ORG CHEM II LAB(MAJOR’S)
KELLY
IR Problem Set 1 ANSWER KEY
1.
K Allyl alcohol
The allyl alcohol OH stretch is indicated by the large, broad absorption between 3200-3500 cm-1.
The double bond is indicated by the C-H stretch in the region just to the left of 3,000 cm-1. This
can be differentiated from benzyl alcohol by the small C=C stretch at about 1650 cm-1. Note that
the C-O stretch absorption at about 1025 cm-1 indicates a 1˚ alcohol.
C-C stretch
Olefinic C-H stretch
CH 2 bend
Aliphatic C-H stretch
C-H OOP
O-H stretch
C-O Stretch (1˚)
2.
N. Cyclooctene
Cyclooctene has an ir spectrum very similar to 1-heptene, but can be differentiated from it by
close examination of the spectrum. Both cyclooctene and 1-heptene have very simple spectra,
with olefinic C-H stretch absorptions above 3,000 cm-1. Both also have very small C=C stretch
absorptions about 1650 cm-1. Cyclohexene has a fairly strong CH2 scissoring bend at about
1470cm-1. It does not, however, have a CH3 symmetric umbrella bend at 1375cm-1. There is a
weak absorption (probably part of the fingerprint) at about 1350cm-1, but this is too low to be the
CH3 bend.
C-C stretch
CH 2 bend
Olefinic
C-H stretch
C-H OOP
Aliphatic C-H stretch
3.
D Heptaldehyde
The intense carbonyl C=O stretch peak at ~1725 cm-1 suggests either an aldehyde, or possibly a
ketone (although it is a little higher frequency than a standard ketone). The aldehyde is indicated
by the two aldehyde C-H stretch absorptions which appear at 2750 & 2850 cm-1. Note that the
peak at 2850 cm-1 appears as a shoulder on the much more intense absorption corresponding to
the symmetric CH2 stretching vibration.
Aldehyde C-H Stretch
CH 2 Bending
Aldehyde C=O stretch
Aliphatic
C-H stretching
4.
M Benzyl Alcohol
Note the broad O-H stretch absorption and the olefinic C-H stretch absorptions. This spectrum
can be differentiated from that for allyl alcohol by observing the combination and overtone bands
from 1600-2000cm-1, which are indicative of a monosubstituted benzene. In addition the C=C
ring stretch at about 1600 + 1475cm-1 indicate that it is an aromatic ring.
C-C stretch
Aromatic ring breath
Aromatic C-H stretch
C-O Stretch (1˚)
C-H OOP
C-O Stretch (1˚)
5
L Butyl acetate
Note the presence of a C=O stretch at about 1750-1760cm-1. This would tend to indicate the
possibility of an ester functional group, or possibly a strained ring ketone. The broad and intense
C-O stretch absorption at about 1250cm-1is a strong indicator of an ester group. This band is the
C-O stretch corresponding to the bond of the ester oxygen and the carbonyl carbon. In addition,
the C-O stretch at about 1050cm-1 is due to the other C-O stretch to a 1˚ carbon.
C-O stretch 1˚
Aliphatic C-H stretch
C-O stretch to
carbonyl carbon
C=O stretch
6
B 3-Pentanone
Note the presence of a C=O stretch at about 1710-1720cm-1. This would tend to indicate the
possibility of an ketone, aldehyde, or possibly a conjugated ester. There are no absorptions at
2850 and 2750cm-1 which would correspond to the aldehyde C-H stretch, thus it is not likely to
be an aldehyde. There are no olefinic C-H stretch absorption bands above 3000cm-1, nor are
there any C=C stretch absorptions around 1600 to 1650cm-1. Thus it is not likely a conjugated
ester (also note the lack of strong C-O stretch absorptions at 1250cm-1 in the ester). The
compound is likely a ketone.
C=O stretch
7.
F Propanoic Acid
The broad O-H absorption from 2400 to 3400cm-1 is a dead giveaway for a carboxylic acid. Note
also the relatively broad C=O at about 1710cm-1 and the C-O stretch at 1250cm-1.
Broad O-H stretch
of acid dimer
C-H stretch
C-O stretch
C=O stretch
8
H 1-Butanol
Note the broad alcohol O-H stretch at 3200-3500cm-1. More importantly, note the 1˚ alcohol CO stretch at ~1050cm-1.
C-O Stretch (1˚)
O-H stretch
9.
I Cyclohexane
Note the simplicity of the spectra, only C-H stretch and bend absorptions. The most important
aspect of this spectrum is what is NOT present. Note the absence of a CH3 bend at 1375 cm-1 .
This would suggest a ring structure.
NO CH 3
SYMMETRIC
BEND
CH 2
Scissoring bend
C-H stretch
10.
E 2-Butanol
Note the broad alcohol O-H stretch at 3200-3500cm-1. More importantly, note the 2˚ alcohol C-O
stretch at ~1100cm-1.
C-O Stretch (2˚)
O-H stretch
11.
C 1-Hexene
1-Hexene has an ir spectrum very similar to cyclooctene, but can be differentiated from it by close
examination of the spectrum. 1-Hexene has a CH3 symmetric umbrella bend at 1375cm-1. Also
note if you will, that the unsymmetric double bond C=C stretch of 1-heptene is more intense than
the symmetric C=C stretch in cyclooctene.
C=C stretch
CH 3 bend
Olefinic C-H
stretch
12.
J 2-Methyl-2-butanol
Note the broad alcohol O-H stretch at 3200-3500cm-1. More importantly, note the 3˚ alcohol C-O
stretch at ~1150cm-1.
C-O Stretch (3˚)
O-H stretch