Conjugation in Alkadienes and 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 Alkenes connected by a single bond are called conjugated dienes.

The Allylic Group Allyl is both a common name and a permissible IUPAC name for the H 2 C=CHCH 2 group. The sp 3 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 Delocalization

In resonance structures that are not equivalent the resonance structures do not contribute equally to the hybrid. Resonance 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 three carbon atoms is generated from the three adjacent p orbitals. Valence 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 S N 1 Reactions Allylic halides react much faster than tertiary alkyl halides. For example:

Since the positive charge is shared between two carbons the water attaches to both carbons to form two products: Allylic Resonance and S N 1 Reactions Intermediate cation

Reaction Equation. Mechanism 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 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 Mechanism Indeed this is the case: Common intermediate cation

For S N 2 reactions we compare reactions of a strong nucleophile with a series of primary alkyl halides including allylic halides. Allylic Halides and S N 2 Reactions

S N 2 reactions occur when allylic chlorides react with good nucleophiles. S N 2 Reactions of Allylic Chlorides

Allylic Free-Radical Halogenation

The allylic radical is stabilized by electron delocalization expressed as resonance between Lewis structures and should be easy to form selectively. Allylic Radical Stablization

Delocalization of the unpaired electron stabilizes the allylic radicals so the bond dissociation enthalpies are lower. Quantifying 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 Chlorination

Mechanism 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 Halogenation

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

If the allylic resonance forms are not equivalent then a mixture of isomeric products is formed. Unselective Allylic Halogenation The allylic radical reacts through the two resonance structures shown below.

Allylic Anions

The allylic anion is planar and stabilized by electron delocalization. In resonance terms: Allylic Anions The electrostatic potential map shows the electron delocalization as compared to propyl anion. Red shows high electron density

A comparison of the pK a ’s of propane and propene give a measure of the relative stability of the two anions. pK a and Stability of Allylic Anions

Classes of Dienes: Conjugated and Otherwise

Dienes may be: Conjugated in which the two alkenes are joined by a C-C single bond; Isolated in which there is at least one sp 3 carbon between the alkenes; Cummulated in which two alkenes share a carbon. Classes of Dienes

Alkadienes are named by replacing the -ane ending and replacing it with –adiene and adding the locant for each alkene. Naming Dienes (E)-1,3pentadiene1,6-heptadiene 1,4-cyclohexadiene

The stabilities of conjugated, cummulated and isolated dienes can be estimated by comparing the heats of hydrogenation. Relative Stabilities of Dienes The conjugated diene is 15 kJ/mol more stable than the isolated diene. Isolated diene = two separate alkenes.

Relative 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 Dienes

Conjugated dienes are more stable than isolated dienes because there is greater electron delocalization. With one sp 3 carbon between alkenes the p orbitals are separated and cannot overlap. Bonding 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 Dienes

Bonding in Allenes

The central carbon which form two  bonds is sp-hybridized and the other two carbons are sp 2 hybridized. Allene in non planar. Bonding in Allenes

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

Thermal dehydrogenation in the presence of catalysts is an industrial process. Preparation of Dienes In the lab dehydration or dehydrohalogenation (elimination reactions) are used.

Rubber is a natural polymer made from isoprene. Diene 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 to Conjugated Dienes

Addition of hydrogen halides to alkenes is a characteristic reaction of alkenes. Addition of HX to Conjugated Dienes

Step 1. Protonation to give an allylic cation. Mechanism 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 formed as a mixture of enantiomers.

Addition of HX to conjugated dienes that form two non equivalent intermediate allylic carbocations yield mixtures of two products. Addition of HX to Conjugated Dienes

The ratio of the two products is temperature dependent. Kinetic and Thermodynamic Products At -80 o C the ratio is 81:19, at 25 o C it is 44:56 and at 45 o C 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 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 is lower so it is more readily formed. For the 1,4-addition product this is reversed. Kinetic and Thermodynamic Products

Halogen Addition to Dienes

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

The 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 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 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 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 Diene Steric effect of the extra methyl destabilizes this diene.

Stereospecific reactions go from stereoisomeric reactants to stereoisomeric products. Stereospecificity in the Diels-Alder Reaction The cis alkene yields the cis product and the trans alkene yields the trans product.

Stereoselective reactions preferentially form one stereo- isomer. Stereoselectivity 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 Analysis and the Diels-Alder Reaction

Target molecules that contain a cyclohexene ring can be synthesized by a Diels-Alder reaction. Retrosynthetic Analysis and the Diels-Alder Reaction Numbering the carbons can help keep track of the diene and dienophile.

Here is a complex target molecule and the disconnection of the molecule corresponding to the Diels-Alder reaction and then the synthesis of the diene. Retrosynthetic Analysis and the Diels-Alder Reaction