chain reaction

Examples of chain reaction in the following topics:

• Nuclear Reactors

  • A nuclear reactor is a piece of equipment where nuclear chain reactions can be controlled and sustained.
  • This is known as a nuclear chain reaction.
  • At least one neutron is required to "strike" a chain reaction, and if the spontaneous fission rate is sufficiently low, it may take a long time before a chance neutron encounter starts a chain reaction—even if the reactor is supercritical.
  • A possible nuclear fission chain reaction.
  • Describe the nuclear chain reaction process utilized in most nuclear reactors

• Radical Chain-Growth Polymerization

  • In practice, larger numbers of moderately sized chains are formed, indicating that chain-terminating reactions must be taking place.
  • Since the concentration of radical species in a polymerization reaction is small relative to other reactants (e.g. monomers, solvents and terminated chains), the rate at which these radical-radical termination reactions occurs is very small, and most growing chains achieve moderate length before termination.
  • Another reaction that diverts radical chain-growth polymerizations from producing linear macromolecules is called chain transfer.
  • Chain transfer reactions are especially prevalent in the high pressure radical polymerization of ethylene, which is the method used to make LDPE (low density polyethylene).
  • Further polymerization at the new radical site generates a side chain radical, and this may in turn lead to creation of other side chains by chain transfer reactions.

• Reactions of Alkanes

  • Alkanes are generally unreactive, but can participate in oxidation, halogenation, and cracking reactions.
  • However, there are a few classes of reactions that are commonly performed with alkanes.
  • The most important reaction that alkanes undergo is combustion.
  • In this reaction, UV light or heat initiates a chain reaction, cleaving the covalent bond between the two atoms of a diatomic halogen.
  • Alkanes can be halogenated at a number of sites, and this reaction typically yields a mixture of halogenated products.

• Cationic Chain-Growth Polymerization

  • Chain growth ceases when the terminal carbocation combines with a nucleophile or loses a proton, giving a terminal alkene (as shown here).
  • At low temperatures, chain transfer reactions are rare in such polymerizations, so the resulting polymers are cleanly linear (unbranched).

• Cyclization by Intramolecular Addition Reactions

  • If a radical is joined to a double bond by a chain of three or more carbons intramolecular addition generates a ring.
  • Likewise, in reaction 3 a six-membered ring is formed preferentially over an alternative seven-membered ring.
  • Note that these reactions tolerate a wide variety of functional groups.
  • The stereoelectronic factor in this reaction is defined by the preferred mode of approach of a radical as it bonds to the pi-electron system of an alkene function.
  • Bonding to the distal carbon is constrained by the structure of the connecting chain.

• Cycloaddition Reactions

  • The most common cycloaddition reaction is the [4π+2π] cyclization known as the Diels-Alder reaction.
  • The stereospecificity of these reactions should be evident.
  • Reaction 3 is an intramolecular Diels-Alder reaction.
  • Since the diene and dienophile are joined by a chain of atoms, the resulting [4+2] cycloaddition actually forms two new rings, one from the cycloaddition and the other from the linking chain.
  • The fourth reaction is a [6+4] cycloaddition.

• Condensation Reactions

  • In a condensation reaction, two molecules or parts thereof combine, releasing a small molecule.
  • When this small molecule is water, it is known as a dehydration reaction.
  • Many condensation reactions follow a nucleophilic acyl substitution or an aldol condensation reaction mechanism (see previous concept for more information).
  • In one type of polymerization reaction, a series of condensation steps takes place whereby monomers or monomer chains add to each other to form longer chains.
  • Recognize the chemical principles of condensation reactions as they relate to polymerization.

• Elimination Reactions

  • Once again, the tolerance of radical reactions for a variety of functional groups is demonstrated.
  • The isomer 1,1-dichloroethane does not undergo an equivalent radical chain elimination.

• Enzyme Catalysis

  • Once the reaction completes, the product(s) leaves the active site, so the enzyme is free to catalyze more reactions.
  • Enzymes can catalyze reactions through a variety of mechanisms.
  • Covalent catalysis: covalent bonding to side chains or cofactors can lower the energy of the transition state.
  • After the reaction has proceeded, the products are released and the enzyme can catalyze further reactions.
  • List the five typical mechanisms used by enzymes to catalyze biological reactions

• Intermolecular Addition Reactions

  • Addition reactions to carbon:carbon double bonds are among the most important free radical reactions employed by chemists.
  • The anti-Markovnikov addition of HBr to alkenes is one such reaction, and the peroxide initiated addition of carbon tetrachloride to 1-hexene is another.
  • As the following equations demonstrate, radical addition to a substituted double bond is regiospecific (i.e. the more stable product radical is preferentially formed in the chain addition process).
  • These inhibitors, or radical scavengers, may themselves be radicals (e.g. oxygen and galvinoxyl) or compounds that react rapidly with propagating radicals to produce stable radical species that terminate the chain.