Conjugated Systems CHEM 2425 Chapter 14

Isolated and Conjugated Dienes

Conjugated systems Benzene Commonly found in nature as pigments, hormones Isoprene

outline Structure

I. Structure Consider molecules with multiple double bonds… Example: pentadienes cumulated (allenes) conjugated isolated separated by: 0 single bonds 1 single bond 2 or more single bonds Which is most stable? Why? Long answer = molecular orbital theory Short answer = orbital overlap

1,3-butadiene stability Unusually strong s bond from greater s-character of C atoms

1,3-butadiene stability Overlap of p atomic orbitals In its most stable conformation, the p orbitals are parallel Allows for p electron delocalization Single bond has partial C=C character, which also contributes to stability

Structure, cont. Conformations of 1,3-butadiene: s-cis (single cis) = double bonds on same side of single bond s-trans (single trans) = double bonds on opposite sides of single bond The two conformations easily interconvert at room temperature Which conformation is more stable?

Conformations of conjugated dienes Mild steric hindrance in s-cis conformation

Butadiene Which conformation appears to be more stable? a. s-cis   a. s-cis b. s-trans c. neither

Estimating the Relative Stability of Resonance Structures The more covalent bonds a structure has, the more stable it is

Structures in which all of the atoms have a complete valence shell of electrons (i.e., the noble gas structure) are especially stable and make large contributions to the hybrid this carbon has 6 electrons this carbon has 8 electrons

Charge separation decreases stability

Alkadienes and Polyunsaturated Hydrocarbons Alkadienes (“Dienes”)

Alkatrienes (“Trienes”)

The Stability of Conjugated Dienes Conjugated alkadienes are thermodynamically more stable than isomeric isolated alkadienes

outline Reactions

Reactions of Isolated Dienes

The Mechanism

Double Bonds can have Different Reactivities

Electrophilic Attack on Conjugated Dienes: 1,4 Addition

Mechanism X (a) (b)

Reactions of Conjugated Dienes

1,2-Addition and 1,4-Addition

Reaction of a Conjugated Diene Mechanism for the Reaction of a Conjugated Diene

A. Kinetic Control versus Thermodynamic Control of a Chemical Reaction

The Diels–Alder Reaction forms a Six-Membered Ring

The Mechanism

Faster if there is an Electron Withdrawing Group on the Dienophile

The Electron Withdrawing Group makes the Electrophile a better Electrophile

Another Diels–Alder Reaction

Alkynes can also be Dienophiles The cyclic product has two double bonds.

The Stereochemistry of the Diels–Alder Reaction The product will be a racemic mixture.

How to Determine the Reactants of a Diels–Alder Reaction

The Diels–Alder Reaction: A 1,4-Cycloaddition Reaction of Dienes

e.g.

A. Factors Favoring the Diels–Alder Reaction Type A and Type B are normal Diels-Alder reactions

Type C and Type D are Inverse Demand Diels-Alder reactions

Relative rate

Relative rate

Steric effects

B. Stereochemistry of the Diels–Alder Reaction The Diels–Alder reaction is stereospecific: The reaction is a syn addition, and the configuration of the dienophile is retained in the product

The diene, of necessity, reacts in the s-cis rather than in the s-trans conformation X

e.g.

Cyclic dienes in which the double bonds are held in the s-cis conformation are usually highly reactive in the Diels–Alder reaction Relative rate

The Diels–Alder reaction occurs primarily in an endo rather than an exo fashion when the reaction is kinetically controlled R is exo longest bridge R is endo

Alder-Endo Rule If a dienophile contains activating groups with p bonds they will prefer an ENDO orientation in the transition state

e.g.

Stereospecific reaction

Stereospecific reaction

Examples

Diene A reacts 103 times faster than diene B even though diene B has two electron-donating methyl groups

Examples

Examples Rate of Diene C > Diene D (27 times), but Diene D >> Diene E In Diene C, tBu group  electron donating group  increase rate In Diene E, 2 tBu group  steric effect, cannot adopt s-cis conformation

Ultraviolet and Visible Spectroscopy UV/Vis spectroscopy provides information about compounds with conjugated double bonds.

An Electronic Transition Only organic compounds with π electrons can produce UV/Vis spectra. A UV spectrum is obtained when UV light (180 to 400 nm) is absorbed. A visible spectrum is obtained when visible light (400 to 780 nm) is absorbed.

A UV Spectrum

UV/Vis Absorption Bands are Broad UV/Vis absorption bands are broad because an electronic state has vibrational sublevels.

All these compounds have the same UV spectrum. Chromophore All these compounds have the same UV spectrum. A chromophore is that part of a molecule that is responsible for a UV/Vis spectrum.

The Beer–Lambert Law A = ε c l A = absorbance of the sample c = concentration of substance in solution l = length of the light path in cm ε = molar absorptivity of the sample (characteristic of the compound)

Cells Used for Taking UV/Vis Spectra

The More Conjugated Double Bonds, the Longer the Wavelength

The More Conjugated Double Bonds, the Longer the Wavelength

Conjugation Makes the Electronic Transition Easier

Colored Compounds Absorb Visible Light (> 400 nm) β-Carotene is found in carrots, apricots, and flamingo feathers. Lycopene is found in tomatoes, watermelon, and pink grapefruit.

Auxochrome An auxochrome is a substituent that alters the position and intensity of the absorption.

Common Dyes

Anthocyanins Responsible for the red, purple, and blue colors of many flowers and fruits.

UV/Vis Spectroscopy Can Be Used to Measure the Rate of a Reaction

UV/Vis Spectroscopy Can Be Used to Measure the Rate of a Reaction

UV/Vis Spectroscopy Can Be Used to Determine a pKa Value The phenolate ion absorbs at 287 nm, but phenol does not.

UV/Vis Spectroscopy Can Be Used to Determine the Melting Temperature of DNA The temperature increases with increasing numbers of G-C base pairs.