Sn1, Sn2, E1, E2 mechanism crash course
Sn1, Sn2, E1, E2 mechanism crash course Chem 372
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This 2 page Class Notes was uploaded by Rachel Taylor on Tuesday January 19, 2016. The Class Notes belongs to Chem 372 at Eastern Michigan University taught by Dr. Friebe in Winter 2016. Since its upload, it has received 25 views. For similar materials see Organic Chemistry in Chemistry at Eastern Michigan University.
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Date Created: 01/19/16
Sn1 and Sn2 Mechanisms Sn2 Sn1 E2 E1 Number of R 1˚ 3˚ 3˚ 3˚ groups favored Rate equation R=k[R-X][Nu] R=k[R-X] R=k[R-X][B] R=k[R-X] Stereochemistry Inversion Mix Antiperiplanar mix Solvent Aprotic Polar Protic Polar Aprotic Polar Protic Nucleophile Strong Weak Strong Weak Best leaving group Iodine Iodine Iodine Iodine Number of Steps 1 2 1 2 Table 1: Overview of Sn1, Sn2, E1, and E2 mechanisms. Number of R groups favored: With Sn1, E1, and E2, the increase of R groups stabilize the carbocation intermediates due to the ability to distribute charge. With Sn2 reactions, since. E2 and Sn2 are very similar, except that Sn2 reactions have steric hindrance with the increase of R groups. When the Nucleophile attacks the molecule, it does not want to be close to other atoms except the carbon where the substitution is taking place (figure 1). Figure 1: Sn2 Nucleophilic substitution. Leaving group: Iodine is always the best leaving group because it is the biggest halogen. The larger the halogen, the better it is at stabilizing a negative charge. Rate Equations: The rate equations of Sn2 and E2 are straight forward: as the concentration of either compound, the rate increases. In Sn1 and E1 reactions, they are two step mechanisms. Because of this, their rate depends on the slowest step in the reaction. This slow step is when the halogen leaves creating a highly unstable carbocation. Thus, the rate equation for both E1 and Sn1 depends on the concentration of the electrophile, not the Base/Nucleophile. Stereochemistry: The most important thing to know about stereochemistry is Sn1 and E1 reactions make mixtures of stereoisomers, and Sn2 mechanisms invert the stereochemistry. Solvent: In Sn1 and E1 mechanisms, polar protic solvents are favored because they hydrogen bond with the leaving group, and it has dipole-dipole interactions with the carbocation, stabilizing the charges. In E2 and Sn2 mechanisms, the anion needs to attack the alkyl halide, so if the charge was stabilized in a polar protic solvent, it would slow down the reaction. Nucleophile: Sn2 and E2 need strong nucleophiles in order to attack the alkyl halide forcing the leaving group to leave. In Sn1 and E1 mechanisms, weak nucleophiles are used to avoid competing Sn2 and E2 mechanisms.
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