Organic 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 p-systems, and this page explores the frontier molecular orbital (FMO) structure of hydrocarbon p-systems, and then of the same p-systems with embedded heteroatoms using Hückel MO theory.
Introduction 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 of sigma-p separability assumes that as p electrons are at a much higher energy than the sigma skeleton electrons, the sigma and p electrons have no influence upon each other. Thus:
The sigma skeleton of an organic molecule can be described using VSEPR theory, with the p system functional groups superimposed on top.
• Heteroatoms are embedded into the hydrocarbon sigma skeleton
• Functional groups have characteristic sets of sigma and p FMOs.
The empirical rule is that FGs assume discrete identities – undergo distinct sets of reactions – when separated by two or more alkyl methylene (–CH2–) carbons:
FG–CH2–CH2–FG
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 substitution
The aromatic ring renders the hydroxy function a stronger Brønsted acid than an alkyl alcohol.
• 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 p 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 p systems.
• VB resonance theory is more qualitative. Resonance structures are interconverted 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 p systems.
• The Hückel and VB approaches are complementary.
The simplest p 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.
Some points:
• Electrons in p HMOs 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 p MO structure there may be many possible VB resonance structures.
• The p MOs form regular patterns. Polyene ribbon p-systems are arranged so that the lowest energy psi1 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 psi2, psi3... 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.
• Frontier molecular 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 p 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 with psi2.
One p-Orbital
Calculated at the Hartree-Fock 6-31G* level using Spartan.
Two p-Orbitals
Three p-Orbitals
Two + Two p-Orbitals
Two p-Orbitals: Cumulenes
Four p-Orbitals
Five p-Orbitals
Six p-Orbitals
Aromaticity
- –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'.
| Polyatomic Species: Hybrid & Molecular Orbitals |
Pericyclic
Reaction Chemistry
|
© Mark R. Leach 1999-2008
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