Study Notes for Chapter 14: Conjugated Compounds and Ultraviolet Spectroscopy

Chapter 14: Conjugated Compounds and Ultraviolet Spectroscopy

Learning Objectives

  • (14.1) Stability of conjugated dienes: Molecular orbital theory

  • (14.2) Electrophilic additions to conjugated dienes: Allylic carbocations

  • (14.3) Kinetic versus thermodynamic control of reactions

  • (14.4) The Diels-Alder cycloaddition reaction

  • (14.5) Characteristics of the Diels-Alder reaction

  • (14.6) Diene polymers: Natural and synthetic rubbers

  • (14.7) Ultraviolet spectroscopy

  • (14.8) Interpreting ultraviolet spectra: The effect of conjugation

  • (14.9) Conjugation, color, and the chemistry of vision

Stability of Conjugated Dienes: Molecular Orbital Theory

  • Formation of Conjugated Dienes:

    • Conjugated dienes are typically formed by the elimination of HX from an allylic halide.

  • Significant Properties of Conjugated Dienes:

    • Possess a shorter central single bond compared to non-conjugated counterparts.

    • Exhibit increased stability as indicated by heats of hydrogenation.

  • Heats of Hydrogenation:

    • Table 14.1 presents heats of hydrogenation for some alkenes and dienes, a key measure of stability among various hydrocarbons.

  • Valence Bond Theory:

    • Orbital hybridization plays a pivotal role in establishing the stability of conjugated dienes.

    • A central C–C single bond results from the σ overlap of sp² orbitals on both carbons.

    • sp² orbitals contain 33% s character, while sp³ orbitals contribute 25% s character.

  • Molecular Orbital Theory:

    • Interaction between π orbitals in a conjugated diene enhances stability.

    • According to this theory, when a π bond forms from the combination of two p orbitals, it creates two π molecular orbitals.

    • One π molecular orbital is bonding and possesses lower energy than the original p orbitals.

    • The other π molecular orbital is antibonding, having a higher energy than the original p orbitals and features a node between nuclei.

  • Illustration of π Molecular Orbitals:

    • Figure 14.2 shows a representation of the four π molecular orbitals in 1,3-butadiene.

  • Worked Example on Stability Ranking:

    • The compound Allene (H2C=C=CH2) has a heat of hydrogenation of -298 kJ/mol (-71.3 kcal/mol).

    • Comparison of stability: Allene is less stable than a conjugated diene, which is, in turn, less stable than a nonconjugated diene.

    • Calculated ΔHhydrog for the allene is -252 kJ/mol through the equation: extΔH</em>exthydrog=−126+(−126)=−252extkJ/molext{ΔH}</em>{ ext{hydrog}} = -126 + (-126) = -252 ext{ kJ/mol}

Electrophilic Additions to Conjugated Dienes: Allylic Carbocations

  • Unique Behavior in Reactions:

    • Conjugated dienes exhibit distinct behaviors in electrophilic addition reactions compared to regular alkenes.

    • When subjected to electrophilic reactions, conjugated dienes yield a mixture of products.

    • The products formed may include both 1,2 and 1,4-addition products due to the action of allylic carbocations.

  • Products of Addition to Delocalized Carbocation:

    • The reaction of Br⁻ with the allylic cation can occur at either C1 or C3, as both carbons share the positive charge in a delocalized manner.

Kinetic versus Thermodynamic Control of Reactions

  • Product Predominance Based on Temperature:

    • At room temperature or lower, reactions typically form a mixture where the 1,2 adduct predominates.

    • Increasing temperature changes the product ratio to favor the formation of the 1,4 adduct over the 1,2 adduct.

  • Mechanics of Kinetic vs Thermodynamic Control:

    • For a reaction yielding products B and C:

    • If B forms more quickly, then ext{ΔG}^
      eqB < ext{ΔG}^ eqC

    • However, if C is more thermodynamically stable, then ext{ΔG}^ ext{°}C > ext{ΔG}^ ext{°}B

  • Kinetic Control Overview:

    • Products in kinetic control depend on the relative rates of formation, irrespective of their stability. Here B would be the major product.

  • Thermodynamic Control Overview:

    • In thermodynamic control, the major product is determined by stability rather than rate. Hence, C would be favored as the major product.

  • Applications Regarding Conjugated Dienes:

    • The principles of kinetic control vs thermodynamic control are applicable particularly in electrophilic addition reactions with conjugated dienes.

  • Worked Example on Stability of 1,4 Adducts:

    • 1,4 adducts of 1,3-butadiene are generally more stable than 1,2 addition products because disubstituted double bonds exhibit greater stability than monosubstituted bonds.

The Diels-Alder Cycloaddition Reaction

  • General Description:

    • The Diels-Alder reaction is where conjugated dienes react with alkenes to form substituted cyclohexene products, which allows for the synthesis of cyclic compounds.

  • Nature of the Reaction:

    • This reaction is categorized as a pericyclic process, implying all interactions occur within a single step during a cyclic redistribution of bonding electrons.

  • Characteristics of the Diels-Alder Reaction:

    • Dienophile: If the dienophile possesses an electron-withdrawing substituent, the reaction proceeds at an elevated rate.

    • Stereospecificity: The reaction is stereospecific, generating a single stereoisomer that retains the stereochemistry of the dienophile.

  • Regiochemistry:

    • The orientation of the diene and dienophile typically favors the formation of endo products over exo products.

    • Endo substituent: Syn to the larger of the two bridges.

    • Exo substituent: Trans to the larger of the two bridges.

  • Steric Overlap:

    • Diels-Alder reactions yield endo products due to the optimal positioning of the reactants above one another, allowing increased overlap of diene and dienophile orbitals.

  • Conformational Restrictions:

    • Dienes involved in a Diels-Alder reaction must adopt an s-cis conformation, allowing Carbons 1 and 4 to be positioned closely enough for reaction, as opposed to the s-trans conformation.

  • Biological Relevance:

    • While rare, biological Diels-Alder reactions do occur, exemplified by the intramolecular Diels-Alder reaction involved in the biosynthesis of lovastatin, which contains a triene.

  • Worked Example to Predict Product:

    • The solution involves rotating the diene into the s-cis conformation to allow for successful reaction completion.

Diene Polymers: Natural and Synthetic Rubbers

  • Polymerization of Conjugated Dienes:

    • Conjugated dienes can undergo polymerization via radical or acid-initiated reactions.

    • Polymerization consists of a 1,4 addition of the growing chain to conjugated diene monomers.

  • Natural Rubber:

    • Primarily derived from the sap of the Hevea brasiliensis tree.

    • Exhibits Z stereochemistry at the double bonds of rubber.

    • Gutta-percha is another natural form related to rubber.

  • Synthetic Rubber:

    • Diene polymerization techniques are crucial for commercial rubber production.

    • For instance, chloroprene polymerization produces neoprene, a synthetic rubber known for its excellent weather resistance.

  • Vulcanization Process:

    • Both natural and synthetic rubbers are typically too soft for practical applications.

    • Vulcanization, which involves heating rubber with sulfur, strengthens the material.

    • Sulfur creates cross-links between hydrocarbon chains, giving rise to the hardness and durability of the rubber.

  • Elastic Properties:

    • The elasticity of rubber arises due to the irregular structure of polymer chains resultant from double bonds, facilitating bending.

  • Worked Example of Polymerization Mechanism:

    • The mechanism for acid-catalyzed polymerization of 1,3-butadiene was addressed.

Ultraviolet Spectroscopy

  • Usage in Molecular Structural Determination:

    • Ultraviolet spectroscopy is particularly important for analyzing the molecular structure of conjugated compounds, although it's less widely used than mass spectrometry, infrared spectroscopy, or nuclear magnetic resonance (NMR).

    • It examines the nature of the conjugated π electron system of a compound.

  • Ultraviolet Spectrum Range:

    • The UV region spans from the short-wavelength end of the visible region to the long-wavelength end of the X-ray region.

    • The preferred area of study for scientists is a narrow range between 2imes10−7extm2 imes 10^{-7} ext{ m} and 4imes10−7extm4 imes 10^{-7} ext{ m}.

  • Principle of UV Spectroscopy:

    • Organic molecules can absorb enough energy from UV radiation to initiate an electron shift from a lower-energy orbital to a higher-energy orbital.

    • This phenomenon is termed π to π* excitation, where UV irradiation causes a π electron to move from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).

  • Recording Ultraviolet Spectra:

    • A sample undergoes irradiation with UV light of varying wavelengths to detect energy absorption.

    • The energy that is absorbed corresponds to that required for electron displacement and is recorded as a chart plotting wavelength against absorbance (A).

  • Understanding Molar Absorptivity (ɛ):

    • The mathematical expression for absorbed UV light is captured as:
      A=extAbsorbanceA = ext{Absorbance}
      A=cimeslimesextɛA = c imes l imes ext{ɛ}

    • Where:

    • A signifies absorbance,

    • c signifies concentration in mol/L,

    • l signifies sample path length in cm.

  • Worked Example:

    • To calculate the concentration of vitamin A, where the absorbance at 325 nm is A = 0.735 in a path length cell of 1.00 cm:

    • Given: extɛ=50,100ext{ɛ} = 50,100

    • The calculation yields:
      Ac=extɛimeslAc = ext{ɛ} imes l

    • Resultant concentration = 1.47imes10−5extL/mol.cm1.47 imes 10^{-5} ext{ L/mol.cm}.

Interpreting UV Spectra: The Effect of Conjugation

  • Dependence on Conjugation:

    • The wavelength of UV radiation causing π to π* excitation in a conjugated molecule varies according to the compound's conjugation structure.

    • The degree of conjugation significantly affects the UV absorption's corresponding wavelength.

    • The energy difference between HOMO and LUMO inversely correlates with the extent of conjugation present in a compound.

  • Worked Example for Absorption in UV Range:

    • Determine if a given compound exhibits UV absorptions in the 200 to 400 nm range.

    • Solution indicates that due to the presence of multiple alternating bonds, the compound indeed exhibits UV absorption within this specified range.

Conjugation, Color, and the Chemistry of Vision

  • Pigmentation Mechanism:

    • The color of compounds arises from the structural characteristics of colored molecules in conjunction with human perception of light.

    • The visible spectrum extends from 400 to 800 nm, lying adjacent to the ultraviolet spectrum.

    • Conjugation systems within certain compounds may lead to UV absorptions extending into the visible spectrum.

  • Visual Functions:

    • Human vision relies on the functionality of rods (operating in low light) and cones (active in bright light) in the eye to perceive color.

Summary of Key Concepts

  • Conjugated dienes consist of alternating double and single bonds, contributing to their increased stability compared to non-conjugated dienes.

  • The Diels-Alder reaction characterizes a unique reaction pathway specific to conjugated dienes, being stereospecific in nature.

  • Ultraviolet spectroscopy proves essential for determining the structure of conjugated π-electron systems, explicating how energy absorption initiates electron transitions from HOMO to LUMO.