PostLab Questions: 1. Review the bromination of an alkene sections in your lectu

Question

PostLab Questions: 1. Review the bromination of an alkene sections in your lecture text. Based on the mechanism presented, use ChemBio3D to predict the product of this reaction. Your answer must include the ChemDraw structures (with stereochemistry labeled) and ChemBio3D models (proper viewing angle, please) for each diastereomer to support your prediction. 02H Br HBr HBr CH2Cl2 trans-Cinnamic Acid threo erythro Fisher Projections of the stereoisomers of 2,3-dibromo-3-phenylpropanoic acid 2. Use Chem3D to determine which one of the molecules shown below undergoes E2 reaction with NaOCH2CHs in CH CH2OH considerably faster than the other. (A review of cyclohexane conformations may be helpful.) Identify this compound and explain. Also, explain what must happen for the slower molecule to react. Be sure to include a printout of all relevant 3D structures (proper viewing angle, please) CH CH3 Br

Explanation / Answer

1. In the bromination of trans-cinnamic acid, first step is the formation of cyclic bromonium ion intermediate. The intermediate can be opened by the attack of nucleophilic Br- species from the opposite side at both carbon ends of the bromonium ion.

The product thus formed would be having an anti-stereochemistry for the two Br added to the alkene. Thus starting with trans-cinnamic acid, the only product formed here would be threo product shown above.

2. Among the two substrates for decalin shown above, the first would undergo an E2 elimination. Only in this case we have a hydrogen anti- in geometry to the Br which will be eliminated in the E2 mechanism. In the second case no such anti hydrogen for elimination in the right geometry exists. For the second molecule to react one must do the reaction under E1 reaction conditions.

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