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Teaching Organic Chemistry through Fiction


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Sn Reactions - Concept Explanation - Pt 2: Sn1 Reactions: Carbocation Stability
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Part Two Concept Explanation:
Sn1 Reactions; Carbocation Stability


An important type of reaction in organic chemistry is a nucleophilic substitution reaction, where a nucleophile replaces another nucleophile on a central carbon atom. Unimolecular nucleophilic substitution reactions, or Sn1 reactions, involve the departure of a leaving group as the first step. The departure of a leaving group forms a carbocation, or a carbon with a positive charge.

Carbocations are innately unstable due to the fact that they have a positively charged carbon; as molecules wish to be neutral, any sort of charge on a molecule results in instability. It is important to note, however, that certain types of carbocations are more stable than others. This occurs due to the positive charge’s ability to be spread out over a larger surface area. Tertiary carbocations have more surface are than secondary carbocations; therefore tertiary carbocations are more stable than secondary carbocations. The least stable are primary carbocations, which have the least surface area. Tertiary carbocations consist of a Carbon bonded to three groups other than hydrogen; a secondary consists of a carbon bonded to two groups other than hydrogen; and primary carbocations are a single carbon bonded one group other than hydrogen.

An important thing to note about carbocation stability is that it’s not an active sharing of the charge (resonance). This type of sharing is called hyperconjugation because it occurs simply due to more groups to help distribute the charge around a greater area. Another way to think of carbocation stability is to have a certain amount of water and two pans of different sizes. A large pan can hold the water better than a smaller pan because the water has more space to spread out.

In the story, Cathy is an unstable singer due to the “charge” placed on her. Despite the fact that she is with Adrian and Brandon, who represent two substituents that are bonded directly to the central carbon, she only feels comfortable singing in the garage. In this case, Cathy is a secondary carbocation and is moderately stable, able to sing in the garage. When Danielle is a part of the group, Cathy is a tertiary carbocation; Cathy becomes stable enough to sing in a competitive environment. The group with Danielle is far more stable than the group without Danielle, reflecting the differences between a tertiary and secondary carbocation’s stability.

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