Drawing Reaction Mechanisms

4 Key excerpts on "Drawing Reaction Mechanisms"

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  eBook - ePub

  Survival Guide to Organic Chemistry

  Bridging the Gap from General Chemistry

  • Patrick E. McMahon, Bohdan B. Khomtchouk, Claes Wahlestedt(Authors)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  Guide to Kineticsand ReactionMechanisms

  9

   

  9.1 GENERAL CONCEPTS

  9.1.1 REACTION MECHANISMS

  1. A reaction mechanism is an accepted sequence of elementary reaction steps which describe all (based on available information) bond-making and bond-breaking events characterizing the change of reactant molecules to product molecules.
  2. A reaction step (or elementary step) is the smallest observable change in molecular bonding, an individual bond-making, bond-breaking , or combination event (simultaneous bond-making and bond-breaking) that can be distinguished experimentally from other such events.
  3. The complete reaction mechanism may be composed of only one step or many steps depending on the overall (complete) reaction and the conditions.
  4. A reaction mechanism, along with the parameters that describe it such as rate, activation energies, and intermediates (described in other sections) is a path function .
     A path function is dependent on the “pathway” or method by which a change occurs
     . Regardless of the numerical value or sign of the free energy change, a reaction can occur only if there exists an available pathway by which reactant molecules can be converted into product molecules.
  5. Path functions must be distinguished from state functions such as ΔG, ΔH, and ΔS. These depend only on the initial and final states of the system: the total energies of the reactants versus the products.
     State functions do not depend on how the reaction changes occur
     .
  6. All reactants and products in a complete reaction or in a single reaction step must exist as an independent species for some measurable amount of time. This existence is due to the presence of energy barriers blocking “instant” decomposition. The compound is considered to be in a “potential energy well” (i.e., a stable energy “valley” similar to a rock sitting in a hole) termed a local energy minimum . A “deep” hole represents a very stable molecule (slow to react) because the energy barriers on each side are high. A “shallow” hole represents a relatively

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  Intermediate Organic Chemistry

  • Ann M. Fabirkiewicz, John C. Stowell(Authors)
  • 2015(Publication Date)
  • Wiley

    (Publisher)

  4 MECHANISMS AND PREDICTIONS

  When planning a new reaction in organic chemistry, we look at the accumulated information on similar reactions in order to predict the best conditions for it. The more we know about the intimate details of the reaction process at the molecular level, the better will be our predictions. A particular reaction may be described as an ordered sequence of bond breaking and making and a series of structures that exist along the way from starting material to product. The description includes the concurrent changes in potential energy. Structures at energetic maxima are called transition states and structures at minima are called intermediates. The complete description is called the mechanism of the reaction.

  4.1 REACTION COORDINATE DIAGRAMS AND MECHANISMS

  The energy-structure relationship is sometimes illustrated with a plot of potential energy versus progress along the pathway of lowest maximum potential energy. This is exemplified in a general way in Figure 4.1 . The plot shows a two-step reaction leading from reactants through a first transition state to intermediates. The intermediates pass through a second transition state to products. The net overall descent for this reaction corresponds to an exothermal process.

  Figure 4.1

  Reaction coordinate diagram.

  A reaction that requires a higher rise to a transition state (activation energy) will be slower than one requiring a lesser rise (if the probability factors are similar) because a smaller fraction of collisions will provide sufficient potential energy to make it.

  The energy values for such plots are derived from measurements of overall exo- or endothermicity and from measurement of the effect of varying the temperature on the rate of the reaction (Section 4.3.6 ).

  Many techniques have been developed for determining mechanisms including complete product (and sometimes intermediate) identification, isotope labeling, stereochemistry, and kinetics [1–3], as are covered in Section 4.3

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  eBook - ePub

  Understanding Advanced Organic and Analytical Chemistry

  The Learner's ApproachRevised Edition

  • Kim Seng Chan, Jeanne Tan;;;(Authors)
  • 2016(Publication Date)
  • WS EDUCATION

    (Publisher)

  CHAPTER 3

  Organic Reactions and Mechanisms

  3.1 Introduction

  What is a chemical reaction? You would say that it is a process whereby substances interact and transform into new ones, usually with different properties. How does the transformation occur? Do the reactant molecules simply collide with each other and therefore combine to form new compounds? If this is so, why do certain reactions occur only when appropriate conditions such as heating or catalyst are imposed?

  A classical view of chemical reactions involves the rearrangement of particles (atoms, ions or molecules), and for this to occur, “old” bonds are broken before “new” bonds are formed. But these rearrangements do not happen by simply bumping molecules around, even though collision is a precursor for a fruitful reaction. When two reacting particles approach each other, there is repulsion between their negatively charged electron clouds. This repulsive force decreases the speed of the approaching particles while they are colliding with each other. If they do not have sufficient kinetic energy to overcome the repulsive force that tries to keep them apart, by the time they “touch” each other, they would simply be “pushed” apart. In such an instance, bonds are neither broken within a particle nor formed between them.

  Let us try a different scenario. This time round, when the approaching particles have sufficient kinetic energy, effectively known as the activation energy (Ea ), they can overcome the repulsive force, which then leads to the inter-penetration of their electron clouds. At this juncture, there can be rearrangement of valence electrons, with the partial breaking of old bonds and the partial formation of new ones — this fuzzy picture is aptly described as the formation of an activated complex in the transition state. In the energy profile diagram (see Fig. 3.1

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  eBook - ePub

  Chemically Reacting Flow

  Theory, Modeling, and Simulation

  • Robert J. Kee, Michael E. Coltrin, Peter Glarborg, Huayang Zhu(Authors)
  • 2017(Publication Date)
  • Wiley

    (Publisher)

  CHAPTER 14 REACTION MECHANISMS

   

  Chapter 13 discussed the theory of elementary reactions. The chemical processes occurring in chemically reacting flows usually proceed by a series of elementary reactions, rather than by a single step. The collection of elementary reactions defining the chemical process is called the mechanism of the reaction. When rate constants are assigned to each of the elementary steps, a chemical kinetic model for the process has been developed.

  Using a chemical kinetic model is one way to describe the chemistry in reacting flow modeling. The chemical kinetic model offers a comprehensive description of the chemistry, but it requires a larger computational effort than simplified chemical models.

  The present chapter discusses the development and use of detailed reaction mechanisms in modeling reacting flows. Developing reaction mechanisms requires attention to some “collective aspects of mechanisms," such as the driving forces for gas-phase chemical processes and the characteristics and similarities of different reaction systems.

  For illustration, selected medium to high temperature gas-phase processes are discussed in some detail. Gas-phase reactions at elevated temperature are important in combustion, incineration, flue gas-cleaning, petrochemical processes, chemical synthesis, and materials production. Although the details of these systems may vary significantly, they share characteristics that are common for all gas-phase reaction mechanisms.

  14.1 Models for Chemistry

  In chemically reacting flow systems, the overall reaction rate may be limited by the mixing rate of the reactants or by the rate of the chemical reaction upon mixing. If mixing is slow compared to chemical reaction, the system is diffusion or mixing controlled, while fast mixing and slow reaction results in a kinetically controlled system (Fig. 14.1 ).

  Figure 14.1

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