Q&A for 30CL

Question:

Thank you for letting us know the answer key as been posted. While going over it, I saw that the answer to 6b is as followed: "b. The 5-position is the position opposite to the OH-group. The t-butyl groups in this position control the approach from the side. If this group is increased in size, this approach pathway is blocked more efficiently. As a result, the e.e.-value should go up slightly." However, I recalled in lecture, it was discussed that this kind of substitution is actually too big for the catalyst, causing it to twist in a way that is not beneficial for the assymetric epoxidation. So, I am wondering which of the two reasoning should be more applicable here. Thank you!

Answer:

Matthew, the one discussed in the lecture was the one in the 3-position which blocks the access from the front. And you are correct in your statement that a larger group in this position is not necessarily an improvement because the ligand starts to change its structure. AB

Question:

Hi Dr. Bacher, in analyzing UV-Vis spectra how do you know if a peak is a n-->pi* transition or a pi-->pi* transition? and also, how can you tell if the transition belongs to a C=O,N, etc. or C=C function?

Answer:

Hi Yi-Jiun, first of all, n-pi* transition are usually at a longer wavelength than pi-pi* transitions because the energy gap for the latter ones is larger. Which type of bond it belongs to is sometimes not easy to say. Generally, pip-pi* transition caused by a benzene ring are below 300 nm, while the corresponding C=N and C=O absorb a little higher, particularly if they are conjugated to a ring or other double bonds. AB

Question:

Hi Dr. BAcher, do we need to know how to analyze DEPT spectra for the final?

Answer:

Hi Yi, you certainly should know how to analyze them. You learned this already in Chem 30BL. If you look at the old exam, each of them has a DEPT spectrum included as part of the spectroscopy problem.

Question:

Hey Dr. Baacher! I'm not sure if you mentioned in class a due date for the nitration post lab or not but could you please tell me when it is due? Monday? Tuesday? Thanks!

Answer:

Hi Sarah, I mentioned that the postlab is due the next meeting after Thanksgiving, so in yoru case on Tuesday morning. AB

Question:

hey professor bacher, i was wondering if we should make tables for the NMR spectra as well. in the guidelines for postlab it only says IR table, is it just enough to discuss about the NMR in the discussion?

Answer:

Hi Terry, I think that preparing table is the easiest way to organize a lot of numbers. AB

Question:

hey prof bacher, where is the nmr spectra for the lidocaine project? i cant seem to find it anywhere. thanks.

Answer:

Hi Sara, follow teh spectroscopy link on the front page. The only spectra that you should find there are the H-NMR and the C-NMR of lidocaine. AB

Question:

In the C-NMR of the Jacobsen's ligand, are there some peaks missing? Or are they not very visible at all? (In the draft, I did mention this, but still dunno how to correctly address this problem.) I counted a total of 14 unique carbons in this compound, and there are only around 7-8 peaks shown in the spectrum. What gives??

Answer:

Matthew, didn't you notice that the baseline is relatively noisy in this spectrum compared to the other spectra? What could be the reason for that? Also, which peaks are you missing?

Question:

Since there is a limit on the paper's length now, I was wondering what format the report should take. I find it very hard to include all needed materials if writing in double space, using font 12. I was wondering if we can use font 10 instead? (Or maybe other tips?)

Answer:

Hi Matthew, if you run 1-2 pages over it is ok. But do not use a font size smaller than 12 since some of these fonts are really tiny and it would be very difficult to read for me. If you are significantly above 20 pages, you should check if you are redundant somewhere. AB

Question:

Hey Professor, for the formal report introduction, do we only need to find an alternate synthesis for the aldehyde and chiral epoxide and not the other products because that's what it seems the reader says. Thanks

Answer:

Hi Terry, one alternative method to prepare the aldehyde that you prepared in the lab and one alternative method to make chiral epoxides is more than sufficient for your paper since you are not asked to writie a review. AB

Question:

Hi, I'm a little confused on trying to figure out how much catalyst and alkene to add to the reaction. The reader says to add 0.5 grams of the alkene and catalyst in 5 ml of dichloromethane. Well, if the alkene has an mw of 120g/mol,and it says approximately 10 mol% based on alkene, 10 mol%=0.1 moles correct? And if so, then that would be 12 grams. That doesn't make any sense because it's obviously not right, and I don't really know what it is I'm missing. And then the catalyst, it says to use 5-10 mol%, which I assume = 0.05-0.1 moles. Which turns out to be a lot. So yeah, there is just something really big that I'm missing here!

Answer:

Hi Amber, 5-10mol% refers to the number of moles of alkene you use. It turns out that you will use about 0.25 g of the catalyst. AB

Question:

Hi Dr Bacher, i know you mentioned in class that heptane was used to lower the solubility of the catalyst, but why was heptane used here? would hexane work as well, since hexane is relatively nonpolar as well?

Answer:

Hi Emily, hexane might work as well, but since the bp of hexane and dichloromethane are much closer, a significant amount of hexane would evaporate away here as well when removing the dichloromethane. AB

Question:

This is more of a review type question... but what is mol%? I am not particular familiar with this concentration unit. Thank you!

Answer:

Hi Matthew, the term mol% refers to a compound that it used in a certain ratio compared to another compound. Assume that you have one mole of compound A and the procedure asks to use 10 mol% of compound B, which would mean that you use 0.1 mol in this case. This terminology is often used in conjunction with catalyst or other promoters. AB

Question:

Hi Dr. Bacher, when preparing for the quiz on Monday what's the best thing to focus on, lecture material or experimental procedures?

Answer:

Both parts are important.

Question:

This is referring to the HW problems for meeting 4. For this meeting, we are not using the manganese acetate tetrahydrate yet, so I was wondering if question 1f should be for the next lab instead.

Answer:

Hi Tina, the question is more a general question, but you are correct it would make more sense for the catalyst. However, please answer the question for the current prelab. AB

Question:

For the optical rotation test, I know pure should be around +12.5. But if my experimental product has a rotation below +12.5, say +11.5, would that be okay? What if it is above it, say around +13.5, would that be okay too? I guess, basically, I want to ask you what the correct "ball park" range should be. Thank you!

Answer:

HI Matthew, an optical rotation of +11.5 sounds reasonable. If the other enantiomer is still in the mixture, or the salt is still wet, the optical rotation would go down compared to the pure enantiomer. A higher optical rotation obviously means that you did not measure/prepare the solution correctly. AB

Question:

For the third step in our lab, it asks which solvent mixture we should use to monitor the progress of the reaction. Shouldn't it be the same mixture (CH2Cl2:hexane= 1:1)?

Answer:

Hi Amber, this is correct. Since you basically have the same compounds in the reaction mixture, the solvent mixture used for TLC in step will do fine here as well. AB

Question:

I was wondering if you can help me with question 1e, why is there a difference and what would I do with each?

Answer:

Hi X, when you look at TLC, you need to look at the polarity of the compounds and the polarity of the stationary phase. The more they are alike, the stronger the interaction and the less movement is observed. AB PS: It would be nice if you would use your name.

Question:

Hi, I have a few questions about our pre-lab write up for this week. First, the reader says to extract our combined organic layers with 2 portions of saturated sodium chloride; I assume this means 2 extractions? Secondly, on the hints section from the 30CL website it says, "The extraction can be performed in one batch since the suspension after addition of the water does not fit entirely into the 125 mL separatory funnel." I don't know if the 'can' should be a 'can't' and that it means we can't do the extraction in one batch because there is too much solution to fit in the separatory funnel, so then we have to divide our mixture in half and then extract? Thanks.

Answer:

Hi Amber, 1. Yes, this means two seprate extractions. 2. Yes. Some students end up adding too much water and get a volume larger than 120 mL. As a result, they will have to split the mixture into two or more parts to do the extraction. AB

Question:

Hey Dr. B. I was wondering for the pre-labs, do we need to include all of the IR spectra data for all of the reagents used, or just for the reactants, (ie in lab meeting 1, would we find it for methanol, formaldehyde, sodium hydroxide and 2,4-ditert-butylphenol or instead just find the data for 2,4-ditert-butylphenol )? I am asking this question mainly because I cannot find the IR data of 2,4-ditert-butylphenol and also just so I know what you expect. Thanks!!! Have a great weekend :)

Answer:

Hi X, did you try the sdbs database for IR spectra? Also, the spectra for many compounds produced in research are not shown anymore, but there are listed in terms of numbers. In this experiment, some data can be found in one of the assigned readings. AB