Chapter 14: Conjugated Compounds and Ultraviolet Spectroscopy
Conjugated dienes are more stable than isolated dienes
A diene contains two carbon–carbon double bonds, and the relationship between them determines the chemistry of the molecule. When double and single bonds alternate, as in buta-1,3-diene or the natural-product building block isoprene (2-methylbuta-1,3-diene), the diene is conjugated: every carbon of the diene unit is sp²-hybridized, and the p orbitals form one continuous π system. When one or more sp³-hybridized carbons intervene, as in penta-1,4-diene, the diene is isolated and each double bond reacts as an ordinary alkene. When the two double bonds share a single carbon, as in…
Four p orbitals combine into four π molecular orbitals
Valence bond pictures treat the two double bonds of buta-1,3-diene separately, but the measured stability of the molecule shows the π electrons are not confined in that way. Molecular orbital theory combines the four p orbitals of the four sp²-hybridized carbons into four π molecular orbitals spanning the entire conjugated unit. The combinations differ in the number of nodes — planes where the wavefunction changes sign — between adjacent carbons. Each combination is labeled ψₙ, the conventional symbol for a molecular-orbital wavefunction, and an asterisk on the label, as in ψ₃, marks an…
Electrophilic addition gives 1,2- and 1,4-products through one allylic cation
Electrophilic addition to a conjugated diene begins exactly as it does for a simple alkene: the π electrons attack the electrophile, and the regiochemistry follows from the stability of the resulting carbocation. For buta-1,3-diene and HBr, protonation at a terminal carbon (C1) produces a carbocation at C2 that is both secondary and allylic — its empty p orbital is adjacent to the remaining C3–C4 double bond, so the π electrons delocalize across three carbons. Protonation at an internal carbon (C2) would instead give a primary carbocation with no resonance stabilization, so that pathway does…
Temperature decides between the kinetic and thermodynamic product
Electrophilic addition of HBr to buta-1,3-diene gives different product ratios at different temperatures. At −80 °C the product mixture is 71% 3-bromobut-1-ene (1,2-addition) and 29% 1-bromobut-2-ene (1,4-addition). At 40 °C the same reagents give 15% of the 1,2-adduct and 85% of the 1,4-adduct. Neither result is an error; the two temperatures probe two different questions. Which product forms faster? And which product is more stable at equilibrium? At −80 °C the answer to the first question controls the outcome. Both products form irreversibly from the shared allylic carbocation, so their…
The Diels–Alder reaction builds a cyclohexene ring in one step
Most polar reactions form one bond at a time through charged intermediates. The Diels–Alder reaction is different in kind: a conjugated diene and an alkene or alkyne — the dienophile, "diene-lover" — combine in a single concerted step to form a six-membered ring. In the cyclic transition state the diene and dienophile approach in parallel planes, and six π electrons shift in concert: the diene's two π bonds and the dienophile's π bond convert into two new σ bonds, joining the diene's termini to the dienophile's carbons, plus one π bond that remains in the ring between the diene's original C2…
What makes a good dienophile and a reactive diene
The Diels–Alder reaction is general, but its rate spans many orders of magnitude depending on the electronic character of the dienophile and the conformational behavior of the diene. Ethylene and other simple alkenes are poor dienophiles: their LUMOs lie too high for effective overlap with a diene HOMO, and the parent cycloaddition needs forcing conditions. Attaching an electron-withdrawing group in conjugation with the double bond — the aldehyde of propenal, the nitrile of acrylonitrile, the ester of methyl acrylate, or the paired carbonyls of maleic anhydride — lowers the LUMO energy and…
Conjugation narrows the HOMO–LUMO gap and lengthens λmax
The fourth spectroscopic tool of this sequence, ultraviolet–visible spectroscopy, reports on conjugated π systems. Irradiating a sample across the 200–400 nm range promotes π electrons from filled to empty molecular orbitals; the lowest-energy such excitation carries an electron from the HOMO to the LUMO, a π → π transition, and appears as a broad band whose maximum defines λmax. For buta-1,3-diene the transition moves an electron from ψ₂ to ψ₃, and λmax = 217 nm. Wavelength converts to energy through E = 1.196 × 10⁵ kJ·nm·mol⁻¹ / λ, whose constant is the product hc·Nₐ of Planck's constant,…
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