reaction mechanism

Examples of reaction mechanism in the following topics:

• Rate Laws for Elementary Steps

  • Every chemical reaction proceeds according to a reaction mechanism, which is a step-by-step description of what occurs during a reaction on the molecular level.
  • It is important to keep in mind that every reaction mechanism is simply a proposed version of what might be occurring at the molecular level; even if a mechanism agrees with experimental results, it is impossible to prove a reaction mechanism for certain.
  • The sum of each elementary step in a reaction mechanism must yield the overall reaction equation.
  • The rate-determining step is the slowest step in a reaction mechanism.
  • Determining the overall reaction rate from the reaction mechanism will be discussed in the next concept.

• Curved Arrow Notation

  • A detailed description of the changes in structure and bonding that take place in the course of a reaction, and the sequence of such events is called the reaction mechanism.
  • A reaction mechanism should include a representation of plausible electron reorganization, as well as the identification of any intermediate species that may be formed as the reaction progresses.
  • It is now common practice to show the movement of electrons with curved arrows, and a sequence of equations depicting the consequences of such electron shifts is termed a mechanism.
  • In general, two kinds of curved arrows are used in drawing mechanisms:
  • The use of these symbols in bond-breaking and bond-making reactions is illustrated below.

• Overall Reaction Rate Laws

  • Rate laws for reactions are affected by the position of the rate-determining step in the overall reaction mechanism.
  • As discussed in the previous concept, if the first step in a reaction mechanism is the slow, rate-determining step, then the overall rate law for the reaction is easy to write, and simply follows the stoichiometry of the initial step.
  • If the rate-determining step is not the first step in the reaction mechanism, the derivation of the rate law becomes slightly more complex.
  • Consider the following reaction:
  • How to determine the rate law for a mechanism with a fast initial step.

• Organic Reactions Overview

  • These reactions are typically much more complex than inorganic reactions, and sometimes occur in many steps.
  • Chemists can analyze starting materials and products to determine structures, but to understand exactly why an organic reaction works, they must determine its mechanism.
  • This sometimes requires experimentation with tests of reaction kinetics, but mechanisms can typically be devised on paper.
  • Organic reaction mechanisms are written using curved arrows that depict transfers of either nonbonding or bonding electrons to form a new bond or exist as nonbonding electrons attached to an atom.
  • Although the number of possible organic reactions is massive and ever-growing, fundamentally, they can be categorized into just five groups based on their mechanisms: addition, elimination, substitution, redox, and rearrangement.

• Rate-Determining Steps

  • However, most chemical reactions occur over a series of elementary reactions.
  • The complete sequence of these elementary steps is called a reaction mechanism.
  • The reaction mechanism is the step-by-step process by which reactants actually become products.
  • A possible mechanism that explains the rate equation is:
  • If the first step in a mechanism is rate-determining, it is easy to find the rate law for the overall expression from the mechanism.

• A Mechanism for Electrophilic Substitution Reactions of Benzene

  • A two-step mechanism has been proposed for these electrophilic substitution reactions.
  • The following four-part illustration shows this mechanism for the bromination reaction.
  • This mechanism for electrophilic aromatic substitution should be considered in context with other mechanisms involving carbocation intermediates.
  • These include SN1 and E1 reactions of alkyl halides, and Brønsted acid addition reactions of alkenes.
  • SN1 and E1 reactions are respective examples of the first two modes of reaction.

• Reaction Rates and Kinetics

  • Among the variables that influence reaction rates are temperature (reactions are usually faster at a higher temperature), solvent, and reactant / reagent concentrations.
  • Useful information about reaction mechanisms may be obtained by studying the manner in which the rate of a reaction changes as the concentrations of the reactant and reagents are varied.This field of study is called kinetics.
  • Many reactions proceed in a stepwise fashion.
  • Departure from this alignment inhibits the reaction.
  • Most reactions are conducted in solution, not in a gaseous state.

• The Rate Law

  • The rate law for a chemical reaction relates the reaction rate with the concentrations or partial pressures of the reactants.
  • For the general reaction$aA + bB \rightarrow C$ with no intermediate steps in its reaction mechanism, meaning that it is an elementary reaction, the rate law is given by:
  • A smaller rate constant indicates a slower reaction, while a larger rate constant indicates a faster reaction.
  • What is the reaction order?
  • A variety of reaction orders are observed.

• Addition Reactions

  • An addition reaction is the opposite of an elimination reaction.
  • In the related addition-elimination reaction, an addition reaction is followed by an elimination reaction; in most reactions, this involves addition to carbonyl compounds in nucleophilic acyl substitution.
  • Most addition reactions to alkenes follow the mechanism of electrophilic addition.
  • An example of this type of reaction is:
  • Polymerization can either proceed via a free-radical or an ionic mechanism.

• Reactions of Ethers

  • This may occur by SN1 or E1 mechanisms for 3º-alkyl groups or by an SN2 mechanism for 1º-alkyl groups.
  • The conjugate acid of the ether is an intermediate in all these reactions, just as conjugate acids were intermediates in certain alcohol reactions.
  • The 2º-alkyl group in example #3 is probably cleaved by an SN2 mechanism, but the SN1 alternative cannot be ruled out.
  • The phenol formed in this reaction does not react further, since SN2, SN1 and E1 reactions do not take place on aromatic rings.
  • The mechanism of peroxide formation is believed to be free radical in nature (note that molecular oxygen has two unpaired electrons).