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Carbocations and factors affecting their stability

A carbocation is a species where a carbon atom bonds to three carbon atoms and has a positive charge. Carbocations are electron deficient species and therefore very reactive and unstable. Anything which donates electron density to the electron-deficient center will help to stabilize them.

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Carbocations and factors affecting their stability

  1. 1. A carbocation is a species where a carbon atom bonds to three carbon atoms and has a positive charge. Carbocations are electron deficient species and therefore very reactive and unstable. Anything which donates electron density to the electron-deficient center will help to stabilize them. Chemistry Net http://chem-net.blogspot.com 1 CARBOCATIONS & STABILITY
  2. 2. Factors that stabilize them are the following: Neighboring carbon atoms (inductive effect) Neighboring carbon-carbon multiple bonds (resonance effect) Neighboring atoms with lone pairs (resonance effect) Chemistry Net http://chem-net.blogspot.com 2
  3. 3. How carbocations are stabilized by neighboring carbons atoms? The stability of carbocations decreases as the number of carbons attached to the C+ decreases. That means that tertiary carbocations are more stable than secondary that in turn are more stable than primary (Fig. 1) Chemistry Net http://chem-net.blogspot.com 3
  4. 4. Fig. 1: Carbocation stability increases as methyl substitution increases around the electron deficient carbon C+. The methyl groups (-CH3) are electron donating and therefore stabilize the positive charge (inductive effect). Chemistry Net http://chem-net.blogspot.com 4
  5. 5. Chemistry Net http://chem-net.blogspot.com 5 An explanation for this is that the methyl group (-CH3) acts as an electron-donor and therefore stabilizes the positively charged cation. Remember that the C atom has an electronegativity of 2.5 and that H 2.2.
  6. 6. Chemistry Net http://chem-net.blogspot.com 6 A better explanation is that electrons are donated from the C-H bonds to the empty p orbital of the C+ therefore stabilizing the carbocation through hyperconjugation (the more the - CH3 groups attached to the C+ the more stable the carbocation becomes).
  7. 7. Chemistry Net http://chem-net.blogspot.com 7 How carbocations are stabilized by carbon- carbon multiple bonds (resonance)? Carbocations where the C+ is adjacent to another carbon atom that has a double or triple bond have extra stability because of the overlap of the empty p orbital of the carbocation with the p orbitals of the π
  8. 8. Chemistry Net http://chem-net.blogspot.com 8 bond. This overlap of the orbitals allows the charge to be shared between multiple atoms – delocalization of the charge - and therefore stabilizes the carbocation.
  9. 9. Chemistry Net http://chem-net.blogspot.com 9 Fig. 2: Carbocation stabilization by multiple bonds adjacent to the C+ atom through p-orbital overlap
  10. 10. This effect is called charge delocalization and is shown by drawing resonance structures where the charge moves from atom to atom. It greatly stabilizes even primary carbocations – normally very unstable – that are adjacent to a carbon- carbon multiple bond. Chemistry Net http://chem-net.blogspot.com 10
  11. 11. Chemistry Net http://chem-net.blogspot.com 11 Fig. 3: Carbocation stabilization by multiple bonds adjacent to the C+ atom
  12. 12. Chemistry Net http://chem-net.blogspot.com 12 How carbocations are stabilized by adjacent atoms with lone pairs? Adjacent atoms with lone pairs act as electron donors to the electron-poor carbocation. This results in forming a double bond (π bond) and the charge is
  13. 13. Chemistry Net http://chem-net.blogspot.com 13 delocalized to the atom donating the electron pair (π donation). Nitrogen and oxygen atoms are the most powerful π donors. However, even halogen atoms stabilize carbocations through donation of a lone pair.
  14. 14. Chemistry Net http://chem-net.blogspot.com 14 Fig. 4: Stabilization of the carbocation by lone pair donation. The O atom donates an electron pair to the C+ atom and a double bond is formed. The positive charge is delocalized to the oxygen atom providing extra stability.

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