Which stereoisomer did you obtain? What does this suggest about the stereochemis
ID: 1063483 • Letter: W
Question
Which stereoisomer did you obtain? What does this suggest about the stereochemistry of the rumination reaction? Which mechanism best accounts for your results? How confident are you in your conclusions? How do your results fit with the material presented in your lecture textbook? Discuss the percent yield for your experiment. State some reasonable explanations why the yield is less than 100 %. If the reaction gave a mixture of products, could you distinguish whether Mechanism 1 or 4 was the correct one? If so, how? In the yield calculation, it is not necessary to calculate the number of moles of bromine added. Why is this so? What product would a student obtain if he attempted the rumination of cis-stilbene (also known as (Z)-stilbene)? Explain your reasoning. A student attempts the chlorination of trans-stilbene. A solid product was obtained. Would you expect it to be the racemic product, the mesa compound or a mixture of the two? Explain your reasoning. Possible mechanisms for rumination: Each mechanism shown here is theoretically possible. Mechanism 1: Brominating is a multi-step process. The pi bond attacks the bromine molecule to form a C-Br bond and generate a carbocation and a bromide (Br^-) ion. The bromide ion then attacks the carbocation to give the product. The bromide ion can attack either face of the carbocation, and this means that both the mesa compound and the racemic mixture would be formed. Mechanism 2: Brominating is a single-step process. The bromine molecule and the double bond form a four-member transition state, and the bonds migrate. The C-Br bonds are formed at the same time that the Br-Br bond and the C-C pi bond break. Both bromine atoms add to the same face of the molecule. When the adding species both attach to the same face, this is called seen addition. The product formed is a racemic mixture. Mechanism 3: Brominating is a multi-step process. One carbon uses the pi electrons to bond to a bromine atom that simultaneous uses a pair of electrons to form a bond to the other carbon. A bromide ion departs at the same time, resulting in a cationic species called a bromonium ion. The bromide ion then attacks one of the carbons from the opposite face from where the bromonium ion is. One C-Br bond breaks while another form. This type of reaction, where one species adds to one face of the molecule while the other species adds to the opposite face is called anti addition. For this reaction, the only isomer formed is the mesa compound. Mechanism 4: Brominating is a multi-step process. In this mechanism, the pi bond attacks the bromine molecule as in Mechanism 1. Sometime after the carbocation is formed, the bromine atom donates a pair of electrons to form the bromonium ion. This step is slow because a strained 3-member ring is formed, and this allows rotation around the C-C bond as shown. This means that two different bromonium ions arc formed, leading to a mixture of products-both the mesa compound and the pair of enantiomers will be formed.Explanation / Answer
Solution:
1). Mechanism 4 is the correct one. Addition of Br2 to the C=C double bond always occurs via cyclic three membered bromonium cation as evident by different techniques. Also cyclic bromonium ion is more stable than that of carbocation formed and thus follows the mechanism via the formation of cyclic bromonium intermediate.
2). In yield calculation, it is not necessary to calculate the number of moles of Br2 added in the reaction since, one mole of Br2 will be added in the reaction to give the desired brominated product and the excess bromine will be left unreacted. So simply by measuring the actual weight of the product obtained and knowing the theoretical weight of the product one can easily find the % yield.
3). The product would be a mixture of meso- and racemic (R,R and S,S) dibromo stilbene as result of anti addition.
4). Chlorination of trans stilbene will follow the mechanism-1 pathway, both anti- and syn- addition is possible to the carbocation intermediate forming meso and + enantiomer (as pre the mechanism shown in mechanism-1)
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