First, a confession … I made a mistake on the answer sheet that I posted last Friday. The very first problem asks for the reaction product of HI + 1-butene in the presence of peroxides. The correct (?) answer is 1-iodobutane (the anti-Markovnikov product), but I drew 2-iodobutane (the Markovnikov product) on the answer sheet. I have corrected and posted a new answer sheet, but because my readers had already marked up the homework assignments, ignore any “wrong” marks.
Second, the great physicist Albert Einstein once said something like this in reference to theories of physics, “Everything should be made as simple as possible, but no simpler.” The same should be said about structural formulas, resonance structures, and chemical reactions.
If a problem asks you to predict the reaction product, please draw only that. If a problem asks you to draw a reagent, draw only that. If a problem asks for all meaningful resonance structures, draw them and follow all of the rules that have been handed down for Lewis structures:
- show ALL valence electrons
- show ALL formal charges
- do not move atoms, only electrons
If you plan to use shortcut formulas like Ph and OAc, use them properly. Ph = C6H5. A “Ph” can only make one bond, never two. Divinylbenzene is not “CH2=CH-Ph-CH=CH2”.
When drawing mechanisms, keep the following in mind:
- a bimolecular mechanistic step corresponds to a physical collision between TWO molecules
- a physical collision between THREE molecules at once is rare to the point of insignificance – do not include trimolecular steps in your mechanism (for example, a diene cannot react with two molecules of Hg(OAc)2 simultaneously)
- energy changes constrain what is possible. Therefore, it is possible for Hg(OAc)2 to add to an alkene and create an unstable cationic intermediate. However, it is not possible that this high-energy intermediate, even if it contains another alkene, would react with a second molecule of Hg(OAc)2 to form an even higher-energy intermediate.
- geometry also constrains what is possible. Any step that requires severe distortion of bond distances, bond angles, dihedral angles, or build-up of extreme steric repulsion, isn’t going to happen
A good rule-of-thumb: all mechanistic problems in this class can be solved by assuming that standard mechanisms operate. You will never have to invent any unusual mechanistic steps. You may, however, need to look out for reactants in unexpected roles. For example, you may become accustomed to water attacking a carbocation, but an alcohol will behave exactly the same way. Or, you might be accustomed to drawing nucleophiles and electrophiles as separate molecules, but they might actually be found in the same molecule.