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rubrics | Back to current Things to keep in
mind ]
Questions posed on this page do not need to be answered or handed in.
However, the issues mentioned here should definitely be kept in mind as you
finish your lab write-up.
Phrases such as "human error" and "instrumental error" are meaningless unless you can come up with concrete examples. What really affected your data and results? Is it realistic to expect that your results could have been better? What definite things (actions, conditions, etc) could have improved them?
Finally, your conclusion should refer back to the pre-lab (Intro.) and address purpose, procedure, and theory. Don't make it long, don't ramble. I want to see well-thought-out and clear, concise writing. (NOTE: I also need to be able to READ the lab in order to asses your work!)
Your lab write-up is not about length! It's about content and your showing me your ability for critical thought and evaluation of data.
In lab #1, you are given only 4 points for yield and 4 points for purity. The significance placed on yield and purity will increase as my expectations for your developing lab skills increase. The first 3 labs are heavy on learning technique, and in future labs your product yields will reflect how you developed your skills early on.
In other words, I am assuming that you are NOT an expert in organic lab right now, so you are getting an opportunity to learn and hone your skills and learn from mistakes, while exercising those thought muscles...
What is the true boiling point of water at the current pressure?
Don't just say "about 100º".
You were given the ambient pressure, now find the temperature:
The following is a curve made by plotting pressure vs boiling point. Values are taken directly from Lab Manual pg. 20.
? What is your thermometer's deviation
-from the "true" boiling point?
-from the "true" melting point?
(note: water is DENSEST at 4ºC, which means for your ice/water mixture, the
temperature of the water at the very bottom of your beaker was likely ~4º,
unless you were stirring frequently and had a significant amount of ice)
The following is a sample calibration curve for a hypothetical thermometer which reads:
1.0º @ 0.0ºC
98.5º @ 760.0mm Hg
(from the BP curve, we can see this is ~100.0ºC)
This means, at 0.0ºC, the thermometer reads 1.0º HIGHER
...and at 100.0ºC, the thermometer reads 1.5º LOWER than the "true" temperature.
• If the thermometer reads 99º, you must subtract -1.5º from this value, giving you a "true" temperature of 100.5º.
• If your thermometer reads 20º, you must subtract +0.5º from this value, giving you a "true" temperature of 19.5º.
Assume that your thermometer's deviation behaves approximately in a linear
fashion.
Also note that if you "rounded" your thermometer reading (ex: 1º
instead of 0.5º), your calibration curve will start to give you trouble as you
extrapolate at higher temperatures..
A COPY OF THIS CALIBRATION CURVE* SHOULD BE KEPT IN YOUR DRAWER WITH YOUR THERMOMETER FOR EASY REFERENCE!
For future labs, you should be able to refer back to this calibration curve and extrapolate "true" values from observed temperatures.
? If your thermometer reads 80.0ºC, what is the "true"
temperature?
If your temperature calibration at a certain temperature is ~0.0º (and thus you do not have to correct your raw data point), note it in writing!
One often-overlooked thing that lets you know how good your numbers are: significant figues.
Did you weigh and record weights to the nearest .01g?
...If you recorded a weight as ~1g, you cannot have a % recovery more precise
than to the nearest 10%!
Did you take temperatures to the nearest .5 (or .2) ºC?
...If you were rounding temperatures to the nearest degree, subjecting a recorded
temperature to a +.15 calibration is meaningless!
...If you were recording temperatures to the nearest .5ºC, your calibration
should also be to the nearest .5 or .2, not the nearest degree.
Note that a melting point spon-on at [literatre value ºC] means very
little if your heating rate was 20ºC/min (~1ºC per 3seconds).
Due to irregular heating and hot spots / cold spots at high heating rates, your
thermometer may not see the same temperature that your capillary tube
does. Slow heating rate will:
1. allow the apparatus to equilibrate, eliminating odd temperature
gradients.
2. be slow enough to allow you to record the temperature to a high degree
of accuracy (instead of the thermometer seeing the capillary's temperature 5º
ago) and precision (to the nearest .5 or .2 degrees).
3. melt slowly enough that you can find two temperature values, a start
and end point: this will help tell you how pure your melting point really is!
Your lab report should include the melting range Δº (Tfinish melting
- Tstart melting).
A perfectly pure material should have a melting range of 0.5-1.5ºC.
A melting range Δº > 3 ~ 4º is usually a sure-fire indication of
impure material.
A melting point depression (Tfinish melting) of more than 2º less
than the literature value also indicates impurities.
You crank up your mel-temp to 50V.
? Will the heating rate (in ºC/min) be the same at 40ºC as the rate
at 80ºC?
? What are good ways to determine your heating rate?
Melting point phase diagram, melting point depression, eutectic, and compositions:
? Do you know what this diagram means? :::
(Taken from G&M pg. 115)
? at 80 mol% A, what will be the melting range? How does this compare to pure A?
? You just completed a synthesis, and your compound has a melting range of 0.5ºC. Can you be sure that it is pure if you do not know the literature melting point?
? What is the Eutectic?
Questions posed on this page do not need to be answered or handed in.
However, the issues mentioned here should definitely be kept in mind as you
finish your lab write-up.
***Concise writing is a very valuable skill! That never means skimping
on content, and avoiding redundant or roundabout wording!***
Take a quick look a these spectra:
DIETHYL ETHER
ETHYL ACETATE
EUGENOL
Eugenol is probably not the only material in your IR spectrum. What
could also be present?
Use your common sense in this assesment. One of your peaks could
potentially be assigned to an acid anhydride. But why would such an assignment
be inappropriate?
Also, consider what happens when solvent evaporates off of a surface.
What happens to the surface (think of rubbing alcohol on your arm), and what
problems could this pose for your IR spectum, especially if it were, say, a wet
and rainy day outside?
Distillations...
What are theoretical plates, and do you want more or less of them when you do a
fractional distillation?
About how many theoretical plates would a simple distillation apparatus have?
Thermometer calibrations..
Did you take into account your thermometer calibration after you recorded
your temperature, to get the values for your distillation curves?
(SERIOUSLY! Let me knjow if you want some graph paper to plot a small
thermometer calibration curve to keep with your thermometer!)
When plotting a curve, points ought to be connected by a smooth line:
while your data was collected periodically, your distillation proceeded more or
less in a continuous fashion!
***DOUBLE-CHECK TO MAKE SURE YOU HAVE ALL YOUR PAGES IN YOUR LAB HANDED IN TO
ME!***
You will need to assign NMR peaks in your lab writeups for prepared compounds.
If you would like to go over NMR and/or IR with me, you are welcome to schedule
a Monday office hour, or see me thursday. If neither of those work, I can
make a coffee-house- or chem-library- appointment with you on the weekend.
NMR and other spectra will become a very important part of your lab writeups in this class, so if you feel like you are struggling, or O-chem was just too long ago... or if your professor just never bothered to teach it in your particular section: it is best to see me NOW rather than later!
• Make sure you understand the E vs Reaction coordinate graph on pg. 35 of
your lab manual, as well as the theory behind it..
* Transition state energies/stabilities & destabilizing
effects
* Stability of your products
* Potential for rearrangement
* Statistical probability of reaction (collisions)
* Chlorination: Did you normalize against C4?
* What is the secondary/primary reactivity per hydrogen in an
UN-substituted hydrocarbon undergoing H-atom abstraction by Cl• atom?
• Are you clear on the mechanisms for this lab? All mechanisms
should be included SOMEWHERE in your lab report.
• Unsaturation Tests:
What are the relevant reactions, and what are the physical
properties (color? Solid? in solution?) of your reactants and products?
Both unsaturation tests and their results should *make sense*
to you!
IR, GC, and NMR.
(bonus spectral materials:)
IR of
2-Methylcyclohexanol
IR of
3-methyl-1-cyclohexene
IR of
Methylenecyclohexane
NMR of
methylenecyclohexane
NMR of
1-methyl-1-cyclohexene
GC of
methylcyclohexenes
GC of
Dichlorobutanes
IR: What seems to be the composition of your methylcyclohexene IR?
What other materials could realistically be present?
Can you find characteristic peaks of said other materials?
GC: While acetone doesn't affect your GC trace for the chlorinated alkane,
the spectrum below is the reason we do not use Acetone to rinse the GC syringe
when running a spectrum of your methylcyclohexenes:
GC of
methylcyclohexenes with Acetone contamination.
***So... what is that tiny third peak in your GC trace with such a short
retention time? (why do you think it has such a short retention time?)
NMR: Bonus point awarded if you can figure out what the extra "junk" is in your
"Compound A" on pg. 42 (yes, I'm serious, you get 1 bonus point! circle or
underline it near the end of the lab report so I make sure to credit you); the
biggest clue is that little peak at δ approx. 4.6ppm.
