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Work, Energy, and Power


The Chemical Bond


Chemical Kinetics

Reactions of Alkenes

Classics of first semester organic chemistry, the electrophilic addition reactions with alkenes need to be mastered before the MCAT. One very important thing to remember is that the electrophilic addition of hydrogen halide to an unsymmetrically substituted alkene occurs by Markovnikov's rule. In the Markovnikov product, the halogen becomes bonded to the more highly substituted of the two carbons.

The mechanism is always key to retention and understanding of the factors determining the structure of products in organic chemistry reactions. To understand the basis for Markovnikov regioselectivity in electrophilic addition of HX, look to the mechanism. The reaction proceeds by proton transfer initially to form a carbocation intermediate. Because alkyl groups are electron releasing by induction, the more highly substituted carbocations are more stable than less substituted carbocations. Highly substituted carbocations have negative charge moving in closely toward the positive carbocation, which is a relative electrostatic potential energy decrease. Therefore, tertiary carbocation intermediates are more stable than secondary carbocations, which are in turn more stable than primary carbocations. If a reaction coordinate has the choice of pathways, the pathway towards the lower energy intermediate will be favored kinetically. This is the mechanistic basis for Markovnikov regioselectivity in electrophilic addition of HX to alkenes.

Another important factor to keep in mind for the MCAT is that because addition of hydrogen halide to an alkene leads to a carbocation intermediate, rearrangements of the carbon skeleton are possible before capture by the halide.

Reactions of Alkenes

Reactions of Conjugated Species

If your first instinct is that the reaction is a straightforward addition of HX to alkene, watch out. The MCAT writers might try to sneak a conjugated diene past you to see if you will forget that the intermediate is an allylic carbocation. Remember that the first step of the reaction, the addition of the proton to a conjugated diene, will form an allylic carbocation intermediate. The major product will reflect either direct (1,2) or conjugate (1,4) addition, depending on reaction conditions.

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