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Chemistry LibreTexts

5: An Introduction to Organic Reactions using Free Radical Halogenation of Alkanes

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  • Learning Objectives

    After reading the chapter and completing ALL the exercises and homework, a student can be able to:

    • recognize and distinguish between the four major types of organic reactions (additions, eliminations, substitutions, and rearrangements) - refer to section 5.1
    • accurately and precisely use reaction mechanism notation and symbols including curved arrows to show the flow of electrons - refer to section 5.2
    • identify nucleophiles and electrophiles in polar reactions - refer to section 5.3
    • perform calculations using the equation \[ΔGº = –RT \ln K = –2.303 RT \log_{10} K \label{eq2}\] and explain the relationship between equilibrium and free energy - refer to section 5.4
    • calculate reaction enthalpies from bond dissociation energies - refer to section 5.5
    • draw Reaction Energy Diagrams from the thermodynamic and kinetic data/information - refer to se5.6
    • use a Reaction Energy Diagram to discuss transition states, Ea, intermediates & rate determining step - refer to section 5.6
    • draw the transition states & intermediates of a reaction - refer to section 5.6
    • describe the structure & relative stabilities of carbocations, free radicals and carbanions - refer to sections 5.7 - 5.9 respectively
    • Explain the mechanism & energetics of the free-radical halogenation of alkanes - refer to section 5.10
    • Predict the products of chlorination & bromination reactions of alkanes based on relative reactivity and selectivity - refer to section 5.11
    • describe the similarities and differences between reactions performed in the lab with biochemical reactions - refer to section 5.12

    • 5.1: Types of Organic Reactions
      The four main classes of organic reactions are additions, eliminations, substitutions, and rearrangements.
    • 5.2: 5.2 Reaction Mechanism Notation and Symbols
      Arrows are used by chemists to communicate electron flow in mechanisms, reaction completion/equilibrium, and resonance relationships.  It is important to use  accuracy when selecting the type of arrow  for reactions and precision in drawing the location of the arrow head and tail for the curved arrows of electron flow.
    • 5.3: the Dance of the Nucleophile and Electrophile
      Sterics and electronics are the underlying driving forces for polar organic reactions.  The electron rich nucleophile (Nu:) reacts with the electron poor electrophile through a variety of pathways that can be limited and/or influenced by steric hindrance.  We explore and learn the polar reaction  pathways in subsequent chapters.
    • 5.4: Describing a Reaction - Equilibrium and Free Energy Changes
      The relationship between equilibrium and free energy is reviewed quantitatively and applied to organic reactions conceptually.
    • 5.5: Homolytic Cleavage and Bond Dissociation Energies
      The products of homolytic cleavage are radicals and the energy that is required to break the bond homolytically is called the Bond Dissociation Energy (BDE) and is a measure of the strength of the bond.
    • 5.6: Reaction Energy Diagrams and Transition States
      Reaction energy diagrams efficiently and effectively communicate the thermodynamics and kinetics of chemical reactions in a single diagram.  They are a useful tool in learning organic chemistry.
    • 5.7: 5.7 Reactive Intermediates - Carbocations
      A carbocation is a cation in which carbon has an empty p orbital and bears a positive charge creating a highly reactive intermediate.  Comparing the relative stability of reaction intermediates helps elucidate reaction mechanisms and predict major and minor products.
    • 5.8: 5.8 Reactive Intermediates - Radicals
      A radical (more precisely, a free radical) is an atom, molecule, or ion that has unpaired valence electron (half filled orbital) creating a highly reactive intermediate.
    • 5.9: Reactive Intermediates: Carbanions and Carbon Acids
      A carbanion is an anion in which carbon has an unshared pair of electrons and bears a negative charge creating a highly reactive intermediate.
    • 5.10: The Free-Radical Halogenation of Alkanes
      Free radical halogenation of alkanes is the substitution of a single hydrogen on the alkane for a single halogen to form a haloalkane. This reaction is very important in organic chemistry because it opens a gateway to further chemical reactions.  We will apply the reaction concepts discussed in this chapter to this reaction to show how empirical data supports these theories.
    • 5.11: Reactivity and Selectivity
      In general, high reactivity correlates with low selectivity and vice versa.  Depending on the structure of the substrate, reaction conditions can be optimized for high reactivity or high selectivity and occasionally for both.
    • 5.12: A Comparison between Biological Reactions and Laboratory Reactions
      Biochemical reactions occur within our body fluids at a typical pH of 7.4 and temperature of 98.6C.  Our biochemistry relies on enzymes to catalyze physiological reactions within this narrow range of environmental conditions.  Synthetic organic chemists can create extreme conditions within reactions flasks to catalyze and promote chemical reactions.
    • 5.13: Additional Exercises
      This section has additional exercises for the key learning objectives of this chapter.
    • 5.14: Solutions to Additional Exercises
      This section has the solutions to the additional exercises from the previous section.