Experiment 14:
IR AND NMR
IDENTIFICATION OF AN
UNKNOWN
Objectives:


To learn how to interpret IR and NMR
spectra.
To use IR and NMR spectra to
propose a structure for an unknown,
given the molecular formula.
Before coming to lab…




Go to the website: www.ochem.com
From the left menu, select TUTORIALS.
From the right column, PRELECTURES,
scroll ¾ of the way down the page.
Watch the following:


SPECTROSCOPY (Part 3 of 4)
SPECTROSCOPY (Part 4 of 4)
(YOU’LL BE GLAD YOU DID! )
IR SPECTROSCOPY
THINGS TO
CONSIDER…
OH
CH3
•What kinds of bonds do
I have?
• If they appeared in the
IR spectrum, where would
they be?
CH3
• Now, look at the
spectrum. Are they
there?
IR SPECTROSCOPY
IR SPECTROSCOPY
Full IR Absorption Correlation Table in Appendix J
Base values for Absorptions of Bonds (cm-1)
O-H
3200-3600
C-O
1000-1200
(Esters have two!)
C-H (sp2)
3000-3100
C-H (sp3)
2850-3000
Aldehyde C-H
2700 & 2800
(there are two!)
Amide N-H
3150-3350
C=O
1650-1740
C-X
500-700
CALCULATING
DEGREE OF UNSATURATION
CcHhNnOoXx
DU = (2c + 2) – (h – n + x)
2
• 1o unsaturation = 1 C=C or 1 ring
• 2o unsaturation = 2 C=C, 2 rings, or CΞC, or combination of C=C & rings
• 3o unsaturation = combination of double bonds, triple bonds, rings
• 4o unsaturation = typically indicates an aromatic ring
13C-NMR
SPECTROSCOPY
Information provided:
A.Functionality (Chemical Shift)
• tells the type of carbon
• via position of signal on x-axis
B.Presence of symmetry
• via # of signals
C.Presence of non-protonated carbons
• via small signals
• sometimes useful, not always!
TYPICAL CHEMICAL SHIFTS

190-220d


160-190d


arenes, alkenes
50-110d


esters, amides, carboxylic acids, acyl halides
110-160d


aldehydes, ketones
alkynes, sp3C attached to functional groups
0-50d

sp3C-Csp3, where 4o>3o>2o>1o
13C
NMR CHEMICAL SHIFT
CORRELATION CHART
R
R
O
C
O
C
H
R
R
R
O
C
O
C
OR R
OH R
O
C
O
C
Fn
NR 2
Csp3
C
X
C
C
220
210
o
o
Csp3
o
o
4 --3 --2 --1
C
0-50d
50-110d
110-160d
190-220d
sp3C
160-190d
200
180
160
140
120
100
p. 118 in lab manual
80
60
40
20
0
1H-NMR
SPECTROSCOPY
Information provided:
A. Functionality



chemical shift
tells the type of hydrogen
Via position on x-axis
B. Presence of symmetry

via the # signals
C. Number of protons of each type per signal

Integration
D. Number of neighboring protons per signal


via the splitting patterns
n+1 rule, where n=# of protons on neighboring carbons
TYPICAL CHEMICAL SHIFTS






10-12d

carboxylic acid

aldehyde

aromatic

alkene

alkyne, hydrogens on carbons attached to
functional groups
9-10d
6.5-8.5d
5.0-6.5d
2.0-4.5d
0-2.0d

sp3C-H, typically 3o>2o>1o
1H
NMR CHEMICAL SHIFT
CORRELATION CHART
SYMMETRY & EQUIVALENCE




Notice that there are 8H
according to the MF, but we
only see 3 signals.
This is because some of the
hydrogens are equivalent.
Number of signals = number
of different types of
hydrogens present
There are 3 different
signals because the
hydrogens are in 3 different
environments.
Ha
O
H b Hb
C
C
C
Ha
O
Ha
Hc
C
Hc
C4H8O2
Hc
INTEGRATION

The integration is
proportional to the #
hydrogens causing that
signal.



There are 3 Ha protons
There are 2 Hb protons
There are 3 Hc protons
2H
O
3H
Ha
C
C
Ha
H b Hb
Hc
C
O
Ha
C
Hc
C4H8O2
Hc
3H
SPLITTING




s
Splitting of a signal occurs b/c
the chemical shift of a signal
can be affected by neighboring
protons.
t
q
Splitting = n + 1, where n = #
neighboring protons. Must be
within 3 bonds or less.
n=3, so
n+1 = 4
(quartet)
Equivalent hydrogens DO NOT
split each other, so Ha would
not split each other.
Protons cannot “see” through
atoms such as oxygens or
nitrogens.
n=0, so
n+1 = 1
(singlet)
Ha
O
H b Hb
C
C
C
Ha
O
Ha
Hc
C
Hc
Hc
n=2, so
n+1 = 3
(triplet)
TABLE 14.1
Name
4acetamidophenol
Molecular
Formula
Look up using
www.chemexper.com
propyl
acetate
benzaldehyde
Structure
Look up using
www.chemexper.com
Degree
of
Unsaturation
Must be
calculated
using MF
COMBINED SPECTRAL
PROBLEMS



Once you have completed Table 14.1, you will
notice 3 questions, followed by 3 sets of
combined spectral problems.
Each page contains a 1H NMR spectrum, a 13C
NMR spectrum, and an IR spectrum pertaining
to one of the compounds in Table 14.1.
You must identify which set of spectra belong
to which compound, and complete the tables by
recording actual chemical shift values and IR
absorptions.
EXAMPLE TABLE
IR Data*
Type of
Absorption
sp3
Structure
Frequency
(cm-1)
CH stretch
sp2 CH stretch
C-O stretch
C=O stretch
OH stretch
Enter ACTUAL IR frequencies
from spectra…NOT base values!
H3
C1
O
C5
H4,5
3
4
C
2
H6
5
1
CH3
6
C2
C6
3
4
C3
C4
Enter ACTUAL NMR chemical shifts from
spectra…NOT base values! Do not include
integration or multiplicity!
PROPOSING A STRUCTURE


The final part of the experiment is to practice
the ability to propose a structure, given only
the molecular formula (MF) and a set of IR and
NMR spectra.
The best place to start is to calculate the
degree of unsaturation using the provided MF,
and use this information as a starting point in
the proposal of possible structures.
PROPOSING A STRUCTURE

Using the MF, calculate the degrees of unsaturation.

Propose possible structures based on the MF and
information gathered from the degrees of unsaturation.




Identify how many signals would appear in the spectra
of each of the possible proposed structures based on
symmetry and equivalent protons.
Identify approximate chemical shifts where these
types of carbons/protons would appear based on
correlation tables.
Identify splitting patterns of each type of proton
present in each of your proposed structures.
Observe the spectra. Using the information you have,
eliminate possible structures until you have identified
the actual structure.
For next lab…



The FINAL LAB REPORT for Experiment 14
will be due at the beginning of class!
The PRE-LAB notebook entry for
Experiment 15 will be due at the beginning
of class!
Safety goggles and closed toe shoes are
MANDATORY!