Chemie en Spectroscopie: IR-spectroscopie
Oefenopgaven
O
1. At what positions will the following compound
absorb in the IR-spectrum and what type of
vibrations are these?
HO
6. The IR-spectrum below is from one of the three isomeric
structures 7, 8 or 9.
C
H
Which structure is the right one? Explain.
2. Part of the IR-spectrum of an
7
unknown compound is given to the
H
C
O
O
right.
What functional groups could be
8 H 3COHC
present? What functional groups
C
H
C
CH
are absent?
9 H 3C
3. Which structure (1 or 2) is not in
C
C
CCH 3
O
accordance with the IR-spectrum to
7. Which structure is best in agreement with the IR-spectrum
the right?
below? Explain.
Explain.
1
2
OH
O
Cl
Cl
Cl
Cl
Cl
4. Which structure (3 or 4) is not in
10
CH 3
Cl
accordance with the IR-spectrum to
12
the right?
Explain.
11
C(CH 3)3
O
HO
C
N
C
3
NH 2
4
8. Which structure is best in agreement with the
IR-spectrum below? Explain.
O
CH 2
O
13
5. Which structure (5 or 6) is in
accordance with the IR-spectrum
O
to the right? Explain.
O
H
C
5
O
H
CH
CH 2
O
H 3COH 2CC
C
14
C
CCH 3
6
15
16
9.
Which IR absorptions would you expect to
17. Below is given the IR-spectrum of an organic compound that contains
see for the following compound:
HC
CCH 2OH
10. Which IR absorptions would you expect to see
C
for the following compound:
only C,H and O.
C
11. What differences would the IR-spectra of the compounds below display
in the wavenumber region 4000-1650 cm-1?
O
H 3CH 2C
C
O
OH
H 3C
C
O
CH 3
H 3CH 2C
C
H
12. What differences would the IR-spectra of the compounds below display
in the wavenumber region 4000-1650 cm-1?
O
C
O
N
C
H
O
N
H
C
O
C
is this compound aromatic or aliphatic?
•
is this compound an alcohol, aldehyde or ketone?
•
is this compound an ester, anhydride, acid?
•
does this compound contain a double or triple bond?
H
18. Below is given the IR-spectrum of an unknown compound.
13. What differences would the IR-spectra of the compounds below display
in the wavenumber region 4000- 1650 cm-1?
C
•
C
C
C
H
14. In what way would the IR-spectra of the
H
H
C
C
H
H
CH 2
following compounds differ?
O
C
H
CH 3
15.
H
C
15. In what way would the IR-spectra
of the following compounds differ?
H 3C
C
H
H2
C
C
O
O
H
16. In which part of the IR-spectra would the
H 3C
following compounds display different
characteristic absorptions?
H 3C
CH 3
CH 3
•
is this compound aromatic or aliphatic?
•
is this compound a C=C or a C∫CH group?
•
does this compound contain a methyl group?
•
does this compound contain a OH, NH or C=O?
19. Below is given the IR-spectrum of an unknown compound.
•
does this spectrum contain information that the unknown compound is
non-aromatic?
•
what does the region 4000-2000cm-1 tell you about the functional
•
groups present? What groups are absent?
what does the region 2000-1200cm-1 tell you about the functional
groups present? What groups are absent?
20. Below is given the IR-spectrum of an unknown compound.
•
21. Below is given the IR-spectrum of a compound C8H8O
•
is this compound aromatic or aliphatic?
what does the region 4000-2000cm-1 tell you about the functional
•
groups present? What groups are absent?
what does the region 2000-1200cm-1 tell you about the functional
groups present? What groups are absent?
•
as part of what kind of functional group is oxygen present?
•
is this compound aromatic?
•
are there aliphatic C-H groups present?
•
what is the most likely substitution pattern for this compound?
1. 3750 - 3000 cm-1 :
3300 - 3000 cm-1 :
3000 - 2700 cm-1 :
O - H stretch
7.
C - H stretch of the aromatic H
vibrations visible (3000 - 2700 cm-1). Structure 10 has no C-H at all,
structure 11 has only aromatic C-H.
C - H stretch of the aldehyde H
1900 - 1650 cm-1 :
C = O stretch
1675 - 1500 cm-1 : C = C stretch of the aromatic
1000 - 650 cm-1 : Ar - H deform (out of plain)
2. groups that could be present:
3750 - 3000 cm-1 :
N - H or O - H stretch
3300 - 2700 cm-1 :
C - H stretch of -CH3, -CH2-, C-H,
Ar-H, >C=C<H, -CC-H, aldehyde H
2400 - 2100 cm-1 :
CC or CN stretch
1475 - 1300 cm-1 : C - H deform
groups that can not be present:
1900 - 1650 cm-1 :
C = O stretch
-1
1675 - 1500 cm
: C = C stretch
3. Structure 1 is not in accordance with the IR-spectrum because there are no
bands present around 3750 - 3000 cm-1 (O-H stretch) and 1675 - 1500 cm1 (C = C stretch).
The absorption in the area 1900 - 1650 cm-1 (C = O stretch) is in
accordance with structure 2.
4. Structure 4 is not in accordance with the IR-spectrum because no C = O
band (1900 - 1650 cm-1) is present. The spectrum displays a C∫N band at
2400 - 2100 cm-1, which is in accordance with structure 3.
5. Peaks are observed above and below 3000 cm-1, which means both
saturated and unsaturated C-H should be present. Structure 5 contains a
>C=C<H group that accounts for the band at 3080 cm-1. Structure 6 would
only give peaks below 3000 cm-1.
6. Structure 8 is the right structure. It can not be structure 9, because there is
a band present at 3300 cm-1 (-CC-H). Furthermore there is no band
present at 1900 - 1650 cm-1 (C = O stretch), again not in accordance with
structures 7 and 9.
The weak band present at 2200 cm-1 (-CC-). and the strong band at
3300 cm-1 (-CC-H) are in accordance with structure 8.
Structure 12 is the right structure, since there are both aromatic C-H
stretch vibrations (above 3000 cm-1) as well as aliphatic C-H stretch
8.
Structure 14 is the right structure. Structure 1 can be excluded because
the spectrum shows unsaturated C-H stretch vibrations above 3000 cm-1.
At 1700 cm-1 a C=O band is clearly visible, which excludes structures 15
and 16.
9.
A strong band at 3750 - 3000 cm-1 (O - H stretch). This band can cover
the band at 3300 of the -CC-H stretch vibration. The -CH2- will give two
absorptions at 2930-2850 cm-1. The –CC- shows a band at 2140 - 2100
cm-1 (symm. stretch). A band at 1475 - 1300 cm-1 is expected for the
aliphatic C - H deformation vibration.
10. A band at 3030 cm-1 (Aromatic C - H stretch). The -CC- is symmetrically
substituted and therefore shows NO band at 2260 - 2190 cm-1 (the stretch
vibration of this symmetrical molecule is not accompanied by a dipole
moment change and therefore is IR-inactive).
In the region 1675 - 1500 cm-1 the aromatic C = C stretch is visible and in
the region 1000 - 650 cm-1 the Ar - H deform (out of plain).
11. All three compounds would display a strong band in the region 17400 1700 cm-1 (C = O stretch). The acid would show a diffuse irregularly
shaped peak at 3000 - 2500 cm-1 (O-H stretch), while the aldehyde would
display a weak absorption at 2720 cm-1.
12. The first compound would give one N-H stretch peak in the region 35003100 cm-1, whereas the second would display two peaks in that region.
The third compound would have no peak at all in this region. All compounds
would display a C=O stretch at around 1640 cm-1. Therefore, the decision
should be based upon the peaks appearing in the N-H stretch region..
13. The first compound would show NO band at 2260 - 2190 cm-1 because the
-CC- is symmetrically substituted.
The second compound would show a weak absorption at 2260 - 2190 cm-1
and in addition a C=O at 1700 cm-1 and a C-H (aldehyde) at 2700 cm-1.
The third compound would also display peaks due to C=O and C-H
(aldehyde), similar to that of the second compound. However, no peak at
2260 - 2190 cm-1 would be observed.
14. Only the first compound would display absorption bands at 3040-3010 cm-1
(>C=C<H stretch) and 1650 cm-1 (-C=C- stretch).
15. The second compound would display the following bands:
one band at 3040-3010 cm-1 (>C=C<H stretch)
two bands at 2960-2870 cm-1 (CH3 stretch)
a weak band at 2700 cm-1 (C-H stretch of aldehyde)
a band in the region 1680-1620 cm-1 (-C=C- stretch)
The above absorption bands would not be present for the first compound.
16. m-Xylene shows one peak at 810-780 cm-1.
p-Xylene shows one peak at 850-800 cm-1.
17. There are no bands visible above 3000 cm-1. Hence, the compound can not
be aromatic. C-H aliphatic are present at 2960 cm-1 and 2930 cm-1.
There is no strong smooth broad band present at 3500-3000 cm-1 and
therefore the compound can not be an alcohol.
The carbonyl band at 1718 cm-1 could be from an aldehyde, ketone or an
acid. It can not be an acid, since the broad OH band is not present. It can
not be an aldehyde, since there is no band at 2720 cm-1.
There are no bands at 2200 cm-1 and 1650 cm-1, indicating that there will
probably be no double or triple bonds and If there are double or triple
bonds, these must be symmetrical.
18. The bands visible above 3000 cm-1 indicate the presence of alkene, alkyne
or aromatic H.
No band at 2200 cm-1 is present, hence -CC-H can be excluded.
The presence of double bonds is clear from the bands at 1650 cm-1 and the
strong bands at 990 and 910 cm-1.
Methyl groups display a characteristic band at 1375 cm-1 and 1450 cm-1,
Methylene groups only display the 1450 cm-1 band. Since only the 1450
cm-1 band is present, the compound does not contain a methyl group.
Since there are NO STRONG bands visible in the region 3500-3000 and
1900-1650 cm-1, there are no OH, NH or C=O groups present.
19. There are no absorptions above 3000 cm-1, 1650 and 1450 cm-1 and no
strong bands in the region 850-700 cm-1, therefore this compound is not
aromatic.
4000-2000 cm-1
The strong broad and smooth band at 3350 cm-1 indicates the presence of
an OH group. The bands at 3000-1850 cm-1 indicates the presence of
aliphatic C-H. Since there are three bands visible there must be two
different aliphatic groups (each C-H displays two peaks). There is no
indication for the presence of C=CH, CCH, Ar-H, C∫N and asymmetrical
C∫C groups.
2000-1200 cm-1
1380 and 1470 cm-1 indicate the presence of a CH3 group. there are no
C=O groups olefinic or aromatic structure parts visible.
20. Peaks above 3000 cm-1 indicate aromatic compound, which is further
strengthened by the presence of peaks at 1620, 1500 and 700 cm-1. No
aliphatic groups are present
Two bands around 3500 cm-1 indicate the presence of an NH2 group. No
OH, C=O (the intensity of the peak at 1600 cm-1 is too low), CN or
asymmetrical CC are present.
21. Double bond equivalents of this compound is 5 (DBE=5)
Oxygen can not be present as OH since no characteristic OH band is
present at 3500-3000 cm-1. The band at 1690 cm-1 indicates the presence
of aldehyde, ketone or amide. Amide is not possible(mol form). therefore
an aldehyde or ketone. Aldehyde can be excluded since no C-H (aldehyde)
at 2700 cm-1 is visible. Probably the compound is a ketone.
Absorptions at >3000, 1600, 1580, 680 indicate aromatic character.
Absorptions at 2920,2960 and 1360 indicate a methyl group.
the two band at 750 and 700 cm-1 indicate mono substitution.
possible structure :
check structure:
DBE = 5
O
CH 3
Guide to tutorial problems IR-spectroscopy (Part I)