Bond breaking or bond fission as it is also known can occur in two common ways; these are:
The image below shows the difference between these two ways in which a covalent bond can be broken.
Heterolytic bond fission or cleavage usually occurs in covalent bonds when the electrons are unequally shared by the atoms involved in forming the covalent bond, this unequal sharing of the two electrons is due to differences in the electronegativity of the two 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 positively and negatively charged ions will be formed.
For example 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 two electrons in the polar covalent bond, so when this type of bond breaks the chlorine atom will take both electrons in the bond forming a negatively charged chloride anion and a positively charged hydrogen cation. This process which results in the formation of ions is called heterolytic bond fission and is outlined below.
Homolytic bond fission tends to occur 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 covalent bond will simply
take back its electron. This means that the "species" formed as a
result of this type of 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 artificial source such as that from a camera flash onto them.
The equation below shows how the bond in a chlorine
molecule can be broken in such a way as to produce two chlorine free radicals
with
unpaired electrons.
The alkanes are a homologous series of hydrocarbons that you met 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 alkane 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 in these explosive and violent reactions; this is covered in detail on the page on free radical substitution reactions.
The table below summaries the main features of homolytic and heterolytic bond fission.
| Feature | Heterolytic bond fission | Homolytic bond fission |
|---|---|---|
| How the bond breaks | Both bonding electrons go to one atom. | One bonding electron goes to each atom. |
| Species formed | Ions (a cation and an anion). | Free radicals (species with unpaired electrons). |
| Type of bond where it is most likely | Polar covalent bonds (electrons already unequally shared). | Non-polar covalent bonds (electrons equally shared). |
| Typical example | H–Cl → H+ + Cl- | Cl2 → 2Cl. |
| What to look for in an answer | Mentions ions, full electron pair moving, electronegativity. | Mentions radicals, one electron each, unpaired electron (dot). |
| Common exam mix-up | Writing radicals instead of ions (missing charges). | Writing ions instead of radicals (missing dot). |
Try to quick quiz below to test your understanding of homolytic and heterolytic bond breaking.