Chapter 14 Conjugated Compounds and Ultraviolet Spectroscopy

Conjugated and Nonconjugated Dienes Compounds can have more than one double or triple bond If they are separated by only one single bond they are conjugated and their orbitals interact 2 The conjugated diene 1,3-butadiene has properties that are very different from those of the nonconjugated diene, 1,5-pentadiene

Preparation of Conjugated Dienes Typically by elimination in allylic halide Specific industrial processes for large scale production of commodities by catalytic dehydrogenation and dehydration 3

Measuring Stability Conjugated dienes are more stable than nonconjugated based on heats of hydrogenation Hydrogenating 1,3-butadiene produces 16 kJ/mol less heat than 1,4- pentadiene 4

Molecular Orbitals We can understand, and predict, many properties of conjugated systems by considering the molecular orbitals Rules for combining orbitals: 1) Always get the same number of molecular orbitals as number of atomic orbitals used to combine 2) As the number of nodes increase, the energy of the molecular orbital increases 3) The molecular orbitals obtained are the region of space where the electrons are reside (on time average)

First consider an isolated alkene (simplest is ethylene) We can separate the  sigma) and  pi) framework With alkenes, we are interested in the  pi) framework since this is what controls the reactivity Molecular Orbitals

With butadiene there are 4 p orbitals in conjugation Realize if we add 4 p orbitals then we must obtain 4 molecular orbitals The difference between the 4 orbitals will be the sign of adjacent atomic orbitals We will obtain 4 molecular orbitals with different number of nodes

Molecular Orbitals We can also predict the relative energy levels of these molecular orbitals by the number of nodes (as nodes increases, energy increases)

9 Molecular Orbitals

Electrophilic Additions to Conjugated Dienes: Allylic Carbocations Review: addition of electrophile to C=C –Markovnikov regiochemistry via more stable carbocation 10

Carbocations from Conjugated Dienes Addition of H + leads to delocalized secondary allylic carbocation 11

Products of Addition to Delocalized Carbocation Nucleophile can add to either cationic site The transition states for the two possible products are not equal in energy 12

Addition to a conjugated diene at or below room temperature normally leads to a mixture of products in which the 1,2 adduct predominates over the 1,4 adduct At higher temperature, product ratio changes and 1,4 adduct predominates 13 What forms faster (kinetic product) and what is more stable (thermodynamic product) need not be the same Kinetic vs. Thermodynamic Products

14 Recall that the rate of a reaction is determined by its energy of activation (E a ), whereas the amount of product present at equilibrium is determined by its stability.

15 The 1,4-product is more stable because it has two alkyl groups bonded to the carbon-carbon double bond, whereas the 1,2-product has only one. The 1,2-product is the kinetic product because of a proximity effect. The proximity effect occurs because one species is close to another.

16 The overall two-step mechanism for addition of HBr to 1,3-butadiene to form both 1,2- and 1,4 addition products is illustrated in the energy diagram below. At low temperature, the energy of activation is the more important factor. Since most molecules do not have enough kinetic energy to overcome the higher energy barrier at lower temperature, they react by the faster pathway, forming the kinetic product. At higher temperature, most molecules have enough kinetic energy to reach either transition state. The two products are in equilibrium with each other, and the more stable compound, which is lower in energy, becomes the major product.

The Diels-Alder Reaction The Diels-Alder reaction is an addition reaction between a 1,3-diene and an alkene (called a dienophile), to form a new six-membered ring. Three curved arrows are needed to show the cyclic movement of electron pairs because three  bonds break and two  bonds and one  bond form. Because each new  bond is ~20 kcal/mol stronger than a  bond that is broken, a typical Diels-Alder reaction releases ~40 kcal/mol of energy.

View of the Diels-Alder Reaction Woodward and Hoffman showed this shown to be an example of the general class of pericyclic reactions Involves orbital overlap, change of hybridization and electron delocalization in transition state The reaction is called a cycloaddition 18

19 View of the Diels-Alder Reaction The reaction occurs because there is a perfect symmetry match between the HOMO of the diene of the LUMO of the alkene and between the HOMO of the alkene and the LUMO of the diene. This perfect symmetry match allows two pairs of electrons to flow to form the two new bonds between the two molecules Since the HOMO of the diene has the highest electron energy the reaction will be the HOMO of the diene with the LUMO of the alkene.

Characteristics of the Diels-Alder Reaction The alkene component is called a dienophile 20 Diels-Alder reaction is favored with electron withdrawing groups on dienophile and electron donating groups on diene

Some examples of Diels-Alder reactions are shown below: All Diels-Alder reactions have the following features in common: 1.They are initiated by heat; that is, the Diels-Alder reaction is a thermal reaction. 2.They form new six-membered rings. 3.Three  bonds break, and two new C-C  bonds and one new C-C  bond forms. 4.They are concerted; that is, all old bonds are broken and all new bonds are formed in a single step.

Stereospecificity of the Diels-Alder Reaction The reaction is stereospecific, maintaining relative relationships from reactant to product –There is a one-to-one relationship between stereoisomeric reactants and products 22

23 Regiochemistry of the Diels-Alder Reaction Reactants align to produce endo (rather than exo) product –endo and exo indicate relative stereochemistry in bicyclic structures –Substituent on one bridge is exo if it is anti (trans) to the larger of the other two bridges and endo if it is syn (cis) to the larger of the other two bridges

In a Diels-Alder reaction, the endo product is preferred, as shown in the two examples below. The transition state leading to the endo product allows more interaction between the electron rich diene and the electron- withdrawing substituent Z on the dienophile, an energetically favorable arrangement.

How endo and exo products are formed in the Diels-Alder reaction

26 Regiochemistry of the Diels-Alder Reaction

27 View of the Diels-Alder Reaction

28 Diene Reactivity “The diene can react only when it adopts the s-cis conformation.” This rotation is prevented in cyclic alkenes. When the two double bonds are constrained to an s- cis conformation, the diene is unusually reactive. When the two double bonds are constrained in the s-trans conformation, the diene is unreactive.

Diene Polymers: Natural and Synthetic Rubbers Conjugated dienes can be polymerized The initiator for the reaction can be a radical, or an acid Polymerization: 1,4 addition of growing chain to conjugated diene monomer 29

Synthetic Rubber Chemical polymerization of isoprene does not produce rubber (stereochemistry is not controlled) Synthetic alternatives include neoprene, polymer of 2-chloro- 1,3-butadiene This resists weathering better than rubber 30

Structure Determination in Conjugated Systems: UV Spectroscopy 31

When electrons in a lower energy state (the ground state) absorb light having the appropriate energy, an electron is promoted to a higher electronic state (excited state). The energy difference between the two states depends on the location of the electron. General Principles

33 Structure Determination in Conjugated Systems: UV Spectroscopy

34 As the number of conjugated  bonds increases, the energy difference between the ground and excited state decreases, shifting the absorption to longer wavelengths. With molecules having eight or more conjugated  bonds, the absorption shifts from the UV to the visible region, and the compound takes on the color of the light it does not absorb.

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38 Homeworks 14.23, , 14.28, 14.33, 14.35, 14.39, , 14.51