2 - Exam 2 Practice Problems and Keys

2 - Exam 2 Practice Problems and Keys - Exam 2 SN1, E1,...

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Unformatted text preview: Exam 2 SN1, E1, SN2, E2 Reactions Markovnikov Addition Ant-Markovnikov Addition Reaction Mechanisms Hoffman and Saytzeff Eliminations Enantiomers and Diastereomers SN1 Reactions: By-Products- Whenever an SN1 reaction occurs there is always the possibility that an E1 by-product will form. This occurs if the nucleophile does not get to the carbocation soon enough. Therefore, if you see a reaction with SN1 conditions always assume that some E1 minor product will form. Racemic Mixtures- In SN1 reactions you also have the possibility of making a racemic mixture of products that is, left and right hand versions of the same molecule. Always check to see if the leaving group is on a chiral carbon, if so, then a racemic mixture will be made. Rearrangements It is also possible for SN1 reactions to rearrange. Carbocations want to be on the most stable carbon, and this means 3 if it is available. Therefore carbocations will rearrange themselves to place the positive charge onto the most stable carbon ONLY IF the most stable carbon is right next door. E2 Reactions: Big Bases For an E2 reaction to take place you must use a big base. Classically this means using the t-butoxide ion (t-ButO- ) but other large bases can be used (more on this later). You do not want to use a base that is so small that an SN2 reaction could occur. If you look carefully, you will see that the conditions of an E2 and an SN2 reaction are nearly identical except for the size of the base. So large bases must be used for E2 reactions. Hoffman vs. Saytzeff - E2 reactions make double and triple bonds by removing an HX from a molecule. But which HX? As can be seen below, you may have a couple of choices; H 3 C C C 2 H 5 Cl H KOH, H 2 O H 3 C C C 2 H 5 OH H H 3 C C C 2 H 5 H OH and H 3 C C C CH 3 H CH 3 H Cl H 3 C C C CH 3 H CH 3 H + Hydride Shift H 3 C C C CH 3 CH 3 H H + H 3 C C C CH 3 CH 3 H H OH- OH H 3 C C C CH 3 H C Cl H H H H Hoffman Product Saytzeff Product The Saytzeff is the inner product and the Hoffman is the outer product. Of the two, Saytzeff is the most stable because it produces a double bond with more carbons around it which can feed electrons to the double bond (by induction). It is possible to select between Saytzeff and Hoffman products by selecting the proper sized base. Hoffman products are always made when very large bases like t-ButO- are used. Large bases are simply to big to grab inner hydrogens and do an elimination. Big bases are force to attack on the outside of the molecule where there is less hinderance, thus the Hoffman product is formed. To get the Saytzeff product a smaller base must be used, but not one that is so small that you risk the possibility of SN2 product formation. For this purpose EtO- is often the base of choice. The EtO- ion sits between the region of large and small bases and will do either E2 or SN2 reactions depending on the substrate used. As long as the substrate is hindered enough, E2 reactions will predominate, but there is always the risk of SN2 by-...
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This note was uploaded on 02/07/2012 for the course CHEM 1 taught by Professor Gelfand during the Spring '11 term at Solano Community College.

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2 - Exam 2 Practice Problems and Keys - Exam 2 SN1, E1,...

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