24 Questions
Which positions in the pyridine ring are most susceptible to nucleophilic substitution?
2 and 4
What is an example of a powerful base that can displace pyridine's hydrogen atom?
Hydride ion
What is the name of the reaction involving amination by sodium amide on pyridine?
Chichibabin reaction
In nucleophilic aromatic substitution, what determines the rate of the overall reaction?
Formation of a negatively charged intermediate
What type of compound can be used for alkylation or arylation in pyridine?
Organolithium compounds
Which type of substitution mechanism for nucleophilic aromatic substitution is analogous to electrophilic substitution?
S_N1 mechanism
Which attack yields a carbanion that is a hybrid of structures IV, V, and VI?
Attack at the 3-position
Why are the structures resulting from attacks on pyridine more stable than the corresponding ones for attack on a benzene derivative?
Due to electron withdrawal by the nitrogen atom
Why is structure III especially stable in nucleophilic substitution reactions?
Due to the location of the negative charge on the electronegative nitrogen atom
Why does nucleophilic substitution occur more rapidly on the pyridine ring than on the benzene ring?
Due to electron withdrawal by the nitrogen atom in pyridine
Which factor of pyridine makes it unreactive toward electrophilic substitution?
Electronegativity of nitrogen
Why is pyridine highly reactive toward nucleophilic substitution?
High electronegativity of nitrogen
Which step in the nucleophilic substitution mechanism for pyridine determines the overall reaction rate?
Formation of the charged intermediate
What type of intermediate is formed in the nucleophilic substitution mechanism for pyridine?
Carbanion
Which reaction involves the use of sodium amide as a nucleophile in pyridine substitution?
Chichibabin reaction
Which positions on the pyridine ring are most susceptible to nucleophilic substitution?
2- and 4-positions
What type of compounds can be used for alkylation or arylation of pyridine through nucleophilic substitution?
Organolithium compounds
Which base is powerful enough to displace the hydrogen atom from pyridine in a nucleophilic substitution reaction?
Hydride ion
Which of the following statements about nucleophilic attack on pyridine is correct?
Attack at the 4-position yields a carbanion that is a hybrid of structures I, II, and III.
Why is structure III especially stable in nucleophilic substitution reactions?
The negative charge is located on the electronegative nitrogen atom.
Which factor contributes to the higher reactivity of pyridine towards nucleophilic substitution compared to benzene derivatives?
The electronegativity of the nitrogen atom.
Which position in the pyridine ring is least susceptible to nucleophilic substitution?
3-position
What is the primary factor that determines the rate of the overall nucleophilic substitution reaction on pyridine?
The stability of the intermediate carbanion.
Why is pyridine unreactive towards electrophilic substitution?
The nitrogen atom is electron-withdrawing.
Study Notes
Nucleophilic Substitution in Pyridine
- Pyridine ring resembles a benzene ring with strongly electron-withdrawing groups, facilitating nucleophilic substitution.
- Nucleophilic substitution occurs readily, particularly at the 2- and 4-positions.
- The reactivity of pyridine is so great that even the powerfully basic hydride ion, H-, can be displaced.
- Two important examples of this reaction are:
- Amination by sodium amide (Chichibabin reaction)
- Alkylation or arylation by organolithium compounds
Mechanism of Nucleophilic Aromatic Substitution
- Reaction proceeds by two steps, with the rate of the first step determining the overall reaction rate.
- Intermediate is negatively charged, unlike electrophilic substitution where it is positively charged.
- The ability of the ring to accommodate the charge determines the stability of the intermediate and the transition state.
Effects of Nitrogen Atom
- Electron withdrawal by the nitrogen atom makes pyridine unreactive toward electrophilic substitution.
- The same electronegativity of nitrogen makes pyridine highly reactive toward nucleophilic substitution.
- The nitrogen atom's ability to accommodate negative charge makes structures more stable.
Nucleophilic Attack at Different Positions
- Attack at the 4-position yields a carbanion that is a hybrid of structures I, II, and III.
- Attack at the 3-position yields a carbanion that is a hybrid of structures IV, V, and VI.
- Attack at the 2-position resembles attack at the 4-position.
- Structure III is especially stable, with the negative charge located on the electronegative nitrogen atom.
Explore the nucleophilic substitution reactions in pyridine, focusing on the 2- and 4- positions. Understand how pyridine's ring structure influences its reactivity compared to benzene.
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