http://orgo.curvedarrow.com/punbb Curved Arrow Press Forum en http://backend.userland.com/rss http://orgo.curvedarrow.com/punbb/viewtopic.php?pid=476#476 476@http://orgo.curvedarrow.com/punbb Topic: Calculating percent composition of a mixture Message: http://organicchemistrymessageboard.yuku.com/topic/1655 wrote:What is the maximum amount of benzaldehyde impurity that could be in a sample of anisole (methoxybenzene) before it "failed" elemental analysis according to JOC standards. I started this by finding the theoretical amounts, but not sure where to go from there. Help pls! This is an interesting question, one that I had not thought about giving to students. Okay, lets just think about it for a minute. If we had pure anisole and benzaldehyde, the values would be:MF: C7H8O (anisole)Molecular Weight: 108.14Elemental Analysis: C, 77.75; H, 7.46; O, 14.80Chemical Formula: C7H6O (benzaldehyde)Molecular Weight: 106.12Elemental Analysis: C, 79.22; H, 5.70; O, 15.08The differences in the percent carbon and hydrogen are 1.47 and 1.76% (7.47-5.70). Those values will change linearly with the amount of benzaldehyde present in the sample. Thus, if the sample contained 10% of benzaldehyde, then the percent composition would be 0.17% low for hydrogen. If the sample contained 100% benzaldehyde, the percent composition would be 5.70% or 1.76% low for hydrogen. In order for the percent composition to be 0.4% low for hydrogen, then 0.22 of anisole must be benzaldehyde. Sun, 04 Oct 2009 08:29:25 -0400 http://orgo.curvedarrow.com/punbb/viewtopic.php?pid=456#456 456@http://orgo.curvedarrow.com/punbb Topic: t-butanol with NH4X to isobutylene and t-butyl halides Message: I really don't know what individual students have done in the experiment. In the absence of the procedure, and what students actually did, I can only guess. Based upon the description, I can imagine the broad results by changing the procedure slightly. Student A: Adds ammonium bromide, sulfuric acid, t-butanol, and ammonium chloride in that order and heats. Major pdt the bromide.Student B: Adds ammonium chloride, sulfuric acid, t-butanol, and ammonium bromide in that order and heats. Major pdt the chloride.Student C: Adds ammonium chloride, ammonium bromide,  t-butanol, and sulfuric acid, in that order and heats. Major pdt equal bromide and chloride.I didn't look up the procedure being done. I just read the student contribution that I originally replied to. It appears to be the same procedure for the SN1 and SN2 reactions. There appears to be an implication that both reactions should occur at the same or similar rates. That is patently incorrect. The SN2 reaction should be much slower than the SN1 reaction. I believe the SN1 reaction will begin upon addition of the reagents. Therefore, I described experiments where the carbocation can form with high concentrations of a single halogen. While contaminated reagent bottles could be possible, I am doubtful. This will depend on the number of reagent bottles available to the class. Let us assume one reagent is contaminated. All experiments from that bottle should give a single result. I have not seen the overall class results, but it sounds as though there is a wider range of student results than can reasonably be explained even by numerous examples of contaminated reagents. Fri, 19 Jun 2009 10:51:14 -0400 http://orgo.curvedarrow.com/punbb/viewtopic.php?pid=455#455 455@http://orgo.curvedarrow.com/punbb Topic: t-butanol with NH4X to isobutylene and t-butyl halides Message: We do this experiment in organic labs here at CCSU, and the results my students obtained varied wildly.  Sometimes the chloride peak would dominate, and sometimes the bromide peak - up to 80:20 ratios in either direction.  Doesn't make any sense to me, unless we have cross-contaminated reagent bottles (which I have to check).  We use t-butanol, which is tertiary, and is thus expected to go by an Sn1 mechanism, so there should be a 50:50 mixture of the two products, but we only saw that result in a minority of cases.  Very odd...I just don't get it!  Will have to go back into the lab myself and see what happens when I do it.  Nice site, by the way... looks like it will be very helpful!  Steve Fri, 19 Jun 2009 09:20:22 -0400 http://orgo.curvedarrow.com/punbb/viewtopic.php?pid=366#366 366@http://orgo.curvedarrow.com/punbb Topic: t-butanol with NH4X to isobutylene and t-butyl halides Message: Q: the experiment is about competing nucleophiles (chlorine and bromine ions) with different alcohols (in this particular case, t-butanol). the solvent -nucleophile mixture (containing the ammonium halides and concentrated sulfuric acid) was refluxed/heated for more than a hour and then purified. t-butanol was added to the to this mixture. the organic layer was extracted and then analyzed via gas chromatography. the link below is a picture of the gas chromatograph that was obtained in the experiment.           i would appreciate it if you can explain this graph to me and its significance with which one is a better nucleophile. in the other part of this experiment, i concluded that bromine was a better nucleophile in the reaction with the ammonium halides and 1-butanol. here is the gc analysis for that reaction.          A: No disrespect to the questioner, but this is really pushing it. I have a lot of experience and know a lot of reactions, but it can be pretty difficult to deduce these problems from this distance, what was given, real procedure, data, analysis, hints, etc. For example, in the first problem, I presume the t-butanol was added first to the mixture and then heated. However, I was able to find this experiment described on a couple of websites. They indicate the bromide and chloride products are to form. From the questioner, I must presume that isobutylene is the first peak, t-butyl chloride and t-butyl bromide as the next peaks. The bromide is the highest boiling and therefore longest retention time. In the lower gc, I am guessing that we are seeing the large peak for the injection followed by 1-chlorobutane and 1-bromobutane. Let us start with the lower example. Since the alcohol is primary, this is going to be an SN2 reaction. Assuming equal amounts of bromide and chloride present in the transition state, since more bromide product is formed, bromide must be the better nucleophile. In the upper example, because the rate limiting step is loss of water, the nucleophilicity should not matter and one might have expected relatively equal amounts of chloride and bromide (reflecting equal concentrations). The tailing of the peaks on the gc's make it difficult to determine an accurate integral from these gc's. Thu, 13 Nov 2008 01:50:21 -0500 http://orgo.curvedarrow.com/punbb/viewtopic.php?pid=281#281 281@http://orgo.curvedarrow.com/punbb Topic: Gc Message: I wouldn't advice trying to GC such a mixture. TLC could work, HPLC would be the best. Fri, 11 Apr 2008 01:05:45 -0400 http://orgo.curvedarrow.com/punbb/viewtopic.php?pid=277#277 277@http://orgo.curvedarrow.com/punbb Topic: Gc Message: If you had the products: benzil, hydrobenzoin, and benzilic acid...What would the GC look like and how would you determine which peaks are each?benzil: 210.23 g/mol; bp= 346 - 340 degrees celsiushydrobenzoin: 228.25 g/mol; bp = 180 degrees celsiusbenzilic acid: 214.26 g/mol; bp = 121-130 degrees celsius Thu, 10 Apr 2008 22:55:58 -0400 http://orgo.curvedarrow.com/punbb/viewtopic.php?pid=224#224 224@http://orgo.curvedarrow.com/punbb Topic: green chem Message: If you calculate this out, I think it is a catalytic amount of hydroxide to do the first step of the aldol condensation. Then an excess is used for the dehydration step and heating. By using vanillin, you have a phenol present. Increase the amount of NaOH to allow the phenol to become completely converted to the phenoxide and add the same amount of additional base to enolize the indanone. Because the vanillin the electron density of the vanillin will have increased, it will react more slowly than the dimethoxybenzaldehyde. From that point, carry out the procedure as described. Fri, 28 Mar 2008 06:51:22 -0400 http://orgo.curvedarrow.com/punbb/viewtopic.php?pid=222#222 222@http://orgo.curvedarrow.com/punbb Topic: green chem Message: How would I modify this reaction if I wanted to use vanillin instead of 3,4-dimethoxybenzaldehyde as the electrophilic partner:Procedure:3,4-dimethoxybenzaldehyde (0.3g) and 1-indanone (0.20g) were mixed together gently with a glass rod until an oil was obtained. NaOH powder (0.1g) was added to the oil until a yellow solid formed. After allowing the solid to harden w/o mixing for 15 mins, 10% NaOH (2mL) was used to acidify the mixture. The crude product was filtered, then recrystallized from 85% ethanol, then filtered again. Thu, 27 Mar 2008 19:04:56 -0400 http://orgo.curvedarrow.com/punbb/viewtopic.php?pid=193#193 193@http://orgo.curvedarrow.com/punbb Topic: reaching -5C temperature Message: I am going to answer your question differently than asked. If you wished to cool something to -5°C, you actually want your cooling bath to reach a temperature lower than -5°C. If you wish to hold something at -5°C, then you want to have a bath that will maintain -5°C. What is the difference? If you are doing a reaction, then as the reaction proceeds, heat is often (generally?) increased in the reaction. That comes from the reaction being exothermic (the most common reason for cooling in the first place) or from addition of another reagent. If your cooling bath did not cool lower than -5°C, then your addition would have be done very slowly because equilibration would be slow to achieve if the temperature differential were small. In that case, you would be better to use a cooling bath that reached lower than -5°C. If you are doing a reaction and you wish to hold something at -5°C for a long period of time, then you want a cooling bath that will maintain -5°C. That is best done with a solvent that melts at -5°C. This is the same principle as ice to maintain 0°C. "The Chemists Companion" contains a table listing several baths that can maintain varying temperatures. A ratio of NaCl to ice would be difficult to maintain unless you were to freeze the brine first so the concentration of the salt in water would not change. Personally, because I was lazy, I generally used acetone to make ice cooling baths for less than 0°C. It is fast and easy. Tue, 18 Mar 2008 00:36:09 -0400 http://orgo.curvedarrow.com/punbb/viewtopic.php?pid=192#192 192@http://orgo.curvedarrow.com/punbb Topic: reaching -5C temperature Message: what should be the ratio of sodium chloride and ice water to reach -5 C temperature? Tue, 18 Mar 2008 00:19:34 -0400 http://orgo.curvedarrow.com/punbb/viewtopic.php?pid=98#98 98@http://orgo.curvedarrow.com/punbb Topic: Ether or ethanol for a recrystallization? Message: Yahoo Answers: Solubility in diethyl ether vs ethanol? Why is diethyl ether used for dissolving organic compounds but not for recrystallization? Why is the opposite true for ethanol? I expect you will get a lot of answers for this question, but probably not many like mine. Most books describing recrystallization use difficult solvents for recrystallizations. I generally used the opposite solvents. For example, ether was a favorite solvent for recrystallizations because I could complete a recrystallization very quickly. This is how I always did them. I wanted to start with a good solvent that would dissolve my sample. If some material didn't dissolve, I filtered it to remove that impurity. I knew it was an impurity if most of the sample dissolved quickly and a small amount did not. Most of my samples were not soluble in hexane so I just started adding hexane while heating on a steam bath. I kept my concentration in the 0.1-10% of solvent range. The ideal was to use a small amount of ether if it would dissolve readily and make up the rest with hexane. Because ether was low boiling, it was easy to evaporate, and the less I had to add, the less I had to evaporate. As the temperature of the flask began to rise, I knew the ether was largely evaporated. At the cloud point, I removed it from the heat, seeded it, and let it crystallize. I could do most recrystallizations in about 10 minutes, start to finish. I did it that way because it was really fast. If I had something that was too polar, I liked acetone-water. Same situation. Acetone dissolves most things and relatively low boiling. Add the least amount possible. Because the boiling points are much higher, it takes longer. From my experience, if you do a lot of recrystallizations and want to get pure samples, most chemists gravitate to mixed solvents. The best pairs are a low boiling solvent in which a sample is easy to dissolve and a higher boiling solvent in which the sample has low solubility. What I described was a dynamic process. What is in the books is a formulaic process. Add 10 mL of 70% ethanol to recrystallize. I tried to show students how I could recrystallize a sample, but I couldn't write a formula or tell exactly how much to use. Tue, 19 Feb 2008 13:42:23 -0500 http://orgo.curvedarrow.com/punbb/viewtopic.php?pid=83#83 83@http://orgo.curvedarrow.com/punbb Topic: Partition of Y in ether in extraction question Message: Compound Y has a solubility of 5.0 g/100 mL in water and 25.0 g/100 mL in ether.  What weight of compound Y would be removed from a solution of 3.0 g of Y in 100 mL of water by a single extraction with 100 mL of ether?Kp = 25/5 = 5x = amount in ether3-x = water; (x/100 mL)/[(3-x)/100 mL] = 5x/(3-x) = 5x = 5 * (3-x) = 15 - 5x6x = 15x = 15/6 = 2.5 g  in ether3- x = 0.5 g in water
What weight of compound Y would be removed from the original water solution if two extractions of 50 mL of ether were used? 5 = (x/50 mL)/[(3-x)/100 mL]; (x/50 mL) * (2/2) = (2x/100 mL) 5 = 2x/(3-x)x = 2.14 g in ether3-x = 0.85 g in water5 = 2x/(0.85-x)x = 0.61 g3-(2.14 + 0.61) = 0.24 g in water
 Tue, 12 Feb 2008 00:32:43 -0500 http://orgo.curvedarrow.com/punbb/viewtopic.php?pid=74#74 74@http://orgo.curvedarrow.com/punbb Topic: Stoichiometry of borohydride reduction of benzil Message: 1) What is the wt of 2.5 mmol of benzil?answer 0.535 is it right2) What is the wt (in grams) of 2.5 mmol of sodium borohydride? 3) What is the theoretical amount of sodium borohydride needed?4) What is the limiting reagent? 5) What is the theoretical yield, assuming you begin with exactly 2.5 mmol of each reagents? 1) Check your calculator, 2.5 mmol x 210.1 mg/mmol = 0.525 g2) 2.5 mmol x 38.03 mg / mmol = 0.95 g3) Since BH4 delivers 4xHs and benzil is a diketone, 2 x benzil (mw 210) +  NaBH4 + 4ROH -> 2 pdt (mw 214) + NaB(OR)4For two moles of benzil, one mole of NaBH4 is needed.2.5 mmol/2 x 38.03 mg / mmol = 0.0475 g4) Benzil because an excess of NaBH4 was used.5) 2.5 mmol x 214.1 mg pdt / mmol = 0.535 g of pdt.To see how a reduction transfers all four hydrogens, see http://www.curvedarrowpress.com/partd/b … dered.html Tue, 05 Feb 2008 14:57:53 -0500 http://orgo.curvedarrow.com/punbb/viewtopic.php?pid=54#54 54@http://orgo.curvedarrow.com/punbb Topic: MS of 3-methyl-1-butanol, 1-pentanol and 2-butyne-1-ol Message: Since I was having difficulty in linking the SDBS Spectral database directly, I snipped the MS spectra here.3-methyl-1-butanol1-pentanol2-butyne-1-ol Sun, 23 Dec 2007 17:42:52 -0500