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Virtual Textbook of Organic Chemistry - Cationic Rearrangements
Comprehensive discussion of many facets cationic rearrangement. Be sure you have the basics down, but the most advanced portions of this tutorial should be treated as an excellent reading comprehension exercise.



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Work, Energy, and Power

Electricity

The Chemical Bond

Thermochemistry

Chemical Thermodynamics and the Equilibrium State

Intramolecular Cationic Rearrangements

Everyone learns in the first semester of organic chemistry that more highly substituted carbocations are more stable (tertiary > secondary > primary) because alkyl groups release negative charge by induction along the bond axes toward the cation. The more alkyl substituents present, the more negative charge density can be drawn in towards the positive charge

Let's practice using our thermochemical language to develop the idea in a bit more depth. Remember from electristatics that the closing of separation between unlike charges represents a decrease in electrostatic potential energy. In the chemical context, this translates to stabilization. The electrostatic potential energy decrease translates to internal energy, enthalpy, and, ultimately, free energy decrease. The energy differences in solution between a tertiary and a secondary and between a secondary and a primary carbocation are about 50 kJ mol-1. Is carbocation rearrangement an endothermic or an exothermic process?




Intramolecular Cationic Rearrangements

Reactions of Alkenes

Rearrangement is an event that is ideal for multiple choice questions, so you want to keep the set of reactions with carbocation intermediates in mind to anticipate their appearance on the MCAT. If you encounter a reaction with a carbocation intermediate on the MCAT, there is a better than even chance that one of the wrong choices in the multiple choice set will involve everything happening properly in the reaction except rearrangement.

Electrophilic addition to alkenes is one important class of reactions that includes several that involve a carbocation intermediate. Many of the electrophilic addition reactions involving alkenes begin by adding a proton to the double bond. This lead to carbocation intermediates and thus possibly rearrangement (addition of HX, sulfuric acid, acetic acid, hydration). Note that the electrophilic addition of halogen molecule is not in this category, proceeding through the triangular halonion ion intermediate.




Intramolecular Cationic Rearrangements

Reactions of Alkyl Halides

The substitution and elimination reactions (SN1 substitution, SN2 substitution, E1 elimination and E2 elimination) are classics of the first semester of organic chemistry and definitely important for the MCAT. You need to know them backwards and forwards before the test. One important aspect to keep in mind is that because carbocation intermediates occur in both E1 elimination and SN1 substitution, rearrangements are possible in these mechanisms. Rearrangement will not occur for SN2 substitution and E2 elimination.



Intramolecular Cationic Rearrangements

Reactions of Alcohols and Ethers

Acid catalyzed dehydration of alcohols is an E1 mechanism. It occurs with a carbocation intermediate and therefor can lead to rearrangement.

We can become overly accustomed to discussing substitution and elimination only in the context of alkyl halides, which is the model form for the reactions in the lecture course, but the MCAT is fond of presenting variations of classic mechanisms. The MCAT writers are just as likely to present a substitution or elimination mechanism involving a different leaving group.




Intramolecular Cationic Rearrangements

Reactions of Aromatic Compounds

In the electrophilic aromatic substitution reaction, Friedel-Crafts alkylation, the electrophile is a carbocation. This means that it cannot be used in synthesis to attach a primary alkyl chain more than two carbons long to an aromatic ring. To accomplish that goal, the alternative reaction of Friedel-Crafts acylation, not alkylation, must be used, followed by reduction. Attempting Friedel-Crafts alkylation of a primary alkyl chain would result in rearrangement.







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