Linear & Aromatic Organic π-Systems
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 π-systems, and this page explores the frontier molecular orbital (FMO) structure of hydrocarbon π-systems, and then of the same π-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 σ-π (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 empirical rule is that FGs assume discrete identities undergo distinct sets of reactions when separated by two or more alkyl methylene (CH2) carbons:
Consider phenol, benzyl alcohol, 2-phenyl ethanol and 3-phenyl propanol:
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.
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. Some points:
One p-Orbital, Alkene, Allyl System, 1,3-Diene, Pentadienyl System, 1,3,5-Triene:
One p-Orbital Functional Groups
Two Conjugated p-Orbital Functional Groups
Iminium Ion & Protonated Carbonyl: Three Conjugated p-Orbital Functional Groups
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".
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:
Cyclopropenyl Cation: Cyclobutadienyl Dication: Cyclooctatetraenide Cation: Methylene Bridged Cyclodecapentaene: Substituted Benzenes and Susceptibility to Electrophilic Aromatic Substitution:
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:
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