Substitution reactions of alkanes Flashcards
what is a substitution reaction?
a reaction in which one atom or group of atoms replaces another atom or group.
why are alkanes not very reactive?
they contain only carbon and hydrogen atoms, being hydrocarbons. They also have single bonds only, and these bonds are not very polar, so do not undergo reactions with more reactive substances such as acids or alkalis.
what is a mechanism?
is a sequence of steps in an overall reaction. they show what happens to the electrons involved in bond breaking and formation
define homolytic fission
when the breaking of a covalent bond occurs and each of the bonding electrons leaves with one species, creating free radicals
what is a species?
any substance that can be represented by a formula
explain what is initiation.
when one molecule (let’s say Chlorine) forms two radicals. This step is called initiation because it starts the overall reaction
what is a radical?
a species which contains an unpaired electron (represented with a dot)
explain the steps of a substitution reaction in halogenation of alkanes
Step 1: initiation, when ultraviolet radiation causes free radicals to form
Step 2: Very reactive halogen free radicals collide with alkane molecules to remove a hydrogen atom from the alkane and produce a hydrochloride molecule and a methyl radical. The methyl radical then reacts with chlorine molecules to form chloro-alkane and a chlorine radical. This step is called propagation.
Step 3: free radicals collide with each other, forming molecules as they both have an unpaired electron to form a covalent bond. This is called termination
explain the propagation step in substitution reactions
in these reactions, free radicals react with molecules to create one molecule and one radical. the reactions taking place in this step are considered together to result in the conversion of the original alkane to the halogenalkane. Eg. CH4 becoming CH3CL
explain why different reactions make up the termination step.
In the reacting mixture, radicals react to form a molecule. However, due to the mix of radicals present in the reaction, different products are produced, however all products are unreactive species.
write the six equations for the mechanism of the reaction between methane and bromine.
Br2 = Br’ + Br’
CH4 + Br’ = CH3’ + HBr
CH3’ + Br2 = CH3Br + Br’
CH3’ + Br’ = CH3Br
Br’ + Br’ = Br2
CH3’ + CH3’ = C2H6
Is this equation showing initiation, propagation or termination? C2H5’ + CH3’ = C3H8
Termination
