Pyridine and Its Derivatives: Structure and Properties
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Questions and Answers

What is the heterocyclic analogue of benzene?

  • Pantoprazole
  • Nicotine
  • Pyridine (correct)
  • Vitamin B6

    • What is the role of pyridine in biological processes?

  • Energy storage molecule
  • Structural component of cell membranes
  • Coenzyme in oxidation/reduction reactions (correct)
  • Hormone regulator

    • Why is pyridine an aromatic heterocycle?

  • It has a nitrogen atom
  • It is stable and reacts by substitution (correct)
  • It has a six-membered ring
  • It is unstable and reacts by addition

    • What is the effect of the N atom on pyridine's electronic structure?

    <p>-I effect via σ bond and -M effect via π bond</p> Signup and view all the answers

    What is a characteristic of pyridine's resonance structures?

    <p>Delocalization of electrons in the ring</p> Signup and view all the answers

    How does pyridine compare to benzene in terms of reactivity to substitution and nucleophilic reactions?

    <p>Pyridine is less reactive to SE reactions and more reactive to SN reactions</p> Signup and view all the answers

    What is a characteristic of pyridine's methyl groups?

    <p>They are highly reactive at the 2-, 4-, and 6-positions</p> Signup and view all the answers

    Why is pyridine more easily reduced than benzene?

    <p>Due to the electronegative effect of the nitrogen atom</p> Signup and view all the answers

    What is the pH at which the reaction is performed?

    <p>pH 8.5</p> Signup and view all the answers

    What is the duration of the reaction?

    <p>rt / 4 days</p> Signup and view all the answers

    What is the reagent used in the reaction?

    <p>HNO3</p> Signup and view all the answers

    What is the functional group attached to the nitrogen atom?

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

    What is the intermediate formed during the reaction?

    <p>Possible intermediate A</p> Signup and view all the answers

    What is the leaving group in the reaction?

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

    Why does the formation of pyridinium cation hinder further SE reactions?

    <p>It is very resistant to further SE reaction</p> Signup and view all the answers

    At which position does SE reaction in pyridine take place?

    <p>At position 3 or 5</p> Signup and view all the answers

    What is the reason for the easy displacement of halogen or nitro substituents at 2, 4, and 6 positions of pyridine by nucleophiles?

    <p>Due to the electron deficiency at these positions</p> Signup and view all the answers

    What is the reason for pyridine being difficult to nitrate?

    <p>It forms a very stable intermediate</p> Signup and view all the answers

    What is the intermediate anion stabilized by during the displacement of halogen or nitro substituents at 2, 4, and 6 positions of pyridine?

    <p>The pyridine ring</p> Signup and view all the answers

    What is the role of HgSO4 in sulphonation of pyridine?

    <p>It acts as a catalyst</p> Signup and view all the answers

    Why are pyridines more easily reduced than benzenes?

    <p>Due to the presence of nitrogen in pyridine</p> Signup and view all the answers

    Why does Friedel-Crafts reaction not occur in pyridine?

    <p>Due to the presence of electron-deficient ring carbons</p> Signup and view all the answers

    What is the effect of acidic solution on the oxidation of pyridine?

    <p>It makes pyridine more resistant to oxidation</p> Signup and view all the answers

    What is the effect of alkaline media on the oxidation of pyridine?

    <p>It makes pyridine more susceptible to oxidation</p> Signup and view all the answers

    At which position does nucleophilic substitution occur in pyridine?

    <p>At position 2, 4, and 6</p> Signup and view all the answers

    Why is nucleophilic substitution favored in pyridine?

    <p>Due to the presence of electron-deficient ring carbons</p> Signup and view all the answers

    Why do pyridines have good leaving ability of the substituents?

    <p>Due to the stability of the intermediate anion</p> Signup and view all the answers

    What is the temperature required for the reaction of pyridine with ammonia?

    <p>160 oC</p> Signup and view all the answers

    What is the mechanism of nucleophilic substitution in pyridine?

    <p>Nucleophilic addition</p> Signup and view all the answers

    What is the reason for the resistance of pyridine to oxidation in general?

    <p>Due to the resonance stabilization of the pyridine ring</p> Signup and view all the answers

    Why is pyridine less basic than corresponding aliphatic amines?

    <p>Its lone pair of electrons occupies sp2 orbital</p> Signup and view all the answers

    What is a characteristic of pyridine as a solvent?

    <p>It is polar and miscible with water and organic solvents</p> Signup and view all the answers

    What is the product of the reaction between pyridine and alkyl halides?

    <p>Quaternary pyridinium salts</p> Signup and view all the answers

    Why is pyridine unreactive to electrophilic substitution reactions?

    <p>Due to the electron-withdrawing effect of the ring nitrogen</p> Signup and view all the answers

    What is the product of the reaction between pyridine and percarboxylic acids?

    <p>Pyridine-N-oxide</p> Signup and view all the answers

    What is the reason for pyridine's ability to catalyze reactions in which acids are eliminated?

    <p>Its ability to form stable salts with acids</p> Signup and view all the answers

    What type of compounds does pyridine form with Lewis acids?

    <p>Stable coordination compounds</p> Signup and view all the answers

    What is the difference between the orbital occupied by the lone pair of electrons in pyridine and aliphatic amines?

    <p>Pyridine occupies sp2 orbital, while aliphatic amines occupy sp3 orbital</p> Signup and view all the answers

    Study Notes

    Six-Membered Ring Heterocycles with One Nitrogen Atom

    Pyridine and Pyridine Derivatives

    • Pyridine (C5H5N) is the heterocyclic analogue of benzene (C6H6), derived by substituting -CH= with -N=.
    • Pyridine is an aromatic heterocycle, stable, and reacts by substitution.
    • Its ring H's are deshielded in the NMR spectrum (δ= 6.8 - 8.5 ppm).

    Importance of Pyridine

    • Pyridine plays a key role in several biological processes (e.g., NAD and pyridoxine (Vitamin B6)).
    • It exists in several alkaloids (e.g., nicotine) and many synthetic drugs (e.g., Pantoprazole sodium).

    Resonance of Pyridine

    • The N atom has –I effect via σ bond and -M effect via π bond.
    • Contributing structures: A, B, C, D, E, and Hybrid structure.

    Chemical Reactions of Pyridine

    • Pyridine is basic, has decreased reactivity to SE reactions, and increased reactivity to SN reactions.
    • It is more easily reduced and more resistant to oxidation reactions.
    • Methyl groups at 2-, 4-, and 6-positions are more reactive.

    Basic Properties of Pyridine

    • Pyridine is basic due to an electron pair on nitrogen available for bond formation with acids or electrophiles.
    • Pyridine is less basic than corresponding aliphatic amines (e.g., piperidine) due to its sp2 orbital being more electronegative.

    Examples of Basic Properties

    • Pyridine reacts with acids to give stable salts.
    • Alkyl halides react with pyridine to form quaternary pyridinium salts.
    • Acyl and arylsulphonyl halides react with pyridine to form quaternary salts.
    • Pyridine forms stable coordination compounds with Lewis acids.
    • Pyridine reacts with percarboxylic acid to give pyridine-N-oxide.

    Electrophilic Substitution (SE) at Ring Carbon Atoms

    • Pyridine is very unreactive to SE reactions due to the -I effect of the N atom.
    • The intermediate σ-complex is destabilized by the -I effect of the N atom.
    • SE reactions occur at position 3- or 5- (β-position) due to the least disfavored σ-complex.
    • Examples of SE reactions: nitration, sulphonation, halogenation, and Friedel-Crafts reactions.

    Nucleophilic Substitution in Pyridine

    • Direct Nucleophilic Substitution occurs at C2, C4, and C6 due to the electron-deficient ring C's.
    • The intermediate anion is stabilized by resonance.
    • Examples of SN reactions: C2 Attack, C4 Attack, and Br/NH2 substitution.

    Reaction of Pyridine with Reducing Agents

    • Pyridines are more easily reduced than benzenes.

    Reaction with Oxidizing Agents

    • Pyridines are resistant to oxidation in acidic solution.
    • In alkaline media, pyridines are more rapidly oxidized than benzenes.
    • Examples of oxidation reactions: CH3CHO and EtOOC-CH2-COOEt.

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    Description

    Learn about the structure and aromatic properties of pyridine, a heterocyclic analogue of benzene, including its stability, reactivity, and NMR spectrum. Discover its importance in various fields.

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