OCR AS Chemistry F332: Chemical Reactions of Halogenoalkanes

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Chemical Reactions of Halogenoalkanes
Reactions of halogenoalkanes involve breaking the carbon-halogen bond.
The bond can break homolytically or heterolytically.
Homolytic Fission
Homolytic fission forms radicals.
One way this can occur is when radiation of the right frequency (visible or ultraviolet) is
absorbed by the halogenoalkanes.
For example, with chloromethane:
We write this in shortened form as:
CH3-Cl + hv CH3· + Cl·
This kind of reaction occurs when halogenoalkanes reach the stratosphere, where they are
exposed to intense ultraviolet radiation.
This is how the chlorine radicals that cause so much trouble for the ozone in the
stratosphere have formed.
Heterolytic Fission
Heterolytic fission is more common under normal laboratory conditions, where reactions of
halogenoalkanes tend to be carried out in a polar solvent such as ethanol, or ethanol and
The carbon ­ halogen bond is already polar and in the right situation it can break forming a
negative halide and a positive carbocation.
For example, with 2-chloro-2-methylpropane:

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The Carbon-Halogen Bond
Halogens are much more electronegative than carbon.
So, the carbon-halogen bond is polar.
The + carbon doesn't have enough electrons. This means it can be attacked by a
A nucleophile is an electron-pair donor.
It donates an electron pair to somewhere without enough electrons.
OH-, NH3 and H2O are all nucleophiles that can react with halogenoalkanes.
Nucleophillic Substitution
Nucleophillic substitution reactions of the halogenoalkanes involve breaking the carbon-
halogen bond.…read more

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Water as a Nucleophile
The water molecule has a lone pair of electrons on the oxygen atom, so water can act as a
nucleophile and attack a halogenoalkanes molecule such as 1-bromobutane ­ though this
reaction is slow than the reaction with OH- ions.
The substitution reaction goes in two stages:
First, H2O attacks the halogenoalkane
Then the resulting ion loses H+ to form an alcohol.…read more

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Using Nucleophilic Substitution to make Halogenoalkanes
When a halogenoalkane reacts with OH- ions, a nucleophilic reaction occurs and an alcohol
is formed.
You can use the reverse of this reaction to produce a halogenoalkane from an alcohol if you
chose the reaction conditions carefully.
It is another nucleophilic substitution reaction, but this time the nucleophile is Hal-.
For example, you can make 1-bromobutane using a nucleophilic substitution reaction
between butan-1-ol and Br- ions.…read more


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