Lec4 | Reactions of pyridine
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Questions and Answers

How does pyridine behave towards electrophilic substitution?

  • Does not undergo any electrophilic substitution (correct)
  • Similar to benzene
  • Highly reactive under mild conditions
  • Forms stable carbonium ions

    • In which position does substitution mainly occur in pyridine during electrophilic reactions?

  • 3- position (correct)
  • 4- position
  • 2- position
  • 1- position

    • What type of reactions involve pyridine acting as a base or nucleophile?

  • Nucleophilic substitution
  • Electrophilic substitution
  • Substitution involving nitrogen directly (correct)
  • Friedel-Crafts reaction

    • Which statement best describes pyridine's reactivity towards various aromatic ring substitutions?

    <p>Similar to a deactivated benzene derivative</p> Signup and view all the answers

    Why does pyridine not undergo the Friedel-Crafts reaction?

    <p>Forms unstable intermediates in such reactions</p> Signup and view all the answers

    How does the nitrogen atom in pyridine affect its reactions?

    <p>Influences electrophilic and nucleophilic reactions due to its unshared pair of electrons</p> Signup and view all the answers

    Which of the following statements about the carbonium ion formed by attacking pyridine at the 4-position is correct?

    <p>It is a hybrid of structures I, II, and III.</p> Signup and view all the answers

    Which of the following statements about the carbonium ion formed by attacking pyridine at the 3-position is correct?

    <p>It is a hybrid of structures IV, V, and VI.</p> Signup and view all the answers

    Which of the following statements about the carbonium ion formed by attacking pyridine at the 2-position is correct?

    <p>It resembles the carbonium ion formed by para attack in benzene.</p> Signup and view all the answers

    Which of the following statements about the stability of the carbonium ions formed in pyridine compared to benzene is correct?

    <p>The carbonium ions formed in pyridine are less stable due to the electron withdrawal by the nitrogen atom.</p> Signup and view all the answers

    Which of the following statements about the substitution reaction in pyridine is correct?

    <p>Pyridine undergoes substitution more slowly than benzene.</p> Signup and view all the answers

    Which of the following statements about the predominant substitution position in pyridine is correct?

    <p>Substitution occurs predominantly at the 3-position.</p> Signup and view all the answers

    Why is a structure with a positive charge on nitrogen especially stable for pyrrole?

    <p>Nitrogen can accommodate the positive charge by sharing four pairs of electrons, allowing it to have an octet.</p> Signup and view all the answers

    Why is a structure with a positive charge on nitrogen especially unstable for pyridine?

    <p>Nitrogen only has a sextet of electrons and resists the removal of electrons due to its electronegativity.</p> Signup and view all the answers

    Why can 2-aminopyridine be nitrated or sulfonated under milder conditions than pyridine itself?

    <p>The amino group activates the pyridine ring, making it more reactive towards electrophilic substitution.</p> Signup and view all the answers

    Why does substitution in 2-aminopyridine occur chiefly at the 5-position?

    <p>The amino group at the 2-position directs the incoming electrophile towards the 5-position due to resonance effects.</p> Signup and view all the answers

    Why is 3-aminopyridine most conveniently made via nicotinic acid?

    <p>Nicotinic acid is a readily available starting material, and its conversion to 3-aminopyridine is straightforward.</p> Signup and view all the answers

    What is the key step in the synthesis of 3-aminopyridine from $\beta$-picoline?

    <p>Oxidation of the methyl group to a carboxylic acid, followed by reduction of the acid to an amine.</p> Signup and view all the answers

    Why does pyridine undergo substitution more slowly than benzene?

    <p>Pyridine has a less stable structure than benzene due to electron withdrawal by the nitrogen atom.</p> Signup and view all the answers

    Why is attack at the 4-position particularly slow in pyridine?

    <p>The electronegative nitrogen atom in the resulting structure has only a sextet of electrons.</p> Signup and view all the answers

    How does the substitution behavior of pyridine differ from that of pyrrole?

    <p>Pyridine substitutes chiefly at the 3-position, while pyrrole substitutes chiefly at the 2-position.</p> Signup and view all the answers

    What characterizes the stability of carbonium ions formed in pyridine compared to those formed in benzene?

    <p>Carbonium ions formed in pyridine are less stable due to electron withdrawal by the nitrogen atom.</p> Signup and view all the answers

    Why does substitution occur predominantly at the 3-position in pyridine?

    <p>The nitrogen atom favors attack at the 3-position over other positions due to its electronegativity.</p> Signup and view all the answers

    What distinguishes substitution behavior between ortho and para attack in pyridine?

    <p>Ortho attack resembles para attack in benzene, leading to similar substitution rates in pyridine.</p> Signup and view all the answers

    Which nitrogen-containing heterocycle forms a structure with a positive charge on nitrogen that is especially stable due to every atom having an octet of electrons?

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

    Why is a structure with a positive charge on nitrogen especially unstable for pyridine compared to pyrrole?

    <p>Pyridine has only a sextet of electrons on nitrogen.</p> Signup and view all the answers

    Why can 2-aminopyridine be nitrated or sulfonated under milder conditions than pyridine itself?

    <p>2-Aminopyridine has more reactive sites for substitution.</p> Signup and view all the answers

    What is the most convenient precursor to synthesize 3-aminopyridine?

    <p>$\beta$-Picoline</p> Signup and view all the answers

    Why is the structure with a positive charge on nitrogen especially stable in pyrrole?

    <p>Every atom in pyrrole has an octet of electrons.</p> Signup and view all the answers

    Which of the following statements best explains the preferential substitution at the 3-position in pyridine during electrophilic reactions?

    <p>The carbonium ion formed at the 3-position is more stabilized by the adjacent nitrogen atom than the carbonium ions formed at other positions.</p> Signup and view all the answers

    Which of the following statements accurately describes the reactivity of pyridine towards electrophilic substitution reactions compared to benzene?

    <p>Pyridine is less reactive than benzene because the nitrogen atom deactivates the ring towards electrophilic attack.</p> Signup and view all the answers

    Which of the following statements best explains why pyridine does not undergo the Friedel-Crafts reaction?

    <p>The nitrogen atom in pyridine acts as a strong electron-withdrawing group, deactivating the ring towards electrophilic aromatic substitution.</p> Signup and view all the answers

    In the electrophilic substitution of pyridine, which of the following statements accurately describes the stability of the carbonium ion intermediates formed at different positions compared to those formed in benzene?

    <p>The carbonium ions formed at the 2- and 4-positions in pyridine are less stable than those formed in benzene, while the carbonium ion formed at the 3-position is more stable.</p> Signup and view all the answers

    Which of the following statements accurately describes the role of the nitrogen atom in pyridine during reactions involving it as a base or nucleophile?

    <p>The nitrogen atom acts as both a base by accepting a proton and a nucleophile by donating its lone pair of electrons.</p> Signup and view all the answers

    Which of the following statements accurately describes the effect of substituting pyridine with an electron-withdrawing group, such as a nitro group, on its reactivity towards electrophilic substitution?

    <p>The electron-withdrawing group activates the pyridine ring, increasing its reactivity towards electrophilic substitution.</p> Signup and view all the answers

    Study Notes

    Electrophilic Substitution in Pyridine

    • Pyridine mainly undergoes electrophilic substitution at the 3-position, due to the nitrogen's electron-withdrawing property.
    • Electrophilic substitution is slower in pyridine compared to benzene, as the aromaticity is disrupted by the nitrogen atom.
    • Pyridine does not undergo Friedel-Crafts reactions because the nitrogen atom coordinates with the Lewis acid, preventing the formation of an active electrophile.

    Role of Nitrogen in Pyridine

    • The nitrogen atom in pyridine acts as an electron-withdrawing group, stabilizing the aromatic system but hindering electrophilic attack.
    • Due to its electronegativity, the nitrogen atom causes a positive charge, which is particularly unstable when formed in pyridine compared to pyrrole.

    Stability of Carbonium Ions

    • Carbonium ions formed at the 2-, 3-, and 4-positions in pyridine have different stabilities, with 4-position being especially slow due to steric hindrance and electronic effects.
    • Carbonium ions in pyridine are generally less stable than those formed in benzene, which can delocalize positive charge over multiple carbons.

    Substitution Reactions with Aminopyridines

    • 2-aminopyridine can undergo nitration or sulfonation under milder conditions due to the activating effect of the amino group at the 5-position.
    • Substitution in 2-aminopyridine is predominantly at the 5-position because the amino group directs electrophiles to this site more effectively.

    Synthesis Insights

    • 3-aminopyridine is most conveniently synthesized from nicotinic acid, providing a straightforward synthetic pathway.
    • The key step in synthesizing 3-aminopyridine from β-picoline involves a simple amination reaction that favors the desired substitution.

    Comparative Reactivity

    • Pyridine shows different reactivity towards ortho and para attacks compared to benzene, with predominant 3-position substitution creating distinct challenges.
    • Structures with a positive charge on nitrogen are particularly stable in pyrrole due to electron delocalization, contrasting with pyridine’s instability under similar conditions.

    Conclusion on Reactivity

    • Pyridine's reactivity towards electrophilic substitution is altered by substituents such as electron-withdrawing groups (e.g., nitro groups), which further decrease its reactivity.
    • Overall, the unique structure of pyridine leads to distinct behaviors in substitution reactions compared to typical aromatic compounds like benzene.

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    Organic Chemistry Heterocyclic Compounds Lec. 4 - PDF

    Description

    Explore the chemical properties and reactions of pyridine in the context of heterocyclic compounds in Organic Chemistry. Learn about electrophilic and nucleophilic substitutions typical of aromatic rings.

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