homolytic and hterolytic bond fission heading

Heterolytic and homolytic bond fission

Bond breaking or bond fission as it is also known can occur in two common ways, these are:

Heterolytic bond fission usually happens in covalent bonds where the electrons are unequally shared by the atoms, this unequal sharing is due to differences in the electronegativity of the 2 atoms involved in the covalent bond. This means that when the covalent bond is broken one atom, the most electronegative atom in the bond takes the 2 electrons in the covalent bond. This will result in one atom gaining an electron and one atom losing an electron, meaning that a positively and negatively charged ion will be formed.

The bond between the two atoms in a hydrogen chloride molecule is a polar covalent one. Chlorine being more electronegative than hydrogen has a greater share of the 2 electrons in the polar covalent bond. When this type of bond breaks, the chlorine atom will take both electrons in the bond forming a negatively charged chloride ion and a positively charged hydrogen ion. This process which reults in the formation of ions is called heterolytic bond fission and is shown below.


Homolytic bond fission

Homolytic bond fission tends to occurs in molecules where there is covalent bonding present between atoms. This means that the two electrons in the covalent bond are equally shared. If this covalent bond is split then each atom involved in forming the bond will simply take back its electron. This means that the "species" formed as a result of this bond breaking will have an unpaired single electron. These atoms with unpaired electrons are called free radicals.

It is possible to break the covalent bonds in halogens such as chlorine and bromine by simply exposing the molecules to bright sunlight or shining light from an artifical source such as that from a camera flash onto them. The diagram opposite shows how the bond in a chlorine molecule can be broken in such a way as to produce two chlorine atoms with unpaired electrons. These atoms with unpaired electrons are called free radicals and they are very very reactive.

When the covalent bond breaks in such a way that each atom simply takes back its electron we call this homolytic bond fission or homolytic bond cleavage.

The alkanes are an homologous series of hydrocarbons that you meet in gcse chemistry. They are generally unreactive molecules, except when they are used as fuels in combustion reactions. They are unreactive with acids, alkalis, electrophiles and nucleophiles, this is simply because they contain non-polar C-H bonds. However free radicals can react with alkanes molecules to produce halogenalkanes or haloalkanes. Here one or more of the hydrogen atoms in the alkane molecule are replaced or substituted by a halogen atom. This is outlined below:

Halogenalkanes

Methane (CH4) is the first member of the alkanes, it is the gas used in Bunsen burners in the lab and for heating and cooking in the home. If a mixture of methane and chlorine are placed in a plastic bottle in a dark room then no reaction will take place. However if a flash is produced from a flash gun then a bright orange flash is seen as a very rapid reaction takes place between the chlorine and the methane gases in the bottle. The products of this reaction are chloromethane (a halogenalkane) and hydrogen chloride gas, an equation for this free-radical subsitution reaction is shown below. This reaction is a chain reaction and occurs through free radical intermediates which are produced by homolytic bond cleavage of a chlorine molecule.

Key Points

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