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19 Cards in this Set
- Front
- Back
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Describe the structure of pyridine.
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A 6-membered aromatic ring with nitrogen as one of the atoms in the ring
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Describe the structure of pyrrole.
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A 5-membered aromatic ring with nitrogen as one of the atoms in the ring. There is a lone pair of electrons on the nitrogen contributing to the aromatic sextet.
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Briefly describe the hybridisation of nitrogen in pyridine and how it differs from the carbons.
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Unlike the carbons, the nitrogen does not need to be excited because of its groud state configuration. It has sp2 hybridisation including 3 sigma bonds (one to hydrogen, two to adjacent carbons) one pi bond and one sp2 orbital to accomodate the lone pair of electrons.
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True or false:
Pyridine can be protonated. |
The lone pair of electrons on the nitrogen is not involved in the aromatic sextet and can therefore be protonated [true]. It is also considered a good ligand.
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A protonated pyridine is called...
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The pyridium ion is the conjugate acid of pyridine.
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Pyrrole is said to be very weakly ______ because....
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Pyrrole is said to be weakly basic because the lone pair of electrons on the nitrogen is part of the aromatic sextet (unlike pyridine)
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Imidazole is a 2N-containing heterocycle. DIscuss.
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This aromatic compound contains a pyridine-like nitrogen (basic) as well as a pyrrole-like nitrogen (weakly basic)
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What amino acid does imidazole belong to?
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Histidine
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What is unique about imidazole at physiological pH?
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This heterocycle and its conjugate acid are in equilibrium and account for constant pH
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Does pyridine undergo electrophilic aromatic substitution? Explain.
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No. The electronegative nitrogen in the ring makes the pi cloud less electron rich and less attractive to electrophiles.
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What reaction does pyridine undergo. Under what conditions and what catalyst must be present?
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This compound undergoes nucleophilic substitution with the reactant NaNH2 and liquid ammonia [NH3] as a catalyst.
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Name the product:
nucleophilic substitution of pyridine with NaNH2 |
The product of this reaction is 2-aminopyridine. [the nucleophile is the amide ion NH2-]
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Does pyrrole undergo electrophilic substitution?
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Yes. This compund undergoes this reaction faster than benzene without heat or catalyst present.
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Explain one reason why pyrrole is highly reactive in comparison to benzene.
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As a 5-membered ring, the aromatic sextet is more densely distributed and more attractive to electrophiles.
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Name several important biological compounds which contains aromatic hetereocyclic rings.
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The nitrogen bases of DNA & RNA, serotonin, melatonin, histamine
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Explain why pyridine and imidazole are basic but pyrrole is not.
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Bronsted-Lowry bases can accept a proton (protonated).
Pyridine is basic because the nitrogen in the ring can be protonated to form the pyridinium ion. Imidazole is basic because it contains a "pyridine-like" nitrogen that can be protonated as well. Pyrrole is not basic because the lone pair of electrons on the nitrogen is part of the aromatic sextet and thus not free to form a bond with hydrogen (can't be protonated) |
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Explain why histidine residues in proteins can act as buffers at physiological pH.
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Histidine contains the imidazole group. Imidazole has a "pyridine-like" nitrogen in the ring which can be protonated. The protonated conjugate acid "consumes" any added bases and the deprotonated conjugate base "consumes" any added acids. This maintains a constant pH and so is called a buffer.
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Explain why pyridine is resistant to electrophilic aromatic substitution.
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Nitrogen is highly electronegative in the aromatic ring. Because of this, it is considered electron withdrawing. This withdrawal of electrons makes the pi cloud less electron rich making the ring as a whole less atractive to electrophiles.
Instead of undergoing electrophilic substitution, pyridine will undergo nucleophilic substitution. |
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Explain why pyrrole undergoes rapid electrophilic aromatic subutitution.
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Pyrrole is highly reactive because the pi electron cloud is more concentrated in a 5-membered ring rather than a more distributed 6-membered ring (like benzene).
Pyrrole reacts with no catalyst or heat unlike benzene. |
