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.

• 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).

• Processes of the Light-Dependent Reactions

  • Because this state of an electron is very unstable, the electron is transferred to another molecule creating a chain of redox reactions called an electron transport chain (ETC).
  • The net-reaction of all light-dependent reactions in oxygenic photosynthesis is: 2H2O + 2NADP+ + 3ADP + 3Pi → O2 + 2NADPH + 3ATP
  • The electrons travel through the chloroplast electron transport chain to photosystem I (PSI), which reduces NADP+ to NADPH.
  • The electron transport chain moves protons across the thylakoid membrane into the lumen.
  • In (b) photosystem I, the electron comes from the chloroplast electron transport chain.

• 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.

• DNA Sequencing Based on Sanger Dideoxynucleotides

  • Sanger sequencing, also known as chain-termination sequencing, refers to a method of DNA sequencing developed by Frederick Sanger in 1977.
  • To each reaction is added only one of the four dideoxynucleotides (ddATP, ddGTP, ddCTP, or ddTTP).
  • This is frequently performed using a denaturing polyacrylamide-urea gel with each of the four reactions run in one of four individual lanes (lanes A, T, G, C).
  • Chain-termination methods have greatly simplified DNA sequencing.
  • Dye-terminator sequencing utilizes labelling of the chain terminator ddNTPs, which permits sequencing in a single reaction, rather than four reactions as in the labelled-primer method.

• 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.

• The Two Parts of Photosynthesis

  • Light-dependent and light-independent reactions are two successive reactions that occur during photosynthesis.
  • Just as the name implies, light-dependent reactions require sunlight.
  • Photosystems consist of a light-harvesting complex and a reaction center.
  • In photosystem I, the electron comes from the chloroplast electron transport chain.
  • Photosynthesis takes place in two stages: light-dependent reactions and the Calvin cycle (light-independent reactions).