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14. Conjugated Compoundsand Ultraviolet Spectroscopy

Based on McMurrys Organic Chemistry , 7 th edition

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Conjugated and NonconjugatedDienes

Compounds can have more than one double or triplebondIf they are separated by only one single bond theyare conjugated and their orbitals interact

The conjugated diene 1,3-butadiene has propertiesthat are very different from those of thenonconjugated diene , 1,5-pentadiene

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Why this Chapter?

Conjugated compounds are common in natureExtended conjugation leads to absorption of visiblelight, producing color

Conjugated hydrocarbon with many double bonds arepolyenes (lycopene is responsible for red color intomatoes)Examine properties of conjugated molecules andreasons for the properties

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14.1 Stability of Conjugated Dienes:Molecular Orbital Theory

Typically by elimination in allylic halideSpecific industrial processes for large scaleproduction of commodities by catalyticdehydrogenation and dehydration

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Measuring Stability

Conjugated dienes are more stable thannonconjugated based on heats of hydrogenation

Hydrogenating 1,3-butadiene produces 16 kJ/molless heat than 1,4-pentadiene

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Molecular Orbital Description of1,3-Butadiene

The single bond between the conjugated doublebonds is shorter and stronger than sp 3 The bonding -orbitals are made from 4 p orbitalsthat provide greater delocalization and lower energy

than in isolated C=CThe 4 molecular orbitals include fewer total nodesthan in the isolated case (See Figures 14-1 and 14-2)In addition, the single bond between the two double

bonds is strengthened by overlap of p orbitalsIn summary, we say electrons in 1,3-butadiene aredelocalized over the bond system

Delocalization leads to stabilization

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14.2 Electrophilic Additions to ConjugatedDienes: Allylic Carbocations

Review: addition of electrophile to C=CMarkovnikov regiochemistry via more stablecarbocation

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Carbocations from ConjugatedDienes

Addition of H + leads to delocalized secondary allyliccarbocation

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Products of Addition to DelocalizedCarbocation

Nucleophile can add to either cationic siteThe transition states for the two possible products arenot equal in energy

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14.3 Kinetic vs. ThermodynamicControl of Reactions

At completion, all reactions are at equilibrium and therelative concentrations are controlled by thedifferences in free energies of reactants and products(Thermodynamic Control )

If a reaction is irreversible or if a reaction is far fromequilibrium, then the relative concentrations ofproducts depends on how fast each forms, which iscontrolled by the relative free energies of thetransition states leading to each ( Kinetic Control )

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Kinetic and ThermodynamicControl Example

Addition to a conjugated diene at or belowroom temperature normally leads to a mixtureof products in which the 1,2 adduct

predominates over the 1,4 adduct At higher temperature, product ratio changesand 1,4 adduct predominates

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14.7 Structure Determination in ConjugatedSystems: UV Spectroscopy

Conjugated compounds can absorb light in theultraviolet region of the spectrumThe electrons in the highest occupied molecularorbital (HOMO) undergo a transition to the lowestunoccupied molecular orbital (LUMO)The region from 2 x 10 -7m to 4 x 10 -7m (200 to 400nm) is most useful in organic chemistry

A plot of absorbance (log of the ratio of the intensityof light in over light transmitted) against wavelengthin this region is an ultraviolet spectrum see Figure14-11

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Ultraviolet Spectrum of 1,3-Butadiene

Example: 1,4-butadiene has four molecular orbitalswith the lowest two occupiedElectronic transition is from HOMO to LUMO at 217nm (peak is broad because of combination with

stretching, bending)

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Quantitative Use of UV Spectra

Absorbance for a particular compound in a specificsolvent at a specified wavelength is directlyproportional to its concentrationYou can follow changes in concentration with time by

recording absorbance at the wavelengthBeers law: absorbance = cl

is molar absorptivity (extinction coefficient) c is concentration in mol/L

l is path of light through sample in cm

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14.8 Interpreting UV Spectra: TheEffect of Conjugation

max : wavelength where UV absorbance for acompound is greatestEnergy difference between HOMO and LUMOdecreases as the extent of conjugation increases

max increases as conjugation increases (lowerenergy)

1,3-butadiene: 217 nm, 1,3,5-hexatriene: 258 nm

Substituents on system increasemax

See Table 14-2

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14.9 Conjugation, Color and theChemistry of Vision

Visible region is about 400 to 800 nmExtended systems of conjugation absorb in visibleregion b -Carotene, 11 double bonds in conjugation, max =

455 nmVisual pigments are responsible for absorbing light ineye and triggering nerves to send signal to brain