Getting Started in Drawing a Mechanism

The term stable is ambiguous in organic chemistry parlance. When a compound is said to be stable, it sometimes means that it has low energy (AG ), i.e., is thermodynamically stable, and it sometimes means that the barrier for its conversion to other species is high (AG ), i.e., is kinetically stable. For example, both benzene and tetra-f-butyltetrahe-drane are surprisingly stable. The former is both kinetically and thermodynamically stable, whereas the latter is kinetically stable and thermodynamically unstable. Certain kinds of compounds, like hemiacetals, are kinetically unstable and thermodynamically stable. In general, stable usually means kinetically stable, but you should always assure yourself that that is what is meant. When in doubt, ask. 

Reagent over arrow ionic product converted into neutral one by aqueous workup 

By-product omitted catalyst over arrow, solvent under arrow 

When reagents are separated by a semicolon, it means that the first reagent is added and allowed to react, then the second reagent is added and allowed to react, and so on. When reagents are numbered sequentially, it may mean the same, or it may mean that the reaction mixture is worked up and the product is isolated after each individual step is executed. 

Reagents added sequentially or reaction mixture worked up after each step 

Three features of the way organic reactions are written sometimes make it difficult for students to figure out what is going on. First, compounds written over or under the arrow are sometimes stoichiometric reagents, sometimes catalysts, 

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As shown above, methenamine is hydrolyzed in aqueous acid to produce formaldehyde and ammonia. Draw a mechanism showing formation of one molecule of formaldehyde (the remaining five molecules of formaldehyde are each released via a similar sequence of steps). The release of each molecule of formaldehyde is directly analogous to the hydrolysis of an acetal. To get you started, the first two steps are provided below ... 

In the space below, draw out a possible mechanism for alkene hydration shown in Model 6. Hints for getting started can be found in the Check Your Work section below. 

The molecule gains back this energy (and more) due to the Coulombic attraction as the atoms move from infinite separation to the experimentally observed bond distance of 267 pm. Coulombic attraction would tend to draw the two ions as close as possible, but we will see later (in Chapters 5 and 6) that quantum mechanics predicts the energy will eventually start to rise if the atoms get too close. Combining all of these concepts gives a commonly used approximate potential for ionic bonds of the form... 

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