Linear & Aromatic Organic π-Systems
chemistry is dominated by the functional group approach. This approach
invokes the experimental observation that ethanal, propanal, butanal, pentanal,
hexanal, etc., all have an "aldehyde", R-CHO, functional group
(FG) and that the spectrum of reactivity of the aldehyde FG is largely
independent of the alkyl group to which it is attached. Thus, it is only
necessary to understand the chemistry of the R-CHO aldehyde FG to predict
the chemistry of a whole range of compounds that possess the R-CHO function.
Most functional groups are associated with π-systems,
and this page explores the frontier molecular orbital (FMO) structure of
and then of the same π-systems
with embedded heteroatoms using Hückel MO theory.
to Organic Functional Groups
MO theory formally assigns
a molecule all encompassing molecular orbitals, however, it is usual and
convenient to regard the structure of larger organic molecules as being
constructed from discrete functional groups or FGs.
The Hückel principle
(sigma-pi) separability assumes that as π-electrons are at a much higher energy than the σ-skeleton electrons,
the sigma and π-electrons have no influence upon each other. Thus:
The sigma skeleton of
an organic molecule can be described using VSEPR theory, with the
π-system functional groups superimposed.
- Heteroatoms are embedded
into the hydrocarbon sigma skeleton
- Functional groups
have characteristic sets of sigma and π FMOs.
The empirical rule is that
FGs assume discrete identities undergo distinct sets of reactions
when separated by two or more alkyl methylene (CH2)
Consider phenol, benzyl alcohol,
2-phenyl ethanol and 3-phenyl propanol:
Phenol, Ph-OH, acts
as a single function because the alcohol OH group is directly
attached to the aromatic ring
- The hydroxy group has
the effect of activating the aromatic ring for electrophilic aromatic
- The aromatic ring renders
the hydroxy function a stronger Brønsted acid than an alkyl
In benzyl alcohol,
PhCH2OH, the aromatic ring has a very small influence
upon the alcohol function and vice versa.
With 2-phenyl ethanol,
PhCH2CH2OH, the benzene
and the alcohol functions have little direct influence (although beta
elimination to styrene, PhCH=CH2, is possible.
The aromatic and alcohol
functions are effectively independent in 3-phenyl propanol, Ph(CH2)3OH.
Test your knowledge of
functional groups using the Chemistry
Tutorials & Drills website.
Hückel MO and VB Resonance Construction of Polyene Ribbons π-System
FGs can be modelled by both
Hückel molecular orbital (HMO) theory and VB resonance models.
Hückel MO theory
is the more sophisticated technique. It can provide quantitative information
about orbital phase, electron energy and electron density for linear,
branched, cyclic, polycyclic, charged and uncharged, hydrocarbon and
heteroatom containing π-systems.
VB resonance theory
is more qualitative. Resonance structures are inter converted by curly
arrows as an aid to predicting where partial charges (which can be equated
with reactive sites) will occur. The VB technique is particularly useful
for heteroatom containing π-systems.
The Hückel and
VB approaches are complementary.
The simplest π-systems are the electronically neutral linear polyene ribbons: ethylene,
1,3-butadiene, 1,3,5-hexatriene, etc. However, the full series includes
the isolated p-orbital, the allyl system of three adjacent p-orbitals
and the pentadienyl system of five adjacent p-orbitals.
Electrons in π-FMOs are delocalised over all adjacent p orbitals.
In the VB model,
localised alkene type double bonds, anion, cation and radical centres
are deemed to exist.
For a given π-MO structure there may be many possible VB resonance structures.
MOs form regular patterns. Polyene ribbon π-systems
are arranged so that the lowest energy ψ1 MOs
have p orbital phases which match right along the "top" and the "bottom"
of the system. As the polyenes grow in length, more electrons are required
to retain near (+1/1) electrical neutrality. Electrons enter the
increasing energy ψ2, ψ3...
MOs in pairs according to the Aufbau principle and Hund's rule.
As energy increases,
each MO has one more node (along the length of the polyene ribbon) than
the previous MO. At a node there is a phase change.
With alkenes, dienes
and trienes all the nodes always occur between atoms, but with
the three p orbital allyl system and the five p orbital pentadienyl
systems, the nodes sometimes occur at an atom. As a node corresponds
to a region of zero electron density these MOs are designated as non-bonding
orbitals. Two points:
- It is common to
show the positions of these nodal atoms with a "dot".
- Bonding MOs always
have corresponding antibonding MOs, but non-bonding MOs do not.
orbital theory stresses the importance of an FG's HOMO, LUMO and SOMO.
- Alkenes, dienes
and trienes are electronically neutral species which contain two,
four and six π-electrons respectively.
- The HOMO and LUMO
phase characteristics of these polyene ribbon systems can be easily
determined from the diagrams.
- Allyl cations, radicals
and anions have their respective LUMOs, SOMOs and HOMOs associated
One p-Orbital, Alkene, Allyl System, 1,3-Diene, Pentadienyl System, 1,3,5-Triene:
One p-Orbital Functional Groups
Calculated at the Hartree-Fock 6-31G* level using Spartan.
Two Conjugated p-Orbital Functional Groups
Ethene, Ethylene, Alkene:
Iminium Ion & Protonated Carbonyl:
Three Conjugated p-Orbital Functional Groups
Enolate Ion :
O-Alkylated Ketone / Vinyl Ether:
Carboxylic Acid, Carboxylate Anion, Carboxylate Dianion:
Two + Two Conjugated p-Orbital Functional Groups
Protonated HCN, Hydrogen Cyanide, Cyanide Ion:
Protonated Alkyl Nitrile, Alkyl Nitrile, Nitrile α-Carbanion:
Protonated Nitrogen, Nitrogen (Dinitrogen)
Two + Two Conjugated p-Orbital Functional Groups: Cumulenes
Alkyl Isocyanate, Alkyl Isothiocyanate, Alkyl Azide, Hydrazoic acid:
Four Conjugated p-Orbital Functional Groups
α,β-Unsaturated Ketone, Aldehyde, Ester, Nitrile, Sulfone & Nitrocompound:
Five Conjugated p-Orbital Functional Groups
β-Diketone, β-Ketoester, Dialkyl Malonate, β-Ketoacid, etc:
Six Conjugated p-Orbital Functional Groups
Cyclic Array of p-Orbitals Containing 4n+2 Electrons: Aromaticity
Some unsaturated organic ring systems, such as benzene, C6H6, are unexpectedly stable and are said to be aromatic. A quantum mechanical basis for aromaticity, the Hückel method, was first worked out by physical chemist Erich Hückel in 1931.
In 1951 von Doering succinctly reduced the Hückel analysis to the "4n + 2 rule".
- Aromaticity is associated a cyclic array of adjacent p-orbitals containing 4n+2 π-electrons, where n is zero or any positive integer.
- Aromaticity is associated with cationic, anionic and heterocyclic π-systems, as well as neutral hydrocarbon structures like benzene and naphthalene.
- Aromaticity can be identified by a ring current and associated down-field chemical shift in the proton NMR spectrum. (Aromatic compounds have a chemical shift of 7-8 in the proton spectrum).
1,2-Diazine (Pyridazine), 1,3-Diazine (Pyrimidine), 1,4-Diazine (Pyrazine) & 1,2,4-Traizine:
1,3,5-Traizine, 1,2,4,5-Tetrazine, Pyrylium Ion & Thiopyrylium Ion:
Some Exotic Aromatic Systems:
- H+ means remove a proton to form the conjugate base.
- Nfg means remove a nucleofugal Lewis base like Cl, TsO, etc. to form the 'conjugate cation'.
Methylene Bridged Cyclodecapentaene:
Substituted Benzenes and Susceptibility to Electrophilic Aromatic Substitution:
- Electron donating functions, such as the -OH group, activate a benzene ring towards SEAr reactions and are ortho, para (2, 4, 6) directing.
- Electron withdrawing groups, such as the -NO2 nitro group, deactivate the benzene ring towards SEAr and are meta (3, 5) directing, but they activate towards SNAr assuming there is a suitable leaving group present, such as a halogen.
Activating and directing effects can be parameterised using carbon-13 NMR data, specifically the 13C chemical shift of the para-carbon (4-position) of a mono-substituted benzene. The effect is more easily visualised if the chemical shift data is quoted with respect to benzene rather than TMS.
Species with para-carbons “up-field” of benzene (negative chemical shift values) are electron rich at the para 4-position and are assumed to be electron rich at the ortho (2 & 6) positions as well. The 2, 4 & 6 positions are correspondingly susceptible to electrophilic aromatic substitution. The more “up-field” the para-carbon is, the more activated the compound is for ortho-para directed SEAr.
Species with para-carbons “down-field” of benzene (positive chemical shift) are electron poor at the ortho-para (2, 4 & 6-) positions making the meta (3 & 5-) positions relatively electron rich. The effect is that the meta positions are the more susceptible to electrophilic substitution, although these positions are deactivated w.r.t. benzene.
If a system with a down-field para-carbon has a nucleofugal leaving group (usually chloride or bromide) ortho or para to the main function, the Nfg functions are susceptible to nucleophilic aromatic substitution, SNAr. Again the greater the down-field shift w.r.t. benzene the more pronounced the effect, assuming that no other reaction pathway predominates.
These effects are additive: two or three electron donating or electron withdrawing groups will have more influence than one.
Why The para-Carbon?
13C NMR spectroscopy, briefly, measures the electronic environment of carbon atoms. For aromatic carbon atoms there are at least three electronic environmental components which contribute to the chemical shift of a particular carbon atom:
- The aromatic ring-current which has the effect of sending aromatic carbons down-field by 100ppm or so.
- Perturbations to the aromatic ring current caused by attached functional groups acting through the π-system.
- Direct, local and “through-space” effects caused by adjacent functional groups. Unfortunately, these cause very large perturbations to the chemical shift of ortho (and to a lesser extent meta) carbons.
|Polyatomic Species: Hybrid & Molecular Orbitals
Functional Group Database
© Mark R. Leach 1999-
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