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We saw in Section 8-7 that bromohydrins are converted into epoxides when treated with base. Propose a mechanism, using curved arrows to show the electron flow.

Metabolism of S-adenosylhomocysteine (Section 11-6) involves the following sequence. Propose a mechanism for the second step.

Reaction of iodoethane withyields a small amount of isonitrile,${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{N}≡\mathrm{C}$, along with the nitrile${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{C}≡\mathrm{N}$as the major product. Write electron-dot structures for both products, assign formal charges as necessary, and propose mechanisms to account for their formation.

One step in the urea cycle for ridding the body of ammonia is the conversion of argininosuccinate to the amino acid arginine plus fumarate. Propose a mechanism for the reaction, and show the structure of arginine.

Methyl esters (${\mathrm{RCO}}_2{\mathrm{CH}}_3$) undergo a cleavage reaction to yield carboxylate ions plus iodomethane on heating with LiI in dimethylformamide:

The following evidence has been obtained: (1) The reaction occurs much faster in DMF than in ethanol. (2) The corresponding ethyl ester (${\mathrm{RCO}}_2{\mathrm{CH}}_2{\mathrm{CH}}_3$) cleaves approximately 10 times more slowly than the methyl ester. Propose a mechanism for the reaction. What other kinds of experimental evidence could you gather to support your hypothesis?

S_N2 reactions take place with inversion of configuration, andS_N1 reactions take place with racemization. The following substitution reaction, however, occurs with complete retention of configuration. Propose a mechanism.

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Which compound in each of the following pairs will react faster in an S_N2reaction with?

a) CH₃Br or CH₃I

Which compound in each of the following pairs will react faster in an S_N2reaction with OH^-?

(b) CH₃CH₂Iin ethanol or dimethyl sulfoxide

Which compound in each of the following pairs will react faster in an S_N2reaction with OH^-?

(c)${\left({\mathrm{CH}}_3\right)}_3\mathrm{CCl}\mathrm{or}{\mathrm{CH}}_3\mathrm{Cl}$

Which compound in each of the following pairs will react faster in an S_N2reaction with OH^-?

(d)${\mathrm{CH}}_2=\mathrm{CHBr}\mathrm{or}{\mathrm{CH}}_2={\mathrm{CHCH}}_2\mathrm{Br}$