Problem set #2 (Meeting 2)
Announcements (Please sure to read them carefully since you will be held to them)
1. You will not be allowed to use any course readers in the lab starting meeting 2. All information that is needed to carry out the lab should be in your pre-lab report. No exceptions! You will not be permitted to perform any experiment without a proper pre-lab because you are deemed unprepared, which poses a significant safety hazard.
2. You will not be permitted to carry out any experiments without proper attire! If you do not show up in proper clothing or/and do not wear goggles, a proper length flame-resistant lab coat and nitrile gloves in the lab (read UCLA PPE policy), you will be excused from the lab with a zero score (lab performance and quiz!) for this lab the first time (you still will have to make up the lab later on without getting any credit for it!). The second time, the consequences will be more severe i.e., dismissal from the course for not following the safety protocol. CAL OSHA and similar offices have been inspecting the department a lot recently (unannounced) and have passed out very heavy fines (i.e., not wearing clothing, a proper lab coat, goggles or nitrile gloves) costs more than $1000 per incident.
3. The infrared assignment for this quarter is posted (see link on front page). The assignment is due by Friday, April 8, 2016 at 12:00 pm in the instructor's office in YH 3077E. Late assignments will not receive any credit. Assignments submitted via email will not be accepted either. If you are not on campus on Friday, you will have to submit the assignment early. Plan your workload accordingly. (The instructor will not allow you to drop the course if you failed to turn in the assignment.) You cannot retrieve the assignments after you submitted it.
4. Make sure that you have access to the online quiz system as soon as possible. If not, inform the instructor asap. This system is largely independently operated from URSA, which means that even if you are enrolled in the course, you might not be in the database for the quiz system.
5. Submit two separate pre-labs during meeting 2: one for the benzoin experiment and one for the elimination experiment.
ATTN: The answers to the questions below are due at the start of meeting 2; these answers should be part of your pre-lab write-up. Show a balanced chemical equation and proper stereochemistry where appropriate.
1. Referring to the benzoin condensation, answer the following questions (include into the benzoin prelab).
a. During the initial stage of the reaction, thiamine hydrochloride is reacted with sodium hydroxide solution. Which purpose does this step serve? Provide a balanced chemical equation for this reaction.
b. Why is the initial reaction perform in water before 95 % ethanol added?
c. Why is it important to store the reaction mixture in a closed 6-dram vial?
d. A student starts with 0.25 g of thiamine hydrochloride and 3.00 mL of 0.10 N sodium hydroxide solution. Afterwards, he adds 2.80 mL of benzaldehyde to the solution. After the work-up, he isolates 2.30 g of white crystals. Determine the percentage yield for the reaction. Show your calculations.
2. Vacuum filtration (sorry the link to the quiz will not be active before 4/1/2016 (around 2 pm) due to enrollment)
Please watch the following video (MP4-format) and read the appropriate chapter in the "Survival Kit Reader". Then take the quiz below.
Hint: In general, if a video does not start, the file should be downloaded to the desktop and opened directly from a video player (i.e., Real Player, VLC player (which can be downloaded here: http://www.videolan.org/vlc/download-windows.html)
Online Quiz
http://bacher.chem.ucla.edu/TakeQuiz/?id=c9f0f895fb98ab9159f51fd0297e236d
To log in, use your last name and your student ID. If you are experiencing problems, contact the instructor via email and include your full name (indicated which one is your last name), your student ID, section and TA. (Hint: Think very careful about each response because many of the questions have more than one answer to them! Many students come up with the most obvious one and miss some of the details, which leads to a zero score for the question because the system is not programmed to give partial credit!) You can open the quiz, print it out and submit the answers later. Even though you can take the quiz until one hour prior to meeting 2 of your section, you should not delay taking it since there might be some problems with the server or the login. There seem to be problems with MAC systems, the Safari Browser and the Google Chrome Browser. The best is using IE 8.0 (or above) or Firefox. After you submit the answers, your score has to appear on your screen. If this does not happen, you will have to retake the quiz. (There will not be any possibility to retake the quiz weeks later since you are supposed to show preparedness at the point in time when you enter the lab!). The quiz is worth 10 points.
3. Referring to the elimination reaction, answer the following questions (include into the prelab for the elimination reaction).
a. Why is it important to use a catalyst in the reaction? Show the key intermediates assuming the reaction uses 2-methylcyclohexanol and follows an E2 mechanism.
b. Why can the use of Amberlyst A15 be considered environmentally more benign than the use of concentrated phosphoric acid?
c. In which way does the student take advantage of the Le Châtelier principle in this reaction? Why is this necessary?
d. Why is it important that the reaction mixture is refluxed for 30 minutes prior to the start of the distillation?
e. Why is it important to observe Rule 9 in the lab?
f. What is the proper concentration of the samples being submitted for GC/MS analysis?
g. Which information does the student obtained from the GC/MS printout?
4. a. Using PC Spartan 2014 (which is only available in the SLC), perform the following calculations for the alkenes: 1-methylcyclohexene, 3-methylcyclohexene, methylenecyclohexane, ethylidenecyclopentane, 1-ethylcyclopentene, and the cations below.
Make sure that all hydrogen atoms are present and there are also no extra hydrogen atoms either.
b. Select Calculations from the Setup menu.
The following window should appear. Select the options shown.
Calculate the energy of the Equilibrium Geometry at Ground State using Semi-Empirical, AM1 method. Verify that the Total Charge is Neutral (for the neutral molecules and Cation (for the cations) and Unpaired Electrons is 0. Do not check any of the other the boxes i.e., IR, Orbitals & Energies and Atomic Charges, because this will cause the calculation to take much more time! Select Submit from Setup menu. When the calculation is completed you will be notified by a pop-up window. It should not take more than a couple of seconds for the calculation to complete. Make sure that you have only one window open at this point. The messages are usually displayed in the first window, and often not in the window you are working in at this point. Under the Display menu, select Properties. Record the value for the Energy.
Note: Please make sure to evaluate the structure that is obtained after the calculation. The carbon atom with the positive charge should be planar (why?). On occassion, the calculation gets "stuck" yielding to non-planar carbocation carbons, hydrogen atoms that are perpendicular to a C-C bond, which is clearly incorrect. In these cases, you will have to redraw the molecule and perform the calculation again.
Data Analysis (include into the elimination prelab):
1. Outline a mechanism to show the formation of the eight cations above from 1-Cyclopentylethanol.
2. Based in the calculated trends in the heat of formation (=energy), which alkene appears to be the most stable based on the calculation? How does this data match with the heats of formation given on the course website (which are the experimental data)? Can you rationalize the differences?
3. Based on the data obtained above, predict the relative product distribution.
4. Which conclusions can be drawn from this simulation? (Hint: Is the used model suitable to predict the outcome of the elimination experiment?)
Problems with calculations: Check here