Conjugation

Question 1

Conjugation: definition?

Answer 1

refers to the sideways-on ( π-type) overlap of two or more adjacent orbitals in a molecule, such that delocalisation of electrons is possible over two or more atoms.

The term conjugation describes the interaction of a π bond with another π bond or a p orbital.

Question 2

What do conjugated molecules contain?

Answer 2

Conjugated molecules contain alternating single and double bonds.

Question 3

What is the simplest example of three p orbitals in conjugation?

Answer 3

The simplest example of three p orbitals in conjugation is the allyl system, which can accommodate either 2, 3, or 4 π-electrons

Question 4

How does conjugation affect electrons and charge?

Answer 4

Conjugation leads to delocalisation of electrons and charge

Question 5

Describe what happens with the conjugation of the allyl cation

Answer 5

We have only 2 electrons to accommodate in the π-system (those in the C=C bond).

This shows us that the LUMO is focused on the termini of the allyl system.

This is the empty orbital that nucleophiles best interact with, so we can predict that nucleophiles will only attack at C(1) or C(3).

We can also now see that the 2 electrons that are present (in the HOMO) are actually spread over the entire allyl system, rather than being constrained to a single bond - the electrons are delocalised.

Question 6

Resonance: definition

Answer 6

A way of illustrating delocalised bonding using only localised Lewis structures

Question 7

What problem occurs when the positive charge and electrons are delocalised over the whole system?

Answer 7

it is no longer obvious what the dotted lines mean in terms of precise bond order.

Question 8

Who introduced the concept of resonance?

Answer 8

Linus Pauling introduced the concept of resonance in the 1930s.

Question 9

What was Linus Pauling’s theory/ concept?

Answer 9

a true structure (resonance hybrid) is an average of two, or more, hypothetical localised structures (canonicals) weighted according to their contribution.

Question 10

What are curly arrows used for in terms of resonance? Are they real or hypothetical?

Answer 10

Curly arrows are used to illustrate bonding changes - to help visualise how one resonance canonical relates to another.

in this context curly arrows only represent hypothetical, as opposed to real, bonding changes (because canonical structures themselves are hypothetical).

Question 11

Rules for drawing resonance structures:

Answer 11

1. Nuclear positions must remain the same (only electrons are in different positions).
2. The maximum number of valence electrons for 1st row (2nd Period) elements is 8 (octet rule). This cannot be exceeded.
3. All resonance forms must have the same number of unpaired electrons

Question 12

What is the extent of contribution of a given canonical structure determined by?

Answer 12

The extent of contribution of a given canonical structure to the resonance hybrid is determined by its thermodynamic stability.

Question 13

What general guidelines should you follow to ascertain the relative stability (contribution) of canonical structures?

Answer 13

1. Minimise the number of electron-deficient atoms [i.e., 1st row (2nd Period) elements with valence electrons less than 8].
2. Minimise the number of formally charged atoms. If unavoidable, ensure the separation for unlike and like charges is minimised and maximised, respectively.
3. Place negative charge, if any, on the most electronegative atoms, and positive charge, if any, on the most electropositive atoms.
4. Do not deviate from idealised bond lengths and angles.

Question 14

Why are conjugated heteroatoms are always sp2 –hybridised?

Answer 14

conjugation of a lone pair with an adjacent p or π orbital is most effective when the lone pair is situated in a pure p orbital.

Question 15

What does conjugating a π-bond with a p-orbital lead to?

Answer 15

conjugating a π-bond with a p-orbital, regardless of the number of electrons in the p-orbital, leads to electronic stabilisation – Estab.

Question 16

What does conjugation of π-bonds with one another lead to?

Answer 16

electronic stabilisation - so conjugated dienes (or related systems) are generally always more thermodynamically-stable than their non-conjugated isomers.

Question 17

How does increasing conjugation affect the HOMO-LUMO gap?

Answer 17

Increasing conjugation lowers the HOMO-LUMO gap ( π -> π*), and means that photons of increasingly longer wavelength are absorbed

Question 18

Basic relationship between conjugation of alkenes and nucleophilicity/electrophilicity:

Answer 18

Conjugation of an alkene with another (simple) alkene increases both nucleophilicity and electrophilicity.

Question 19

What happens to alkene reactivity if we conjugate the C=C bond with a methoxy group to give an enol ether?

Answer 19

The oxygen lone pair is now conjugated with the C=C bond, so the lone pair occupies a p-orbital in order to maximise overlap.

The MOs for the enol ether are analogous to those for the allyl anion, but the symmetry is broken and all of the MOs are lowered in energy due to the electronegative oxygen.

Conjugation of alkenes to lone groups makes the C=C bond more nucleophilic.

Question 20

What happens then to alkene reactivity if we conjugate the C=C bond with a carbonyl group (C=O)?

Answer 20

The introduction of an electronegative oxygen into the system lowers the energies of all the MOs, including the HOMO and the LUMO.

Conjugation of alkenes to carbonyl (C=O) groups makes the C=C bond more electrophilic (lowers the LUMO).

Question 21

What are Mesomeric Effects?

Answer 21

electron-donation or withdrawal effects can also operate in the π-network, and that these can be qualitatively described using resonance structures.

These effects are termed mesomeric effects, and given the symbols +M (for π-electron donors) or -M (for π-electron acceptors).

Question 22

What is the Inductive effect?

Answer 22

based on electronegativity - refers to electron-donation ( +I) or withdrawal ( -I) effects operating in the σ-network.

Question 23

What is hyperconjugation?

Answer 23

It is possible for σ-orbitals to engage in conjugation also, either by side-on overlap with p or π orbitals, or with other σ-orbitals.

This type of conjugation is called hyperconjugation

Question 24

Another name for hyperconjugation?

Answer 24

σ-conjugation

Question 25

is π-conjugation more or less stabilising than σ-conjugation? Why?

Answer 25

σ-conjugation is much less stabilising than π-conjugation.

because the donor orbital (a σ(C-H) bonding orbital) is far lower in energy than a bonding orbital or a non-bonded pair, and so the orbital-orbital interaction is less strong.

Question 26

What is positive hyperconjugation?

Answer 26

A +M mesomeric effect (positive hyperconjugation) occurs when a filled σ-orbital serves as a donor (electron source) in the stabilising interaction, the effect being more powerful when the σC-M orbital is higher in energy (typically “M” more electropositive).

Question 27

What is negative hyperconjugation?

Answer 27

A -M mesomeric effect (negative hyperconjugation) occurs when an empty σ-orbital serves as an acceptor (electron sink) in the stabilising interaction, the effect being more powerful when the σC-X orbital is lower in energy (typically “X” more electronegative).

Question 28

What are the effects of hyperconjugation on stability?

Answer 28

hyperconjugation accounts for the stability ordering of carbocations: CH3 < 1° < 2° < 3° .

more heavily-alkylated alkenes are more thermodynamically-stable than less-alkylated alkenes.

Question 29

What are C-Substituents?

Answer 29

they are simple conjugated systems of carbon atoms.

They may be donors or acceptors, depending upon what they are conjugated with, responding to and stabilising electron demand or electron excess, as appropriate.

Question 30

What are Z-Substituents?

Answer 30

conjugated systems that withdraw electrons from double bonds that they are conjugated with.

most of them have electronegative heteroatoms, they are strong acceptors and usually weak, but occasionally, strong π-acceptors, especially for substituents like nitro and sulfonyl, where an electronegative heteroatom is the point of attachment.

Question 31

What are X-Substituents?

Answer 31

They are π-donors and σ-acceptors.

typically electronegative atoms like N, O or S with a lone pair

they donate their lone pairs to a π-system, and those based on electronegative atoms withdraw electrons from the σ-framework.