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O-H and N-H Stretching
• Both of these occur around 3300 cm-1, but
they look different.
– Alcohol O-H, broad with rounded tip.
– Primary amine (RNH2), broad with two sharp
spikes.
– Secondary amine (R2NH), broad with one sharp
spike.
– No signal for a tertiary amine (R3N)
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Alcohols have characteristic IR absorptions
associated with both the O-H and the C-O stretching
vibrations.
•O–H stretch, hydrogen bonded 3500-3200 cm-1 and
is a very intense, broad band
•C–O stretch 1260-1050 cm-1 (s)
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An Alcohol IR Spectrum
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Amines
The N–H stretches of amines are in the region
3300-3000 cm-1. (weaker and sharper than alcohol
O–H stretches).
-In primary amines (RNH2),
Secondary amines (R2NH) show only a single weak
band , only one N–H bond.
-Tertiary amines (R3N) do not show any band in this
region since they do not have an N–H bond.
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Stretching vibrations (Carbonyl groups)
Structural unit
Range, cm-1
Aldehydes and ketones
1710-1750
Carboxylic acids
1700-1725
Acid anhydrides
1800-1850 and 1740-1790
Esters
1730-1750
Amides
1680-1700
C
O
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Aldehydes and ketones
A useful diagnostic band for aldehydes
is the O=C–H stretch, in the region
-1
2800-2700cm .
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Carboxylic Acids
Carboxylic acids show a strong, wide band for the
O–H stretch. Unlike the O–H stretch band
observed in alcohols, the carboxylic acid O–H
stretch appears as a very broad band in the region
3300-2500 cm-1, centered at about 3000 cm-1.
This is because carboxylic acids usually exist as
hydrogen-bonded dimers.
O–H stretch from 3300-2500 cm-1
C=O stretch from 1700-1725 cm-1
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O-H Stretch of a Carboxylic Acid
This O-H absorbs broadly, 2500-3500 cm-1, due to
strong hydrogen bonding.
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Esters
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An Amide IR Spectrum
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A Nitrile IR Spectrum
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1-Explain which functional group is present in the compound with the following IR
spectrum. Show to which class these compounds belong.
3100 -- The broad intense absorption band seen here is characteristic of a carboxylic acid dimer.
2960 -- CH aliphatic stretch
2870 -- CH aliphatic stretching.
1720-- CO
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The compound is aliphatic carboxylic acid
3080 -- Aromatic CH absoprtion
2760 -- Aldehyde CH
1735 -- C=O absorption
1615 -- The 1600 and 1500 bands for the aromatic nucleus are variable.
The compound is aromatic aldehyde.
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2995 --Aliphatic CH stretching vibration absorption.
1765 & 1740 -- C=0 Note the two minima of this band. Two carbonyls.
1045 -- Symmetric strectch for the C-O-C of the ester.
The compound is aliphatic ester
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3350 -- The broad intense absorption band seen here is characteristic of -OH
2950 -- CH aliphatic stretch
------- no C=O absorption
The compound is aliphatic alcohol
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2229
3080 -- Aromatic CH absoprtion
2950-2850 -- CH aliphatic stretch
C N
2229
-The compound is aromatic nitrile substituted with alkyl
group
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1716
2900-2850 --Aliphatic CH stretching vibration absorption.
1716
-- C=O absorption
No other absorption
Simple aliphatic ketone
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