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Part 2 Chemistry Extended Summary:


Sn1 Reactions; Carbocation Stability (Hyperconjugation)

An Sn1 reaction involves 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 generally 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 chargeís ability to be spread out over a larger surface area. Tertiary carbocations have more surface area 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.

Cathy is an unstable singer due to the charge placed on her, the carbocation, by the leaving group's departure; atoms that are charged are unstable, and want to become neutral. Because she is with Adrian and Brandon, who represent two substituents bonded directly to the central carbon, she only feels comfortable singing in the garage. Cathy is similar to a secondary carbocation and is moderately stable. When Danielle joins of the group, Cathy becomes a tertiary carbocation and stable enough to sing in a competitive environment. The group with Danielle, a three substituent carbocation, is more stable than the group without Danielle, reflecting the differences between a tertiary and secondary carbocation's stability.

An important thing to note about carbocation stability is that itís 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. Sigma bonds on the adjacent carbons are in the same plane as the empty pi orbitals of the carbon with the positive charge, resulting in this charge distribution.

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