carbocation

Examples of carbocation in the following topics:

• Rearrangement of Carbocations

  • The formation of carbocations is sometimes accompanied by a structural rearrangement.
  • Such rearrangements take place by a shift of a neighboring alkyl group or hydrogen, and are favored when the rearranged carbocation is more stable than the initial cation.
  • This surprising result may be explained by a carbocation rearrangement of the initially formed 2º-carbocation to a 3º-carbocation by a 1,2-shift of a methyl group.
  • Another factor that may induce rearrangement of carbocation intermediates is strain.
  • As shown in the following equation, this rearrangement converts a 3º-carbocation to a 2º-carbocation, a transformation that is normally unfavorable.

• Wagner-Meerwein Rearrangements

  • As noted in the carbocation stability order shown below, such carbocations are relatively unstable and are formed slowly.
  • In the case of the neopentyl cation, however, the initially formed 1º-carbocation may be converted to a more stable 3º-carbocation by the 1,2-shift of an adjacent methyl group with its bonding electrons.
  • Although a 3º-carbocation is initially formed, the angle and torsional strain of the four-membered ring is reduced by a methylene group shift resulting in ring expansion to a 2º-carbocation.
  • Protonation of the double bond gives a 3º-carbocation.
  • An adjacent hydrogen atom (colored blue) shifts as a hydride moiety to create a new 3º-carbocation, which in turn induces the shift of a methyl group (colored green) with formation of yet another 3º-carbocation.

• A Mechanism for Electrophilic Substitution Reactions of Benzene

  • This mechanism for electrophilic aromatic substitution should be considered in context with other mechanisms involving carbocation intermediates.
  • To summarize, when carbocation intermediates are formed one can expect them to react further by one or more of the following modes:
  • The cation may rearrange to a more stable carbocation, and then react by mode #1 or #2.
  • The second step of alkene addition reactions proceeds by the first mode, and any of these three reactions may exhibit molecular rearrangement if an initial unstable carbocation is formed.
  • The carbocation intermediate in electrophilic aromatic substitution (the benzenonium ion) is stabilized by charge delocalization (resonance) so it is not subject to rearrangement.

• Elimination

  • In unimolecular elimination (E1), a leaving group first detaches itself from a carbon atom, leaving behind a carbocation.
  • A base then deprotonates a carbon adjacent to the carbocation, and the electrons from the previous C-H bond then go to form a C=C bond at the carbocation.
  • E1 is favored with more substituted carbocations, which are less accessible to nucleophiles.
  • Thus, if there are hydrogen atoms attached to multiple carbon atoms vicinal to the carbocation, the double bond will be formed between the former carbocation and the most substituted vicinal carbon.
  • A stronger base is required than in E1, because in E2, the hydrogen is less acidic at the point of removal (there is no vicinal carbocation).

• The Nonclassical Carbocation Hypothesis

  • The role of carbocation intermediates in many organic reactions is well established.
  • By comparison, the endo-isomer ionizes to a classical 2º-carbocation, which is rapidly converted to the more stable nonclassical ion.
  • Some acetate anion may bond to the 2º-carbocation before it changes, accounting for the residual optical activity in this reaction.
  • Are there other relatively stable nonclassical carbocations?
  • In the first diagram below, the simplest hypervalent carbocation, methanonium, is drawn on the left in the gray shaded box.

• Anchimeric Assistance

  • It is apparent that in both cases an initially formed 1º-carbocation has rearranged prior to product formation, as depicted in the second diagram below.
  • The resulting phenonium ion would immediately open to a 3º-carbocation, in which the assisting phenyl group has shifted to an adjacent position.
  • The activation energy for this step is larger for neopentyl chloride because it leads to a discrete 1º-carbocation.
  • The second step in the neopentyl chloride solvolysis is a rapid rearrangement of the 1º-carbocation to an isomeric 3º-carbocation.
  • In both cases, the 3º-carbocation intermediate finally disproportionates to a mixture of substitution and elimination products.

• Pinacol Rearrangement

  • The resulting 3º-carbocation is relatively stable, and has been shown to return to pinacol by reaction in the presence of isotopically labeled water.
  • A 1,2-methyl shift generates an even more stable carbocation in which the charge is delocalized by heteroatom resonance.
  • or Which intermediate carbocation is more stable?
  • Under mild acid treatment, the diol rearranges rapidly to an aldehyde by way of a 1,2-hydrogen shift to the initially formed diphenyl 3º-carbocation.
  • Treatment with cold sulfuric acid should produce the more stable diphenyl 3º-carbocation, and a methyl group shift would then lead to the observed product.

• Tiffeneau-Demjanov Rearrangement

  • Ambiguity in determining the initial site of carbocation formation presented a problem in the analysis of many pinacol rearrangements.
  • Cyclic ketones have two alpha-carbon atoms, each of which might shift to the nascent 1º-carbocation.
  • The initial stage of an aryl group shift to an adjacent carbocation site may be viewed as an intramolecular electrophilic substitution of the Friedel-Crafts type.

• Brønsted Acid Additions

  • Of all the reagents discussed here, these strong acid additions (E = H in the following equation) come closest to proceeding by the proposed two-step mechanism in which a discrete carbocation intermediate is generated in the first step.

• Addition Reactions Initiated by Electrophilic Halogen

  • We can account both for the high stereoselectivity and the lack of rearrangement in these reactions by proposing a stabilizing interaction between the developing carbocation center and the electron rich halogen atom on the adjacent carbon.
  • The stabilization provided by this halogen-carbocation bonding makes rearrangement unlikely, and in a few cases three-membered cyclic halonium cations have been isolated and identified as true intermediates.
  • The positive charge is delocalized over all the atoms of the ring, but should be concentrated at the more substituted carbon (carbocation stability), and this is the site to which the nucleophile will bond.