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Questions and Answers
What type of intermediate is formed during the reaction with HBr?
Reactions involving HBr exhibit anti-Markovnikov selectivity.
False
What is added to the substrate during the reaction with HBr?
H+ and Br-
In reactions with HBr, rearrangements are __________.
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Match the following terms with their descriptions:
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What type of regioselectivity is observed with H3O+?
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The reaction with H3O+ involves a carbocation intermediate.
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What species are added to the substrate when using H3O+?
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In reactions involving H3O+, rearrangements are __________.
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What is the stereoselectivity of reactions involving Br2/CCl4?
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Markovnikov's rule states that hydrogen will add to the least substituted carbon in electrophilic addition reactions.
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What type of intermediate is formed in reactions with mercurinium ions?
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In the reaction with HBr under peroxide conditions, the regioselectivity is __________.
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Match the following reagents with their respective stereoselectivity:
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What are the products formed when alkenes undergo oxidative cleavage under reducing conditions?
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The product of ozonolysis followed by hydrolysis generally produces carboxylic acids.
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How many products are typically formed when a reaction results in 2 chiral centers?
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In reactions involving acids and alcohols, the addition is __________.
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Which of the following substrates would lead to a radical intermediate?
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Study Notes
Reagents and Their Effects
- Reagents: Hydrohalic acids such as HBr, HCl, and HI are commonly used in reactions involving alkene additions.
- What's Added: During the reaction, protons (H+) and halide ions (Br-) are introduced to the substrate.
Regioselectivity
- Markovnikov's Rule: The addition of HBr (or similar acids) follows Markovnikov's principle, which states that the more substituted carbon of an alkene receives the hydrogen, leading to the formation of the more stable carbocation.
Stereo Selectivity
- Stereo Selectivity: The addition process does not exhibit a specific stereo preference, allowing for potentially multiple stereoisomers.
Intermediate Formation
- Carbocation Intermediate: The formation of a carbocation is a key intermediate step during the addition reaction, characterized by an electron-deficient carbon.
Rearrangements
- Possible Rearrangements: Carbocation intermediates can undergo rearrangements, such as hydride shifts or alkyl shifts, to form more stable carbocations if applicable.
Reagents and Reactions
- HBr, HCl, HI: These are strong acids that provide protons (H+) and halide ions (Br-) for reactions with alkenes.
- H3O+H_3O^+H3O+ (Hydronium ion): Serves as a source of protons and hydroxide ions (OH−OH^-OH−) in acid-catalyzed reactions.
Regioselectivity
- Markovnikov's Rule: During the addition of HBr, HCl, or HI to alkenes, the more substituted carbon receives the proton (H+), leading to greater stability.
Stereo Selectivity
- No Preference: The reactions do not show stereo selectivity, meaning they do not favor cis or trans configurations in the products.
Intermediates
- Carbocation Formation: The reaction involves the formation of a carbocation intermediate, which is critical for the subsequent substitution reactions.
Rearrangements
- Possible Rearrangements: Rearrangements of the carbocation may occur, resulting in different product distributions depending on the stability of the carbocation formed.
Reactions with Hydrogen Halides and Hydronium Ion
- Reagents: HBr, HCl, and HI are common hydrogen halides used in reactions with alkenes.
- What's Added: These reagents provide H+ ions and the corresponding halide ion (Br-, Cl-, or I-).
- Regioselectivity: The addition follows Markovnikov's rule, meaning the H+ adds to the carbon atom with the most hydrogen atoms already attached, while the halide adds to the other carbon.
- Stereo Selectivity: These reactions generally do not exhibit stereo selectivity, implying that both possible orientations of the product can form.
- Intermediate: The reaction proceeds through the formation of a carbocation intermediate, a positively charged species that can undergo further reactions.
- Rearrangements: Carbocation intermediates can rearrange, leading to more stable structures, affecting the final product distribution.
Reactions with Hydronium Ion
- Reagent: Hydronium ion, noted as ( H_3O^+ ), acts similarly to hydrogen halides in reactions with alkenes.
- What's Added: In the presence of ( H_3O^+ ), the reaction generates H+ and OH- ions for addition to the alkene.
- Regioselectivity: Follows Markovnikov's rule, ensuring the hydroxyl group (OH) is attached to the more substituted carbon atom.
- Intermediate: The process also involves the formation of a carbocation as an unstable intermediate that can undergo further transformations.
- Rearrangements: As with hydrogen halides, there is a potential for rearrangements of the carbocation intermediate.
Electrophilic Addition Reactions to Alkenes
- Various reagents can initiate electrophilic addition to alkenes, leading to different products.
- HBr, HCl, HI: Introduce H+ and Br; follow Markovnikov's rule; form a carbocation; possible rearrangements occur.
- H3O+: Adds H+ and OH-; follows Markovnikov's rule; carbocation involved; possible rearrangements.
- H+, ROH: Adds H+ and OR; Markovnikov addition; carbocation formation; rearrangements are possible.
- Halogen Reagents (Br2, Cl2): Produce anti-addition products; bromonium or chloronium ions formed; no rearrangements.
- Br2/CCl4 or Cl2: Add Br+ and Br; show anti stereochemistry; no rearrangements.
- Br2/H2O: Adds Br+ and OH; follows Markovnikov's rule; anti stereochemistry; no rearrangements.
- (1) Hg(OAc)2, H2O (2) NaBH4: Adds H+ and OH; follows Markovnikov's rule, anti stereochemistry; mercurinium ion is the intermediate; no rearrangements.
- (1) BH3 THF (2) H2O2, OH, H2O: Provides anti-Markovnikov addition; adds H+ and OH; features syn stereochemistry; no rearrangements.
- H2 with a catalyst (Pd/C, Pt/C, Ni): Adds H and H in syn orientation; no rearrangements.
- HBr/ROOR (peroxide): Leads to anti-Markovnikov addition of H and Br with syn stereochemistry; involves radical intermediate; no rearrangements.
- RCO3H (MCPBA): Introduces O in syn addition; no rearrangements.
- KMnO4 (cold, dilute)/ OH or OsO4 (followed by H2O2): Results in syn addition of OH.
- Formation of Chiral Centers: Number of products correlates with chiral centers formed; from 0 (1 product) to 2 (4 products).
Oxidative Cleavage of Alkenes
- Reducing Conditions: Utilize O3 at -78°C followed by (CH3)2S or Zn/H2O for cleavage.
- Oxidizing Conditions: Use O3 followed by H2O2 or KMnO4 under hot, concentrated conditions for cleavage.
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Description
Test your knowledge on the regioselectivity and stereo selectivity of organic reactions involving HBr, HCl, and HI. This quiz focuses on identifying the added reagents, intermediates, and the possibility of rearrangements in these reactions. Perfect for chemistry students looking to reinforce their understanding of reaction mechanisms.
