40 Questions
Which type of hybridization is exhibited by each carbon atom in pyrrole?
sp2-hybridized
How many nitrogen atoms are present in an imidazole molecule?
Two nitrogen atoms
In imidazole, how many electrons does the sp2-hybridized nitrogen atom in the double bond contribute to the aromatic system?
One electron
What type of bond does the second sp2-hybridized nitrogen atom in imidazole have?
Single bond
How many lone pairs does the sp2-hybridized nitrogen atom in pyrrole contribute to the p orbital?
Two lone pairs
Where are the bases covalently attached in nucleic acids?
1' position of a pentose sugar ring
Which nitrogenous base is composed of two nitrogen-containing heterocycles, imidazole, and pyrimidine?
Adenine
In DNA, what is the sugar component that replaces the hydroxyl group at the 2'-position with a hydrogen?
Deoxyribose
Which nitrogenous base is 5-methyluracil?
Thymine
Which nitrogenous base has a purine structure and exists as the 'carbony' tautomer?
Guanine
What type of orbital is involved in pyrimidine's sp2 hybridization?
$sp$ orbital
Which aromatic compound has nitrogens that are both sp2-hybridized and contribute electrons to the aromatic system?
Pyridine
What type of orbital does the nitrogen atom in pyrrole contribute its lone pair electrons to the aromatic system?
p orbital
How many !electrons are there in the aromatic system of imidazole contributed by the nitrogen atom not involved in a double bond?
Two
In pyrrole, how many !electrons does each sp2-hybridized carbon contribute to the aromatic system?
One
Which heterocycle has a lone pair of electrons on nitrogen in an sp2 orbital?
Imidazole
Which compound has a delocalized lone pair of electrons in a p orbital perpendicular to the ring?
Pyrrole
What type of hybridization do both nitrogens have in imidazole?
sp2
Which of the following nucleotides is a purine with a bicyclic structure?
Adenine
In RNA, which base replaces thymine in DNA?
Uracil
Which nucleotide base is not found in DNA?
Uracil
Which type of structure do adenine and guanine nucleotides exhibit?
Bicyclic purines
What type of rings do cytosine, thymine, and uracil contain?
Monocyclic pyrimidines
Which of the following nucleotide bases are not heterocyclic?
Cytosine and thymine
Which molecular biology topic will Dr. Mark Carlile cover in Week 25?
Nucleic acid biochemistry
What advice does Dr. Carlile give to students regarding studying biological subjects?
Read an introductory chapter in a Biology, Biochemistry or Physiology textbook
Which institution offers the Royal Institution Christmas Lectures that are recommended by Dr. Carlile?
University of Oxford
What topic is covered in the Royal Institution Christmas Lecture titled 'Where do I come from?'
DNA and gene function
If a molecule has sp²-hybridization, what geometric shape does its electron domain arrangement resemble?
Trigonal planar
Which of the following nitrogenous heterocycles is found in the structure of imidazole?
Pyrrole
What type of hybridization is exhibited by the carbon atoms in pyrrole?
sp2
How many pi electrons does each sp2-hybridized nitrogen atom in imidazole contribute to the aromatic system?
1
In purine, what are the hybridization states of both nitrogen atoms?
sp2
What type of orbital does the sp orbital in pyrimidine contain?
p orbital
How many nitrogen atoms are present in an imidazole molecule?
2
Describe the aromatic compound formed by purine.
Bicyclic
Which purine base is a stronger base than purine and protonates at N-1 only?
Adenine
Which nitrogenous base exists as the 'carbony' tautomer?
Guanine
In purine, where does guanine attach to the base?
N-9 position
What type of orbital do both nitrogens in purine contribute to the aromatic system?
sp2
Study Notes
Pyrrole and Imidazole
- Pyrrole and imidazole are five-membered, nitrogen-containing heterocycles with six !electrons, making them aromatic compounds.
- In pyrrole, each of the four sp2-hybridized carbons contributes one electron, and the sp2-hybridized nitrogen atom contributes two electrons from its lone pair, which occupies a p orbital.
- Imidazole has two nitrogen atoms in a five-membered, unsaturated ring, with both nitrogens being sp2-hybridized.
- One nitrogen in imidazole is in a double bond, contributing one electron to the aromatic system, while the other is not in a double bond, contributing two electrons from its lone pair.
Nucleic Acids
- Nucleic acids are composed of heterocyclic (carbon- and nitrogen-containing) aromatic rings, with a variety of substituents.
- The bases of DNA and RNA are adenine (A), guanine (G), cytosine (C), thymine (T), and uracil (U).
- Adenine and guanine are purines, with bicyclic structures (two fused rings), while cytosine, thymine, and uracil are monocyclic pyrimidines.
- In RNA, the thymine base is replaced by uracil.
- Nucleosides are formed when the bases are covalently attached to the 1 position of a pentose sugar ring.
Purine Bases
- The archetype of purine bases is purine, composed of two nitrogen-containing heterocycles – imidazole and pyrimidine.
- Adenine (6-aminopurine) is a stronger base than purine and protonates at N-1 only.
- Guanine (2-amino-6-hydroxypurine) exists as the 'carbony' tautomer.
Tautomerism
- Tautomerism is a reaction involving the intramolecular transfer of a proton.
- Tautomerism can be catalyzed by both acid and base.
- Tautomerism occurs in heterocyclic systems, such as purines, including adenine and guanine.
Learn about the structures of pyrrole and imidazole, including the hybridization of carbons and nitrogens, as well as the contribution of electrons to the aromatic systems. Understand the differences in the nitrogen atoms' contributions in these heterocyclic compounds.
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