Textbook Question
Name the following alkynes according to the IUPAC rules of nomenclature.
(b)
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5. Chirality
R and S Configuration
Problem 60a
Textbook Question
In Chapter 12, we introduce the SN2 reaction, a nucleophilic substitution reaction that proceeds with inversion. Confirm that inversion has occurred in each of the following examples by determining the absolute configuration of the chiral center in the reactants and products.
(a) 
Verified step by step guidance1
Step 1: Identify the chiral center in the reactant molecule. The carbon atom bonded to the bromine (Br), hydrogen (H), and two other groups is the chiral center.
Step 2: Assign priorities to the substituents around the chiral center based on the Cahn-Ingold-Prelog rules. Bromine (Br) has the highest priority due to its atomic number, followed by the isopropyl group, the methyl group, and finally hydrogen (H) with the lowest priority.
Step 3: Determine the absolute configuration of the chiral center in the reactant. Arrange the molecule so that the lowest priority group (H) is pointing away from you, then trace the path from the highest priority group to the lowest. If the path is clockwise, the configuration is R; if counterclockwise, it is S.
Step 4: Repeat the process for the product molecule. Assign priorities to the substituents around the chiral center in the product, where the cyanide group (N≡C) replaces bromine. Cyanide has a higher priority than bromine due to its atomic number.
Step 5: Compare the absolute configurations of the reactant and product. If the configuration has changed (e.g., from R to S or vice versa), inversion has occurred, confirming the SN2 reaction mechanism.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
S_N 2 Reaction
The S_N 2 reaction is a type of nucleophilic substitution where a nucleophile attacks an electrophile, resulting in the simultaneous displacement of a leaving group. This reaction is characterized by a single concerted step, leading to the inversion of configuration at the chiral center. The rate of the reaction depends on the concentration of both the nucleophile and the substrate, making it bimolecular.
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Inversion of Configuration
Inversion of configuration refers to the change in the spatial arrangement of atoms around a chiral center during a reaction. In S_N 2 reactions, the nucleophile approaches the electrophile from the opposite side of the leaving group, resulting in a switch of the configuration from R to S or vice versa. This is crucial for determining the stereochemical outcome of the reaction.
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Absolute Configuration
Absolute configuration describes the specific three-dimensional arrangement of atoms around a chiral center, designated as either R (rectus) or S (sinister) based on the Cahn-Ingold-Prelog priority rules. To confirm inversion in S_N 2 reactions, one must compare the absolute configurations of the reactants and products. This comparison helps in understanding how the stereochemistry is altered during the reaction.
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