Textbook Question
Which reaction in each of the following pairs takes place more rapidly? (EtOH is ethyl alcohol; Et2O is diethyl ether.)
d.
e.
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7. Substitution Reactions
Substitution Comparison
3:55 minutesProblem 39
Textbook Question
When (R)-(1-bromoethyl)benzene is treated with sodium cyanide, a single enantiomer is produced. However, upon treatment of the same molecule with water, a mixture of two enantiomers is obtained.
(a) Explain these results.
(b) Why is only partial racemization sometimes observed?
Verified step by step guidance1
Step 1: Analyze the reaction with sodium cyanide (NaCN) in diethyl ether (Et2O). The reaction involves a nucleophilic substitution (SN2) mechanism. In an SN2 reaction, the nucleophile (CN⁻) attacks the carbon bonded to the bromine atom from the opposite side, leading to inversion of configuration. This results in the formation of a single enantiomer of the product.
Step 2: Examine the reaction with water (H2O). Water acts as a weak nucleophile and the reaction proceeds via an SN1 mechanism. In an SN1 reaction, the bromine atom leaves first, forming a carbocation intermediate. This intermediate is planar and can be attacked by water from either side, leading to the formation of both enantiomers.
Step 3: Explain the observation of partial racemization. Partial racemization occurs because the carbocation intermediate formed during the SN1 reaction is not perfectly symmetrical. Steric and electronic effects from the phenyl group and the ethyl group can slightly favor attack from one side over the other, resulting in an unequal mixture of enantiomers (70:30 ratio).
Step 4: Highlight the stereochemical outcome. In the SN2 reaction, the stereochemistry is inverted due to the backside attack of the nucleophile. In the SN1 reaction, the planar carbocation intermediate allows for attack from both sides, leading to a racemic mixture or partial racemization.
Step 5: Summarize the key difference between the two reactions. The SN2 mechanism produces a single enantiomer due to stereochemical inversion, while the SN1 mechanism produces a mixture of enantiomers due to the planar nature of the carbocation intermediate.
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3mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Nucleophilic Substitution
Nucleophilic substitution is a fundamental reaction in organic chemistry where a nucleophile replaces a leaving group in a molecule. In the case of (R)-(1-bromoethyl)benzene reacting with sodium cyanide, the cyanide ion acts as a nucleophile, leading to the formation of a single enantiomer due to the stereochemistry of the reaction. This process typically follows either an SN1 or SN2 mechanism, influencing the stereochemical outcome.
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Enantiomers and Racemization
Enantiomers are pairs of molecules that are non-superimposable mirror images of each other, often differing in their optical activity. When (R)-(1-bromoethyl)benzene is treated with water, the reaction can lead to racemization, where both enantiomers are formed due to the reversible nature of the reaction. This occurs because water can act as a nucleophile, allowing for the formation of both configurations at the chiral center.
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Partial Racemization
Partial racemization refers to the situation where a reaction produces a mixture of enantiomers, but one enantiomer is favored over the other. This can occur due to the reaction conditions, such as temperature or solvent effects, which can stabilize one transition state over another. In the case of (R)-(1-bromoethyl)benzene with water, the presence of competing pathways and the stability of intermediates can lead to a predominance of one enantiomer while still allowing some formation of the other.
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