23 2 Stereospecific reactions
Stereospecific Reactions
Definition: A reaction is stereospecific if it leads to products with well-defined and easily predictable stereochemistries.
Example: Reaction of Trans Alkene with Bromine
Outcome: Produces an anti-addition product.
Stereocenters:
Side 1:
1, 2, 3 configuration: Clockwise rotation → R stereocenter.
Side 2:
1, 2, 3 configuration: Clockwise rotation with hydrogen pointed out → S stereocenter.
Conclusion: As the groups are the same and there is an inversion of stereocenter, the product is a meso compound. Therefore, this reaction yields a single meso product.
Comparison: Reaction of Cis Alkene
Example: Cis Butane Reaction
Results in two different products:
RR stereoisomer:
1, 2, 3 clockwise rotation → R stereocenter.
SS stereoisomer:
1, 2, 3 counterclockwise rotation → S stereocenter.
Conclusion: Produces a pair of enantiomers (RR and SS).
Mechanism of Reaction
Step 1: Electrophilic Attack
The alkene as an electrophile donates a pair of electrons to bromine, while bromine donates one of its lone pairs back, forming a bromonium ion with a formal charge of +1.
Step 2: Nucleophile Attack
Nucleophile attacks:
From one side: Kicks out bromine, resulting in an RR stereoisomer.
From the other side: Produces an SS stereoisomer due to different orientations.
Reaction with Trans Alkene
Start: Similar process of electron donation leading to a bromonium ion.
Attack Outcomes:
Attack from one side leads to:
Carbon 1: R
Carbon 2: S
Attack from the opposite side gives:
Carbon 1: R
Carbon 2: S
Symmetry Consideration
Due to symmetry, the two carbons can be seen as equivalent.
An internal mirror plane in the molecule confirms it as a meso compound.
Stereospecific Reactions
Definition
A reaction is considered stereospecific if it results in products with well-defined and easily predictable stereochemistries. This specificity is crucial in organic chemistry as it influences the properties and reactivity of the compounds formed.
Example: Reaction of Trans Alkene with Bromine
Outcome: The reaction of a trans alkene with bromine results in the formation of an anti-addition product.
Stereocenters:
Side 1:
1, 2, 3 configuration: Clockwise rotation leads to the designation of an R stereocenter.
Side 2:
1, 2, 3 configuration: Clockwise rotation with hydrogen directed outward indicates an S stereocenter.
Conclusion: Since the groups attached to the stereocenters are the same but there is an inversion of the stereocenter, the resultant product is a meso compound. Consequently, this reaction yields a single meso product, which is crucial for predicting the outcome of synthesizing complex molecules.
Comparison: Reaction of Cis Alkene
Example: Cis Butene Reaction
Outcome: This reaction results in two distinct products:
RR stereoisomer:
1, 2, 3 configuration indicates clockwise rotation, leading to an R stereocenter.
SS stereoisomer:
1, 2, 3 configuration shows counterclockwise rotation, resulting in an S stereocenter.
Conclusion: This reaction generates a pair of enantiomers (RR and SS), highlighting the importance of stereochemistry in determining the optical activity and reactivity of the products.
Mechanism of Reaction
Step 1: Electrophilic Attack
In this initial step, the alkene acts as an electrophile, donating a pair of electrons to the bromine molecule. Simultaneously, bromine donates one of its lone pairs back to the alkene, resulting in the formation of a bromonium ion with a formal charge of +1, establishing a critical intermediate in the reaction pathway.
Step 2: Nucleophile Attack
The nucleophile (bromide ion) then attacks the bromonium ion:
From one side: This attack will displace bromine, resulting in the formation of the RR stereoisomer.
From the opposite side: This orientation produces the SS stereoisomer, showcasing the influence of sterics and electronic factors on the final stereochemical outcome.
Reaction with Trans Alkene
The reaction mechanism is similar, initiating with the formation of a bromonium ion through electron donation.
Attack Outcomes:
An attack from one side results in:
Carbon 1: designated as R
Carbon 2: designated as S
An attack from the opposite side yields:
Carbon 1: designated as R
Carbon 2: designated as S
Symmetry Consideration
Due to the symmetrical nature of the trans molecules, the two carbons can be viewed as equivalent, allowing for analysis of stereoisomers.
An internal mirror plane present in the molecule further confirms that it is indeed a meso compound, which is significant as it indicates the compound is achiral despite containing stereocenters.
Understanding these principles of stereospecific reactions is essential for predicting the behavior of organic compounds during various chemical reactions, influencing both synthetic strategies and practical applications in pharmaceutical development and materials science.