Proton removal writing reaction mechanisms

Cyclic-AMP is formed from ATP in a reaction catalyzed by the enzyme adenylate cyclase. Assume that adenylate cyclase acts as a base to remove a proton from the 3 -hydroxyl group of ATP and write a mechanism for the formation of cAMP. 

The aryne intermediate is usually written with a triple bond and a delocalized aromatic system, as shown in 3-44. The anion in the side chain reacts as a nucleophile with the electrophilic aryne. The resulting anion, 3-45, can remove a proton from ammonia to give 3-46. Because the product has been reached, we usually stop writing the reaction mechanism at this point. However, in the reaction mixture, amide will remove a proton from the carbon a to the cyano group of 3-46. Only during workup will the anion be protonated to give back 3-46. 

Referring to the mechanism for the acid-catalyzed hydrolysis of methyl acetate, write the mechanism—showing all the curved arrows— for the acid-catalyzed reaction of acetic acid and methanol to form methyl acetate. Use HB and =B to represent proton-donating and proton-removing species, respectively. 

Development of a carbocation center due to loss of chloride ion can account for the loss of stereochemistry at this carbon. It seems reasonable to write this step after removal of a proton to give the anion because, if chloride loss occurs before anion formation, the carbocation would be expected to react with solvent before the anion could be formed. In that case one might expect to isolate some of the compound in which chloride had been replaced by methoxyl. The effect of the solvent on the stereochemistry of the reaction is due to its ability to solvate the intermediate charged species involved in a stepwise mechanism. 

The first example we will consider is the aldol reaction, in which two molecules of ethanal, on treatment with a base such as NaOH, react together to form aldol, 3-hydroxybutanal. Write down the mechanism for the removal of a proton from ethanal to give the carbanion, and also indicate the stabilisation that is available for this ion. 

The reduction of nitrobenzene to aniline is a typical oxidation-reduction reaction in which tin metal, Sn°, is oxidized to stannic ion, Sn, in the form of stannic chloride, SnCl4 hydrochloric acid serves as the source of protons. A plausible mechanism of this reaction is outlined in Scheme 21.3. Generally, the reduction occurs by a sequence of steps in which an electron is first transferred from a tin atom to the organic substrate to give an intermediate radical ion that is then protonated. The oxygen atoms on the nitro group are eventually removed as water molecules. It is left as an exercise to write a balanced equation for the overall reaction and to provide a mechanism for the reduction of N-phenylhydroxylamine (11) into aniline (see Exercises 13 and 14 at the end of this section). 

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