Carbocations are key players in organic reactions. Their stability depends on factors like substitution, hybridization, and resonance. Understanding these influences helps predict reaction outcomes and mechanisms in organic synthesis.

Primary, secondary, and tertiary carbocations differ in stability, affecting their reactivity. Inductive effects and hyperconjugation explain why more substituted carbocations are more stable. This knowledge is crucial for predicting and controlling organic reactions.

Carbocation Structure and Stability

Structure and stability of carbocations

• Carbocation stability influenced by several structural factors
  • Substitution: number of carbon or alkyl groups bonded to positively charged carbon
    • Increasing alkyl substitution enhances carbocation stability
    • Alkyl groups electron-donating and stabilize positive charge through hyperconjugation and inductive effects (methyl, ethyl, propyl)
  • Hybridization: hybridization state of positively charged carbon
    • $sp^3$ hybridized carbocations less stable than $sp^2$ and $sp$ hybridized carbocations
    • $sp^2$ and $sp$ hybridized carbocations stabilized by resonance (allyl, benzyl)
    • Stability trend: $sp$ > $sp^2$ > $sp^3$
  • Resonance stabilization: delocalization of positive charge across multiple atoms
    • Increases overall stability of the carbocation

Primary vs secondary vs tertiary carbocations

• Carbocations classified based on degree of substitution at positively charged carbon
  • Primary carbocations: one alkyl group attached to positively charged carbon (ethyl carbocation)
  • Secondary carbocations: two alkyl groups attached to positively charged carbon (isopropyl carbocation)
  • Tertiary carbocations: three alkyl groups attached to positively charged carbon (tert-butyl carbocation)
• Experimental evidence supports stability trend: tertiary > secondary > primary
  • Solvolysis reactions of alkyl halides in polar protic solvents proceed through carbocation intermediates
    • Tertiary alkyl halides react faster than secondary alkyl halides, which react faster than primary alkyl halides (tert-butyl bromide > isopropyl bromide > ethyl bromide)
    • Reaction rates: tertiary > secondary > primary
  • Rearrangement of carbocations during nucleophilic substitution reactions
    • Secondary and tertiary carbocations can rearrange to more stable carbocations before capturing a nucleophile (neopentyl carbocation rearranges to tert-pentyl carbocation)
    • Primary carbocations do not typically undergo rearrangement due to low stability

Inductive effects and hyperconjugation

• Inductive effects: electron-donating ability of alkyl groups stabilizes carbocations
  • Alkyl groups more electron-donating than hydrogen atoms
  • Positive charge on carbocation dispersed by electron-donating alkyl groups
  • Inductive effects more pronounced when alkyl group closer to positively charged carbon (neopentyl carbocation more stable than tert-butyl carbocation)
• Hyperconjugation: delocalization of electron density from adjacent $\sigma$ bonds to empty p orbital of carbocation
  • Hyperconjugation involves overlap of filled $\sigma$ orbital (usually C-H or C-C) with empty p orbital of carbocation
  • Overlap stabilizes carbocation by delocalizing positive charge
  • More alkyl groups attached to carbocation, more hyperconjugation can occur, leading to increased stability (tert-butyl carbocation more stable than isopropyl carbocation)
  • Hyperconjugation more effective than inductive effects in stabilizing carbocations

Carbocation reactions and rearrangements

• Carbocation rearrangement: structural reorganization to form more stable carbocations
  • Can involve hydride or alkyl shifts to achieve greater stability
• Nucleophilic addition: reaction of carbocations with nucleophiles to form new covalent bonds
• Elimination reaction: loss of a proton from a carbocation to form an alkene