16.4
π Molecular Orbitals of the Allyl Cation and Anion
An allyl group is a three-carbon conjugated system where the sp³-hybridized allylic carbon is bonded to a CH=CH2 group via a single bond. Allyl anions…
Recall that 1,3-butadiene is a four-carbon conjugated system. In comparison, the allyl cation and anion are three-carbon conjugated systems.
A linear combination of the three unhybridized atomic p orbitals in allylic systems gives three π molecular orbitals.
The lowest energy molecular orbital, ψ1, is formed by an in-phase overlap of all p orbitals and is entirely bonding.
ψ2 has one node passing through the central carbon. Since there is no overlap between adjacent carbons, this is a nonbonding orbital and energetically similar to the atomic orbitals.
ψ3 is entirely antibonding with three out-of-phase p orbitals forming two nodes.
The two π electrons of the allyl cation fill up ψ1, which becomes the HOMO, making ψ2 the LUMO.
In comparison, the allyl anion has four π electrons distributed between ψ1 and ψ2, where ψ2 is the HOMO and ψ3 the LUMO.
Interestingly, in both systems, the charge is concentrated on the end carbons, which agrees with the resonance picture.
View the full transcript and gain access to JoVE Core videos
Q1: How many molecular orbitals form from the p orbitals in an allyl system?
Three molecular orbitals form from a linear combination of the three unhybridized atomic p orbitals in allylic systems. These are designated ψ1, ψ2, and ψ3, each with distinct bonding characteristics and energy levels that determine electron distribution in allyl cations and anions.
Q2: What is the difference between bonding, nonbonding, and antibonding orbitals in the allyl system?
ψ1 is entirely bonding with in-phase p orbital overlap. ψ2 is nonbonding with a node through the central carbon and no adjacent orbital overlap. ψ3 is entirely antibonding with out-of-phase p orbitals forming two nodes. These orbital types determine electron stability and reactivity in allyl species.
Q3: How do allyl cations and anions differ in electron distribution across molecular orbitals?
The allyl cation has two π electrons filling ψ1, making ψ1 the HOMO and ψ2 the LUMO. The allyl anion has four π electrons distributed between ψ1 and ψ2, with ψ2 as the HOMO and ψ3 as the LUMO. Despite different electron counts, both systems share identical molecular orbital frameworks.
Q4: Why is charge concentrated on the end carbons in allyl cations and anions?
Charge concentration on end carbons reflects the molecular orbital electron distribution. In both cations and anions, the central carbon's p orbital contributes minimally to charge density due to the nonbonding nature of ψ2, which has a node at that position, leaving positive or negative charge localized at the terminal carbons.
Q5: What hybridization change occurs when an allyl group forms from propene?
The sp³-hybridized allylic carbon in propene converts to sp² hybridization when an allyl cation or anion forms. This change generates three sp²-hybridized carbons, each with an unhybridized p orbital available for π bonding and molecular orbital formation in the conjugated system.
Q6: How are allyl cations and anions generated in organic reactions?
Allyl cations form as intermediates during substitution reactions involving allylic halides. Allyl anions are obtained by treating propene with a strong base that deprotonates methyl groups. Both processes create three-carbon conjugated systems with sp² hybridization and pi molecular orbitals available for bonding.
Q7: How do allyl systems compare to 1,3-butadiene in terms of conjugation?
The allyl cation and anion are three-carbon conjugated systems, while 1,3-butadiene is a four-carbon conjugated system. Both involve linear combinations of unhybridized p orbitals to form molecular orbitals, but allyl systems have three π molecular orbitals compared to butadiene's four, affecting their electronic properties and reactivity.