCHING REGION

• This region stretches from about 1800 to 1650
  cm-1 - RIGHT IN THE MIDDLE OF THE SPECTRUM
• The base value is 1715 cm-1 (ketone)
• The bands are very strong !!! due to
• the large CO dipole moment.

• CO is often one of the strongest peaks
• in the spectrum

5
KETONE
2-Butanone
BASE 1715
overtone
2x CO
C-H
CH bend
CO
6
CO IS SENSITIVE TO ITS ENVIRONMENT
EACH DIFFERENT KIND OF CO COMES AT A DIFFERENT
FREQUENCY
acid chloride
carboxylic acid
ester
aldehyde
amide
ketone
1690
1710
1715
1725
1735
1800
anhydride
1810 and 1760
THESE VALUES ARE WORTH LEARNING all are /-
10 cm-1
( two peaks )
7
CO BOND LENGTHS IN CARBONYL COMPOUNDS
shorter
longer
1.225 A
1.231 A
1.235 A
1.248 A
acid chloride
ester
ketone
amide
1780 cm-1
1735 cm-1
1715 cm-1
1680 cm-1
8
FACTORS THAT INFLUENCE THE CO
ABSORPTION
9
INDUCTIVE AND RESONANCE EFFECTS ON THE
CARBONYL FREQUENCY
O
Electron-donating groups
C
R
weaken the carbonyl and
A
lower its absorption frequency
R Me, Et, etc.
O
Electron-withdrawing groups
B
C
X
strengthen the carbonyl and
raise its absorption frequency
X F, Cl, Br, O
10
INDUCTIVE AND RESONANCE EFFECTS ON THE
CARBONYL FREQUENCY
Resonance
weakens the carbonyl and
lowers its absorption frequency
Y N, O, or CC
C
(Note the lengthening of the CO bond! )
O
H
O
Hydrogen bonding
D
lengthens and weakens the CO bond and
C
R
lowers its absorption frequency
R
11
HOW THE FACTORS AFFECT CO
STRETCHING VIBRATIONS
acid chloride
carboxylic acid
ester
aldehyde
amide
ketone
B
A
C
1690
1710
1715
1725
1735
1800
D
anhydride
1810 and 1760
( two peaks )
A
C
E-donating
Resonance
B
D
E-withdrawing
H-bonding
12
SUMMARY
Ketones are at lower frequency than Aldehydes
because of the second electron-donating alkyl
group.
Acid chlorides are at higher frequency than
ketones because of the electron-withdrawing
halide.
Esters are at higher frequencies than ketones
due to the electron-withdrawing oxygen atom. This
is more important than resonance with the
electron pair on the oxygen.
Amides are at lower frequencies than ketones due
to resonance involving the unshared pair on
nitrogen. The electron-withdrawing effect of
nitrogen is less important than the resonance.
Note the electronegativity difference, O versus
N, weights the two factors (resonance/
e-withdrawal) differently in esters than in
amides.
Acids are at lower frequency than ketones due to
H-bonding.
13
CONFIRMING PEAKS
14
CONFIRMATION OF FUNCTIONAL GROUP
Every type of carbonyl compound has other
places you can look to confirm your conclusion
based on frequency alone.
CO at 1710 cm-1
CO at 1725 cm-1
also look for OH (H-bonded) and C-O 1200 cm-1
also look for aldehyde CH 2850 and 2750 cm-1
CO at 1735 cm-1
CO at 1690 cm-1
also look for two C-O at 1200 and 1000 cm-1
also look for two NH peaks at 3400 cm-1
Ketones have CO at 1715 cm-1 and no NH,
OH, C-O or -CHO
Anhydrides have two CO peaks near 1800 cm-1
and two C-O
15
SELECTED SPECTRA
16
2-Butanone
KETONE
overtone of strong CO peak
BASE 1715
1719 x 2 3438
overtone
C-H
CH bend
CO
3438
17
Nonanal
ALDEHYDE
BASE 1725
CHO
gt4C
CH bend
CO
3460
18
Dodecanoyl Chloride
ACID CHLORIDE
BASE 1800
CH bend
CO
C-H
3608
19
Ethyl Butanoate
ESTER
BASE 1735
C-O
C-H
CO
3482
20
2-Methylpropanoic Acid
CARBOXYLIC ACID
BASE 1710
O-H
C-O
C-H
CO
21
CARBOXYLIC ACID DIMER
lowers frequency of CO
and also of O-H
Strong hydrogen-bonding in the dimer weakens the
O-H and CO bonds and leads to broad peaks at
lower frequencies.
22
AMIDE
BASE 1690
two peaks sym / asym
Propanamide
C-H
CO
NH2
CH bend