40 Questions
What is the first condition for a molecule to be considered aromatic?
The molecule must be cyclic
What type of hybridized atom 'kills' conjugation in a molecule?
Sp3 hybridized atom with four bonds to atoms
What is necessary for aromaticity to exist in a molecule?
A continuous ring of p-orbitals
Why is (Z)-1,3,5-hexatriene not aromatic?
It lacks a cyclic structure
What is the significance of an atom having an available p orbital in a ring?
It means the atom can participate in resonance
Which of the following atoms can also be considered as sp2 hybridized?
An atom attached to an sp2 atom with 1 or more lone pair electrons
What is the general formula for the number of pi electrons in an aromatic hydrocarbon?
4n+2
Which type of hydrocarbon has a similar stability to its open-chain fully conjugated hydrocarbon of the same number of carbon atoms?
Nonaromatic hydrocarbon
What is the condition for a hydrocarbon to be considered aromatic?
Cyclic, planar, and fully conjugated
How many pi electrons are counted in a pure double bond?
2
What is the characteristic of an antiaromatic hydrocarbon?
Especially unstable
What is the purpose of the Hückel (4n+2) rule?
To determine the number of pi electrons in an aromatic hydrocarbon
What is the main condition for a molecule to be considered aromatic?
Fully conjugated and cyclic
What type of orbital does a lone pair occupy in a molecule that is considered aromatic?
2p orbital
Why do the π electrons in the double bond outside of the ring not count towards the π electrons for aromaticity?
They are not part of the ring
What is the value of n in the Hückel (4n + 2) rule when the molecule has 6 π electrons?
1
What is necessary for a molecule to be considered nonaromatic?
Not being conjugated all around
How many lone pairs are counted as π electrons in a molecule that is considered aromatic?
Only one lone pair
What is the total number of pi electrons in the Cyclopentadiene anion?
6
Why does the nitrogen lone pair in pyrrole contribute to the pi system?
It is not involved in a pi bond
How many porbitals can an atom contribute to the pi system in Furan?
One
What is the orientation of the p-orbitals in benzene?
At right angles (90°) to the plane of the ring
Which of the following molecules is aromatic?
All of the above
How many electrons are in each p-orbital in benzene?
One
What is the total number of pi electrons in a benzene molecule?
6
Where is the lone pair located in the benzene anion?
At 90 degrees to the pi system, in the plane of the ring
What is the consequence of delocalization of p electrons in a planar, conjugated molecule with 4n p electrons?
Decreased stability
How are the p electrons arranged in cyclobutadiene?
Localized in two double bonds
What is the number of p electrons in cyclooctatetraene?
8
Why is pyridine not antiaromatic?
Its lone pair is not participating in the pi system
Which type of electrons does the nitrogen atom contribute to the pi system in pyrrole?
π electrons
Why does the oxygen atom in furan contribute only one p-orbital to the pi system?
Because one lone pair is in the plane of the ring
What is the total number of pi electrons in a molecule that satisfies the Hückel (4n + 2) rule with n = 1?
6
Why does the Cyclopentadiene anion satisfy the Hückel (4n + 2) rule?
Because it has 6 pi electrons
What is the orientation of the p-orbitals in a planar, conjugated molecule?
At 90° angles to the plane of the ring
Why does the benzene anion not satisfy the Hückel (4n + 2) rule?
Because it has 4 pi electrons
Which molecule has a similar stability to its open-chain fully conjugated hydrocarbon of the same number of carbon atoms?
Benzene
Why does the Cyclopentadiene anion have a planar structure?
Because it has a sp2 hybridized atom
What is the purpose of the Hückel (4n + 2) rule?
To determine the aromaticity of a molecule
Which atom can contribute a maximum of one p-orbital to the pi system in a molecule?
Any atom
Study Notes
Aromaticity and Hückel (4n+2) Rule
- Aromatic hydrocarbon: a cyclic, planar, fully conjugated hydrocarbon with 4n+2 pi electrons (2, 6, 10, 14, 18, etc)
- Aromatic hydrocarbon is especially stable relative to an open-chain fully conjugated hydrocarbon of the same number of carbon atoms
Conditions for Aromaticity
- Condition-1: The molecule must be cyclic
- Condition-2: Every atom in the ring must be conjugated
- Every atom in the ring must have an available p orbital
- Every atom in the ring must be able to participate in resonance
- Condition-3: The molecule must have [4n+2] pi electrons
- Condition-4: The molecule must be flat (planar)
Counting pi Electrons
- π- bonds are simply the second bond made in a double bond
- Any pure double bond is one sigma/σ and one pi/ π bond
- Count two π electrons per double bond and ignore the σ electrons
- If lone pairs are present, consider the molecular geometry, and only the π electrons that are in the ring count towards aromaticity
Examples of Aromatic Compounds
- Cyclopentadiene anion: 2 (from the lone pair) + 4 (from the two pi bonds) = 6 pi electrons, which is a Hückel number
- Pyrrole: nitrogen bears a lone pair that contributes to the pi system, giving a total of 6 pi electrons
- Furan: oxygen bears two pairs of lone electrons, but only one contributes to the pi system, giving a total of 6 pi electrons
Anti-Aromatic Compounds
- A monocyclic, planar, fully conjugated hydrocarbon with 4n pi electrons (4, 8, 12, 16, 20...)
- Anti-aromatic hydrocarbon is especially unstable relative to an open-chain fully conjugated hydrocarbon of the same number of carbon atoms
- Cyclobutadiene: 4 pi electrons, which is an anti-aromatic hydrocarbon
- Cyclooctatetraene: 8 pi electrons, which is not an aromatic hydrocarbon
Test your understanding of aromaticity, antiaromaticity, and the Hückel (4n + 2) rule, including the conditions for a molecule to be cyclic and aromatic. Learn about the importance of rings and pi bonds in determining aromaticity.
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