Conjugation in Alkadienes and Allylic Systems. A double bond can act like a substituent and give...

Conjugation in AlkadienesConjugation in Alkadienesand Allylic Systemsand Allylic Systems

A double bond can act like a substituent and give other groups special properties and reactivity.

For example carbocations, radicals and anions connected to alkenes are called allylic carbocations, allylic radicals and allylic anions.

Conjugation in Alkadienes and Allylic Systems Conjugation in Alkadienes and Allylic Systems

Alkenes connected by a single bond are called conjugated dienes.

The Allylic GroupThe Allylic Group

Allyl is both a common name and a permissible IUPAC name for the H2C=CHCH2 group.

The sp3 hybridized carbon of an allyl carbon is the allylic carbon.

Each of the following allyl groups are resonance stabilized and the charge or unpaired electron is shared between the two end carbons.

Resonance Forms and DelocalizationResonance Forms and Delocalization

In resonance structures that are not equivalent the resonance structures do not contribute equally to thehybrid.

Resonance StructuresResonance Structures

The positive charge is shared between the two carbons that have positive charge in the individual resonance structures.

In valence bond theory a -bond that covers all threecarbon atoms is generated from the three adjacent p orbitals.

Valence Bond Theory and Allyl CationsValence Bond Theory and Allyl Cations

The resonance stabilized carbocation is much more stable so the rate determining ionization step is much faster.

Allylic Halides and SAllylic Halides and SNN1 Reactions1 Reactions

Allylic halides react much faster than tertiary alkyl halides.For example:

Since the positive charge is shared between two carbonsthe water attaches to both carbons to form two products:

Allylic Resonance and SAllylic Resonance and SNN1 Reactions1 Reactions

Intermediate cation

Reaction Equation.

Mechanism of Hydrolysis of Allylic ChloridesMechanism of Hydrolysis of Allylic Chlorides

Step 1. Ionization.

Step 2a. Addition of water to one end.

Step 2b. Addition of water to the other end.

Mechanism of Hydrolysis of Allylic ChloridesMechanism of Hydrolysis of Allylic Chlorides

Step 3a. Deprotonation.

Step 3b. Deprotonation.

The major product corresponds to the more stable resonance structure.

Any other allylic chloride that forms the same stabilized cation should form exactly the same mixture of products.

Confirmation of MechanismConfirmation of Mechanism

Indeed this is the case:

Common intermediate cation

For SN2 reactions we compare reactions of a strong nucleophile with a series of primary alkyl halides including allylic halides.

Allylic Halides and SAllylic Halides and SNN2 Reactions2 Reactions

SN2 reactions occur when allylic chlorides react with good nucleophiles.

SSNN2 Reactions of Allylic Chlorides2 Reactions of Allylic Chlorides

Allylic Free-Radical HalogenationAllylic Free-Radical Halogenation

The allylic radical is stabilized by electron delocalizationexpressed as resonance between Lewis structures andshould be easy to form selectively.

Allylic Radical StablizationAllylic Radical Stablization

Delocalization of the unpaired electron stabilizes the allylic radicals so the bond dissociation enthalpies arelower.

Quantifying the Stability of Allylic RadicalsQuantifying the Stability of Allylic Radicals

The allylic radical is 55 kJ/mol more stable than propyl radical.

Free radical reaction of propene with chlorine at high temperatures is an industrial process.

Allylic ChlorinationAllylic Chlorination

Mechanism of Allylic ChlorinationMechanism of Allylic Chlorination

Step 1: Bond dissociation. Initiation

Step 2: Hydrogen atom abstraction. First propagation step.

Step 3: Chlorine atom abstraction. Second propagation step.

The two propagation steps repeat over and over.

Allylic brominations are usually carried out with N-bromo-succinimide.

Allylic HalogenationAllylic Halogenation

Allylic halogenation is a selective reaction provided that all the allylic hydrogens are equivalent and lead to equivalent resonance forms.

Allylic HalogenationAllylic Halogenation

alkene equivalent radical resonance forms

If the allylic resonance forms are not equivalent then a mixture of isomeric products is formed.

Unselective Allylic HalogenationUnselective Allylic Halogenation

The allylic radical reacts through the two resonance structures shown below.

Allylic AnionsAllylic Anions

The allylic anion is planar and stabilized by electron

delocalization. In resonance terms:

Allylic AnionsAllylic Anions

The electrostatic potential map shows the electrondelocalization as compared to propyl anion.

Red showshigh electrondensity

A comparison of the pKa’s of propane and propene give a measure of the relative stability of the two anions.

pKpKaa and Stability of Allylic Anions and Stability of Allylic Anions

Classes of Dienes:Classes of Dienes:Conjugated and OtherwiseConjugated and Otherwise

Dienes may be:

Conjugated in which the two alkenes are joined by a C-Csingle bond;

Isolated in which there is at least one sp3 carbon between

the alkenes;

Cummulated in which two alkenes share a carbon.

Classes of DienesClasses of Dienes

Alkadienes are named by replacing the -ane ending and replacing it with –adiene and adding the locant for each alkene.

Naming DienesNaming Dienes

(E)-1,3pentadiene 1,6-heptadiene

1,4-cyclohexadiene

The stabilities of conjugated, cummulated and isolated dienes can be estimated by comparing the heats of hydrogenation.

Relative Stabilities of DienesRelative Stabilities of Dienes

The conjugated diene is 15 kJ/mol more stable than the isolated diene. Isolated diene = two separate alkenes.

Relative Stabilities of DienesRelative Stabilities of Dienes

The cummulated double bonds of allene are relatively high energy. The heat of hydrogenation of allene is 45 kJ/mol more than twice the heat of hydrogenation of propene!

The single bond that separates the two conjugated dienes is relatively short.

Bonding in Conjugated DienesBonding in Conjugated Dienes

Conjugated dienes are more stable than isolated dienes because there is greater electron delocalization. With onesp3 carbon between alkenes the p orbitals are separated and cannot overlap.

Bonding in Conjugated DienesBonding in Conjugated Dienes

conjugated isolated

There is maximum orbital overlap and electron delocaliza-tion if the two dienes are coplanar. The two coplanar conformations are called s-cis and s-trans.

Conformations of Conjugated DienesConformations of Conjugated Dienes

Bonding in AllenesBonding in Allenes

The central carbon which form two bonds is sp-hybridized and the other two carbons are sp2 hybridized. Allene innon planar.

Bonding in AllenesBonding in Allenes

In these diagrams the p-orbitals are shown in different colors to better show their mutually orthogonal spatial arrangement.

Bonding in AllenesBonding in Allenes

p orbitals C1C2

p orbitals C2C3 p orbitals C1C2 and C2C3

The nonplanarity of allenes means that 1,3-disubstituted allenes are chiral.

Chirality with AllenesChirality with Allenes

Thermal dehydrogenation in the presence of catalysts is an industrial process.

Preparation of DienesPreparation of Dienes

In the lab dehydration or dehydrohalogenation (eliminationreactions) are used.

Rubber is a natural polymer made from isoprene.

Diene PolymersDiene Polymers

The polymer consists of isoprene units connected together.

Rubber has(Z)-alkenes.

Gutta Percha has (E) alkenes and is more durable and was used to insulate undersea communication cables.

Addition of Hydrogen Halides Addition of Hydrogen Halides to Conjugated Dienesto Conjugated Dienes

Addition of hydrogen halides to alkenes is a characteristic reaction of alkenes.

Addition of HX to Conjugated DienesAddition of HX to Conjugated Dienes

Step 1. Protonation to give an allylic cation.

Mechanism of Addition of HXMechanism of Addition of HX

Step 2. Attack of chloride.

In this reaction both resonance forms of the allylic cation are equivalent so only one product is formedas a mixture of enantiomers.

Addition of HX to conjugated dienes that form two nonequivalent intermediate allylic carbocations yield mixturesof two products.

Addition of HX to Conjugated DienesAddition of HX to Conjugated Dienes

The ratio of the two products is temperature dependent.

Kinetic and Thermodynamic ProductsKinetic and Thermodynamic Products

At -80 oC the ratio is 81:19, at 25 oC it is 44:56 and at 45 oC it is 15:85.

At low temperatures the fastest formed product is preferentially formed – the kinetic product.

At high temperatures the more stable product is formed –the thermodynamic product. More substituted double bond!

Kinetic and Thermodynamic ProductsKinetic and Thermodynamic Products

kinetic

The bromide is generated close to C-2 so addition to C-2 is faster. Kinetic product.

For the ratio to change with temperature the reactions have to be reversible so the 1,2 addition product must ionize to reform the allylic cation and then form the 1,4-addition product.

thermodynamic

The activation energy to form the 1,2-addition product islower so it is more readily formed. For the 1,4-addition product this is reversed.

Kinetic and Thermodynamic ProductsKinetic and Thermodynamic Products

Halogen Addition to DienesHalogen Addition to Dienes

1,4-Addition predominates with (E) products preferentially formed.

Halogen Addition to DienesHalogen Addition to Dienes

The Diels Alder ReactionThe Diels Alder Reaction

The Diels-Alder reaction is a cycloaddition reaction of a diene and an alkene (called a dienophile). It is an example of a pericylclic reaction (cyclic transition state).

The Diels-Alder ReactionThe Diels-Alder Reaction

No catalyst is needed.

Electron withdrawing groups on the alkene enhance the rate of reaction. Carbonyls (C=O) are electron withdrawing groups.

Substituents and the Diels Alder ReactionSubstituents and the Diels Alder Reaction

Two electron withdrawing groups makes the dienophile even more reactive.

The product is a cyclohexene so the product will always contain one more ring than the reactants.

Products of Diels-Alder ReactionsProducts of Diels-Alder Reactions

The diene must have the s-cis conformation to react.Dienes that are cannot be s-cis do not react. If the s-cis conformation is not favored then the reaction will be very slow.

Conformation of the DieneConformation of the Diene

Steric effect of the extra methyl destabilizes this diene.

Stereospecific reactions go from stereoisomeric reactants to stereoisomeric products.

Stereospecificity in the Diels-Alder ReactionStereospecificity in the Diels-Alder Reaction

The cis alkene yields the cis product and the trans alkeneyields the trans product.

Stereoselective reactions preferentially form one stereo-isomer.

Stereoselectivity in the Diels-Alder ReactionStereoselectivity in the Diels-Alder Reaction

Here there are two cis products: endo and exo.

Endo is favored (3:1) so the reaction is stereoselective as well.

Retrosynthetic AnalysisRetrosynthetic Analysisand the Diels-Alder Reactionand the Diels-Alder Reaction

Target molecules that contain a cyclohexene ring can besynthesized by a Diels-Alder reaction.

Retrosynthetic Analysis and the Retrosynthetic Analysis and the Diels-Alder ReactionDiels-Alder Reaction

Numbering the carbons can help keep track of the diene anddienophile.

Here is a complex target molecule and the disconnectionof the molecule corresponding to the Diels-Alder reactionand then the synthesis of the diene.

Retrosynthetic Analysis and the Retrosynthetic Analysis and the Diels-Alder ReactionDiels-Alder Reaction