addition reaction

Examples of addition reaction in the following topics:

• Addition Reactions

  • An addition reaction is the opposite of an elimination reaction.
  • There are two main types of polar addition reactions: electrophilic addition and nucleophilic addition.
  • Two non-polar addition reactions also exist: free radical addition and cycloadditions.
  • 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.

• Substitution Reactions of Benzene and Other Aromatic Compounds

  • The chemical reactivity of benzene contrasts with that of the alkenes in that substitution reactions occur in preference to addition reactions, as illustrated in the following diagram (some comparable reactions of cyclohexene are shown in the green box).
  • Many other substitution reactions of benzene have been observed, the five most useful are listed below (chlorination and bromination are the most common halogenation reactions).
  • Since the reagents and conditions employed in these reactions are electrophilic, these reactions are commonly referred to as Electrophilic Aromatic Substitution.
  • The specific electrophile believed to function in each type of reaction is listed in the right hand column.

• Addition Reactions of Alkenes

  • The most common chemical transformation of a carbon-carbon double bond is the addition reaction.
  • A large number of reagents, both inorganic and organic, have been found to add to this functional group, and in this section we shall review many of these reactions.
  • Consequently, if the bond energies of the product molecules are greater than the bond energies of the reactants, the reaction will be exothermic.
  • The following calculations for the addition of H-Br are typical.
  • Note that by convention exothermic reactions have a negative heat of reaction.

• By Structural Change

  • This is best accomplished by perceiving the reaction pathway or mechanism of a reaction.
  • In an addition reaction the number of σ-bonds in the substrate molecule increases, usually at the expense of one or more π-bonds.
  • Some common reactions may actually be a combination of reaction types.
  • The reaction of an ester with ammonia to give an amide, as shown below, appears to be a substitution reaction ( Y = CH3O & Z = NH2 ); however, it is actually two reactions, an addition followed by an elimination.
  • The addition of water to a nitrile does not seem to fit any of the above reaction types, but it is simply a slow addition reaction followed by a rapid rearrangement, as shown in the following equation.

• Changes in Temperature

  • Reactions can be classified by their enthalpies of reaction.
  • A diagram of the reaction coordinate for an exothermic reaction is shown in .
  • Le Chatelier's Principle predicts that the addition of products or the removal of reactants from a system will reverse the direction of a reaction, while the addition of reactants or the removal of products from a system will push the reaction towards the formation of products.
  • Applied to temperature, Le Chatelier's Principle predicts that the addition of heat to a system will cause an opposing reaction in the system to remove heat.
  • Exothermic reactions will be shifted toward the reactants.

• Product Selectivity

  • It is often the case that addition and elimination reactions may, in principle, proceed to more than one product.
  • However, rearrangements are known to occur during some reactions.
  • If the reaction products are such that stereoisomers may be formed, a reaction that yields one stereoisomer preferentially is said to be stereoselective.
  • In the addition of bromine to cyclohexene, for example, cis and trans-1,2-dibromocyclohexane are both possible products of the addition.
  • Since the trans-isomer is the only isolated product, this reaction is stereoselective.

• Reactions of Alkylidene Complexes

  • The alkylidene complexes described above undergo many interesting and synthetically useful reactions.
  • The next concept (Fischer Carbene Reactions) incorporates four diagrams of reactions.
  • The first diagram displays a few carbonyl-like reactions, which mimic organolithium addition to ketones (eq. # 1), ester amination (eq. # 2), the Wittig reaction (eq. # 3) and alpha-carbon alkylation (eq. # 4).
  • The second diagram of reactions begins with an equation illustrating the reductive and oxidative removal of the metal component of the carbene.

• The Effect of a Catalyst

  • Reactions can be sped up by the addition of a catalyst, including reversible reactions involving a final equilibrium state.
  • However, it is very important to keep in mind that the addition of a catalyst has no effect whatsoever on the final equilibrium position of the reaction.
  • If the addition of catalysts could possibly alter the equilibrium state of the reaction, this would violate the second rule of thermodynamics; we would be getting "something for nothing," which is physically impossible.
  • Try running the reaction with and without a catalyst to see the effect catalysts have on chemical reactions. 1.
  • A catalyst speeds up a reaction by lowering the activation energy required for the reaction to proceed.

• Pericyclic Reactions

  • An important body of chemical reactions, differing from ionic or free radical reactions in a number of respects, has been recognized and extensively studied.
  • The four principle classes of pericyclic reactions are termed: Cycloaddition, Electrocyclic, Sigmatropic, and Ene Reactions.
  • Corresponding intramolecular reactions, which create an additional ring, are well known.
  • All these reactions are potentially reversible (note the gray arrows).
  • The electrocyclic reaction shown above is a ring forming process.

• Ene Reactions

  • The reverse process is called a retro ene reaction.
  • The following examples illustrate some typical ene reactions, with equation 3 being an intramolecular ene reaction.
  • Reaction 4 is drawn as a retro ene reaction, although this has not been demonstrated to be general for all reactions of allylic alcohols with thionyl chloride.
  • The third diagram above displays two additional examples.
  • A similar acid-catalyzed reaction of simple aldehydes with alkenes to give allylic alcohols, 1,3-diols or 1,3-dioxanes is known as the Prins reaction.