Susanna Liang ([email protected])
Nathan Nellessen ([email protected])
Chem 51LB
Dr. Link
http://sites.uci.edu/ochemtutors/
Chem 51LB Practical Review Packet
Fundamental Lab Technique
• TLC: Thin Layer Chromatography
Label the parts of the TLC plate:
Solventfront
Silica Plate: polar / nonpolar
If compound A is more polar than
compound B, it will move less / more.
Compound A
- When checking the progress of a reaction,
how should you spot your plate?
Origin
1 line for starting material, 1 line for starting
material and rxn (cospot), and 1 line for rxn
50%
EtOAc/hexanes
Draw what is expected to happen to Compound Draw how to spot a TLC plate when checking
A in each situation:
the progress of a reaction:
Ifthesolvent
islesspolar,
allthespots
willmove
less(lower
onthe
plate).Ifit’s
morepolar,
itwillmove
more
(higheron
10%
75%
EtOAc/hexanes EtOAc/hexanes theplate)
1 line for starting material,
1 line for starting material
and rxn (cospot), and 1
line for rxn
Rf:
!"#$%&'( !"#$ !"#$#% !" !"#$
!"#$%&'( !"#$ !"#$#% !" !"#$%&' !"#$%
Notes:
-
Place filter paper in your TLC chamber, and don’t let the TLC plate touch the filter paper
-
Place the TLC plate in the chamber straight why??
So that the spots don't run crooked in the solvent
Susanna Liang ([email protected])
Nathan Nellessen ([email protected])
•
Chem 51LB
Dr. Link
http://sites.uci.edu/ochemtutors/
Acid-Base Extraction:
1. When do you use acid-base extraction?
Use for separation of organic compounds that differ in their solubility in water based on
their acid-base properties. If one compound can become ionic, then it will become more
soluble in water than in organic solvent.
2. How do you determine which layer is which?
Water drop test
3. What determines which layer goes on top/bottom?
Density
a. More dense = bottom, less dense = top
b. Most organic solvents are less dense than water except DCM
Given the following compounds, draw what is expected to be in each layer after adding NaOH?
COOH
OH
NaCl
NaOH
Aqueous (water)
Na+
Organic (Ether)
In order to isolate the acid, once you
perform the extraction, you will need to
add acid to reprotonate the acid and
perform another extraction (if that’s
what you’re trying to isolate) because
otherwise it will remain an ion and stay
in the aqueous layer.
ClCOO-
OH
Notes:
-
Make sure the stopcock is closed before you add your mixture
-
After you shake to mix the layers, take off the stopper and let the layers separate before
extracting
Susanna Liang ([email protected])
Nathan Nellessen ([email protected])
•
Chem 51LB
Dr. Link
http://sites.uci.edu/ochemtutors/
Melting Point:
1. What effect do impurities have on the melting point of a compound?
Impurities break the ordered arrangement of molecules of a pure compound and lower the
melting temperature and widen the melting range.
2. What is melting point used for?
Identifying compounds by comparing melting points with a known compound
3. How do you determine the identity of an unknown compound?
Perform a mixed melting point with the compound you think it is, and if the melting point
stays the same then it indicates that they are the same compound.
Notes:
-
Try and pack the same amount into each capillary tube and tap it down completely
-
To perform a mixed melting point experiment, pack the same amount of known and
unknown compound
•
Recrystallization:
1. What is recrystallization used for?
Purification of a compound
2. How does recrystallization work?
It lowers the solubility of the desired compound with organic solvent and water so that it
can precipitate out of solution while impurities remain in the solvent as the solution is
heated and then cooled in an ice bath.
Notes:
-
When recrystallizing, make sure to add a MINIMAL amount of solvent so that your
product can precipitate
Susanna Liang ([email protected])
Nathan Nellessen ([email protected])
Chem 51LB
Dr. Link
http://sites.uci.edu/ochemtutors/
Stoichiometry Calculations:
A student carries out a reaction using 5 ml of 1-chloropropane and 0.200 g of sodium hydroxide
forming 0.260 g of propanol. What is the limiting reagent? What is the theoretical yield? What is
the percent yield? (The density of 1-chloropropane is 0.89 g/cm3)
NaOH
Cl
78.529 g/mol
OH
39.9971 g/mol
60.10 g/mol
1000 cm3 = 1 L, 1 mL = 1 cm3
density = mass / volume
moles of 1-chloropropane:
5 mL x (0.89 g / 1 mL g) = 4.45 g
4.45 g x (1 mol / 78.539 g) = 0.0567 moles of 1-chloropropane
moles of NaOH:
0.200 g x (1 mol/ 39.9971 g) = 0.005 moles of NaOH
NaOH is limit
Theoretical Yield:
0.005 mol x (60.10 g / 1 mol) = 0.301 g
Percent Yield:
(0.260 / 0.301) x 100% = 86.38 %
Mechanisms:
Draw the mechanism for each of the following reactions:
OH
Br
R
NaOH
EtOH
O
R
R
O
R
O-
H
OH-
R
Br
O
Susanna Liang ([email protected])
Nathan Nellessen ([email protected])
O
Br
N+
Br
+ Br Br
O
H
OH
N+
Chem 51LB
Dr. Link
http://sites.uci.edu/ochemtutors/
Br-
H
Br
OH
Br
Br-
N+
H
+ Br
OH
Br
Br-
Br-
O
Br
O
Br+
KOtBu
Br
-O
H
Br
+
OH
Br
Br
KOtBu
O
OH
Br
Br
+E
+
Susanna Liang ([email protected])
Nathan Nellessen ([email protected])
OH
Chem 51LB
Dr. Link
http://sites.uci.edu/ochemtutors/
H2SO4
H2SO4
+
+
OH
Critical Thinking
1. A student is performing an elimination reaction using a secondary alcohol and sulfuric
acid. The only available technique for the student to identify their product is IR. The
student decided to take an IR before and after the reaction was complete. How will the
student know if the reaction has gone to completion?
The student should see a strong broad OH stretch around 3000-3500 before the reaction takes
place. Then the student should not see that peak anymore once the reaction is complete. They
should also see the emergence of a Csp2-H peak around 3100 showing that the reaction has been
completed. The presence of Csp3-H does nothing for telling how the reaction has progressed.
Susanna Liang ([email protected])
Nathan Nellessen ([email protected])
Chem 51LB
Dr. Link
http://sites.uci.edu/ochemtutors/
2. A student is running an extraction and has forgot to label their layers. How could the
student determine what layer is the organic layer and what layer is the aqueous layer?
The student should add a drop of water to the flask and see what layer it goes to. Whatever layer
the water drop goes to will be the aqueous layer, meaning that the other layer will be the organic
layer.
3. When filtering their final product a student does not see as much crude product as they
would like to see. They decide to run a TLC plate with the filtrate. In lanes 1, 2, and 3
they run: one of the original reactants, standard product, and the filtrate. After finishing
the TLC it showed that lanes 1 and 3 both had an Rf value of 0.7. Lane 2 had an Rf value
of 0.5. What can be concluded about the experiment?
Many things can be speculated about this experiment but the data collected from the TLC can
mean any of the following:
1. The reaction did not run to completion (left over reactant)
2. The student added too much of one reactant and not enough of the other reactant
3. No product fell into the filtrate
4. A stockroom manager buys a large container of tert-butoxide that has a shelf life of 10
years. The stockroom employees give out the tert-butoxide for 5 years for an elimination
reaction lab. Over time students start to get elimination products that would be favored by
a strong tiny base, not a strong bulky base such as tert-butoxide. The stockroom manager
hires you to come investigate this issue because students are losing their minds trying to
analyze their results. What is the most plausible explanation for this problem? (Hint: the
container of tert-butoxide has to be opened to the atmosphere to pour into smaller
containers for the lab rooms)
Believe it or not this is what happened to your lab sections this year for your elimination labs.
The large container we bought whenever exposed to the atmosphere would react with the water
vapor, which would create hydroxide anions. Over time it has built up enough to a level where it
can react with 2-bromobutane to form a product that is favored by a small strong base.
Susanna Liang ([email protected])
Nathan Nellessen ([email protected])
Chem 51LB
Dr. Link
http://sites.uci.edu/ochemtutors/
Spectroscopy
Strategy
Step 1: Solve for Degrees of Unsaturation
Step 2: Look for peaks that would use up unsaturation (carbonyl in IR, protons in the 6-8 range
in NMR to show benzene ring)
Step 3: Identify easy pieces of the NMR (isopropyl, carboxylic acids, aldehydes)
Step 4: Put the pieces together
Step 5: Put it all together and check using the chemical formula, IR, and NMR
Susanna Liang ([email protected])
Nathan Nellessen ([email protected])
Chem 51LB
Dr. Link
http://sites.uci.edu/ochemtutors/
Find the structures of the following unknown compounds given molecular formula, HNMR, and
IR spectra:
1. C9H12
Susanna Liang ([email protected])
Nathan Nellessen ([email protected])
2. C
C10H13NO2
9H11NO2
Chem 51LB
Dr. Link
http://sites.uci.edu/ochemtutors/
Susanna Liang ([email protected])
Nathan Nellessen ([email protected])
Space to work on IR/NMR:
Chem 51LB
Dr. Link
http://sites.uci.edu/ochemtutors/
Susanna Liang ([email protected])
Nathan Nellessen ([email protected])
Chem 51LB
Dr. Link
http://sites.uci.edu/ochemtutors/
Lab Safety
This is a new section being added to the practical this year to get across some basic lab
safety/disposal techniques before you start 51LC. Every problem she could throw at you can be
answered by reading the labels on the pictures given.
Some safety tips to remember that she has mentioned multiple times throughout this quarter:
-
No gloves when going to stock room
-
Do not touch face when gloves are on, even if they are clean (prevents bad habits)
-
All glass goes into glass waste. Such things include: Melting point tubes, broken
glassware, capillary tubes
-
USED TLC plates will go into the waste container with the USED TLC plate label on it
-
All chemicals should go into the chemical waste container
-
No running