Substitution Reactions of Alkanes
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why are alkanes considered to be fairly unreactive
• because they contain only carbon and hydrogen atoms - and only have single bonds
why do alkane’s single bonds make them unreactive
• they are not very polar • so they dont undergo reactions with substances that are reactive • such as acids, alkalis and reactive metals
what is a simple example of a substitution reaction between methane and chlorine gas
CH4 + Cl2 = CH3Cl + HCl
what is happening during the substitution reaction and what is it called
• one of the hydrogen atoms from methane have been replaced by a chlorine atom • in this case, it would be called chlorination
what is the broader name given to substitution reactions using halogens
halogenation
what does a mechanism do in chemistry
• it tries to explain the actual changes that happen during a reaction • especially with the bonding between the atoms
what is a mechanism in chemistry (definition)
• a sequence of two or more steps • each one represented by an equation • that shows how a reaction takes place
what happens when you mix chlorine with methane at room temperature
no reaction occurs
what are the two conditions that need to be met in order for a reaction to occur
• there needs to be a sufficient rise in temperature - or it needs to be exposed to ultraviolet radiation (sunlight)
what does the ultraviolet radiation affect
• the chlorine - not the alkane (methane)
what does the ultraviolet light do to the chlroine
• it breaks the chlorine molecule (Cl2) into chlorine atoms
how are the outer electrons of this broken chlorine molecule distributed
• the bond in a chlorine molecule consists of a shared pair of electrons • which are equally shared between the two atoms • so when broken, each chlorine atoms takes one electron from the shared pair
what is this kind of bond breaking between chlorine molecules, for example, called
homolytic fission
what equation shows the chlorine molecule being broken down into two chlorine atoms and what is this step called
• Cl2 = Cl. + Cl. (notice the dots) - this step is called initiation
what does the dot represent
an unpaired electron
if the dot represents and unpaired electron, what does that make and not make Cl.
• it doesnt make it an ion or a molecule - but instead it is called a free radical
what is a free radical
• a species with an unpaired electron • ‘species’ refers to any substance that can be represented by a formula • which includes atoms, molecules, ions and radicals
how would chlorine radicals and methane molecules interact and why
• when they collide the chlorine radical will remove a hydrogen atom from the methane molecule • this is because radicals are very reactive
what is the equation for this process and what is this step called
• Cl. + CH4 = CH3. + HCl - this called propagation (1)
what would the newly created molecular radical now react with, along with this formula
• it would react with chlorine molecules to produce chlorine radicals • CH3. + Cl2 = CH3Cl + Cl.
what is that step called
propagation (2)
why are the two steps called propagation
• because both equations involve one radical reacting with molecule • which produces one radical and one molecule
what is the overall product when the two propagation steps are combined
CH4 = CH3Cl
what is the last reaction that can take place
• two radicals colliding and reacting with each other - to form a stable molecule