Notes0616 - Experiment 3 Qualitative Organic Analysis Alkyl...

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Experiment 3: Qualitative Organic Analysis Alkyl halides Alkyl halides can undergo nucleophilic substitutions through two different mechanisms, S N 1 and S N 2. S N 1 test – AgNO 3 / Ethanol In S N 1 reactions the rate-determining step is the formation of the carbocation. Therefore, the more stable the carbocation that is formed the faster the reaction occurs. Table 1: Carbocation Stability 3 o benzylic 3 o allylic > 2 o benzylic 2 o allylic ~ C R R R > CH 2 ~ CH 2 CH CH 2 Benzylic Allylic H 3 C CH H 3 C Tertiary Secondary > CH 3 CH 2 Primary > CH 3 CH 2 CH Methyl Vinyl ~ ~ Why are tertiary carbocations more stable then secondary, and secondary more stable than primary? Tertiary carbocations are stabilized by the presence of nearby alkyl groups. Alkyl groups are electron-donating and donate electrons to help stabilize the positive charge. Why are benzylic and allylic carbocations stable enough to undergo S N 1 reactions? Resonance stabilized carbocations - practice drawing the different resonance structures. The formation of Silver chloride, AgCl, as a white solid indicates a positive test. Mechanism: 13
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Cl Ag + AgCl OEt H EtO H EtO OEt EtOH EtOH EtOH EtOH Stereochemistry: If starting with a chiral (four different groups on one carbon) alkyl halide, the stereochemistry of the final product will be a racemic mixture. The reason for this is the fact that the carbocation intermediate is planar so nucleophilic attack can occur from either side. This reagent can only be used for alkyl halides that are soluble in alcohol. Primary halides, vinylic halides, and phenyl halides do not react. Secondary halides react slowly, sometimes requiring heating and 15 minutes, and tertiary halides react rapidly. S N 2 test – NaI/ Acetone 14
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An S N 2 reaction is a bimolecular nucleophilic substitution that occurs in one step. A positive test is the appearance of a precipitate or turbidity due to the formation of NaCl. A false positive test is possible if NaI is not soluble in the alkyl halide and the NaI precipitates out. This can be prevented by adding more acetone. A false negative test is possible if water is present because NaCl is soluble in water. Mechanism: Cl I - I Cl - (R)-1-chloro-1-phenylethane (S)-1-iodo-1-phenylethane Stereochemistry: This is a one-step, concerted-mechanism that results in the inversion of configuration because of backside nucleophilic attack. The inversion of configuration also causes a change in stereochemistry if the starting material and product are chiral.
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