Radical
A highly reactive molecular entity that has an unpaired electron.
Radical species can be an atom, molecule, cation or anion.
Radicals and the ozone layer
Ozone layer provides an important role in absorbing UV radiation from the Sun. Chlorofluorocarbons (CFCs) are easily broken down by UV radiation to form radicals.
The radicals react with the ozone, causing the ozone layer to deplete, creating ‘holes’.
Holes result in greater levels of UV radiation reacting with the Earth’s surface.
Common radicals (4)
Chlorine, hydroxide, oxide (O2-), methyl
Explain the reactivity of radicals.
Since electrons of opposite spins have the tendency to pair, an unpaired electron in a radical makes it highly reactive and unstable.
A radical species forms a more stable covalent bond upon interaction with an electron from another substance.
Outline how radicals are formed.
A radical species can be formed from species that contain covalent bonds through homolytic fission.
The energy provided by UV radiation is enough to cause the homolytic fission of some covalent bonds that have a relatively low bond enthalpy. (e.g. Cl-Cl, Br-Br, O=O)
Homolytic fission
When a covalent bond breaks evenly with each atom ending up with one of the electrons from the covalent bond.
What is the homolytic fission step called in a chain reaction/reaction mechanism
Initiation
Is initiation endothermic or exothermic
Endothermic
Describe the reactivity of alkanes
Alkanes are very unreactive and stable due to non-polar and relatively strong bonds.
(Non-polar bonds; relatively symmetrical distribution of electrons within the covalent bond, no partial charges in the molecule.)
(C-C and C-H bonds have a high bond enthalpy, requiring a lot of energy to break.)
Radical substitution reaction
A radical substitution reaction is a chemical reaction where a radical species replaces another atom, usually hydrogen, in a molecule.
Reaction between radicals and alkanes
Radicals are highly reactive and so are able to react with alkanes.
Alkanes can undergo a substitution reaction, meaning part of the alkane is substituted by a radical.
Steps in radical substitution reactions
- Initiation
The formation of a radical (endothermic). - Propagation
Radical species react with a covalent species, forming a new radical and a new covalent species (endo or exothermic depending on bond energies). - Termination
Two radicals form a single covalent species, terminating the chain reaction (exothermic; formation of a stable bond from unstable reactive species, usually a halogen radical).
Products in radical substitution reactions
There is a mixture of products formed, since both the initial radical and the radical produced are able to react.
The formation of a hydrogen radical is rare, and the combination of hydrogen with a halogn radical to form a stable covalent bond is a much more probable process.
Significance of radical substitution reactions
Useful/helpful in the synthesis of molecules with specific functional group.
Keywords
Fission
Homolytic
Heterolytic
Free radical
Nucleophile
Electrophile
Cation
Anion
2 main types of isomers
Structural isomer and stereoisomer
Structural isomer
Molecules that share the same molecular formula but possesses different structural formulas (different connectivity)
Stereoisomer
Molecules that share the same molecular formula and connectivity (structural arrangement) but different spatial arrangements (3-dimensional orientations).
2 main types of stereoisomerism
Configurational and Conformational
Conformational stereoisomers
Molecules that share the same molecular formula and connectivity but different spatial arrangements which are interconverted by rotation around a sigma bond.
Configurational stereoisomers
Molecules that share the same molecular formula and connectivity but different spatial arrangements which are interconverted by breaking/reforming a bond(s).
2 types of configurational stereoisomerism
Optical and geometric
Optical isomers
Non-superimposable, shows chirality
Enatiomer or diastereomer
Geometric isomers
Isomerism that occurs due to restricted rotation around double bonds or cyclic structures, where substituents are fixed in space.
