Showing a Mechanism

From an arrow-pushing point of view, all acid-base reactions are the same. It goes like this  

EXERCISE 3.38 Show the mechanism for the following acid-base reaction  

Answer Remember—2 arrows. One from the base to the proton and the other from the bond (that is losing the proton) to the atom (currently connected to the proton)  

Induction The negative charge on the left is destabilized by an electron-donating alkyl group. The one on the right is not destabilized in this way. 

Based on factor 3, we conclude that the one on the right is more stable, and therefore the equilibrinm lies to the right. We show this in the following way  

There are always two arrows. One is drawn coming from the base and grabbing the proton. The second arrow is drawn coming from the bond (between the proton and whatever atom is connected to the proton) and going to the atom currently connected to the proton. That s it. There are always two arrows. Each arrow has a head and a tail, so there are four possible mistakes you can make. You might accidentally draw either of the heads incorrectly, or you might draw either of the tails incorrectly. With a little bit of practice you will see just how easy it is, and you will realize that acid-base reactions always follow the same mechanism. 

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However, the switchover from an A2 to an A1 hydrolysis is a very common mechanistic pathway in strong acid media, probably more common than the pure A2 mechanism. Excess acidity analyses have shown that thioacetic acid, several thiobenzoic acids, and many thiolbenzoate and thionbenzoate esters show this sort of mechanism switch.179 Acylals and thioacylals also show this behavior,116 with thioacylals using two water molecules and acylals one. Many hydroxamic acids react this way,127,216 as do esters of various types,41,217,218 episulfoxides219 and aryloxatriazoles.220 Acylhydrazines can also show a mechanism switch of this sort, although with these substrates the situation is somewhat more complex.221... 

Scheme 16 shows a mechanism for the catalysis that is consistent with the kinetics and other data. 1H-NMR experiments in toluene-d8 at 250 K... 

Samples of intermediates and finished products taken for analysis are recorded, stating the time, date, and conditions for these samples. Deviations in operating conditions and out of specification (OOS) conditions in samples are reported and investigated. Figure 9.2 shows a mechanism for production and in-process controls. 

Ibuprofen is an interesting case, in that the (5)-(+)-form is an active analgesic, but the ( )-(—)-enantiomer is inactive. However, in the body there is some metabolic conversion of the inactive (I )-isomer into the active (5 )-isomer, so that the potential activity from the racemate is considerably more than 50%. Box 10.11 shows a mechanism to account for this isomerism. 

Maurice Mitchell, Jr. The answer to Dr. Wender s question, in my opinion, is that where solubility is an equilibrium phenomenon, Heredy s work shows a mechanism for solubilizing coal. Thus, although the differences in... 

Scheme 48 shows a mechanism for the Rh(I)-catalyzed reaction proposed by Wilkinson (11a, 107). The reaction starts with the insertion of coordinated olefin into the metal-hyidrogen bond in the hydrido-... 

Methylpropene reacts with ethene and hydrogen chloride under polar conditions to yield 1-chloro-3,3-dimethylbutane. Show a mechanism for this reaction that is consistent with the reactants, conditions, and product. Give your reasoning. 

The dendrimer tubes prepared by this multilayer process have only one open end and probably show a mechanical stability gradient, indicating that the effective number of layer-deposition steps in the proximity of the closed end is lower than elsewhere because the ionic dendrimer to be precipitated has limited access there. Production of uniform tubes would therefore require templates with two openings. 

In addition to the reaction shown on p. 353, diphenhydramine can also be prepared by heating bromodiphenylmethane and 2-(dimethylamino)-l -ethanol in a polar solvent. Show a mechanism for this reaction ... 

The reaction of an alkene with bromine in an alcohol as solvent produces an ether as the product. Show a mechanism for the following reaction and explain the stereochemistry of the product ... 

Figure 11.5 shows a mechanism that has been postulated for this reaction. First, an electrophilic mercury species adds to the double bond to form a cyclic mercurinium ion. Note how similar this mechanism is, including its stereochemistry and regiochemistry, to that shown in Figure 11.4 for the formation of a halohydrin. The initial product results from anti addition of Fig and OH to the double bond. In the second step, sodium borohydride replaces the mercury with a hydrogen with random stereochemistry. (The mechanism for this step is complex and not important to us at this time.) The overall result is the addition of H and OH with Markovnikov orientation. 

H-NMR spectrum. Show the structure of this product, show a mechanism for its formation, and explain the regiochemistry of the reaction (see Section 11.2). 

When benzene is mixed with deuterated sulfuric acid, deuterium is slowly incorporated onto the ring. Show a mechanism for this process ... 

The sugar fructose is an isomer of glucose. Like glucose, fructose forms a cyclic hemiacetal, but in this case the ring is five membered rather than six membered. Show the structure for the hemiacetal formed from fructose and show a mechanism for its formation in acidic solution. 

When acetate ion is treated with basic water that has been enriched with l80, the heavy oxygen atoms are incorporated into both oxygens of the acetate ion. Show a mechanism for this reaction. 

Show a mechanism for the conversion of GBL to GHB. Explain why the equilibrium favors GHB in this reaction. Explain why GHB is converted back to GBL when treated with acid. 

When proteins are analyzed, it is often necessaiy to cleave them into their individual amino acid components. This is accomplished by refluxing the protein in acidic water. Show a mechanism for this process using the following model reaction, the hydrolysis of a dipeptide ... 

Show a mechanism for the interconversion of glyceraldehyde-3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP) in basic solution ... 

Deuterium can be incorporated at the positions a to a carbonyl group by reaction with D20 in the presence of acid. Show a mechanism for this process. If the reaction were continued, what is the maximum number of deuterium atoms that would be incorporated into a single molecule ... 

The reaction of alkenes with a small amount of a radical initiator is a common method used to prepare polymers (see Chapter 24). An intermediate stage in the formation of polyethylene is shown in this equation Show a mechanism for the formation of this intermediate. 

Show a mechanism for the conversion of D-fructose to a furanose. Do not worry about the stereochemistry of the reaction. 

What aldohexose could be formed from D-galactose on treatment with base Show a mechanism for the formation of this aldohexose. 

D-Glyceraldehyde, dihydroxyacetone, or a mixture of these two isomers reacts in the presence of sodium hydroxide to form fructose, along with other products. Show a mechanism for this reaction. 

Show a mechanism for nucleoside hydrolysis in aqueous acid. 

Show a mechanism for the elimination reaction that occurs in the first step of the process shown in Figure 28.9. 

One way to accomplish the isomerization of a double bond into conjugation with a carbonyl group, one of the reactions needed for the last part of the synthesis of progesterone from diosgenin, is to treat the compound with base as shown in the following equation. Show a mechanism for this reaction. 

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