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Substitution Reactions
- Alkanes
- Alkanes - inert compounds - lack of
polarity. C + H have v. similar
electronegativities - alkanes non-polar
molecules. Not readily attacked by
common chemical reagents.
- E.g. water, acids, alkalis + many
oxidisiding + reducing agents are polar +
usually initiate reactions by attraction to
polar groups in other compounds. Such
polar reagents don't react with alkanes.
- Some non-polar reagents will react
with alkanes - halogens. In prescence
of UV light - will subsitute H atoms in
alkane with halogen atoms. E.g. when
Cl mixed with methane + exposed to
sunlight, chloromethane formed +
hydrogen Cl gas evolved:
- CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)
- Because reaction requires
ultraviolet light - called
photochemical reaction.
- Further substitution possible, producing
dichloromethane, trichloromethane +
tetrachloromethane. Other halogens e.g.
Br produces similar substitution products.
E.g. hexane, Br produces bromohexane:
- C₆H₁₄(l) + Br₂(l) → C₆H₁₃Br(l) + HBr(g)
- C₆H₁₄(l) + Br₂(l) → C₆H₁₃Br(l) + HBr(g)
- CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)
- Alkanes - inert compounds - lack of
polarity. C + H have v. similar
electronegativities - alkanes non-polar
molecules. Not readily attacked by
common chemical reagents.
- Substitution Mechanism
- Sequence of stages known as mechanism of
reaction. E.g. mechanism of UV light
sufficient to break Cl-Cl bond. Absorption
of light energy causing bond to break
known as photodissociation.
- Homolytic fission occurs + 2
Cl atoms formed, each having
7 electrons in outer shell. Cl
atoms each have 1 unpaired
electron + thus free radicals.
- Free radicals react v. rapidly with
other molecules/chemical species.
As homolytic fission of Cl product
molecule must occur before any
chloromethane can be formed.
- Initiation
- Cl₂(g) → Cl∙(g) + Cl∙(g)
- Cl₂(g) → Cl∙(g) + Cl∙(g)
- Sequence of stages known as mechanism of
reaction. E.g. mechanism of UV light
sufficient to break Cl-Cl bond. Absorption
of light energy causing bond to break
known as photodissociation.
- Propagation
- Reaction of Cl free radical with methane
molecule produces hydrogen chloride + CH₃∙
(methyl) radical. Dot indicates unpaired
electron. C atom in this CH₃∙ fragment also has
7 electrons in its outer shell.
- Methyl free radical can
react with Cl molecule to
produce chloromethane +
new Cl free radical:
- Cl∙(g) + CH₄(g) → HCl(g) + CH₃∙(g)
- CH₃∙(g) + Cl₂(g) → CH₃Cl(g) + Cl∙(g)
- CH₃∙(g) + Cl₂(g) → CH₃Cl(g) + Cl∙(g)
- Cl∙(g) + CH₄(g) → HCl(g) + CH₃∙(g)
- 2 steps enable reaction to continue. 1st
step, Cl radical used up. 2nd step releases
new Cl free radical - allows repetition of
1st step. Reaction will continue as long as
there's supply of methane molecules +
undissociated Cl molecules.
- 2 steps constitute chain reaction
+ known as propagation steps of
reaction. More propagation steps
can occur; involving products of
earlier propagation steps:
- Cl∙(g) + CH₃Cl(g) → HCl(g) + CH₂Cl∙(g)
- CH₂Cl∙(g) + Cl₂(g) → CH₂Cl₂(g) + Cl∙(g)
- CH₂Cl∙(g) + Cl₂(g) → CH₂Cl₂(g) + Cl∙(g)
- Cl∙(g) + CH₃Cl(g) → HCl(g) + CH₂Cl∙(g)
- Reaction of Cl free radical with methane
molecule produces hydrogen chloride + CH₃∙
(methyl) radical. Dot indicates unpaired
electron. C atom in this CH₃∙ fragment also has
7 electrons in its outer shell.
- Termination
- Reaction to form various substituted
chloromethanes + hydrogen chloride
ceases when supply of reagents is
depleted. Variety of possible
termination steps - take place when 2
free radicals meet:
- Cl∙(g) + CH₂Cl∙(g) → CH₂Cl₂(g)
- CH₂Cl∙(g) + CH₂Cl∙(g) → C₂H₄Cl₂(g)
- CH₂Cl∙(g) + CH₂Cl∙(g) → C₂H₄Cl₂(g)
- Cl∙(g) + CH₂Cl∙(g) → CH₂Cl₂(g)
- Also include recombination of 2 Cl
free radicals to form Cl molecules.
Alternatively, 2 methyl free radicals
can combine to form ethane molecule:
- Cl∙(g) + Cl∙(g) → Cl₂(g)
- CH₃∙(g) + CH₃∙(g) → CH₃CH₃(g)
- CH₃∙(g) + CH₃∙(g) → CH₃CH₃(g)
- Cl∙(g) + Cl∙(g) → Cl₂(g)
- These/any other termination steps will
remove free radicals + disrupt propagation
steps - stopping chain reaction. Mixture of
products obtained, making free radical
reactions unsuitable for preparation of pure
product.Reaction = free-radical substitution.
- Reaction to form various substituted
chloromethanes + hydrogen chloride
ceases when supply of reagents is
depleted. Variety of possible
termination steps - take place when 2
free radicals meet:
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