Conjugation Flashcards
Conjugation: definition?
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.
What do conjugated molecules contain?
Conjugated molecules contain alternating single and double bonds.
What is the simplest example of three p orbitals in conjugation?
The simplest example of three p orbitals in conjugation is the allyl system, which can accommodate either 2, 3, or 4 π-electrons
How does conjugation affect electrons and charge?
Conjugation leads to delocalisation of electrons and charge
Describe what happens with the conjugation of the allyl cation
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.
Resonance: definition
A way of illustrating delocalised bonding using only localised Lewis structures
What problem occurs when the positive charge and electrons are delocalised over the whole system?
it is no longer obvious what the dotted lines mean in terms of precise bond order.
Who introduced the concept of resonance?
Linus Pauling introduced the concept of resonance in the 1930s.
What was Linus Pauling’s theory/ concept?
a true structure (resonance hybrid) is an average of two, or more, hypothetical localised structures (canonicals) weighted according to their contribution.
What are curly arrows used for in terms of resonance? Are they real or hypothetical?
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).
Rules for drawing resonance structures:
- Nuclear positions must remain the same (only electrons are in different positions).
- The maximum number of valence electrons for 1st row (2nd Period) elements is 8 (octet rule). This cannot be exceeded.
- All resonance forms must have the same number of unpaired electrons
What is the extent of contribution of a given canonical structure determined by?
The extent of contribution of a given canonical structure to the resonance hybrid is determined by its thermodynamic stability.
What general guidelines should you follow to ascertain the relative stability (contribution) of canonical structures?
- Minimise the number of electron-deficient atoms [i.e., 1st row (2nd Period) elements with valence electrons less than 8].
- Minimise the number of formally charged atoms. If unavoidable, ensure the separation for unlike and like charges is minimised and maximised, respectively.
- Place negative charge, if any, on the most electronegative atoms, and positive charge, if any, on the most electropositive atoms.
- Do not deviate from idealised bond lengths and angles.
Why are conjugated heteroatoms are always sp2 –hybridised?
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.
What does conjugating a π-bond with a p-orbital lead to?
conjugating a π-bond with a p-orbital, regardless of the number of electrons in the p-orbital, leads to electronic stabilisation – Estab.
What does conjugation of π-bonds with one another lead to?
electronic stabilisation - so conjugated dienes (or related systems) are generally always more thermodynamically-stable than their non-conjugated isomers.
How does increasing conjugation affect the HOMO-LUMO gap?
Increasing conjugation lowers the HOMO-LUMO gap ( π -> π*), and means that photons of increasingly longer wavelength are absorbed
Basic relationship between conjugation of alkenes and nucleophilicity/electrophilicity:
Conjugation of an alkene with another (simple) alkene increases both nucleophilicity and electrophilicity.
What happens to alkene reactivity if we conjugate the C=C bond with a methoxy group to give an enol ether?
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.
What happens then to alkene reactivity if we conjugate the C=C bond with a carbonyl group (C=O)?
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).
What are Mesomeric Effects?
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).
What is the Inductive effect?
based on electronegativity - refers to electron-donation ( +I) or withdrawal ( -I) effects operating in the σ-network.
What is hyperconjugation?
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
Another name for hyperconjugation?
σ-conjugation
is π-conjugation more or less stabilising than σ-conjugation? Why?
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.
What is positive hyperconjugation?
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).
What is negative hyperconjugation?
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).
What are the effects of hyperconjugation on stability?
hyperconjugation accounts for the stability ordering of carbocations: CH3 < 1° < 2° < 3° .
more heavily-alkylated alkenes are more thermodynamically-stable than less-alkylated alkenes.
What are C-Substituents?
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.
What are Z-Substituents?
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.
What are X-Substituents?
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.
