Seminary of the Southwest Chemistry Questions

CHEM 416 Unknown Elucidation Review1. Use the spectra given below to determine the identity of the compound. Answer questions a – e.
a. Using the mass spectral data, what is the m/z ratio for the molecular ion (M+)?
What is the chemical formula for a hydrocarbon with this molar mass? (hint: use the rule of 13)
ii. What is the chemical formula for a compound with the same molar mass but contains one oxygen
iii. Write the equation used to calculate the degree of unsaturation, then calculate this value for the
oxygen containing substance from part ii.
b. Draw a molecular structure consistent with these spectra.
c. Label the key identifying peaks directly on the IR spectrum.
d. Label the protons found in the NMR spectrum and on the structure in part b.
e. Explain the fragmentation pattern of the mass spectrum.
2. An unlabeled sample has the following 13C NMR: δ 219.6, 38.0, 23.1 ppm. Identify the structure (A – D)
that best fits the data.
3. Complete the table with this following list of carbon types:
a) C ; b) CH; c) (+) CH2 (i.e. above the baseline); d) (–) CH2 ( i.e. below the baseline); e) CH3
Carbon signal detected
4. Draw the molecular structure of a compound that matches the following DEPT 13C NMR spectra. The
molecular formula of the compound is C4H8O.
5. Draw the molecular structure of an unknown compound using the data given below.
Molecular formula: C6H12O
IR: 1705 cm – 1
H NMR: no absorptions greater than δ 3 ppm
C NMR: δ 24.4, δ 26.4, δ 44.2 and δ 212.6 ppm
6. Give the expected 13C NMR signal (in ppm) for deuterated chloroform (CDCl3):
7. Write instructions to prepare two solutions using hexane as the solvent: a) 10 mL of a 0.2% (v/v) solution
of an unknown liquid; and b) 0.2% (m/v) solution of an unknown solid.
8. What volume (in 𝜇L) of a 0.2% (v/v) stock solution is needed to make 10 mL of a 1.0 × 10 -4 M solution?
What volume of a 0.2% (m/v) stock solution is needed to make 10 mL of a 1.0 × 10 -4 M solution?
(Assume any molar mass between 100 – 200 g/mol and density = 1 g/mL for a liquid unknown) Show your
volume of 0.2% (v/v) stock = __________ 𝜇L
volume of 0.2% (m/v) stock = __________ 𝜇L
Spectral Determination of an
Unknown Compound
CHEM 416 – Experiment 14
Instrumentation – Identical to Experiment 13
Analyze your unknown sample using the same instrumentation and settings as
well as sample prep steps as your terpene samples.

FTIR (functional groups)
All instrumentation has been used in
GCMS (molar mass)
previous experiments
UV spectroscopy (conjugation)
H NMR (number of hydrogen atoms)
C, DEPT-90 and DEPT-135 NMR spectra (carbon types) – provided
Note: include all labeled spectra with your lab report for this experiment
FTIR Analysis of Unknown Compound
FTIR analysis steps the same as Experiment 13:

Clean the diamond surface, then run a background
using the Nicolet iS10 with ATR compartment and
○ NS = 10
○ Resolution = 4 cm-1
Apply your sample directly to the ATR surface, 1-2
drops of unknown liquid or sprinkle small amount of
solid directly on the ATR diamond surface
Leave the sidearm off to the side for a liquid sample
(unless it is volatile), or lower to secure a solid sample
Run the IR scan to determine functional groups present
and print resulting spectrum to include in your writeup
GCMS Analysis Sample Prep
GCMS analysis steps the same as Experiment 13:
Note: some unknowns require
diethyl ether instead of hexane as
the solvent for the GCMS analysis

Liquid unknown: prepare a stock 0.2% (v/v) solution in hexane (or diethyl ether as
directed) for analysis by GCMS
○ 20 µL diluted to 10 mL (note: 0.2% v/v = 0.2 mL/100 mL)

Solid unknown: prepare a stock 0.2% (m/v) solution in hexane for analysis by GCMS
○ 20 mg dissolved to 10 mL (note: 0.2% m/v = 0.2 g/100 mL)
GCMS Instrumentation
Model: Agilent HP 6890/5973 GCMS
capillary column
Column: HP-5MS capillary column
● (5%-phenyl)-methylpolysiloxane (nonpolar column)
● length = 30 m
● i.d. = 0.25 mm
● film thickness = 0.25 𝜇m
Ionization Method: Electron Impact (EI)
Mass Analyzer: Quadrupole
GCMS Instrument
Print the resulting chromatogram and mass spectrum to include with your write-up
GCMS Instrument Conditions
Oven conditions:
0 – 0.50 min
0.50 – 7.0 min 30℃/min to 270℃
Injection volume = 1 𝜇L
Flow rate = 1 mL/min
Solvent delay* = 3 min
Rate (℃
* Note: some unknowns require diethyl ether instead
of hexane as the solvent for the GCMS analysis AND
you must override the solvent delay
Temperature (℃)
Hold Time (min)
Analysis of Mass Spectral Data
The mass spectrum is used to establish the molar mass of the unknown substance as well as
help determine number/type of atoms present and degree of unsaturation (DoU) following the
Determine the molar mass of the unknown and key fragments (note: confirm the unknown
molar mass with your instructor)
Use the Rule of 13 to establish the formula unit:
○ (m/z of molecular ion)/13 = number of carbon atoms
○ (m/z of molecular ion) – (12)(number of carbon atoms) = number of hydrogen atoms
○ Include other elements (i.e. O, N, Br, Cl) in the formula unit as identified by IR and
mass spectra
Determine the degrees of unsaturation for more insight about the structure:
DoU = [(2C + 2) + N – H – X]/2
UV Analysis of Unknown Compound
UV analysis analysis steps the same as Experiment 13:
Prepare a 10-mL solution with a concentration of 1 x 10-4 M using a hexane solvent

Liquid unknown: dilute 0.2% (v/v) solution to 1 x 10-4 M
○ Use the molar mass from the GCMS analysis to convert from % v/v to corresponding
molarity (M) value (assuming about 1 g/mL density value)
○ Then make the appropriate dilution

Solid unknown: dilute the 0.2% (m/v) solution to 1 x 10-4 M
○ Use the molar mass from the GCMS analysis to convert from % m/v (g/mL) to
corresponding molarity (M) value.
○ Then make the appropriate dilution
Sample Calculation

Liquid unknown: prepare a 1 x 10-4 M solution in hexane

0.2% v/v = 0.2 mL/100 mL
For example, a sample calculation for a hypothetical molar mass = 170 g/mol
(Assumption: density = 1 g/mL)
Solid unknown: prepare a 1 x 10-4 M solution in hexane

Note: you will need to make a NEW 0.2% stock
solution with hexane if you used diethyl ether during
the GCMS analysis
0.2% m/v = 0.2 g/100 mL
For example, a sample calculation for a hypothetical molar mass = 170 g/mol
Use MV = MV to calculate the volume needed to make a 10 mL solution of 1 x 10-4 M
Evolution 300 UV-VIS Spectrophotometer
Wavelength range = 190 – 300 nm
Data interval = Normal
Scan speed = Intelliscan
Bandwidth = 2 nm
Cuvette: Quartz
Evolution 300 spectrophotometer
Handle with care & rinse several times with hexane solvent between uses ($45)
Run a blank with hexane in the cuvette, then your sample. Label the 𝝀max value,
print the resulting spectrum to include with your write-up.
UV Spectra for Diene and Enone Chromophores
UV data is most often used to reaffirm a conclusion about
the presence or absence of a chromophore previously
identified by NMR or IR.
Conjugated diene and enone chromophores give strong
UV absorption peaks (about 200 – 300 nm). The geometry
and substitution pattern influence the position.
The 𝝅 electrons in these systems undergoes a 𝝅 → 𝝅*
transition with the extent of the conjugation and
substituent types shifting 𝝀max values to longer
Wavelength (nm)
UV Spectra of Substituted Benzenes
Substituted benzenes have two main
band types between:
● 180 – 200 nm (E)
● 200 -240 nm (K)
Substituted benzenes causes these 𝝀max
values to shift to longer wavelengths.
UV Spectrum for Benzene
NMR Analysis of Unknown Compound
Proton NMR analysis steps the same as Experiment 13:

Prepare one NMR tube containing your unknown, add
○ 2 – 3 drops of liquid unknown in 0.6 mL CDCl3
○ About 8-10 mg of solid unknown in 0.6 mL in CDCl3
4 cm solution height inside tube
unknown + CDCl3
brought to 4 cm
CLEAN the NMR tube when finished ($15)
Mix solid and solvent in vial for
transfer to the NMR tube
4 cm
Bruker 400 MHz NMR supported by Top Spin software
Run a single 1H NMR for the unknown
Instrument settings:
● Number of scans (NS) = 16
● Time domain (TD) = 65k
● Sweep width (SW) = 20.5
Bruker 400 MHz Autosampler
Integrate all peaks to establish the proton environments and numbers. Print the
spectrum to include with your write-up.
Note, a 13C NMR will be provided for your unknown sample.
Decoupled 13C NMR
Carbon-13 has a low natural abundance (1%), therefore,
requires a more concentrated sample and a greater
number of scans for acceptable signal-to-noise (S/N)
Chemical shifts follow the same rough trends as seen
in proton NMR
Peaks heights are not uniform and can not be used to
determine the number of carbons at a given signal
Makes it possible to count the number of different
carbon environments in a structure
C NMR of Methyl Propanoate in CDCl3
Identifies four non-equivalent carbon atoms
CDCl3 at 77 ppm
C NMR of Toluene in CDCl3
Identifies five non-equivalent carbon atoms even though the molecule contains seven carbon
atoms. This is due to a plane of symmetry that makes ring carbons equivalent.
DEPT (Distortionless Enhancement by Polarization Transfer) NMR allows for the
determination of how many hydrogen atoms are bound to each carbon:

Quaternary carbons
DEPT-90 and DEPT-135
● only CH resonances appear
● signals due to CH3, CH2 and quaternary carbons are absent
● CH3 and CH resonances appear as positive signals
● CH2 resonances appear as negative signals (below the baseline)
● quaternary signals are absent
Identifying Carbon Types
Putting information from the 13C, DEPT-90 and DEPT-135 NMR spectra together
makes it possible to tell the number of hydrogen atoms attached to each carbon.
Carbon Signal
Decoupled 13C NMR
C, CH, CH2 and CH3
CH & CH3
CH2 (negative)
Include the 13C, DEPT-90 and DEPT-135 spectra and analysis with your write-up.
Carbon NMR Chemical Shifts
Recruiting for Fall 2022
For those of you who are graduating, CONGRATULATIONS!!
For anyone returning in the fall, consider the following:

TA for CHEM 416 lab
Any interest? Apply for the position when posted for the fall.

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