Topic 10 Flashcards
(9 cards)
What is an Alkane?
They contain only single C-C bonds and so are bonded to the maximum number of H atoms possible. They show little tendency to undergo chemical reactions due to the strong bonds.
Combustion Reactions - Alkanes
Alkanes undergo readily combustion. An alkane with an abundance of oxygen will undergo complete combustion whereas an alkane with not enough oxygen or heat will undergo incomplete combustion. Incomplete combustion is undesirable due to its products.
What is Free Radial Substitution?
Another reaction alkanes will undergo in mild conditions. A free radical is a chemical species with an unpaired electron, and are highly reactive because of their tendency to pair electrons.
FRS - alkanes
Alkanes react with F2, Cl2 or Br2 to produce a halogenoalkane. This requires UV light as a catalyst or heat to reach Ea(min).
Steps of FRS
- Initiation - undergo hemolytic fission (dividing the electrons in the bond equally between the 2 atoms so that the 2 free radicals are formed). Must be catalyzed with UV light.
- Propagation - produce more radicals that allow the reaction to continue.
- Termination - no radicals are produced so the Chan reaction stops.
Homolytic fission vs. heterolytic fission
Homolytic - dividing the electrons in the bond equally between the 2 atoms.
Heterolytic - the electrons are divided unequally between the 2 atoms
Alkenes
Hydrocarbons containing a double bond. They do not contain as many H’s as possible). The double bond is the site of unsaturation. They form a homologous series with the general formula C(n)H(2n). Also undergo combustion reaction similarly to alkanes.
Hydrogenation (H(2))
Hydrogen will add to a double bond in the presence of a Ni catalyst at 180 degree. The product is the corresponding alkane.
Halogenation (X(2))
Alkanes react easily with the halogens to form a di-halo alkane. The halogen atoms will be on adjacent carbon atoms. (Non-polar molecules so a non-polar solvent is used for these reactions)
