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What type of reaction is Bonnemann cyclization classified as?
What is the temperature required for the Diels-Alder reaction in the given reaction?
What is the pKa of pyridine?
What type of reactions does pyridine undergo due to its electronic structure?
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What is the nature of pyridine?
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What happens when pyridine reacts with strong acids?
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What is the name of the reaction that involves the reaction of 1,3-butadiene?
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What is the role of pyridine in some reactions?
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What is the most common example of heterocyclic compounds?
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What is the hybridization state of the carbon atoms and nitrogen atom in pyridine?
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What is the orientation of the lone pair of electrons of the nitrogen atom in pyridine?
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What is the condition for a compound to be aromatic, according to Huckel's rule?
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Why is pyridine basic?
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How can pyridine be represented?
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What determines the rate of the overall reaction in electrophilic and nucleophilic substitution?
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What is the charge of the intermediate in electrophilic substitution?
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Why are the carbanions more stable in nucleophilic attack on a pyridine derivative compared to a benzene derivative?
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What is the effect of the nitrogen atom on the carbanions in nucleophilic attack?
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What is the hybrid of structures resulting from nucleophilic attack at the 4-position?
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Why is the rate of the reaction affected by the ability of the ring to accommodate the charge?
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What is the charge of the intermediate in nucleophilic substitution?
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What is the effect of nucleophilic attack at the 2-position compared to the 4-position?
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What is the other name of the compound shown in the figure?
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What is responsible for the basicity of Piperidine?
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How would you classify Pyridine?
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What type of reactions is Pyridine nucleus highly deactivated towards?
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What is the Kb value of Piperidine?
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What is the name of the compound with a cyclic structure shown in the figure?
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What is the characteristic of Pyridine towards electrophilic substitution reactions?
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What is the name of the compound with an unsaturated ring shown in the figure?
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Why is structure Il especially stable?
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What is the reason for pyridine being highly reactive towards nucleophilic substitution?
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What is the result of the reaction of pyridine with potassium hydroxide?
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What is the reactivity of pyridine towards electrophilic substitution?
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What is the nature of pyridine as a nucleophile?
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What is the product of the reaction of pyridine with butyllithium?
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What is the role of the nitrogen atom in pyridine?
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What is the result of the reaction of pyridine with sodium amide?
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Attack at the 3-position in pyridine is especially slow.
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Nitrogen in pyridine shares electrons readily and resists the removal of electrons.
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A structure in which nitrogen bears a positive charge is especially stable in pyridine.
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Substitution in pyrrole occurs predominantly at the 4-position.
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Pyridine is more reactive towards nucleophilic substitution than pyrrole.
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In pyrrole, nitrogen accommodates the positive charge by sharing three pairs of electrons.
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Attack at the 4-position is less favored in pyridine.
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Pyrrole is less reactive towards electrophilic substitution due to its electronic structure.
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The nitrogen atom in pyridine has an octet of electrons.
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Substitution in pyridine occurs predominantly at the 2-position or 4-position.
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Study Notes
Structure and Properties of Pyridine
- Pyridine is a planar molecule, similar to benzene, due to sp2-hybridized carbon and nitrogen atoms.
- The lone pair of electrons on the nitrogen atom lies in the plane of the ring.
- Pyridine is an aromatic compound, following the Huckel rule of aromaticity (4n+2 π-electrons).
- The nitrogen's lone pair of electrons is in an sp2 orbital, perpendicular to the p orbitals of the ring, making it basic.
Synthesis of Pyridine
- Pyridine can be represented as a resonance hybrid of several structures.
- It can be synthesized through Bonnemann cyclization and Diels-Alder reaction.
Reactions of Pyridine
- Pyridine undergoes electrophilic and nucleophilic substitution reactions typical of aromatic rings.
- Pyridine acts as a base or nucleophile, with reactions involving the nitrogen atom directly.
Basic Character of Pyridine
- Pyridine is basic in nature, with a pKa of 8.75.
- It reacts with strong acids to form salts and is also known as piperidine.
Electrophilic Substitution Reactions
- Pyridine is less aromatic than benzene and pyrrole, making it a highly deactivated aromatic nucleus towards electrophilic substitution reactions.
- The reaction proceeds in two steps, with the rate of the first step determining the overall reaction rate.
Nucleophilic Substitution Reactions
- Nucleophilic attack at the 4-position yields a carbanion that is a hybrid of structures, which is especially stable due to the negative charge on the nitrogen atom.
- Attack at the 3-position yields a carbanion that is also a hybrid of structures.
- Pyridine is more reactive towards nucleophilic substitution than benzene, and more reactive at the 2- and 4-positions than at the 3-position.
Other Reactions of Pyridine
- Reaction with potassium hydroxide yields 2-hydroxypyridine through keto-enol tautomerism.
- Reaction with organometallic compounds, such as butyllithium, yields 2-butylpyridine.
- Reaction with sodium amide yields pyridinium chloride.
Pyridine as a Nucleophile
- As a tertiary amine, pyridine has nucleophilic properties, reacting with electrophiles to form products such as pyridinium chloride.
Pyridine Synthesis
- Hantzsch Synthesis: condensation of beta-dicarbonyl compound, ammonia, and aldehyde leads to 1,4-dihydropyridine derivative, which on oxidation with HNO3 yields pyridine derivative.
- From Pyrrole: Pyrrole heated with methylene chloride in presence of sodium ethoxide forms pyridine.
- From Picoline: Beta-picoline on oxidation with potassium dichromate and sulfuric acid gives nicotinic acid, which on decarboxylation with calcium oxide gives pyridine.
Electrophilic Substitution in Pyridine
- Pyridine is less reactive towards electrophilic substitution reactions due to:
- Higher electronegativity of nitrogen atom reduces electron density on nitrogen atom.
- Pyridine forms pyridinium cation with electrophile, decreasing electron density on nitrogen atom.
- Substitution reactions occur mainly at the 3-position (or β-position) due to the stability of the resulting intermediate.
Comparison with Pyrrole
- In pyrrole, nitrogen bears a positive charge, which is a stable state due to every atom having an octet of electrons.
- In pyridine, nitrogen bearing a positive charge is unstable due to nitrogen having only a sextet of electrons.
Reactivity of Pyridine
- Pyridine undergoes nitration, sulfonation, and halogenation only under vigorous conditions.
- Pyridine does not undergo Friedel-Crafts reaction.
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Description
This quiz covers the properties of pyridine, a heterocyclic compound, including its planar molecular structure and aromaticity. It also discusses the hybridization of carbon and nitrogen atoms and the arrangement of electrons.
