HALOGENOALKANES
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What are halogenoalkanes?
•Halogenoalkanes are similar to alkanes but with
one or more of the hydrogen atoms replaced by a
halogen.
trichloromethane
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chloro-pentafluoroethane
Naming halogenoalkanes
A prefix is added to the name of the alkane depending on
what halogens are attached.
halogen
fluorine
chlorine
bromine
iodine
prefix
fluorochlorobromoiodo-
no. halogen atoms
one
two
three
four
five
prefix
–
ditritetrapenta-
Another prefix is used to indicate how many atoms of each
halogen is present.
Numbers are used, where necessary, to indicate to which
carbon atom(s) each halogen is attached.
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What’s the halogenoalkane?
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Classification
A chain of carbon atoms can be represented by R when
drawing the structure. This is referred to as an R group.
Primary (1°)
halogenoalkanes have one R
group attached to the carbon
linked to the halogen.
Secondary (2°)
halogenoalkanes have two R
groups attached to the carbon
linked to the halogen.
Tertiary (3°) halogenoalkanes
have three R groups attached to
the carbon linked to the halogen.
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Primary, secondary or tertiary?
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Reactions of Halogenoalkanes
The carbon–halogen bond in halogenoalkanes is polar
because all halogens are more electronegative than carbon.
δδ+
δ+
δThe polar bond means that the carbon atom has a small
positive charge (δ+), which attracts substances with a lone
pair of electrons. These are nucleophiles, a species
(molecule or negative ion) which can donate an electron pair
in a chemical reaction
•Halogenoalkanes commonly undergo nucleophilic substitution
and elimination reactions
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Hydrolysis
•In the presence of water, halogenoalkanes undergo hydrolysis.
•Water acts as a nucleophile
•As a result , the -OH group substitutes for the halogen, giving
an alcohol and a hydrogen halide.
•Rate of Hydrolysis can be measured using Silver nitrate
•Silver ions react with halide ions formed from hydrolysis to
give a ppt
•We can tell how fast a hydrolysis occurs by observing the rate
at which ppt forms
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Various halogenoalkanes are treated with a solution of
silver nitrate in a mixture of ethanol and water. Nothing
else is added. After varying lengths of time precipitates
appear as halide ions (produced from reactions of the
halogenoalkanes) react with the silver ions present.
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original precipitate
observation
AgCl
precipitate dissolves to give a
colourless solution
AgBr
precipitate is almost unchanged using
dilute ammonia solution, but dissolves
in concentrated ammonia solution to
give a colourless solution
AgI
precipitate is insoluble in ammonia
solution of any concentration
Same structure Different Halogen
Fastest
Slowest
In these reactions, bond
strength is the main
factor deciding the
relative rates of
reaction.
•The C–I bond is the weakest and so most readily undergoes nucleophilic
substitution. The rate of reactions involving iodoalkanes is the highest.
Same Halogen Different structure
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Reaction With Alkali
Reagent: KOH/NaOH
Condition : Heat Under Reflux
Reaction: Nucleophilic Substitution
Reaction With Cyanide
Reagent: KCN
Condition : Heat Under Reflux
Reaction: Nucleophilic Substitution
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Reaction With Ammonia
Reagent: NH3
Condition : Heat in sealed tube
Reaction: Nucleophilic Substitution
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Nucleophilic Substitution
• Reaction between Haloalkane + OH-/CN-/NH3
• There are 2 different mechanism(SN2 or SN1) depending on
Type of Haloenogalkane
SN2 Mechanism
•
•
•
•
•
•
Dominant for primary Halogenoalkanes
S stands for substitution
N stands for nucleophile
2 indicates there are 2 species in the beginning of reaction
One step reaction
Secondary can undergo SN2 as well
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For example, using 1-bromopropane as a typical primary halogenoalkane:
•The transition state occurs when the O—C bond is half formed and C-halogen
bond half broken
•The energy released in the Formation of O—C bond is enough to provide the
energy to break C—Br bond
•The weaker the C-halogen bond the faster the reaction is
C—I > C—Br > C—Cl
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SN1 Mechanism
•
•
•
•
•
Dominant for secondary and Teriary Halogenoalkanes
S stands for substitution
N stands for nucleophile
1 indicates there is one specie in the beginning of reaction
2 step reaction
Step 1: C—Halogen bond is broken heterolytically forming a carbocation
and halide ion
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Step 2 : Carbocation Attacked by Nucleophile in a faster reaction
CW
Draws the Mechanism for
a. NH3 + CH3CH2CH2Cl
b. KCN + CH3I
c. (CH3)C-Cl + NaOH
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Elimination Reaction
Reagent: Conc NaOH
Condition : Heat under reflux in solution of ethanol
Reaction: Elimination reaction
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Nucleophilic substitution
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What’s the keyword?
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