Electron Density, Inductive Effect, Delocalisation

Question 1

What is an electron cloud? How does it relate to orbitals?

Answer 1

An electron cloud is the region of space in which the electrons of a molecule are most likely to be found.

All electrons are in an orbital, the region of space where it is most likely to be found. All orbitals in combination, determine the electron cloud of a molecule.

Question 2

What is electron density? Is it evenly distributed?

Answer 2

Electron density of a region in a molecule is the likelihood of finding an electron in that region of the molecule.

Regions with higher likelihood of finding an electron there are considered to have higher electron density and vice versa.

It is not evenly distributed. Some atoms/regions have more electron density than others due to certain interactions.

Note: Electron density is a continuous distribution, not discrete regions.

Question 3

How does electron density affect reactivity?

Answer 3

Regions with increased electron density exhibit stronger nucleophilic behavior, and regions with lower electron density exhibit stronger electrophilic behavior.

Question 4

Explain partial charges (δ+, δ-) and full charges in the context of electron density.

Answer 4

A δ+ occurs when there is decreased electron density of an atom, while a δ- occurs when there is increased electron density of an atom.

An atom with a full + charge will have low electron density as it is electron-deficient. An atom with a full - charge will have high electron density as it is electron-rich.

Question 5

1. What electrostatic attraction exists between partial charges?
2. How do partial charges affect reactivity?

Answer 5

1. Opposite charges attract and like charges repel.
2. Atoms with δ- act as nucleophiles, and atoms with δ+ act as electrophiles.

Note: pd-pd interactions are a result of #1.

Question 6

Explain how charged atoms cause partial charges in bonded atoms.

Answer 6

A positively charged atom attracts electrons to itself. Thus, it attracts electron density towards itself, and atoms bonded to it lose electron density and gain δ+ charges.

A positively charged atom repels electrons electrons from itself. Thus, it repels electron density away from itself, and atoms bonded to it gain electron density and gain δ- charges.

Note: Alkyl groups are still inductively withdrawing despite a δ+ on an alpha carbon.

Question 7

What is the inductive effect and how does it cause partial charges? When is it not significant?

Answer 7

Consider an atom with a δ+ charge (perhaps as a result of a polar bond).

The atom now attracts electron density towards itself, reducing electron density of its neighbours, propagating δ+ charges to its neighbours. This propagation continutes in a chain reaction spreading outwards from the initial atom with δ+ charge. This propagation is termed the inductive effect.

It is not significant in alkyl groups, and thus should be omitted in them.

Note: Alkyls are weakly electron withdrawing inductively without hyperconjugation.
Note: Electron density transfer is usually denoted with arrows»_space;> or δ+ > δδ+

Question 8

Does the δ+ induced through the inductive effect become stronger or weaker with more bonds away from a electronegative atom? Up to how many bonds away is the inductive effect significant?

Answer 8

The gained δ+ decreases with more bonds away from the electronegative atom as each atom with some δ+ induces a smaller δ+ in its neighbours. Thus, the inductive effect is usually only significant up to three bonds away.

Question 9

When the difference in electronegativity is larger, is there an effect on the strength of the inductive effect?

Answer 9

A bigger difference in electronegativity leads to greater shifting of electron density, leading to stronger δ- and δ+ charges formed.

Question 10

Why do regions of high or low electron density destabilise a molecule?
Name ways in which high/low electron density is reduced to acheive more stability, from more to less significant.

Answer 10

Regions of high electron density are electron rich act as strong nucleophiles, and regions of low electron density are electron poor and act as strong electrophiles. This high reactivity makes the molecule less stable.

Ways to reduce high/low electron density to acheive more stability:
- Delocalisation (Resonance, hyperconjugation)
- Inductive effect, polar bonds, bonds with charged atoms

Note: Reactivity is more important than electron-electron repulsion when high electron density is involved.

Question 11

Explain how the inductive effect can reduce high/low electron density and stabilise a molecule.

Answer 11

An opposing partial charge can be created on the charged atom to reduce the charge of the atom, reducing its reactivity and making the molecule more stable. (Etc. δ+ created on a negatively charged O- by an attached —CF3 group).

Question 12

What is delocalisation? When does it occur?

Answer 12

Delocalised electrons not confined to an atomic orbital (one atom) or a bonding orbital (2 atoms). They belong to a larger orbital which spans across multiple atoms.

It occurs when a molecule has a delocalised pi system or hyperconjugation interactions.

Question 13

1. What is hyperconjugation? How is it understood in terms of electron density?
2. Is there electron flow between sigma bonds and pi systems?

Answer 13

1. Hyperconjugation is the delocalisation of electrons in the bonding orbital of a sigma bond to an aligned and adjacent pi system/energetically accessible pi orbital. This donates electron density from the bonding orbital of a sigma bond to the p orbital/pi system.
2. Hyperconjugation does not involve actual electron flow between sigma bonds and pi systems. The orbitals only interact.

Note: Imagine, where 8 represents a lobe of a p orbital, and ⇿ represents a hyperconjugation interaction.:

           H
  8  ⇿|
=C—CH2
  8 

Question 14

How does hyperconjugation cause partial charges?

Answer 14

The atoms involved in the sigma bond which donates electron density develops δ+ charges. If a specific atom accepts the electron density, the atom is said to develop δ- charges.

Question 15

Explain how hyperconjugation can stabilise a electron deficient atom.

Answer 15

Electron density can be donated to the p orbital of an electron deficient atom from adjacent aligned sigma bonds through hyperconjugation.

Question 16

What is a delocalised pi system? When does a delocalised pi system occur?

Answer 16

A delocalised pi system is a set of p orbitals that share electrons through delocalisation. It occurs in a molecule at a conjugated system.

A conjugated system is formed with connected atoms, all with energetically accessible unhybridized p orbitals. The orbitals also have to be aligned side-by-side, thus the system must be planar.

This creates a delocalised pi system where electrons are delocalised among all the p orbitals in the conjugated system.

Example illustration, where 8 is a lobe of a p orbital and ⇿ is a delocalisation.:

     8⇿8 ⇿  8 ⇿  8
H2C=CH—CH=CH2
     8⇿8 ⇿  8 ⇿  8

Note: This can occur when double bonds and single bonds alternate in a chain.

Question 17

Is H2C=C=CH2 one conjugated system?

Answer 17

No. The p orbitals of the two distinct systems are at a 90° angle to each other.

Question 18

Briefly, how do you derive resonance structures? What is the resonance hybrid and how is it derived?

Answer 18

Starting from one possible lewis structure of the molecule, the resonance structures of a molecule can be derived through arrow-pushing of the pi electrons in the system. All valid lewis structures created this way are resonance structures.

The resonance hybrid represents the true structure of the molecule. It is a weighted average of all resonance structures, where the bonds/charges/lone electrons are distributed. More stable resonance structures contribute more to the resonance hybrid.

Question 19

How to do arrow-pushing to generate resonance structures?

Answer 19

1. A pi bond forms a new pi bond to an adjacent atom
2. A lone pair forms a pi bond to an adjacent atom
3. Conversely, A pi bond forms a lone pair on adjacent atom

In these, +/- charges and lone electrons may be generated.

If octet rule is exceeded, the atom has to give up a pi bond, or the arrow-pushing is not possible.

Question 20

Are all the bond lengths in ozone equal? Explain your answer using resonance structures.

Answer 20

The bond lengths are equal. Its resonance hybrid is an average of its two resonance structures. As both bonds are an average between a single bond and double bond, they are the same bond and thus have the same bond length. They are considered to have bond order 1.5.

Question 21

Explain how delocalised pi systems can stabilise a molecule with a charged atom.

Answer 21

If the charged atom in a delocalised pi system has a p orbital which has an additional or missing electron, sufficiently stable resonance structures may exist where the charge is delocalised to other atoms in the system. If such structures exist, it means that the charge is distributed among atoms in the system, and thus the electron deficiency/richness is spread out allowing the molecule to be less reactive and more stable

Note: Similar to radicals and lone pairs.

Question 22

Explain how delocalised pi systems can stabilise a molecule with a lone electron.

Answer 22

If an atom in a delocalised pi system has a lone electron in its p orbital, sufficiently stable resonance structures may exist where the lone electron is delocalised to other atoms in the system. If such structures exist, it means that the lone electron is distributed among atoms in the system, and thus the electron deficiency is spread out, allowing the molecule to be less reactive and more stable.

Note: Similar to charged atoms and lone pairs.

Question 23

Explain how delocalised pi systems can stabilise a molecule with a lone pair.

Answer 23

If an atom in a delocalised pi system has a lone pair in its p orbital, sufficiently stable resonance structures may exist where the lone pair is delocalised to other atoms in the system. If such structures exist, it means that the lone pair is distributed among atoms in the system, and thus the electron deficiency is spread out, allowing the molecule to be less reactive and more stable.

Note: Similar to radicals and charged atoms.

Question 24

How does delocalisation of electrons stabilise a molecule? (Assume there are no charged atoms/lone pairs/lone electrons/electron-rich C=C etc.)

Answer 24

When electrons are delocalised, they are not constrained to smaller atomic or bonding orbitals. Being less confined in a larger orbital formed in delocalisation leads to less electron-electron repulsion, leading to less potential energy and greater stability.

Note: Is less significant than reducing electron-deficiency or electron-richness.

Question 25

Rank the relative stabilising effects of delocalised pi systems, hyperconjugation and the inductive effect.

Answer 25

Delocalised pi systems >> hyperconjugation > inductive effect

Question 26

State the relative strengths of hyperconjugation, resonance and inductive effects.

Answer 26

Inductive effects are easily overshadowed by hyperconjugation and resonance. Resonance > hyperconjugation

Question 27

Is there free rotation about sigma bonds between atoms of a conjugtated system?

Answer 27

In the resonance hybrid, there is a partial pi bond between all atoms in a conjugated system. Thus, the bonds are stiffer than a lone sigma bond and rotation is restricted. The higher the pi character, the more restricted rotation is. Additionally, rotation is energetically unfavourable as it would disrupt conjugation by causing p orbitals to be misaligned. Note: In bonds with low pi character like a conjugated diene, rotation happens spontaneously through a low energy pathway, similar to radical inversion.

Question 28

Why are more electronegative atoms better able to stabilise a negative charge?

Answer 28

It holds the extra electron pair from the charge closer to its nucleus, allowing it to be more tightly held by electrostatic forces of attraction, less easily lost and thus more stable.