Orgamics
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Questions and Answers

What is the result of the reaction between R-OH and PBr3 in the presence of pyridine?

  • R-OH
  • R-Br (correct)
  • R-Br2
  • R-O-PBr2

    • The reaction of PBr3 with 3° alcohols proceeds through an SN2 mechanism.

    False

    What is the expected outcome when R-OH is treated with PBr3 in the presence of pyridine?

  • Formation of an intermediate
  • No reaction (correct)
  • Dehydration of the alcohol
  • Formation of R-Br

    • PBr3 can react with primary alcohols to produce alkyl bromides.

    <p>True</p> Signup and view all the answers

    What type of mechanism does the reaction between PBr3 and primary alcohols involve?

    <p>SN2</p> Signup and view all the answers

    PBr3 can react efficiently with 3° alcohols.

    <p>False</p> Signup and view all the answers

    Which of the following statements about the conversion of alcohols to alkyl chlorides is true?

    <p>1° and 2° alcohols can be converted to alkyl chlorides using SOCI2/pyridine.</p> Signup and view all the answers

    The reaction between SOCI2 and phenols proceeds through an SN2 mechanism.

    <p>False</p> Signup and view all the answers

    What types of alcohols can R-OH be in the reaction with SOCI2 and pyridine to produce R-Cl?

    <p>1° or 2° alcohols</p> Signup and view all the answers

    In the conversion of alcohols to alkyl chlorides, SOCI2 and pyridine do not work on _____ alcohols.

    <p>3°</p> Signup and view all the answers

    What is the purpose of converting an OH group into a sulfonate ester?

    <p>To enhance its leaving group ability</p> Signup and view all the answers

    The conversion of alcohols to alkyl sulfonates does change the stereochemistry at the C-OH bond.

    <p>False</p> Signup and view all the answers

    Match the following reactions with their products:

    <p>R-OH + TsCl + Pyridine = R-OTs R-OH + MsCl + Pyridine = R-OMs</p> Signup and view all the answers

    What is the first step in the acid-catalyzed dehydration of alcohol?

    <p>Protonation of the hydroxyl group</p> Signup and view all the answers

    The dehydration of alcohols can occur without an acid catalyst.

    <p>False</p> Signup and view all the answers

    What product is formed after the deprotonation step in the acid-catalyzed dehydration of alcohol?

    <p>Alkene</p> Signup and view all the answers

    In the dehydration of alcohol, the reaction proceeds through the loss of _____ to form a carbocation.

    <p>water</p> Signup and view all the answers

    Match the following steps with their correct descriptions in the dehydration of alcohol:

    <p>Step 1 = Protonation of the hydroxyl group Step 2 = Loss of water and carbocation formation Step 3 = Deprotonation and alkene formation Step 4 = Formation of a leaving group</p> Signup and view all the answers

    What type of mechanism do primary alcohols use in dehydration reactions?

    <p>E2</p> Signup and view all the answers

    Secondary alcohols undergo dehydration through an E2 mechanism.

    <p>False</p> Signup and view all the answers

    Match the following types of alcohols with their corresponding dehydration mechanisms:

    <p>Primary = E2 mechanism Secondary = E1 mechanism Tertiary = E1 mechanism</p> Signup and view all the answers

    Which of the following is a strong oxidizing agent used to oxidize a primary alcohol to a carboxylic acid?

    <p>Na2Cr2O7/H2SO4</p> Signup and view all the answers

    What is the result of chromium oxidation on a 3° alcohol?

    <p>No reaction occurs.</p> Signup and view all the answers

    Chromium oxidation can work on 3° alcohols if they are treated with a strong oxidizing agent.

    <p>False</p> Signup and view all the answers

    What type of alcohols do not undergo oxidation with chromium reagents?

    <p>Tertiary alcohols</p> Signup and view all the answers

    For a reaction involving 3° alcohols with Na2Cr2O7/H2SO4, the result is _____ .

    <p>no reaction</p> Signup and view all the answers

    What is the oxidation product of a primary alcohol when treated with Na2Cr2O7 or H2CrO4?

    <p>Carboxylic acid</p> Signup and view all the answers

    PCC or periodinane can oxidize a primary alcohol to a carboxylic acid.

    <p>False</p> Signup and view all the answers

    Name one reagent that can oxidize a primary alcohol to an aldehyde.

    <p>PCC or periodinane</p> Signup and view all the answers

    Match the reagent with the corresponding product of primary alcohol oxidation:

    <p>PCC = Aldehyde Na2Cr2O7/H2SO4 = Carboxylic acid</p> Signup and view all the answers

    What is the product when a 3° alcohol is treated with PCC or periodinane?

    <p>No reaction</p> Signup and view all the answers

    What type of alcohol is characterized by having a tertiary carbon with a hydroxyl group?

    <p>3° alcohol</p> Signup and view all the answers

    Both 3° alcohol and phenol yield _____ when treated with PCC or periodinane.

    <p>no rxn</p> Signup and view all the answers

    What is the product of oxidizing a primary alcohol?

    <p>Aldehyde</p> Signup and view all the answers

    Tertiary alcohols can be oxidized to ketones.

    <p>False</p> Signup and view all the answers

    Name one reagent used to oxidize primary alcohols to aldehydes.

    <p>DMSO</p> Signup and view all the answers

    Secondary alcohols are oxidized to __________.

    <p>ketones</p> Signup and view all the answers

    What is formed when an alcohol (R-OH) reacts with NaH, Na, or K?

    <p>Alkoxide</p> Signup and view all the answers

    The Williamson ether synthesis proceeds through an SN1 mechanism.

    <p>False</p> Signup and view all the answers

    In the Williamson ether synthesis, the alkoxide reacts with an _______ to form an ether.

    <p>alkyl halide</p> Signup and view all the answers

    Match the following components of the Williamson ether synthesis with their roles:

    <p>Alcohol = Reactant that produces an alkoxide Alkoxide = Nucleophile that reacts with alkyl halide Alkyl halide = Electrophile in the reaction Ether = Final product of the synthesis</p> Signup and view all the answers

    In Williamson ether synthesis, which of the following is NOT one of the three required reactants?

    <p>Aldehyde</p> Signup and view all the answers

    The reaction mechanism for Williamson ether synthesis is SN1.

    <p>False</p> Signup and view all the answers

    What type of base can be used in the Williamson ether synthesis?

    <p>NaH, Na, K, or NaOH</p> Signup and view all the answers

    In the Williamson ether synthesis, the alkoxide reacts with an _______ to form an ether.

    <p>alkyl halide</p> Signup and view all the answers

    Match the components of Williamson ether synthesis with their roles:

    <p>Alkyl halide = Reactant that forms ether Alcohol = Source of alkoxide Base = Deprotonates alcohol Ether = Product of the reaction</p> Signup and view all the answers

    What is the first step in the reaction mechanism when HI is added to the ether?

    <p>Protonation of the ether oxygen</p> Signup and view all the answers

    The reaction proceeds through an SN2 mechanism.

    <p>False</p> Signup and view all the answers

    What are the products formed after the reaction with excess HI?

    <p>H3C-I and HO-CH2CH3</p> Signup and view all the answers

    What is the primary reagent used in the reaction with cyclohexene to form an epoxide?

    <p>mCPBA</p> Signup and view all the answers

    Alkenes can react with peroxy acids to form epoxides.

    <p>True</p> Signup and view all the answers

    Name the product formed when cyclohexene reacts with mCPBA.

    <p>cyclohexene oxide</p> Signup and view all the answers

    An alkene reacts with __________ to form an epoxide.

    <p>mCPBA</p> Signup and view all the answers

    Under basic conditions, which species acts as the nucleophile?

    <p>Nuc</p> Signup and view all the answers

    The presence of protons (H+) in acidic conditions can facilitate the nucleophilic attack on an alcohol.

    <p>True</p> Signup and view all the answers

    What happens to the OH group under acidic conditions during nucleophilic substitution?

    <p>It gets protonated.</p> Signup and view all the answers

    Study Notes

    Alcohols and Phenols to Alkyl Halides

    • Alcohols (R-OH) can be converted to alkyl halides using phosphorus tribromide (PBr3) in the presence of pyridine, resulting in the formation of R-Br (alkyl bromide).
    • The alkyl group (R) can be a methyl group (CH3) or primary (1°) and secondary (2°) alcohols.
    • Tertiary (3°) alcohols and phenols do not react with PBr3/pyridine for this conversion.
    • The reaction mechanism for this conversion is SN2, which is not favorable for 3° alcohols due to steric hindrance.

    Overview of Chemical Reaction

    • The reaction depicted involves the compound PBr3 and a hydroxy-alkene (indicated by OH).
    • The reaction between PBr3 and the hydroxy-alkene results in "no reaction," suggesting stability of the starting material under the given conditions.

    Structural Features

    • The structure contains multiple double bonds (C=C) indicative of alkenes, which may react under specific conditions.
    • The presence of cyclic structures is implied, which often affects reactivity and sterics in chemical reactions.

    Role of Pyridine

    • Pyridine is included in the reaction framework, often acting as a base or solvent in organic reactions.
    • Its role in stabilizing intermediates or facilitating reactions is crucial to understanding reaction pathways.

    Conclusion

    • The absence of reaction underscores the need for additional reagents or conditions to activate reagents in similar organic syntheses.
    • Understanding the molecular framework and functional groups is essential for predicting chemical behavior in organic chemistry.

    Conversion of Alcohols and Phenols to Alkyl Halides

    • Alcohols (R-OH) can be converted to alkyl bromides (R-Br) using phosphorus tribromide (PBr3) in the presence of pyridine.
    • The conversion is effective for methyl (CH3), primary (1°), and secondary (2°) alcohols but not for tertiary (3°) alcohols or phenols.
    • PBr3 and pyridine utilize an SN2 mechanism, which is unsuitable for 3° alcohols due to steric hindrance.
    • The reaction results in inverted stereochemistry at the carbon atom bearing the hydroxyl group.

    Mechanism Considerations

    • The SN2 reaction mechanism involves a single transition state where the nucleophile attacks the carbon as the leaving group departs.
    • Transition state formation is difficult for 3° alcohols, leading to no reaction when PBr3/pyridine is applied.
    • The stereochemical outcome (inversion) is crucial in synthetic applications, impacting product configurations.

    Reaction Conditions

    • PBr3 must be used carefully, requiring reactions to be conducted under anhydrous conditions to prevent hydrolysis.
    • Pyridine acts as a base, facilitating the departure of bromide ions and stabilizing the intermediate formed during the reaction.

    Alcohols to Alkyl Halides

    • Alcohols can be transformed into alkyl chlorides and bromides using efficient methods.
    • One effective method involves the reaction of alcohols with thionyl chloride (SOCl2) in the presence of pyridine.

    Reaction Details

    • The general reaction formula: R-OH + SOCl2 ---(pyridine) ---> R-Cl, where R represents the alkyl group.
    • Suitable alkyl groups (R) for this reaction include methyl (CH3), primary (1°), and secondary (2°) alcohols.

    Limitations

    • Tertiary (3°) alcohols and phenols cannot be converted using the SOCl2/pyridine method due to the nature of the reaction mechanism.
    • The mechanism proceeds through an S_N2 pathway, which is not favorable for sterically hindered tertiary alcohols.

    Comparison with PBr3

    • Similar limitations apply when using phosphorus tribromide (PBr3) for bromide formation; it also fails with tertiary alcohols and phenols.

    OH Groups

    • OH groups are classified as poor leaving groups in substitution reactions.
    • To effectively replace an OH group, it must be converted into a sulfonate ester.

    Sulfonate Esters

    • Common sulfonate esters include tosylate (OTs) and mesylate (OMs).
    • Conversion reactions utilize chlorides and pyridine as a base to facilitate the transformation.

    Reactions

    • Reaction with tosyl chloride:
      • R-OH + TsCl + Pyridine yields R-OTs
    • Reaction with mesyl chloride:
      • R-OH + MsCl + Pyridine yields R-OMs

    Stereochemistry

    • The substitution process does not alter stereochemistry around the C-OH bond, preserving the original spatial configuration of the molecule.

    Acid-Catalyzed Dehydration of Alcohol

    • The dehydration process converts an alcohol into an alkene using acid as a catalyst.
    • Starting alcohol features three R groups (R1, R2, R3), which can be alkyl groups or hydrogen.

    Reaction Steps

    • Protonation of the Hydroxyl Group:

      • Strong acids like H2SO4 or H3PO4 protonate the hydroxyl (-OH) group of the alcohol.
      • This generates a better leaving group: water (H2O).

    • Loss of Water:

      • The generated water molecule is expelled, resulting in the formation of a carbocation.
      • The structure transitions from R3R2R1OH to R3R2R1+.

    • Deprotonation to Form Alkene:

      • A base removes a proton from a neighboring carbon atom adjacent to the carbocation.
      • This step leads to the formation of the double bond, resulting in the final alkene structure.

    Key Points

    • The protonation step enhances the leaving ability of the -OH group.
    • Formation of a carbocation is a critical intermediate in this reaction.
    • The newly formed alkene is the final product after deprotonation.
    • Reactions of this nature typically require heat to proceed efficiently.

    Alcohol Dehydrations

    • Primary alcohols undergo dehydration via an E2 mechanism, resulting in the elimination of a water molecule and the formation of a double bond.
    • Secondary and tertiary alcohols favor an E1 mechanism for dehydration, characterized by the formation of a carbocation intermediate followed by elimination.
    • In elimination reactions, the Zaitsev rule is observed, favoring the formation of the more substituted alkene product, leading to greater stability.
    • The E1 mechanism allows for carbocation rearrangements, which can result in different products due to shifts in the molecular structure.

    Oxidation Process

    • Primary alcohols can be oxidized to carboxylic acids via strong oxidizing agents.
    • Common oxidizing agents include:
      • Sodium dichromate in sulfuric acid (Na2Cr2O7/H2SO4)
      • Chromium trioxide (H2CrO4)
      • Chromic acid (CrO3/H2SO4)

    Reactants and Products

    • Starting reactant:
      • Primary alcohol represented as [R]-CH2OH, where R is an alkyl group.
    • Resulting product:
      • Carboxylic acid depicted as [R]-CO2H, maintaining the same alkyl group (R) in the transition.

    Structural Transformation

    • The oxidation converts the hydroxyl group (-OH) of the primary alcohol into a carboxyl group (-CO2H).
    • The general reaction can be illustrated as:
      • [R]-CH2OH → [R]-CO2H

    Chromium Oxidation of Alcohols

    • Chromium oxidation is ineffective on tertiary (3°) alcohols that lack a hydrogen atom bonded to the alcoholic carbon.
    • Tertiary alcohol structure features a carbon atom connected to three other carbon groups and an -OH (hydroxyl) group: 
         HO
   ⎜
R - C - R
   ⎜
    R
    • Key oxidizing agents include:
      • Sodium dichromate (Na2Cr2O7) in combination with sulfuric acid (H2SO4)
      • Hexavalent chromium species like chromic acid (H2CrO4)
      • Chromium trioxide (CrO3) with sulfuric acid (H2SO4)
    • The reaction with 3° alcohols and these oxidizing agents results in no reaction, highlighting the stability of 3° alcohols against oxidation under these conditions.

    Conversion of Alcohols and Phenols to Alkyl Halides

    • Reaction of an alcohol (R-OH) with phosphorus tribromide (PBr3) in pyridine yields bromides (R-Br) for primary and secondary alcohols.
    • Tertiary alcohols and phenols do not react with PBr3/pyridine due to the SN2 mechanism, which is not compatible with sterically hindered substrates.
    • Alternative method: Alcohols can be converted to alkyl chlorides using thionyl chloride (SOCl2) in pyridine, effective for primary and secondary alcohols.
    • The SN2 mechanism also applies to the conversion using SOCl2/pyridine, rendering it ineffective for tertiary alcohols.

    OH Groups as Poor Leaving Groups

    • Hydroxyl (OH) groups require conversion to better leaving groups, such as sulfonate esters (tosylates or mesylates), for effective substitution.
    • Tosylation process: Alcohol reacts with tosyl chloride (TsCl) in pyridine, forming a tosylate (R-OTs).
    • Mesylation process: Alcohol reacts with mesyl chloride (MsCl) in pyridine, producing a mesylate (R-OMs).
    • Stereochemistry remains unchanged at the carbon undergoing the conversion.

    Acid-Catalyzed Dehydration of Alcohols

    • Dehydration transforms alcohols into alkenes, facilitated by strong acids like H2SO4 or H3PO4.
    • The process initiates with the protonation of the hydroxyl group, forming a more favorable leaving group, water.
    • Loss of water leads to the formation of a carbocation, which is a key intermediate.
    • Deprotonation of an adjacent carbon to the carbocation finalizes the formation of the alkene.

    Mechanism of Dehydration

    • Primary alcohols undergo dehydration via an E2 mechanism.
    • Secondary and tertiary alcohols typically follow an E1 mechanism.
    • Elimination reactions prefer the formation of more stable, substituted alkenes (Zaitsev's rule).
    • E1 intermediates may rearrange to yield more stable carbocations.

    Oxidation of Primary Alcohols

    • Strong oxidizing agents allow the conversion of primary alcohols into carboxylic acids.
    • Common oxidizing agents include sodium dichromate (Na2Cr2O7) in sulfuric acid (H2SO4), chromic acid (H2CrO4), and chromium trioxide (CrO3) in sulfuric acid.
    • Primary alcohol [R]-CH2OH can be oxidized to carboxylic acid [R]-CO2H.

    Limitations in Oxidation

    • Primary alcohols can be oxidized to aldehydes using pyridinium chlorochromate (PCC) or periodinane, but oxidation does not occur for tertiary alcohols due to lack of hydrogen on the alcoholic carbon.

    Chemical Reaction Schemes

    • 3° Alcohol Structure

      • Contains a tertiary carbon with three substituents (labeled as "R")
      • Has a hydroxyl group (OH) attached to the tertiary carbon
      • Indicated as "3° alcohol" in accompanying text

    • Reagents for 3° Alcohol

      • Reacted with PCC (pyridinium chlorochromate) or periodinane
      • Both reagents produce "no rxn", showing that no chemical transformation occurs

    Phenol Structure

    • Phenol Chemical Structure

      • Features a benzene ring with a hydroxyl group (OH) directly attached
      • Labeled as "phenol" in the accompanying text

    • Reagents for Phenol

      • Similar to tertiary alcohol, reacted with PCC or periodinane
      • Results in "no rxn", indicating again that no change happens

    Key Takeaways

    • Both 3° alcohols and phenols do not undergo reactions when treated with either PCC or periodinane
    • No chemical change suggests stability of these compounds under specified conditions

    Oxidation Reactions Overview

    • Oxidation reactions involve the conversion of alcohols into carbonyl compounds through the loss of hydrogen and gain of oxygen.

    Primary Alcohols to Aldehydes

    • Primary alcohols can be oxidized to aldehydes.
    • Reaction formula: R-CH2OH + [O] → R-CHO + H2O
    • Common reagents for this oxidation include DMSO, (COCl)2, and NEt3.

    Secondary Alcohols to Ketones

    • Secondary alcohols can be oxidized to ketones.
    • Reaction formula: R2CHOH + [O] → R2CO + H2O
    • The same reagents used for oxidizing primary alcohols (DMSO, (COCl)2, NEt3) are effective for secondary alcohols.

    Tertiary Alcohols

    • Tertiary alcohols cannot be oxidized to carbonyl compounds.
    • Lack a hydrogen atom on the carbon bonded to the hydroxyl group (–OH), making oxidation impossible.

    Williamson Ether Synthesis Overview

    • Ethers can be synthesized through the Williamson ether synthesis method.
    • Requires an alcohol (R-OH) as the starting material which reacts with sodium hydride (NaH), sodium (Na), or potassium (K).

    Formation of Alkoxide

    • The reaction of alcohol with metallic sodium or potassium leads to the formation of an alkoxide (R-O-).
    • Alkoxides are strong nucleophiles, making them suitable for further reactions.

    Reaction with Alkyl Halide

    • The alkoxide subsequently reacts with an alkyl halide (R-Br) to produce an ether (R-O-R).
    • This step is key and relies on the alkoxide attacking the carbon linked to the halogen.

    Mechanism Details

    • The synthesis proceeds via an SN2 mechanism, characterized by a backside attack on the carbon atom.
    • The result of this mechanism is the inversion of stereochemistry at the carbon center where substitution occurs, which is important for stereochemical outcomes in synthesis.

    Williamson Ether Synthesis Overview

    • Involves three reactants: alkyl halide, alcohol, and base.
    • Alkyl halides are reactive substrates that participate in the synthesis process.
    • The alcohol acts as a nucleophile, contributing the alkoxide ion after reaction with the base.
    • Common bases used include sodium hydride (NaH), sodium (Na), potassium (K), and sometimes sodium hydroxide (NaOH).

    Key Mechanism Features

    • The mechanism of Williamson Ether synthesis follows an SN2 pathway, which involves a single concerted step leading to the formation of the ether.
    • Larger alcohols than ethanol-ethoxide can still undergo the SN2 reaction; preoccupations with ethanol-ethoxide rule are not applicable.
    • SN2 mechanism is characterized by backside attack, requiring good leaving groups (like halides) for effective reaction.

    Important Considerations

    • The choice of base is critical, as it should effectively deprotonate the alcohol to form the alkoxide.
    • Steric hindrance in the alkyl halide can influence reactivity; less steric bulk favors faster SN2 reactions.
    • The overall reaction yields an ether, distinguished by the alkyl group from the halide and the alkoxy group from the alcohol.

    Chemical Reaction Overview

    • Reactants include H3C-O-CH2CH3 (an ether compound).
    • Reagents utilized in this reaction are HI (hydroiodic acid).
    • Products formed are H3C-I (alkyl iodide) and HO-CH2CH3 (an alcohol).

    Reaction Conditions

    • The reaction is conducted in the presence of excess HI to drive the reaction forward.

    Reaction Scheme

    • The reaction transforms H3C-O-CH2CH3 when treated with HI into:
      • H3C-I (alkyl iodide)
      • HO-CH2CH3 (alcohol)
    • This reaction scheme emphasizes the conversion processes and the participation of iodide.

    Mechanism of the Reaction

    • The reaction follows an SN1 mechanism, characterized by two key steps:
      • First Step: Protonation of the ether oxygen occurs, facilitating the departure of the ether carbon as a leaving group. This step generates a carbocation and water.
      • Second Step: The iodide ion attacks the carbocation, resulting in the formation of the alkyl iodide product.

    Reversibility and Equilibrium

    • The first step of the reaction is reversible; however, using excess HI can shift the equilibrium to favor product formation.

    Chemical Reaction Overview

    • Reactants: Cyclohexene, an example of an alkene, is the starting material in this reaction.
    • Reagent: mCPBA (meta-chloroperoxybenzoic acid) is a peroxy acid used as the oxidizing agent for the reaction.
    • Product: The reaction results in the formation of cyclohexene oxide, which is classified as an epoxide.

    Reaction Details

    • Mechanism: The reaction proceeds through the epoxidation of the alkene, where mCPBA adds an oxygen atom across the double bond of cyclohexene.
    • Significance: Epoxides are valuable intermediates in organic synthesis, often used in further transformations due to their reactivity.

    Reactions Under Basic Conditions

    • Nucleophiles (Nuc) are active participants in reactions under basic conditions.
    • Typically involves deprotonation processes with hydroxide ions (OH).
    • Order of reactivity and competitive interactions must be carefully monitored.
    • Processes often yield two primary products from the reactants.

    Reactions Under Acidic Conditions

    • Acidic conditions promote protonation reactions, influencing nucleophilicity.
    • H ions play a crucial role in altering the reaction pathway.
    • The order of products may differ significantly from basic conditions.
    • Final outcome includes the restoration of hydroxides (OH) and nucleophilic entities, suggesting a shift in reaction dynamics.
    • Careful consideration required for the balance of reactants and products during transformations.

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    Description

    This quiz covers the process of converting alcohols and phenols to alkyl halides using PBr3 in pyridine. Emphasis is placed on the suitability of primary and secondary alcohols for this reaction mechanism, which is crucial for understanding SN2 reactions. Test your knowledge on reaction conditions and outcomes!

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