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Everything posted by Horza2002

  1. Yer sorry, I got mixed up at the start. Your new mechanism seems more plausible to me...the second step is definately feaseable and probable. However, I don't like the first step. It just doesnt look right to me really....I'm not sure sure if titanium can activate an alkene in this manor.
  2. Propably no much...they don't think we should wear gloves all the time.
  3. Nope, we don't have a biotage, I did them all by hand...turned out to be 6 as I forgot I had one extra to do. Just to point out, you have one to many carbon atoms in your product thatn you have in your starting material. There should only be four carbons between the alcohol and the thioacetal. I still can't work out why they use titatium. I found this paper in which they do this reaction but they do it a different way. They use borontriflouride as the Lewis acid but pretreat the 2,3-dihydrofuran with tosic acid and methanol to get an intermediate which then undergoes the reaction with the dithiol. Maybe this will help you. Asymmetric synthesis of the macrolide (−)-aspicilin , Guy Solladie, Inmaculada Fernandez and Carmen Maestro, Tetrahedron: Asymmetry, Volume 2, Issue 8, 1991, Pages 801-819
  4. What source did you hear that the extinction rate is very high from?
  5. The thing with text books is that they are designed to get poeple to the next level of basic understanding (i.e. get you to a basic level to pass a degree). As Hypervalent has just said, sometimes certain processes are just glossed over because either the mechanism is not know/fully understood or it simply just too complicated and would omly serve to confuse people who are trying to learn about it...too much information in one go is often far more problematic when learning a new area than too little I have found in my experience. That is the of the wonders of science...the more you learn, the more questions it raises. Don't be downheartened at all! That is a completely normal response to people, especially when they are just starting out to learn about science as it appears that you are. I am currently a PhD student in chemical biology, it has taken me 4 years of training to get this far...it is certainly not a quick fix. It will also depend on the subject you are studying about whether you just have to memorise stuff. To some extent, all subjects require you to memorise stuff. However in my experience, biology is just simply remember all of this stuff and be able to use it. Chemistry is more remember these few basic rules and then use them to understand situations that you havn't seen before. Physics is also similar to chemistry, you just need to understand some basics to be able to move on.
  6. There is no chemical reaction. It is simply the polystyrene dissolving in the DCM. DCM (dichloromethane) is a very solvating organic solvent (i.e. it dissolves a lot of things very well). The majority of block polystyrene is actually only air, there is very little that is actually polymer fibers. So simply, the polymer is dissolving in the DCM.
  7. Oopps sorry...given you overt love of methathesis reactions, I only read the title and assumed you wanted to know why 1st generation Grubbs catalysts required a titanium cocatalyst while 2nd/3rd generationones didn't. Sorry it was late when I posted! From looking at your proposed mechanism, I don't think the titatnium would become covalently attatched to the substrate. The doesn't look right to me. Also, I very much doubt the last step goes stepwise. The secondary cation generated won't be that stable. I would actually guess that the thiol attakcs the thio-acetal carbon and then breaks the C-O(H)+ in a concerted process. I've got a set of 5 flash columns I need to do today, so I'll have plenty of time to have a think and come up with a reason as to why you use the titanium.
  8. You will actuallyget two competing reactions here; the one deprotonation of the alcohol followed by cyclisation and the secondwill be a SN2 substitution of the bromide with hydroxide. This sounds like a typical early degree level question because they often just pick a reaction to make a point and not worry about the side reactions. In practise, I would actually guess that the substitution would actually go much fast; hydroxide isn't really strong enough to completely deprotonate an alcohol. You usually use something much more basic (e.g. sodium hydride (NaH) or potassium t-butoxide (KO-tBu)). Both of these bases are actually non-nucleophilic so you would get minor susbtitution. Also, there is no such compound as 5-bromobutanol....the number refers to which carbon atom the bromoide is attatched and butanyl means there are only four carbons. I assume you mean either 5-bromopentanol or 4-bromobutanol.
  9. If you can wait until the morning, I have a paper on my desk at work outlining exactly why you use titanium in methathesis reactions
  10. Basically, you can't get it off. I wouldn't recommend using anything to remove the stain as it would simply take your skin away too. Just wait and in a few weeks/months it'l disappear.
  11. Power and influence over their neighbours and enemies would be the obvious one. Also to aquire the resources that those new lands provided would be another. It's difficult to say whether the world benefited from it because there's nothing to compare it too. If I had to guess, I would tentatively say yes, it did make the world better. Yes granted entire populations were wiped out because they didn't want to be ruled; but they also spread the value of education, paved the way for world-wide travel and increased trade around the world. I think you also need to remember that it wasn't just the British who colonised everything. The French colonised large portions of Africa, the Dutch colonised loads of island chains, the Spanish colonised south America, the Turks (then the Ottamon empire) conqured the middle east. And then there were all the empire before the 1800's your talkign about....the Romans managed to conquer and colonies the whole of Europe, the Holy Roman Empire also tried to....The Mongolians had a fair stab at trying it as well and the Mayan controlled south America before the Spanish defeated them. Please be careful here. It was not the English that colonised, but was the British; there is a difference.
  12. Another piece of adivce for using TLC; wear gloves. I managed to get a lovely permanganate stain on my thumb print on my TLC plate today from where I'd accidently touched the plate. And of course, the thumb print covered where I wanted to look....
  13. In theory, every reaction possible is an equilibrium. The reaction can simply just go back the same way in came (which is why catalysts don't alter the position of equilibrium). However, some reactions lie so far to one side of the reaction that you might as well just say that they are complete. Burning hexane for example is a reaction where the reverse basically doesn't happen. If you think about it, trying to get 6 carbon dioxide molecules are 12 water moelcuels to meet at exactly the right time is very unlikely.
  14. I would check with your professor first because they oftne have their own way that they want the students to follow. There is also a different way to write them up depedning on what subject you have.
  15. Tissue engineers will not be engineers int he convential sence...they will be biochemists....just to clear that up for you
  16. The same as any catalysis really, there are a number of disadvantages. The main one is that are the two ends of the double bonds that you want to cyclise get further and further away from each other (i.e. to make larger and larger rings) the efficieny of the reaction is lost. Heres a review that talks about it. Don't get me wrong, methathesis is a very powerful tool....you just need to give several subtle tweaks to your individual reaction to get it too work (i.e. concetration, use of catalysts, use reduced pressue, etc.) Schuster, M. and Blechert, S. (1997), Olefin Metathesis in Organic Chemistry. Angewandte Chemie International Edition in English, 36: 20362056. doi: 10.1002/anie.199720361 Enzymes can be used in total synthesis, or in general synthesis yes. The problem is, however, the majoritiy of enzymes are specific for one substrate and only one substrate. However, there are a lot of enzymes that can be used. One example I have used is an acetylation enzyme. Basically I made a non-natrual amino acid racemically and then treated it with this enzyme. This enzyme then only reacted with the L-form of my racemate to convert it to the N-acetyl functional group. At the end of the reaction, I simply did an acid wash to remove the D-form and then hydrolyised the amide again to giev me nice enatiomerically pure L-amino acid. This type of resolution can be useful, but it often only works on very small molecules; normally ones that resemble amino acids. You can also used a compination of this enzyme and a racemise enzyme for amino acid like molecules. The racemise enzyme racemises the remaining D-amino acid to the L which is then reacted so you actually get a 100% yeild of enatiomerically pure compound and not just the 50% you put in. Heres a review on the matter: Enzymes for chemical synthesis, Kathryn M. Koeller & Chi-Huey Wong, Nature, 409, 232-240, 2001 I'd like to highlight the enzyme aldolase from that review. Using that enzyme, it is possible to get enatioselective forming of carbon-carbon bond via an aldol reaction. Very cool! The regioselectivity of the the sugar moeities will arise from the enzyme specificitiy when it catalsyses the reaction. There are other types of Ethromycin compunds (A,B, C, D and I beleive E). These basically differ from the number of suagrs, the regiochemistry and some further modifications to the macrolid ring.
  17. Might I point out that Obama was not the one who put this plan forward at all....it was the British and the French. And it took Obama a hell of a long time to decide what he was actually going to do come the UN vote and for most of it was very relucatant to do anything. As well as that, Obama as made it quiet clear that it is not an American led plan. It was only after the UN made it clear that this was an international endevaour that anything happened at all. And don't forget its not only the European countries hes got on board, there are a lot of the Arab League who are involved as well. The fact that they are working with the west is a sure sign that it not some American plan to steal oil. Do you really think the Arab League would help the Americans steal oil as you so claim?
  18. Basically thats what I meant: I would advice against doing such a specilises subject so early in your education. I would advice starting with a broader subject area and then specialise afterwards once you get to the PhD level. If anything, I would say that having a wider range of subject knowledge would actually help your career because it shows you are veristile and can do many things...not just one set subject. You will also be able to look at the same problem from multiple angles....something that is very useful indeed. There are PhD's currently working in the same lab as me (I'm a synthetic organic chemist/microbiologist) who have come to this area of research from a maths background with no scientific training whatso ever. I would say that a biochemistry degree would be far more useful to you now, than having such a specilised subject.
  19. There are lots of other aspects of their physiology that are easier to exaplin why they are mammals. One obvious one is that they don't posses gills. Both dolphins and whales need to come to the surface to breath using lungs and then they hold their breath while swimming. Granted bluw whales can hold their breath for a VERY long gitme with a single gulp of air. Another exampel would be that they give birth to their young in a very similiar way to land mammals. Fish tend to lay eggs that eventually hatch to release the young...although not all fish do this.
  20. Now thats an interesting question actually. It really depends on what area of inorganic chemistry you are talking about. If you are talking about the catalytic properties of a metal (e.g. palladium metal in hydrogenation reactions), then we usually only refer to the top most layer of atoms. This is because only the atoms on the surface can do anything in the reaction that we are interested in. In almost all cases, this is what people refer to when they say the surface of the metal; the outter most layer of atoms. If we are talking about the structure of a crystal, then there we can use several layer of atoms to fully describe the structure of the crystal. These are normally expressed in Miller index. While using Miller indices, you are actually only looking at planes through the unit cell of the substance which sometimes need several levels of atoms to express them. I use these as an example, because the structure of a metal (i.e. its Miller index) has an effect on the reactivity of a metal. For example with the palladium example I used above, the three different crystal structures (001), (010) and (100) all have different reactivities.
  21. Yes I agree with you that the microscopic argument doesn't apply greatly on the macroscopic scale. Although that would only apply if you where trying to predict something that was entirely on the macroscopic level. And if you to predict something with absolute certiantity, you would have to take into account microscopic properties. ...and no, just because you can't predcit with absolute certianity what will happen, theres no reason at all to thing that everything is pre-determined. If anything, it implies that nothing is pre-determined. If it was, then you should be able to predict what would happen.
  22. First of all, this is a very interesting topic if research that you want to get into...good for you doing all the research and knowing the best places to do it. In my opinion, I would advice you to do the Biochemistry one and not the engineering one for a couple of reasons: Diversity - A degree in biochemistry will give you a larger knowledge base than doing a Chem/Bio engineering course. You would learn not only the intergral parts of chemistry and biology, but also how they are interlinked. From what I've read about the engineering option, you would only learn the "working knowledge" of how chemistry and biology work. You might not get taught the fundamentals that are essential for fully understand certain processes Specilisation - A Chem/Bio engineering is a very specalised area which could cause you issues in the future. If, 10 years down the line, you decide that you don't actually want to do work in that area, you might find ir difficult to change to another subject area. In contrast, a biochemistry degree will open alot more avenues for you. I would also imagine that using the biochemistry as a basis, then you should be able to move into the engineering subject much easier In my experience, it is much easier to move from a broader subject to a more specialised subject than the reverse.
  23. While yes, if you knew what everything in existance was doing at one time, you could predicted exactly how they'll interact for the rest of time; although it would be incredabley hard! However, the Hiensenburg Uncertainity principle means that you can't know both a particles position and momentum simulatenously...so the best you can do it predict what might happen.
  24. Yep...if you ever needed a reason not to wear contacts in the lab thats it....just get percription safety specs like mine...not only are they safe, they damn sexy too!
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