322B_Prelab_Answers

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 more. Cf Solm p 557ff. 2. (a) Iodine helps start a Grignard by (1) etching surface of metal, (2) very likely acting as a radical chain carrier (1 2 easily dissociates I . atoms) . formation probably involves some kind reaction at metal surface. (b) R-MgBr + 1 2 -----> R-I + MgBrI (c) Add a conc solution R-Br initially but only a small part total amount halide. Once starts, dilute mixture ether then add rest soln slowly so that any instant unreacted in rxn mixt is low. (d) For desired reaction: + Mg R-MgBr coupling reaction, R-MgBr reacts yet R-Br: R-MgBr + R-R + MgBr2 Thus, overall sum 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, molar stoichiometry ~s 1:1 RBr:Mgi for coupling rxn, it is 2:1. Since one taking SM's 1:1, if only rxn occurs, all metal just consumed. The reaction con- sumes half as much metal. 3. (a) product alcohol will dehydrate to alkenes: (b) With HCI, obtain corresponding alkyl chloride. (c) Magnesium(II) plus hydroxide produces a slurry of Mg(OH)2' i.e., milk of magnesia (MOM). Separation from this gooey semi-solid be difficult. needs acid keep the Mg+ 2 dissolved. (d) problem with having any residual ether soln arises during distillation. While cold, dilute not a problem, distilla- tion becomes hot, (more) conc acid. distillation then steam alkenes; cf (a). (e) K 2C03 not only a DA, also basic. Thus which escaped earlier washings neutralized and precipitated its potassium salt. This an extra safeguard; use of a basic DA assures removal ~
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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 those using alcohol and a protonic in that the key intermediate, a carbocation, is same, i.e., reactions mechanistically similar, broadly speaking. 2. (a) See cited Solm problem. EtOH much more nucleophilic than anisole. (b) form irreversibly, but the The Ph 3COH/EtOH product must reversibly. Ph 3COH/HOAC If) (c) (1) no other reasonable structures. Ph 3C-O-C-Me· ~ d ' <----> Ph 3C-O-C-Me II HO (£) Second ion stable; only it can delocalize charge. Whole mechanism analogous of (a). ,.J ,.oJ (d) Assuming overall reaction AG' s comparable, llG activation, AG*, both formation .a.ru1 decomposition ester, will be less ether, Ph 3C-OAC, because greater 1M stability. Under Ph 3C-OEt, conditions reaction, ester not ether furnish required Ph 3C+.
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Page 2 - Chern 322/Sb Answers to Pd 2 (F-C Alkylation) 3. (a) question S(a) (b) from the Appendix: The rate ratio = exp[(-E a,rxn1 + E a,rxn2)/RT], which taking 2 rxn1 have lower E gives e (-7.4) at T a, 1 and e (-2.7) higher T, i.e., a given difference in E a makes a bigger T.
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322B_Prelab_Answers - Chern 322bL/325bL Period 1 JBE...

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