Bond fission and attacking species
Breaking bonds and attacking species
- A covalent bond can break in two ways.
- The fragments may be radicals or ions.
- Reactions are driven by nucleophiles and electrophiles.
Homolytic vs heterolytic fission
- Homolytic fission: the bond splits evenly, one electron to each atom → two free radicals (unpaired electron).
- Heterolytic fission: the bond splits unevenly, both electrons to one atom → two ions.
Practice
Homolytic fission of a covalent bond produces:
Homolytic = even split, one electron each → two radicals; heterolytic = uneven → two ions.
Practice
Heterolytic fission produces:
In heterolytic fission both bonding electrons go to one atom, giving a positive and a negative ion.
Nucleophiles and electrophiles
- a nucleophile has a lone pair and is attracted to an electron-poor (positive) centre.
- an electrophile is attracted to an electron-rich (negative) centre, such as a C=C double bond.
Practice
A nucleophile is a species that:
Nucleophiles use a lone pair to attack electron-poor carbons; electrophiles seek electron-rich centres.
Practice
An electrophile is attracted to:
Electrophiles are electron-seeking; a C=C double bond is electron-rich and attracts them.
You've got it
Key idea
- homolytic fission → two free radicals (one electron each); heterolytic fission → two ions
- a nucleophile (lone pair) attacks an electron-poor centre
- an electrophile is drawn to an electron-rich centre (e.g. C=C)