Stretching Carbonyl Base Value
1715
-1 cm
IR Exercises
Given the following IR spectra, write the formula of the compound and assign the infrared bands to the appropriate functional group(s)
2-Butanone
Normal Base Values for C=O Stretching
Functional Group Anhydride (band I) Acid chloride Anhydride (band II) Ester Aldehyde Ketone Carboxylic acid Amide Frequency (cm ) 1810 1800 1760 1735 1725 1715 1710 1690
-1
Dodecanoyl Chloride
Ethyl Butanoate
Propanamide
2-Methylpropanoic Acid
Nonanal
O CH3 CH 2 CH2 CH2 CH 2 CH2 CH 2 CH2 C H
4-Methyl-3-penten-2-one
Acetophenone
Cyclic Ketones (C=O Stretch)
O C O 1815 O 1780 R O C R O 1705 O 1745 1715 C O -1 2140 cm 1715 cm -1
Cyclopentanone
1,3,3-Trimethylbicyclo[2.2.1]heptan-2-one
Cyclobutanone
4-Methylcyclohexene
Toluene
Anisole
A summary of the principle infrared bands and their assignments. R is an aliphatic group.
Funct. Group C-H Type sp hybridized 2 sp hybridized sp hybridized aldehyde C-H primary amine, amide secondary amine, amide tertiary amine, amide alcohols, phenols carboxylic acids nitriles acetylenes
3
N-H
O-H
CN CC
Frequencies cm-1 R3C-H 2850-3000 =CR-H 3000-3250 C-H 3300 H-(C=O)R 2750, 2850 RN-H2, RCONH 2 3300, 3340 RNR-H, RCONHR 3300-3500 RN(R3), RCONR2 none free O-H 3620-3580 hydrogen bonded 3600-3650 R(C=O)O-H 3500-2400 RCN 2280-2200 R-CC-R 2260-2180 R-CC-H 2160-2100
Peak Intensity M(sh) M(sh) M-S(sh) M(sh) S,S(br) S(br) W(sh) S(br) S(br) S(sh) W(sh) M(sh)
A summary of the principle infrared bands and their assignments. R is an aliphatic group.
C=O aldehydes ketones esters carboxylic acids amides (Amide I) (Amide II) (Amide II) amide anhydrides carboxylates olefins R(C=O)H R(C=O)R R(CO2)R RCO2H RCONH2, RCONHR RCONH2 RCONHR RCONR2 R(CO2CO)R R(CO2)-, M+ R2C=CR2 R2C=CH2 R2C=C(OR)R RNO2 1740-1720 1730-1710 1750-1735 1720-1680 1670-1640 1650-1620 1550 1650-1620 1820, 1750 1600, 1400 1680-1640 1600-1675 1600-1630 1550, 1370 S(sh) S(sh) S(sh) S(sh) S(sh) S(sh) S(sh) S(sh) S, S(sh) S,S(sh) W(sh) M(sh) S(sh) S,S(sh)
C=C
-NO2 nitro groups
IR Exercises
Given the following IR spectra and the condensed formula or the molecular weight of the compound, assign the infrared bands to the appropriate functional group(s) and figure out the formula of the compound.
C8H16O2
C8H8O
C7H6O2
Molecular weight: 168.24
C7H12O4
Molecular weight: 422.82 g/mol
Factors affecting the frequency of infrared peaks
1. Resonance and conjugation 2. Ring strain: A: on carbonyl frequencies B. on C-H stretching frequencies Halogens A: on carbonyl frequencies B. on C-H stretching frequencies 4. Chirality Phase: solid, liquid and gas (fundamentals in the gas phase are shifted to higher frequencies) ie. solvent or interactions solute lead to weakening of force constants; effects of H-bonding.
1. Effect of resonance and conjugation on infrared frequencies
+ ON O OC O C O + N O O O
NR2 C O C
+ NR2
O -
What about the effect of conjugation?
+ O O-
Do the facts support this interaction?
OC O C
O
O -
Carbonyl frequency: 1720-1680 (acid) 1700 cm-1 Ether frequency = 1000 to 1400 cm-1 1200 cm-1 Average of the two fundamentals = 1450 cm-1
Resonance
100 80 % Transmittance 60 40 20 0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers, cm -1
1550
1400
Sodium benzoate, KBr pellet
Carbonyl frequency: 1720-1680 (acid) 1700 cm-1 Ether frequency = 1000 to 1400 cm-1 1200 cm-1 Average of the two fundamentals = 1450 cm-1 Sodium benzoate: (1550, 1400 cm-1 ); Average : 1475 cm-1
O
O-
R
NR2
R
NR2 +
How important is resonance in amides?
O
O-
R
NR2
R
NR2+
How important is resonance in amides?
O
In
H N
CH3
the barrier to rotation of the two CH 3 groups is approximately 18 kcal/mol
CH3
(A C-C bond is worth about 60 kcal/mol)
The effect of conjugation on carbonyl frequencies.
Non-conjugated Frequency Conjugated Compound cm-1 Compound butanal 2-butanone 1725 1717 2-butenal methyl vinyl ketone propenoic acid ethyl acrylate 2-butenoic anhydride 1-cyclohexene1,2-dicarboxylic anhydride Frequency cm-1 1691 1700, 1681 1702 1727 1782, 1722 1844, 1767 benzaldehyde acetophenone Frequency cm-1 1702 1685
propanoic acid ethyl propionate butanoic anhydride cis-cyclohexane1,2-dicarboxylic anhydride
1715 1740 1819, 1750 1857, 1786
benzoic acid ethyl benzoate benzoic anhydride phthalic anhydride
1688 1718 178...