State the effects of aromaticity and antiaromaticity on stability
If a conjugated system is aromatic, it experiences increased stability. If a conjugated system is antiaromatic, it experiences increased instability.
How do you apply hückel’s rule to identify aromaticity and antiaromaticity?
In a conjugated system, consider a single ring which does not share atoms with another ring. Count the electrons of all conjugated p orbitals in the ring, excluding exocyclic bonds.
Where n is a positive integer, if:
- The number of electrons is 4n+2, the ring is aromatic.
- The number of electrons is 4n, the ring is antiaromatic.
- Else, the ring is non-aromatic.
Warn: Yes, hückles rule is only applicable to monocyclic systems.
How do you use hückel numbers to identify aromatic bicycles?
In a conjugated system, consider two fused rings which do not share atoms with another rings. Count the electrons of all conjugated p orbitals in the two rings, excluding exocyclic bonds.
Apply the same criteria as hückel’s rule to figure out if it is aromatic/antiaromatic/not aromatic.
Note: This is NOT hückel’s rule.
Source: McMurry; MOC - Aromatic, Non-Aromatic, or Antiaromatic?
Note: This does not apply to tricyclic systems and above. See: Biphenylene, pyrene etc.
State the 3 requirements of a clar structure
A clar formula is constructed by placing aromatic sextet rings, represented by circles, in some hexagons of the polycyclic conjugated system. There are three requirements:
1. Two circles must not be drawn in neighbouring hexagons
2. The formula must contain the maximum number of circles.
3. The circles must be arranged in such a way that the other hexagons in the system can still be drawn with bonds and double bonds.
Like resonance structures, rotationally equivalent structures are considered distinct.
Note: 3 is the reason anthracene ⌬⌬⌬ cannot have the clar structure of ⏣⬡⏣.
How do the clar structures of a molecule relate to its true structure?
A system’s true structure is an average of all of its clar structures. (Similar to resonance structures)
In the true structure, more circle character is indicative of greater aromatic character in the ring. The complete lack of a circle in a sextet indicates very low but present aromatic character. It can be used to predice where alkene reactions and aromatic reactions prefer to occur.
Source: Ivan Gutman - Clar formulas and Kekule structures https://match.pmf.kg.ac.rs/electronic_versions/Match17/match17_75-90.pdf
Is cyclooctatetraene antiaromatic? Is biphenyl a single conjugated system?
Cyclooctatetraene prefers a non-planar conformation which is not destabilised by antiaromaticity. The most stable conformation of biphenyl places the two benzene rings perpendicular to each other.
State and explain the common pattern for resonance-withdrawing and resonance-donating groups.
Where the groups are aligned and conjugated with where it is attached,
1. In —L(…) where L has a lone pair and is not electron deficient (ie. it is electron rich), L is willing to donate electrons through resonance. Eg. —OH, —halogen, —NH3
2. In —D(…) where D is electron deficient, D prefers to accept electrons through resonance. It can be caused by a more electronegative group being bonded or a positive charge: resonance-withdrawing. Eg. Cyanide, C=O
State the common pattern for inductive-withdrawing and inductive-donating groups. State the pattern for alkyls.
Inductive (includes hyperconjugation):
⁃ Alkyl: Weakly donating to p orbitals through hyperconjugation. Larger alkyls donate more.
⁃ Consider other groups by the polar bond between group and atom it is attached to.
Compare the relative strength of resonance and inductive effects for electron density withdrawal.
Resonance effects often outweigh inductive effect. The exception is halogens, where the inductive withdrawal outweighs resonance donation.
What makes a group activating or deactivating? Why?
If a group has a net electron donating effect to a ring, it allows the carbocation intermediate to be more stabilised, allowing it to be more reactive for EAS and thus activating. Similarly, a net withdrawing effect is deactivating and makes the ring less reactive for EAS.
State the activating and deactivating effects of common groups?
Recall that resonance effects are stronger than inductive effects (except halogens). You can infer the following:
- —L(…) where L has a lone pair and is not electron deficient: activating
- —D(…) where D is electron deficient: deactivating
- Halogens: As inductive withdrawal is stronger than resonance donation in halogens, it is deactivating
- Alkyls: activating due to hyperconjugation
State and explain the relative importance of resonance and inductive effects in determining directing effects.
The effect of resonance in determining directing effects is always more important than the inductive effect. Resonance allows the charge to be spread out across more atoms, while inductive effect merely reduces the charge at a position slightly.
State where the positive charge is formed during an EAS and how it delocalises in the ring.
During EAS, a positive charge is formed at a carbon adjacent to the position the substituent is attached to. Through resonance, a charge can delocalise to atoms in the ring that are 2 bonds away.
Explain how resonance causes directing effects.
A positively charged carbocation intermediate is created during EAS.
If the charge is able to delocalise to a ring carbon with a resonance-donating group attached, it is then able to delocalise into the group and benefit from resonance stabilisation. Thus, substitution at the ortho and para positions have a stabilised intermediate, and are directed to.
If the charge is able to delocalise to a ring carbon with a resonance-withdrawing group attached, it causes two adjacent positive charges, which is unstable due to electrostatic repulsion. There is also little resonance stabilisation as the attached atom is electron-deficient and does not prefer to donate electrons to delocalise the charge into the group. Thus, substitution at the ortho and para positions have a destabilised intermediate, and meta is directed to.
Explain the directing effects of the alkyl group.
The alkyl group is weakly inductively donating (hyperconjugation). It does not participate in resonance.
If the charge is able to delocalise to a ring carbon with an alkyl attached, it is then able to be stabilised by hyperconjugation donation from the sigma bonds in the alkyl group. Thus, substitution at the ortho and para positions have a stabilised intermediate, and are directed to.
What factor affects the strength of the inductive donating/withdrawing effect of groups?
The more electron deficient or electronegative the attached atom is, the more withdrawing it is and the less donating it is. (Eg. Cyanide vs -Ac)
State and explain the factors which affect the strength of the resonance donating/withdrawing effects of groups.
For resonance withdrawers: The more electron deficient the attached atom is, the more willing it is to accept an electron pair and the more withdrawing it is.
For resonance donators:
- If the atom with the lone pair can donate not only to the benzene ring but also to another atom, its donating effect is diminished. (Eg. -OR vs -OAc)
- As the principal quantum number of the p orbital with the lone pair increases, the size of the orbital increases, causing a larger size mismatch with the 2p orbitals in the carbons. This causes a less effective overlap with the pi system and thus weaker resonance donation. Eg. (-SH vs -OH, -Br vs -Cl)
What is the relative strength of alkyl and halogen activating/deactivating and directing effects compared to other groups?
Alkyls are the weakest activators and ortho para directors.
Halogens are the weakest deactivators and the second weakest ortho para director after alkyls.
How do activating/deactivating and directing effects of halogens change down the group? Explain.
Down the group, directing effects decrease in strength.
Down the group, inductive withdrawal decreases as electronegativity decreases. Resonance donation decreases. As the principal quantum number of the p orbital with the lone pair increases, the size of the orbital increases, causing greater size mismatch with the 2p orbitals of the carbons, reducing effectiveness of resonance donation.
Deactivating effects are comparable. Inductive withdrawal decreases and resonance donation also decreases. These two decreases happen to roughly the same extent.
How do activating/deactivating and directing effects of alkyls change as the alkyl gets bigger? Explain.
Bigger alkyls have more extensive hyperconjugation donation to the p orbital of the atom it is bonded to as there are more sigma bonds that can be donated from. Thus, inductive donation increases, causing activating effects and ortho para directing effects to increase in strength.
Explain how to handle multiple groups directing differently.
We prioritise the effects of group with the strongest influence first.
Then, we prioritise less sterically hindered positions.
If there is already a group at ortho and para, is it favoured to substitute a group at meta? How is such a product usually created?
No, steric hindarance makes this unfavourable. Such a product can be obtained through substituting at ortho and meta then at para.
If there is an ortho para director on a benzene ring, and there are two ortho sites compared to one para site, why is it that both products are obtained in comparable amounts?
It is because steric hindarance with the group disfavours ortho substitution.
Is this framework of activating/deactivating and directing applicable to pi systems in general?
YES. These same ideas generalise and can be applied to dienes, polycyclic aromatics etc.
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Isomers, cis/trans & CIP & E/Z, misc. (Lecture 1)17
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Chirality (Lecture 1)15
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Conformations (Lecture 1)8
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Hybridization (Lecture 1)10
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Electron Density, Inductive Effect, Delocalisation28
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Organic Chem Fundamentals - Electro/neucleophiles, Arrows, Mechanism & Reaction Types (Lecture 1)17
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Carbocations & carbon radicals13
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Alkenes Characteristics (Chapter 7)8
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Extra acids & bases10
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SN1, SN2, E1, E225
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Aromaticity & EAS24
