Alkanes

• In alkanes, the covalent bonds around each atom form a tetrahedral structure with bond angles of 109.5 degrees.
• All bonds in alkanes are a type of single covalent bond called sigma bonds.
• Sigma bonds form when electron orbitals from adjacent atoms directly overlap. The bonds contain one electron from each atom either side of the bond.
• Sigma bonds are fully rotational so c atoms can move relative to one another.
• Alkanes are non-polar as carbon and hydrogen atoms have very similar electronegativities.
• Alkanes are insoluble in water and other polar solvents.
• Alkanes are generally unreactive due to their non-polar nature and their strong covalent bonds between atoms.
• As the length of an alkane chain increases, their boiling point increases as the intermolecular forces are getting stronger or more numerous.
• Alkanes are non-polar so induced dipole - dipole intermolecular forces form between them.
• Short chain alkanes have a lower boiling point as there are very few london forces between them.
• As the length of the alkane chain increases, there are more electrons and a larger surface area on the molecule, so the strength and amount of london forces increases.
• Branched chain alkanes have a lower boiling point as branches prevent molecules getting as close together, hence decreasing the amount and strength of london forces formed.
• alkanes are soluble in non-polar solvents such as cyclohexane
• The more points of contact between alkane chains, the more intermolecular forces will form.
• packing describes how closely alkane chains can get together.
• Ultraviolet light has enough energy to break the sigma bonds in a molecule to form free radicals.
• Free radicals are any species with an unpaired electron.
• Free radicals are highly reactive species.
• A substitution reaction occurs when one species/group is replaced by another species.
• There are 3 stages of free radical substitution:

  • Initiation
  • Propagation
  • Termination
• In initiation, UV light is shined onto a reaction mix causing the breaking of sigma bonds and the formation of free radicals.
• When a covalent bond is split forming free radicals, this process is called homolytic fission (homolysis)
• In propagation, a free radical reacts with another molecule, forming a product and a new free radical. There are two steps of propagation and this causes a chain reaction.
• The substitution reaction continues until termination, which occurs when two free radicals react to form a molecule with no unpaired electrons; stopping the chain reaction.