322B_Prelab_Answers - Chern 322bL/325bL Period 1 JBE...

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Chern 322bL/325bL Period 1 JBE ANSWERS to QUESTIONS for GRIGNARD SYNTHESIS OF 2-METHYL-2-HEXANOL 1. One could make the title product from 2-hexanone and methyl bromide. But usually among aliphatics, species with four or fewer CIS are more readily available than those with more. Cf Solm p 557ff. 2. (a) Iodine helps start a Grignard by (1) etching the surface of the metal, and (2) very likely by acting as a radical chain carrier (1 2 very easily dissociates to I . atoms) . Grignard formation probably involves some kind of radical chain reaction at the metal surface. (b) R-MgBr + 1 2 -----> R-I + MgBrI (c) Add a conc solution of R-Br initially but only a small part of the total amount of halide. Once reaction starts, dilute the reaction mixture with ether and then add the rest of R-Br soln slowly so that at any instant the conc of unreacted R-Br in the rxn mixt is low. (d) For the desired reaction: R-Br + Mg -----> R-MgBr For the coupling reaction, the R-MgBr reacts with as yet unreacted R-Br: R-MgBr + R-Br -----> R-R + MgBr2 Thus, the overall reaction for the coupling is the sum of these two reactions:
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Page 2 - Chern 322bL/325bL Grignard (Pd 1) Answers 2 R-Br + Mg -----> R-R + MgBr2 Thus, in the desired reaction, the molar stoichiometry ~s 1:1 RBr:Mgi for the coupling rxn, it is 2:1. Since one is taking the SM's 1:1, if only the desired rxn occurs, all the metal is just consumed. The coupling reaction con- sumes only half as much metal. 3. (a) The product alcohol will dehydrate to alkenes: (b) With HCI, one will obtain the corresponding alkyl chloride. (c) Magnesium(II) plus hydroxide produces a slurry of Mg(OH)2' i.e., milk of magnesia (MOM). Separation of the product from this gooey semi-solid will be difficult. Thus, one needs acid to keep the Mg+ 2 dissolved. (d) The problem with having any residual acid in the ether soln of the product arises during the distillation. While cold, dilute acid is not a problem, during the distilla- tion it becomes hot, (more) conc acid. The distillation then becomes the steam distillation of alkenes; cf (a). (e) K 2C03 is not only a DA, it is also basic. Thus any acid which escaped the earlier washings will be neutralized and precipitated as its potassium salt. This is an extra safeguard; use of a basic DA assures removal of ~ acid.
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Chern 322b/325bL Period 2 JBE ANSWERS to QUESTIONS for FRIEDEL-CRAFI'S PREP 1. Alkylations with an alkyl halide plus a Lewis acid are simi- lar to those using an alcohol and a protonic acid in that the key intermediate, a carbocation, is the same, i.e., the reactions are mechanistically similar, broadly speaking. 2. (a) See cited Solm problem. EtOH is much more nucleophilic than anisole. (b) form irreversibly, but the The Ph 3COH/EtOH product must form reversibly. Ph 3COH/HOAC product must If) (c) (1) no other reasonable structures. Ph 3C-O-C-Me· ~ d ' <----> <----> Ph 3C-O-C-Me II HO (£) Second ion is more stable; only it can delocalize charge.
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