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Unit 2
 Alkanes are not particularly reactive due to the non-polar
nature of their bonds.
 They can, however, react with halogens in the presence of
sunlight or UV light where halogenoalkanes are produced
along with steamy fumes of the corresponding hydrogen
halide.
 In this reaction an atom of hydrogen has been replaced
with an atom of a halogen, and is an example of a
substitution reaction.
 This substitution reaction is thought to occur by a chain
reaction which has 3 main steps: Propagation, Initiation and
Termination.
 CH4 + Cl2  CH3Cl + HCl
 This reaction will not take place in the dark, it requires UV
light to provide energy to break the Cl-Cl bond. This splits
the chlorine molecules into chlorine atoms.
 Cl-Cl  Cl● + Cl● (the dot represents an unpaired electron).
 This type of bond breaking is known as homolytic fission
and usually occurs when the bond is non-polar or very
slightly polar.
 In homolytic bond fission one electron from the bond goes
to one atom while the other electron goes to the other
atom.
 Atoms with unpaired electrons are known as radicals
which are incredibly unstable and are therefore incredibly
reactive.
 The initiation step in a chain reaction produces radicals.
 If a bond were to split unevenly, ions are formed.
 The atom that got both electrons would be –ve, the other
+ve.
 This is called heterolytic bond fission and will be favoured
when the bond is polar.
 Reactions proceeding via heterolytic bond fission tend to
produce far fewer products and are therefore better suited
for synthesis.
 In reactions involving heterolytic bond fission, attacking
groups are classified as “nucleophiles” or “electrophiles”.
 Electrophiles are chemical species that are electron
deficient and are therefore “electron loving” species.
 Electrophiles are molecules or positively charged ions
which are capable of accepting an electron pair.
 They will seek out electron rich sites in organic molecules.
 Examples include NO2+ and SO3H+
 Nucleophiles are chemical species that are rich in
electrons and are “electron donating” species.
 Nucleophiles are molecules or negatively charged ions
which have at least one lone pair of electrons that they can
donate and form dative bonds.
 They will seek out electron-deficient sites in organic
molecules.
 Examples include H2O, NH3 and halide ions.
 Double headed curly arrows are used to indicate the
movement of electron pairs in a reaction.
 The tail of the arrow shows where the electrons originate
from and the head shows where they end up.
 An arrow starting at the middle of a covalent bond
indicates that heterolytic bond fission is occurring.
 When an arrow is drawn with the head pointing to the
space between two atoms, this indicates that a covalent
bond will be formed between these two atoms.
 A single headed curly arrow indicates the movement of a
single electron.
 These are useful in discussions about radical chemistry
mechanisms.
 Haloalkanes can be regarded as substituted alkanes where
one or more of the hydrogen atoms have ben replaced with
a halogen atom.
 In naming haloalkanes the halogen atoms are treated as
branches and naming is done in the same way as for
branched alkanes.
 2-bromo-2-chloro-1,1,1-trifluoroethane
 Remember, branches are named in alphabetical order.
 Have three different structural types which are primary,
secondary and tertiary.
 These are determined by the number of alkyl groups (R)
attached to the carbon atom directly attached to the
halogen atom (X).
 Due to the polar nature of the Carbon-Halogen bond,
haloalkanes are susceptible to nucleophilic attack.
 The presence of the slight positive charge on the carbon
atom makes haloalkanes susceptible to nucleophilic attack.
 The nucleophile donates a pair of electrons forming a bond
with the carbon atom of the C-X bond.
 The halogen is “thrown out” and substituted by the
nucleophile.
 The mechanism for this will be covered later.
 Reactions of monohaloalkanes with alkalis produce
alcohols.
 A solution of aqueous KOH or NaOH is used.
 Reactions with alcoholic potassium alkoxides (Potassium
methoxide in methanol CH3OK) produces ethers.
 Reactions of ethanolic potassium cyanide or sodium
cyanide (KCN or NaCN in ethanol) produces nitriles.
 The end nitrile contains one more carbon than the original
haloalkane.
 This is very useful in synthetic organic chemistry as a way
of increasing the chain length of an organic compound.
 The nitrile can be converted into the corresponding
carboxylic acid through acid hydrolysis.
 Monohaloalkanes can undergo elimination reactions to
form alkenes.
 This is achieved by heating the monohaloalkane under
reflux with ethanolic potassium or sodium hydroxide.
 In this reaction a hydrogen halide is removed from the
original monohaloalkane and for some it can result in two
different alkenes being produced.
 This is due to the availability of more than one H atom that
can be removed in the formation of the hydrogen halide.
 For example, 2-chlorobutane can result in but-1-ene and
but-2-ene, of which but-2-ene is the major product.
 Zaitsev's rule
 "The alkene formed in greatest amount is the one that
corresponds to removal of the hydrogen from the β-carbon
having the fewest hydrogen substituents."
 For example, when 2-iodobutane is treated with
alcoholic KOH, but-2-ene is the major product and but-1ene is the minor product.