Download IR Spectroscopy and Mass Spectroscopy

KOT 222 ORGANIC CHEMISTRY II
CHAPTER 12
INFRARED SPECTROSCOPY
and
MASS SPECTROSCOPY
Part II
Infrared Spectroscopy
IR Spectroscopy of
Alcohols and Amines
¾ The O-H and N-H bonds are strong and stiff to give
vibration at higher frequencies ~ 3300 cm-1.
™ alcohol O-H
™
acid O-H
™ amine N-H
R
O
3300 cm-1, broad
3000 cm-1, broad
3300 cm-1, broad with spikes
H
H
R
N
R''
H
H
R
N
R'
R
N
R'
IR Spectrum of Alcohol
¾The broad shape at around 3300 cm-1 is due to the diverse
nature of the hydrogen bonding interactions of alcohol
molecules.
¾Band centered near 1050 cm-1 is assigned to C-O stretching.
Other functional groups also absorb at 1000 – 1200 cm-1 !!
The position and the breadth of the O–H absorption
band depend on the concentration of the solution
It is easier to stretch an O–H bond if it is hydrogen
bonded
IR Spectrum of Amine
Depends on no.
of N-H bond
2o amine
1o amine
IR Spectroscopy of
Carbonyl Compounds
¾ C=O double bond has a large dipole moment
which produce intense absorption.
¾ The C=O stretching vibrations of simple ketones,
aldehydes, and carboxylic acids occur at
frequencies around 1710 cm-1.
Unusual strong hydrogen bonding in carboxylic
acids, shift the broad O-H absorption band to
about 3000 cm-1.
Participation of C=O in hydrogen bonding also
results in broadening of it absorption band.
Variations in C=O Absorption
¾ The position of the absorption band depends on
electron delocalization, the electronic effect of
neighboring substituents, and hydrogen
bonding.
¾ As conjugation lower the C=C stretching
frequency, it also lower the C=O absorption
frequency to ~ 1680 cm-1.
The weak C=C absorption band may not appear.
If presence, however, can be inferred from:
™ its effect on the C=O absorption frequency.
™ absorption of =C-H above 3000 cm-1.
The C=O group of an amide absorbs at an even
lower frequency, 1640-1680 cm-1.
For amide:
• strong C=O
absorption.
• prominent N-H
stretching
absorption.
For alkene:
• weak C=C
absorption.
• appearance of
unsaturated C-H
stretching.
Carbonyl absorption above 1710 cm-1
¾ The C=O of an ester absorbs at a higher
frequency, ~1730-1740 cm-1.
¾ Carbonyl groups in small rings (5 C’s or less)
absorb at an even higher frequency.
Absorption of C-N Bonds
¾ Carbon-nitrogen bonds are more polar than
carbon-carbon bonds, thus give stronger
absorption.
¾ C-N single bonds absorb around 1200 cm-1.
• Close to C-C and C-O absorptions
¾C=N double bonds absorb at 1660 cm-1.
• Same region as C=C, but stronger absorption.
¾C≡C triple bonds strongly absorb just
above 2200 cm-1.
• C≡C signal is much weaker and is just below 2200
cm-1
IR Spectrum of Nitrile
Summary of IR Spectroscopy
IR spectrum can:
1. Indicate the functional group in the compound.
2. Show the absence of other functional groups that would give
strong absorption.
3. Confirm the identity of an unknown through fingerprint region.
IR Spectroscopy
Strengths and Limitations
¾IR alone cannot determine a structure.
¾Some signals may be ambiguous.
¾The functional group is usually indicated.
¾The absence of a signal is definite proof
that the functional group is absent.
¾Correspondence with a known sample’s
IR spectrum confirms the identity of the
compound.