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Teaching Organic Chemistry through Fiction


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Sn Reactions - Concept Explanation - Pt 1: Sn1 Reactions: Step 1-Leaving Group Departure
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Part One Concept Explanation
Sn1 Reactions; Step 1-Leaving Group Departure




An important type of reaction in organic chemistry is a nucleophilic substitution reaction. A nucleophilic substitution reaction occurs when a nucleophile (either a single atom or several atoms bonded together) is attracted to a positive charge or a nucleus. A nucleophile is usually a negatively charged group, and in a nucleophilic substitution reaction, it is replaced by another. A nucleophile can be a single atom such as a halide (such as chlorine or iodine), denoted by X, or a molecule with a negative charge (alcohol functional groups (OH) or a cyanide group (CN)). Some nucleophiles are more negatively charged, and therefore, better nucleophiles than others.

There are two types of nucleophilic substitution reactions: unimolecular and bimolecular, refered to as Sn1 and Sn2, respectively. In this case, we are talking about an Sn1 reaction, which takes place in several steps. The word Sn1 comes from the fact that the rate determining step for an Sn1 reaction consists of only one substance; an Sn1 reaction consists of only one nucleophile interacting with the central carbon atom at all times.

In an Sn1 reaction, the first step is when the nucleophile leaves. In this case, the nucleophile is called a leaving group (LG). A leaving group will usually leave because it is stable enough on its own. Leaving can be assisted through the use of heat and certain solvents—generally a leaving group will leave when in a protic solvent that contains free protons that help stabilize the leaving group. Additionally, an Sn1 reaction only occurs when the carbon that the leaving group is attached to will still be stable enough even after the leaving group leaves. The molecule that forms from the departure of the leaving group is a positively charged carbon, or a carbocation; while still not completely stable, it is stable enough to exist in solution.

In the case of this story, Lisa Green is the leaving group—such as a halide or OH group—in the molecule. Due to the high electronegativity of the leaving group, the leaving group is strained in the molecule, just as Lisa feels uncomfortable in the group. Therefore, Lisa attempts to leave, just as the leaving group will leave; Lisa is able to leave due to the fact that she is stable enough on her own—she’s a strong enough singer—that she can handle being on her own. Her leaving, however, places a great deal of pressure on Cathy, who represents the central carbon atom. Cathy therefore becomes positively charged, a carbocation.

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Continue to Part Two
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