Substitution reactions for alkanes Flashcards
Define radical substitution.
Radical substitution is a type of substitution reaction in which a radical replaces a different atom or group of atoms.
Define mechanism
Mechanism is a sequence of steps showing the path taken by electrons in a reaction.
Define intiation
Initiation is the first step in a radical substitution in which free radicals are generated by ultraviolet radiation.
What conditions are required for alkanes to react with halogens?
Alkanes react with halogens in the presence of ultraviolet radiation or at temperatures of about 300C.
Explain what happens in a reaction of an alkane and a halogen.
The reaction is radical substitution:
- Covalent bonds are broken by homolytic fission to form radicals with an unpaired electron.
- A hydrogen atom in the alkane is substituted by a halogen atom.
Give the halogenation reaction for methane with chlorine.
CH4 + Cl2 ——> CH3Cl + HCL
Explain step 1 : Initiation
of radical substitution
The Cl-Cl bond in a chlorine molecule is broken by homolytic fission, forming two chlorine radicals. Ultraviolet radiation provides the energy for this bond fission.
Cl-Cl ——–> Cl* + Cl*
Quick note:
After initiation, the reaction can continue without the need for further energy or more radicals.
Explain step 2 : Propagation
of radical substitution
The propagation step has two steps:
1) Methane reacts with a chlorine radical. A single C-H bond is broken by homolytic fission, forming a methyl radical CH3. Hydrogen chloride, HCl is also formed.
CH4 + Cl ——–> *CH3 + HCl
2) Methyl radical reacts with a chlorine molecule. The organic product chloromethane, CH3Cl is formed together with a further chlorine radical.
CH3 + Cl2 ——-> CH3Cl + Cl
Explain why propagation reactions are rapid?(of radical substitution)
Propagation is a chain reaction. Although a chlorine radical is used up in the first propagation step, another chlorine radical is generated in the second step. This chlorine radical can now react with another methane molecule. This continues until all the chlorine has been used or the termination stage has removed all the radicals.
Define propagation.
Propagation is the two repeated steps in radical substitution that build up the products in a chain reaction.
Explain step 3: termination
of radical substitution
Two radicals combine to form a molecule. There are a number of possible termination steps. This is because of the large numbers of radicals in the reaction mixture. The following molecules are produced:
Cl+Cl——>Cl2
CH3+CH3*—–> C2H6
CH3+Cl—–>CH3Cl
Define termination
Termination is the step at the end of radical substitution when two radicals combine to form a molecule.
