Reaction Intermediates: Radical, Carbocation, Carbanion

Posted on November 26th, 2012

It is important to know the hierarchy of Reaction Intermediates such as Radicals, Carbocations, Carbanions.
Here we present a quick guide to Reaction Intermediate hierarchies.
The Big Picture: Radicals and Carbocations prefer a greater degree of alkyl substitution. While, Carbanions are the opposite: Carbanions prefer a lesser degree of alkyl substitution. However, all three prefer the allylic position the most!
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    1. Allylic position
      1. Definition: The position immediately next to a double bond
      2. Image: The arrow points to the allylic position: 
    2. Reaction Intermediates
      • Radical
        • Typically electrons come in pairs. However there are unpaired electrons known as radical electrons. These are usually just called radicals.
        • Radical stability
          1. Radicals prefer a greater degree of alkyl substitution. Even more so, radicals prefer to be in the allylic position.
          2. Therefore here is the hierarchy of radical intermediate stability:

      • Carbocation
        • Carbocations serve as electrophiles in reactions. They will attract electrons easily as the carbon is deficient in electrons.
        • Carbocation stability
          1. Carbocations prefer a greater degree of alkyl substitution. Even more so, carbocations prefer to be in the allylic position. Therefore here is the hierarchy of carbocation intermediate stability:

      • Carbanion
        • Carbanions serve as nucleophiles in reactions. They will donate electrons easily as the carbon has excess electrons.
        • Carbanion stability
          1. Carbanions prefer a lesser degree of alkyl substitution. Even more so, carbanions prefer to be in the allylic position. Therefore here is the hierarchy of carbanion intermediate stability:

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