radical

Examples of radical in the following topics:

• Radical Chain-Growth Polymerization

  • Virtually all of the monomers described above are subject to radical polymerization.
  • When radical polymerization is desired, it must be started by using a radical initiator, such as a peroxide or certain azo compounds.
  • Because radicals are tolerant of many functional groups and solvents (including water), radical polymerizations are widely used in the chemical industry.
  • The 1º-radical at the end of a growing chain is converted to a more stable 2º-radical by hydrogen atom transfer.
  • 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.

• Intermolecular Addition Reactions

  • 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).
  • The following diagram provides other examples of radical addition to double bonds.
  • The first two equations show how different radicals may be generated selectively from the same compound.
  • Indeed, free radical polymerization of simple substituted alkenes is so facile that bulk quantities of these compounds must be protected by small amounts of radical inhibitors during storage.
  • 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.

• The Configuration of Free Radicals

  • Since the difference in energy between a planar radical and a rapidly inverting pyramidal radical is small, radicals generated at chiral centers generally lead to racemic products.
  • Initial formation of a carboxyl radical is followed by loss of carbon dioxide to give a pyramidal bridgehead radical.
  • This radical abstracts a chlorine atom from the solvent, yielding the bridgehead chloride as the major product.
  • Rapid decomposition to other radicals may occur, but until one or both of these radicals escape the solvent cage a significant degree of coupling (recombination) may occur.
  • Cage recombination of radicals may be sufficiently rapid to preserve the configuration of the generating species.

• Solving Problems with Radicals

  • Roots are written using a radical sign, and a number denoting which root to solve for.
  • Roots are written using a radical sign.
  • Any expression containing a radical is called a radical expression.
  • You want to start by getting rid of the radical.
  • Do this by treating the radical as if it where a variable.

• Adding, Subtracting, and Multiplying Radical Expressions

  • An expression with roots is called a radical expression.
  • To add radicals, the radicand (the number that is under the radical) must be the same for each radical, so, a generic equation will have the form:
  • Multiplication of radicals simply requires that we multiply the variable under the radical signs.
  • the value under the radical sign can be written as an exponent,
  • Then, the fraction under the radical sign can be addressed, and the radical in the numerator can again be simplified.

• Elimination Reactions

  • The use of thionoesters, such as a xanthates, as radical generating functions was described above, and these groups may also serve as excellent radical leaving groups.
  • Once again, the tolerance of radical reactions for a variety of functional groups is demonstrated.
  • An industrial preparation of vinyl chloride from 1,2-dichloroethane, made by adding chlorine to ethylene, proceeds by elimination of a chlorine atom from an intermediate carbon radical.
  • The isomer 1,1-dichloroethane does not undergo an equivalent radical chain elimination.

• Radical Recombination Reactions

  • Radical coupling (recombination) reactions are very fast, having activation energies near zero.
  • The only reason radical coupling reactions do not dominate free radical chemistry is that most radicals have very short lifetimes and are present in very low concentration.
  • Consequently, if short lived radicals are to contribute to useful synthetic procedures by way of a radical coupling, all the events leading up to the coupling must take place in a solvent cage.
  • The oxy radical abstracts a hydrogen atom from a nearby carbon, and the resulting radical couples with •NO to give a nitroso compound.
  • Photolysis generates an oxy radical that is located close to the 18-methyl group.

• Fractions Involving Radicals

  • In mathematics, we are often given terms in the form of fractions with radicals in the numerator and/or denominator.
  • When we are given expressions that involve radicals in the denominator, it makes it easier to evaluate the expression if we rewrite it in a way that the radical is no longer in the denominator.
  • You are given the fraction $\frac{10}{\sqrt{3}}$, and you want to simplify it by eliminating the radical from the denominator.
  • Recall that a radical multiplied by itself equals its radicand, or the value under the radical sign.
  • Therefore, multiply the top and bottom of the fraction by $\frac{\sqrt{3}}{\sqrt{3}}$, and watch how the radical expression disappears from the denominator:$\displaystyle \frac{10}{\sqrt{3}} \cdot \frac{\sqrt{3}}{\sqrt{3}} = {\frac{10\cdot\sqrt{3}}{{\sqrt{3}}^2}} = {\frac{10\sqrt{3}}{3}}$

• Background & Introduction

  • A radical is an atomic or molecular species having an unpaired, or odd, electron.
  • Early chemists used the term "radical" for nomenclature purposes, much as we now use the term "group".
  • The resonance structures drawn here may give the impression that the triphenylmethyl radical is planar (flat).
  • Other relatively stable radicals, such as galvinoxyl have been prepared and studied.
  • The term "free radical" is now loosely applied to all radical intermediates, stabilized or not.

• Simplifying Radical Expressions

  • A radical expression that contains variables can often be simplified to a more basic expression, much as can expressions involving only integers.
  • Expressions that include roots are known as radical expressions.
  • A radical expression is said to be in simplified form if:
  • For example, let's write the radical expression $\sqrt { \frac { 32 }{ 5 } }$ in simplified form, we can proceed as follows.
  • This follows the same logic that we used above, when simplifying the radical expression with integers: