Radical disproportionation
Encyclopedia
Radicals
in chemistry are defined as reactive atoms or molecules that contain unpaired electrons in an open shell. The unpaired electrons cause radicals to be unstable and reactive. Reactions in radical chemistry can generate both radical and non-radical products
. Radical disproportionation encompasses a group of reactions in organic chemistry
in which two radicals react to form two different non-radical products. These reactions can occur with many radicals in solution
and in the gas phase. Due to the unstable nature of radical molecules, disproportionation
proceeds rapidly and requires little to no activation energy
. The most thoroughly studied radical disproportionation reactions have been conducted with alkyl radicals, but there are many organic molecules that can exhibit more complex, multi-step disproportionation reactions.
to form a double bond
. Other atoms such as halogens may also be abstracted during a disproportionation reaction. Abstraction occurs as a head to tail reaction with the atom that is being abstracted facing the radical atom on the other molecule.
play a significant role in disproportionation with ethyl
radicals acting as more effective acceptors than tert-butyl radicals. Tert-butyl radicals have many hydrogens on adjacent carbons to donate and steric effects often prevent tert-butyl radicals from getting close to abstracting hydrogens.
of the products increases by one over the reactants. Thus the reaction is exothermic
(ΔH = 50 – 95 kcal/mol) and proceeds rapidly.
, disproportionation proceeds at a slightly slower rate whereas the rate of recombination remains the same. Thus disproportionation is weakly affected by the kinetic isotope effect
with kH/kD = 1.20 ± 0.15 for ethylene. Hydrogens and deuterons are not involved in recombination reactions. However, deuteron abstraction during disproportionation occurs more slowly than hydrogen abstraction due to the increased mass and reduced vibrational energy of deuterium, although the experimentally observed kH/kD is close to one.
which removes electron density from the donor carbon atom thereby facilitating hydrogen abstraction. The rate of disproportionation is also aided by the more electronegative oxygen on the acceptor molecule.
s or chain propagation with disproportionation and recombination occurring in the terminal step of the reaction. Terminating chain propagation is often most significant during polymerization as the desired chain propagation cannot take place if disproportionation and recombination reactions readily occur. Controlling termination products and regulating disproportionation and recombination reactions in the terminal step are important considerations in radical chemistry and polymerization
. In some reactions (such as the one shown below) one or both of the termination pathways can be hindered by steric or solvent effects
.
is also formed, and thus the chain reaction does not readily proceed. During living free radical polymerization
, termination pathways for a growing polymer chain are removed. This can be achieved through several methods, one of which is reversible termination with stable radicals. Nitroxide radicals and other stable radicals reduce recombination and disproportionation rates and control the concentration of polymeric radicals.
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
in chemistry are defined as reactive atoms or molecules that contain unpaired electrons in an open shell. The unpaired electrons cause radicals to be unstable and reactive. Reactions in radical chemistry can generate both radical and non-radical products
Product (chemistry)
Product are formed during chemical reactions as reagents are consumed. Products have lower energy than the reagents and are produced during the reaction according to the second law of thermodynamics. The released energy comes from changes in chemical bonds between atoms in reagent molecules and...
. Radical disproportionation encompasses a group of reactions in organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
in which two radicals react to form two different non-radical products. These reactions can occur with many radicals in solution
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
and in the gas phase. Due to the unstable nature of radical molecules, disproportionation
Disproportionation
Disproportionation, also known as dismutation is used to describe a specific type of redox reaction in which a species is simultaneously reduced and oxidized so as to form two different products....
proceeds rapidly and requires little to no activation energy
Activation energy
In chemistry, activation energy is a term introduced in 1889 by the Swedish scientist Svante Arrhenius that is defined as the energy that must be overcome in order for a chemical reaction to occur. Activation energy may also be defined as the minimum energy required to start a chemical reaction...
. The most thoroughly studied radical disproportionation reactions have been conducted with alkyl radicals, but there are many organic molecules that can exhibit more complex, multi-step disproportionation reactions.
Mechanism of Radical Disproportionation
In radical disproportionation reactions one molecule acts as an acceptor while the other molecule acts as a donor. In the most common disproportionation reactions, a hydrogen atom is taken, or abstracted by the acceptor as the donor molecule undergoes an elimination reactionElimination reaction
An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism...
to form a double bond
Double bond
A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...
. Other atoms such as halogens may also be abstracted during a disproportionation reaction. Abstraction occurs as a head to tail reaction with the atom that is being abstracted facing the radical atom on the other molecule.
Disproportionation and Steric Effects
Radical disproportionation is often thought of as occurring in a linear fashion with the donor radical, the acceptor radical, and the atom being accepted all along the same axis. In fact, most disproportionation reactions do not require linear orientations in space. Molecules that are more sterically hindered require arrangements that are more linear, and thus react more slowly. Steric effectsSteric effects
Steric effects arise from the fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds , and this may affect the molecule's preferred shape and reactivity.-Steric...
play a significant role in disproportionation with ethyl
Ethyl group
In chemistry, an ethyl group is an alkyl substituent derived from ethane . It has the formula -C2H5 and is very often abbreviated -Et.Ethylation is the formation of a compound by introduction of the ethyl functional group, C2H5....
radicals acting as more effective acceptors than tert-butyl radicals. Tert-butyl radicals have many hydrogens on adjacent carbons to donate and steric effects often prevent tert-butyl radicals from getting close to abstracting hydrogens.
Alkyl Radical Disproportionation
Alkyl radical disproportionation has been studied extensively in scientific literature. During alkyl radical disproportionation, an alkane and an alkene are the end products and the bond orderBond order
Bond order is the number of chemical bonds between a pair of atoms. For example, in diatomic nitrogen N≡N the bond order is 3, while in acetylene H−C≡C−H the bond order between the two carbon atoms is also 3, and the C−H bond order is 1. Bond order gives an indication to the stability of a bond....
of the products increases by one over the reactants. Thus the reaction is exothermic
Exothermic
In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...
(ΔH = 50 – 95 kcal/mol) and proceeds rapidly.
Cross Disproportionation of Alkyl Radicals
Cross disproportionation occurs when two different alkyl radicals disproportionate to form two new products. Different products can be formed depending on which alkyl radical acts as a donor and which acts as an acceptor. The efficiency of primary and secondary alkyl radicals as donors depends on the steric effects and configuration of the radical acceptors.Competition with Recombination
Another reaction that can sometimes occur instead of disproportionation is recombination. During recombination, two radicals form one new non-radical product and one new bond. Similar to disproportionation, the recombination reaction is exothermic and requires little to no activation energy. The ratio of the rates of disproportionation to recombination is referred to as kD/kC and often favors recombination compared with disproportionation for alkyl radicals. As the number of transferable hydrogens increase, the rate constant for disproportionation increases relative to the rate constant for recombination.Kinetic Isotope Effect on Disproportionation and Recombination
When the hydrogen atoms in an alkyl radical are displaced with deuteriumDeuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
, disproportionation proceeds at a slightly slower rate whereas the rate of recombination remains the same. Thus disproportionation is weakly affected by the kinetic isotope effect
Kinetic isotope effect
The kinetic isotope effect is the ratio of reaction rates of two different isotopically labeled molecules in a chemical reaction. It is also called "isotope fractionation," although this term is somewhat broader in meaning...
with kH/kD = 1.20 ± 0.15 for ethylene. Hydrogens and deuterons are not involved in recombination reactions. However, deuteron abstraction during disproportionation occurs more slowly than hydrogen abstraction due to the increased mass and reduced vibrational energy of deuterium, although the experimentally observed kH/kD is close to one.
Polar Effects and Alkoxy Radical Disproportionation
Alkoxy radicals which contain unpaired electrons on an oxygen atom display a higher kD/kC compared to alkyl radicals. The oxygen has a partial negative chargeIon
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
which removes electron density from the donor carbon atom thereby facilitating hydrogen abstraction. The rate of disproportionation is also aided by the more electronegative oxygen on the acceptor molecule.
Termination of chain processes
Many radical processes involve chain reactionChain reaction
A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events....
s or chain propagation with disproportionation and recombination occurring in the terminal step of the reaction. Terminating chain propagation is often most significant during polymerization as the desired chain propagation cannot take place if disproportionation and recombination reactions readily occur. Controlling termination products and regulating disproportionation and recombination reactions in the terminal step are important considerations in radical chemistry and polymerization
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
. In some reactions (such as the one shown below) one or both of the termination pathways can be hindered by steric or solvent effects
Solvent effects
In chemistry, Solvent effects is the group of effects that a solvent has on chemical reactivity. Solvents can have an effect on solubility, stability and reaction rates and choosing the appropriate solvent allows for thermodynamic and kinetic control over a chemical reaction.-Effects on...
.
Reducing Disproportionation in Living Free Radical Polymerization
Many polymer chemists are concerned with limiting the rate of disproportionation during polymerization. Although disproportionation results in formation of one new double bond which may react with the polymer chain, a saturated hydrocarbonHydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
is also formed, and thus the chain reaction does not readily proceed. During living free radical polymerization
Living free radical polymerization
Living free radical polymerization is a type of living polymerization where the active polymer chain end is a free radical. Several methods exist...
, termination pathways for a growing polymer chain are removed. This can be achieved through several methods, one of which is reversible termination with stable radicals. Nitroxide radicals and other stable radicals reduce recombination and disproportionation rates and control the concentration of polymeric radicals.