Ch_322a_10.012 - (A(B The alkyl radical is locally trigonal...

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Generation of a Second Stereocenter When one stereocenter is already present, the generation of a second stereocenter may occur with unequal probabilities for the two possible configurations (R or S). As is shown below, the reaction pathways leading to the (R) and (S) configurations are diastereomerically related , and, therefore, may have different values for the E a of the product forming step. Example Photochlorination of (S)-2-chloropentane leads to a complex product mixture. Among the products are the 2,3-dichloropentanes. CH 3 CH Cl CH 2 CH 2 CH 3 Cl 2 h ! CH 3 CH Cl CH Cl CH 2 CH 3 * * * (2S) (2S3R) and (2S3S) two diastereomeric products produced in unequal amounts
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A Stereochemical Analysis C C CH 3 H Cl H H C 2 H 5 * (S)-2-chloropentane Abstraction of either H leads to the same alkyl radical. Cl . (-HCl) C C CH 3 H Cl H C 2 H 5 * . Cl 2 Cl 2 C C CH 3 H Cl H C 2 H 5 * Cl C C CH 3 H Cl H C 2 H 5 * Cl (2S,3R)-2,3-dichloropentane * * (2S,3S)-2,3-dichloropentane
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Unformatted text preview: (A) (B) The alkyl radical is locally trigonal planar, but the intermediate is overall chiral because of the stereocenter at C-2. Reaction by paths (A) and (B) are diastereomerically related and may have different values for E a , producing the products in unequal amounts. Racemic Forms If the photochlorination reaction is carried out on a racemic form of 2-chloropentane, racemic forms of each diastereomer are produced. A parallel reaction pathway involving the (R) enantiomer produces two diastereomeric products in relative amounts that mirror those produced from the (S) enantiomer. CH 3 CH Cl CH 2 CH 2 CH 3 Cl 2 h ! CH 3 CH Cl CH Cl CH 2 CH 3 * * * (2S) (2S,3R) and (2S,3S) (2R) (2R,3S) and (2R.3R) enantiomers enantiomers enantiomers...
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