CHAPTER 6 Helpful Hints

• Pi bonds cannot rotate, so alkenes have different stereoisomers. Alkenes with substituents can be named using the cis/trans method (if one non-H substituent is on each sp₂ carbon atom) or the more general E-Z system.
• Remember that E-Z priorities are based upon the atomic number of attached atoms. Also remember that priorities are not based upon total size. It is the FIRST point of difference that matters.
• Alkene double bonds are more stable (lower total energy) with more alkyl groups attached. These stabilities can be measured experimentally by comparison of heats of hydrogenation or heats of combustion.
• THE PI BOND OF AN ALKENE IS WEAKLY NUCLEOPHILIC, SO IT REACTS WITH STRONG ELECTROPHILES. PI BONDS ARE SLIGHTLY NUCLEOPHILIC BECAUSE THE ELECTRON DENSITY IS NOT ENTIRELY BETWEEN ATOMIC NUCLEI, MAKING IT ACCESSIBLE TO ELECTROPHILES.
• Markovnikov's rule - in H-X reactions: The H ends up on the less-substituted carbon atom. This is explained mechanistically by carbocation stabilities, since the lowest energy carbocation intermediate has the lower activation energy.
• We can estimate relative activation energies from relative intermediate stabilities due to Hammond's Postulate.
• Carbocation intermediates can rearrange by shifting a hydrogen or alkyl group. This will only occur if a new carbocation of equal or greater stability is generated by the rearrangement.
• When learning new reactions: learn the overall transformation, the reagents used, and the mechanism.
• Reactions are like sentences with a subject (the reactant), a verb (the reagent that transforms the subject into the object), and the object (the product of the reaction).
• If you know any two of the three components of a reaction (reactant, reagent, products), you should be able to predict the identity of the third.