Question 1:Use chair conformers to discuss the stereochemistry of: a.Cis-1,2-diG-cyclohexane b.Trans-1,3-diG-cyclohexane With G is a substituted group. You will discuss about the possibility separating two enantiomers (resolution) from each other in each case. Question 2:Draw all (cis- and trans-)conformations of 1-methylbicyclo[4.3.0]nonane ; Then, compare their stability! Explain! If the question is not clearly, you can request, and we will discuss, ok!!! Note that: It have three conformation (2 cis- and 1 trans-), after draw all conformation, you will compare their stability! Have fun!! :danhmay ( :hiphop (
- a.b cis- & trans- deu ton tai 1 cap doi quang!!!. mac du a ko wang hoat, va n
sach' cho no' lameso???,b wang hoot ro~ rang.
Có lẽ các bạn chưa hiểu câu hỏi lắm, nên trả lời lung tung quá! BM sẽ translate một chút nhé!
Question 1:Use chair conformers to discuss the stereochemistry of: a.Cis-1,2-diG-cyclohexane b.Trans-1,3-diG-cyclohexane With G is a substituted group. You will discuss about the possibility separating two enantiomers (resolution) from each other in each case.
Bình thường chúng ta dùng cấu trúc phẳng (flat) để giải quyết các bài toán về lập thể với hệ thống vòng, nhưng bây giờ đề bài chúng ta yêu cầu dùng dạng cấu trạng ghế để giải thích về đồng phân lập thể (stereochemistry), discuss ở đây không có nghĩa chỉ nói suông đáp án, mà còn phải nói lời giải thích! Ngoài ra trong đề còn yêu cầu xét khả năng phân giải (hay phân tách) (resolution) hai chất đối quang (enantiomer) nữa!
Question 2:Draw all (cis- and trans-)conformations of 1-methylbicyclo[4.3.0]nonane ; Then, compare their stability! Explain! If the question is not clearly, you can request, and we will discuss, ok!!! Note that: It have three conformation (2 cis- and 1 trans-), after draw all conformation, you will compare their stability!
Ở question 2, đề yêu cầu vẽ tất cả các cấu trạng của 1-methylbicyclo[4.3.0]nonane, sau đó so sánh độ bên của các cấu trạng đó, và luôn luôn nhớ có giải thích! BM đã gợi ý cho các bạn có hai cấu trạng cis và một cấu trạng trans (đều là cấu trạng dung hợp). Bây giờ nói đến câu trả lời của tieuphong_kt: Phải nói là rất đáng phê bình! Không biết bạn lười hay bạn sao đó, mà BM nhìn vào câu trả lời của bạn cảm thấy bạn chẳng coi ai ra gì cả, một câu trả lời cho một cuộc thi không phải như vậy! BM đề nghị bạn sửa gấp lại đi! Và chú ý lần sau trả lời nhớ giải thích nhé! Điều đó mới chứng tỏ bạn thật sự hiểu câu hỏi!
bạn trả lời câu này là sao? nhiều sách cho nó là meso, bạn đặt rất nhiều dấu hỏi, vậy là bạn không đồng ý với sách, hay vẫn chưa hiểu!
b quang hoạt rõ ràng, rõ quá đến nổi không cần giải thích mà phản xạ là thấy không quang hoạt liền đúng không!
Xin hỏi bạn thấy hay và đồng ý chỗ nào, có thể thảo chia sẽ được không! Bạn phải nói ra câu trả lời của mình, để từ đó BM sẽ giúp bạn so với đáp án, sẽ có những nhận xét bổ sung cho bạn, ok! Chúc các bạn học tốt!!! :pocolo (
My conputer is being repaired, so I can not draw the structure. I hope you will accept my answers. Thanks a lot ! 1/ a. Cis- 1,2-diGcyclohexane, a meso compound, has one substitution group in axial position, one in e position. Two substitution groups can change each other and the required energy is so small. For this reason, it is difficult to resolute two conformaion of cis- 1,2-diGcyclohexane Trans- 1,2-diGcyclohexane has two enatiomers. Both two sustitution groups are in e-posiotion. If both of them are in a- posiotions ,this molecule need to be supplied a large energy.So two enantiomers can not change ech ether easily and they can be resoluted.
b.Similar to 1,3-diGcyclohexane: Cis-,a meso compound, can be resoluted Trans-,hasing two enatiomers, nan not ne resoluted.
2/ 1-methylbicyclo[4.3.0]nonane,hasing two fused rings-six members ring and five members ring, has three conformations Trans-:the methyl group anh H are both in axial position(A) Cis-: + the methyl group is in e- position (B) + the methyl group is in a- position (C) Because methyl, the large group, is in e-position, B is the stablest conformations In A and C ,methyl is in a- position. But in A , two methyliene(-CH2-) of the five ring are in e- position and the five ring is rather planar,so methyl is pushed back to the six –members ring. There are the repulsion between methyl ang H in the cyclohexane. Certainly, B is more stable than A. :phuthuy ( :xuong ( :cool ( :doivien( :ninja1 ( :quyet ( :bidanh( :nhanmat( :nhau ( :dantoc (
Bạn có thể dùng bộ chemoffice hay bất cứ chương trình nào vẽ công thức hóa, như chemwindow, isisdraw… nhưng theo BM, vẽ bằng chemoffice là dễ nhất, bây giờ chemoffice đã ra bộ 2005, bạn có thể mua ở Bùi Thị Xuân (TPHCM), hoặc các quầy soft khác. Nếu bạn đang học KHTN TPHCM, bạn ko tìm mua được thì cứ liên hệ với BM, BM sẽ cho mượn. Số fone là: 0986713422 Nói chung soft này mua cũng dễ!!!Mà dùng cũng dễ luôn, về mò một chút là được! Chúc bạn học tốt!!! :quatang(
Question 1:Use chair conformers to discuss the stereochemistry of: a.Cis-1,2-diG-cyclohexane b.Trans-1,3-diG-cyclohexane With G is a substituted group. You will discuss about the possibility separating two enantiomers (resolution) from each other in each case.
In figure 1, we easily see these conformers are enantiomers, they change each other rapidly, so it is difficult to separate them from each other. Whereas in figure 2, the (e,e) conformer has lower energy, so we can ignore (a,a) conformer. The (e,e) conformer exists as a resovable racemate (figure 3). In figure 4, the (e,e) conformer is more stable and there is a plane of symmetry. The higher energy (a,a) conformer has a symmetry plane too. So these have not stereoisomer. Figure 5 describe equilibrium of the trans-1,3-diX-cyclohexane, each conformer has an enantiomer and hence exists in a resovable racemic form (figure 6)
Question 2:Draw all (cis- and trans-)conformations of 1-methylbicyclo[4.3.0]nonane ; Then, compare their stability! Explain! If the question is not clearly, you can request, and we will discuss, ok!!! Note that: It have three conformation (2 cis- and 1 trans-), after draw all conformation, you will compare their stability!
The order of stable conformations of 1-mythylbicyclo[4.3.0]nonane: B>C>A Explain: the large substituted mythyl group is e- position that the most stable conformation. Two conformers remaining are the mythyl group is a- position, where exists repulsion with the a- H of C2 and C4, made the conformation is less stable. The more repulsive, or the mythyl group is nearer the Hidrogen, the less is stable. See the cis- conformer, one of the methylenes (-CH2-) is a- position, so the five membered ring trend down, the repulsion of the five membered ring with the mythyl is less than. Whereas, the trans- conformer have two methylenes is e- position, so the five membered ring trend up, the repulsion with the mythyl is more than, the conformer is the least stable conformer. Have fun!!! :phuthuy (
, certainly, C ismore stable than A, ok! Excellent!!! Your answer is very clear!Now, I will add the several figure illustrated, and expand about ability separating (resolution), to make clearly about ability of resolution in stereochemistry! The method resovable diastereomers: As we know, diastereomers have different physical properties such as melting and boiling points, refracting, solubilitis in different solvents, crystalline structures, and specific rotations…Because of their difference in solubility, so they often can be separated from each other by fractional cystallization. Because of their slight differences in molecular shape and polarity, so they often can be separated from each other by chromatography. Biside, diastereomers also can be separated by basing on chemical properties. They have different chemical properties toward both chiral and achiral reagents, so the transition states are not mirror images of each other and so will no necessarily have the same energies, the enthalpy of the transition states are somewhat different and thus the rates of reaction will differ. The method resovable enantiomers: The chemical properties of enantiomers are the same toward achiral reagents, solvents, catalysts, and conditions. Towards chiral reagents, solvents, catalysts, and conditions, eantiomers react at different rates, because at the transtition states, the enantiomers are not mirror images of each other, they are diastereomeric, and hence have enthalpies. The method resovable an enantiomer and its racemic form: This case is as was remarked above, the chemical properties are the same toward achiral reagents, but chiral reagents react at different rates. Have fun! :chautroi
Method of resolution" 1.Conversion to diastereomers 2.Different absorption 3.Chiral recognition 4.Biochemical processes 5.Mechanical separation 6.Kinetic resolution 7.Deracemization Now, let’s discuss about these methods: Conversion to diastereomers Conditions: The racemic mixture to be solved contains acarboxyl group. Method: The steps of resolution are crystallizing in a suitable solvent, filtration, and recover the enantiomer and chiral reagent. Ex: Supplement of this method:
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When a molecule does not contain a carboxyl group, it is converted to a carboxylic acid before resolution is attempted.
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Racemic bases can be converted to diastereomeric salfs with active acids.
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Alcohols can be converted to diastereomeric esters.
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Aldehydes ti duastereomeric hydrazones.
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Hydrocarbons can be converted ro diastereomeric inclusion compounds, with urea (cage structure).
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Chiral Crown ethers have been used to separated mixtures of enantiomeric alkyl- and arylammonium ions, by the formation of diastereomeric complexs.
Differential absorption
Method: When a racemic mixture is placed on a chromatographic column, if the column consists of chiral subtances, then in principle the enantiomer should move along the column at different rates and should be separable. Explain: As the solution passes through the column, the enantiomers form weak complexes, usually through hydrogen bonding, with the chiral colymn packing. The diastereomeric complexes have different physical properties, they also have different binding energy and equilibrium constants for complexation. Chiral recogniton Method: The use of chiral hosts to form diastereomeric inclusion compounds was mentioned above. But in some case it is possible for a host to form an inclusion compound with one enantiomer of racemic guest, but not other. Difficult: More often, both diastereomers are formed, but one forms more rapidly than the other. Ex: Use of the chiral Crown ether (A) partially to resolve the racemic amine salt, when an aqueous solution of (B) was mixed with a solution of optically active (A) in CHCl3, and the layers separated, the CHCl3 layer contained about twice as much of the complex between A and (R)-B as of the other diastereomeric complex. Biochemical processes Method 1: A certain bacterium may digest one enantiomer but not the other. Difficult: It is necessary to find the proper oganism, and since one of the enantiomer is destroyed in the process. Method 2: Enzymes is a group of enantioselective catalyst. Enzymes is highly efficent and selective catalyst and can carry out a variaty of transformations Explain: The reason is that the interaction of the enzyme with one enantiomer is diastereomeric to its interaction with the other. Because enzyme catalysis is usually based on a specific fit to an “active site”, the degree of selection between the two enantiomer is often very high. Mechanical separation Method : This is the method by which Pasteur proved that racemic acid was actually a mixture of (+) and (-) tartaric acids. In the case of racemic soldium ammonium tartrate the enantiomers crustallize separately all the (+) molecules going into one crytal and all the (-) into another. Since the crystals too are nonsuperimposable, their apparence is not identical and a trained crystallographer can separate them with tweezers. Kinetic resolution
- Principle: The resolution depend on the difference in rates of reaction of two enantiomers with a chiral reagent. The transition-state and intermediates (R-substrate…R-reactant) and (S-substrate…R-reactant) are diastereomeric.
- Method: Stopping the reaction before completion.
- Ex:
- Resolution of racemic alkenes allenes by treatment with optically active diisopinocampheylborane, since alkenes do not easily convert to diastereomers if not other functional groups are present. Another example is the resolution of allylic alcohols with one enantiomer of a chiral expoxidation agent. In this case, the discrimination was extreme, one enantiomer was converted to the epoxide and the other was not, the rate ratio being >100 times. Deracemization
- Method: one enantiomer is converted to the other, so that racemic mixture is converted ro a pure enantiomer, or to a mixture enriched is one enantiomer +Ex: After 28 days the solution contained 89% of one enantiomer and 11% of the other I hope to receive the active discussion from you about the methods, the difference between principle and practical when use these methods to resolute the diastereomers or enantiomers… Have fun!!! :spam ( :rockon (
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