HALOALKANE
Chapter 4
TIMBERLAKE LECTUREPLUS 1999
1
LEARNING OUTCOMES
1. Introduction to haloalkanes
2. Nomenclature & classification of haloalkanes
3. Chemical properties of haloalkanes
4. Application of haloalkanes
2
INTRODUCTION
Haloalkanes or alkyl halides
- compounds that contains halogen atom bonded to an sp3
hybridized carbon atom.
General formula : R-X
CnH2n-1X (cyclic)
where X : halogen atom (F, Cl, Br or I)
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CLASSIFICATION OF HALOALKANES
Haloalkanes are classified according to the nature of
carbon atom bonded to the halogen.
General Formula
Classification
CH 3 X
Alkyl halide
- halogen is bonded to Alkyl
group
R CH 2 X
Primary (10) halide
- halogen is bonded to 10
carbon atom
R
R CH X
Secondary (20) halide
- halogen is bonded to 20
carbon atom 4
General Formula
R
R C X
R
X
Classification
Tertiary (30) halide
- halogen is bonded to 30
carbon atom
-
Aryl halide
halogen is bonded
aromatic ring
5
to
EXAMPLE :
Classify the following haloalkanes :
No.
i.
Haloalkanes
Classification
CH 3 CH 2 Br
ii.
CH 3 CH(Cl)CH 3
iii.
(CH 3 ) 3 C(Br)
H3C
Cl
iv.
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IUPAC NOMENCLATURE
Haloalkanes are named as alkanes with halogen as substituents.
Locate and number the parent chain from the direction that
gives the substituent encountered first the LOWER NUMBER.
Show halogen substituents by the prefixes flouro-, chloro-,
bromo- and iodo-, and list them in ALPHABETICAL ORDER
along with other substituents.
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EXAMPLE :
i.
CH 3 CHCH 2 CH 3
Br
2-bromobutane
Cl
ii.
BrCH 2 CH 2 CHCHCH 2 CH 3
CH 3
1-bromo-3-chloro-4-methylhexane
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EXAMPLE :
iii.
CH 2 CH 2 F
CH 3 CH 2 CH 2 CHCH 2 CH 2 CH 3
4-(2-flouroethyl)heptane
H3C
iv.
CH 3
Cl
2-chloro-1,1-dimethylcyclopentane
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EXAMPLE :
v.
Br
4-bromocyclohexene
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CHEMICAL PROPERTIES
1. Nucleophilic Substitution Reaction
Haloalkanes undergo nucleophilic substitution reactions in which
the halogen atom is replaced by a nucleophile.
In this reaction, the nucleophile attacks the partially positive
charge (δ+) carbon atom bonded to the halogen (δ-).
General
reaction :
_
_
R X + Nu:
R _ Nu
+
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_
X:
(A): HYDROLYSIS OF HALOALKANE WITH
AQUEOUS
SOLUTION OF NAOH
(H 2 O/NAOH)
Alkaline hydrolysis is carried out by boiling R-X with NaOH(aq) to
form alcohol.
_
R X + NaOH
Example :
H2O
CH 3
H2O
_
_
CH 3 C Br + NaOH
CH 3
R_ OH + NaX
CH 3
CH 3 _ C _OH + NaBr
CH 3
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(B): REACTION OF HALOALKANE WITH
POTASSIUM CYANIDE (KCN)
When R-X is refluxed with KCN in alcohol, the halogen atom is
substituted by the CN- to produce a nitrile compound.
_
_
R X + CN
alcohol
reflux
_
_
R CN + X
Example :
CH 3 CH 2 Br + KCN
alcohol
reflux
CH 3 CH 2 CN + KBr
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(C): REACTION OF HALOALKANE WITH
AMMONIA (NH 3 )
When R-X is heated with excess concentrated NH3, the halogen
atom is replaced by the amino group, NH2-.
R _X
NH3
R_ N H 3 + X
_
NH3
R _ NH
2 + NH4 + X
_
(amine)
Example :
CH 3 CH 2 Cl + excess NH 3
_
+
CH 3 CH 2 NH 2 + NH 4 Cl
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MECHANISMS OF NUCLEOPHILIC
SUBSTITUTION REACTION
They are 2 important mechanisms for the substitution
reaction:
1. Unimolecular Nucleophilic Substitution Reaction (SN1)
2. Bimolecular Nucleophilic Substitution
Reaction
(SN2)
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1. UNIMOLECULAR NUCLEOPHILIC
SUBSTITUTION REACTION (S N1)
The term unimolecular means there is only ONE MOLECULE involved in
the transition state of the rate-limiting step.
SN1 reactions are governed mainly by the relative stabilities of
carbocations.
Relative reactivities of haloalkanes in an SN1 reaction :
R-X < R-X < R-X
1o
2o
3o
increasing reactivity
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The mechanism of SN1 reaction involves 2 steps.
Step 1 : Formation of a carbocation (rate determining step)
R
R_ C _ X
slow
R
_
_
R C+ + X
R
R
3o alkyl halide
carbocation
halide ion
Step 2 : Nucleophilic attack on the carbocation
R
_
_
R C + + Nu:
R
fast
R
R_ C _ Nu
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R
Example 1 :
Reaction of 2-bromo-2-methylpropane with H2O.
CH 3
CH 3 _ C _Br + H 2 O
CH 3
CH 3
CH 3 _ C _ OH + HBr
CH 3
SN1 mechanism :
Step 1 : Formation of a carbocation
CH 3
CH 3 _ C _ Br
CH 3
slow
CH 3
CH 3 _ C +
CH 3
+ Br
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_
Step 2 : Nucleophilic attack on the carbocation
CH 3
CH 3 _ C +
+ H 2 O fast
CH 3
CH 3
H
_
_
CH 3 C O
+ H
CH 3
+
H2O
CH 3
H
_
_
CH 3 C O
+ H
CH 3
CH 3
CH 3 _ C _ OH + H 3 O +
CH 3
Loss of proton, H+ to solvent
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Example 2 :
Write the mechanism for the following reaction.
CH 3
CH 3 _ C _CH 2 Br + NaOH(aq)
CH 3
CH 3
CH 3 _ C _CH 2 CH 3 + NaBr
OH
SN1 Mechanism :
Step 1 : Formation of carbocation
CH 3
CH 3 _ C _ CH 2 _Br
CH 3
slow
CH 3
_
_
_
CH 3 C CH 2 + Br
+
CH 3
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Rearrangement :
CH 3
CH 3 _ C _ CH 2
+
CH 3
1,2-methyl shift
Step 2 : Nucleophilic attack on the carbocation
CH 3
CH 3 _ C _ CH2
+
CH 3
+ OH
_
fast
CH 3
CH 3 _ C _ CH 2
+
CH 3
CH 3
CH 3 _ C _ CH 2 CH 3
OH
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Exercise 1 :
Write a reasonable structures of products formed
when 1-iodobutane reacts with
i.
ii.
iii.
KCN
NaOH/H2O
excess NH3
Write the mechanism for the reaction in (ii).
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Exercise 2 :
The structure of compound A is as follows:
CH3
CH3 C Br
CH3
i.
ii.
Give IUPAC name for A
Compound A react with OH- forming an alcohol. Write the
mechanism for the formation of this alcohol and name the
reaction.
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2. BIMOLECULAR NUCLEOPHILIC
SUBSTITUTION
REACTION (S N2)
The term bimolecular means that the transition state of the rate
limiting step involves the collision of two molecules.
SN2 reactions are governed mainly by steric factors (steric
effect).
Steric effect
- is an effect on relative rates caused by the space-filling
properties of those parts of a molecule attached at or near to
the reacting site.
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The reactivity on SN2 reaction depends on the size of atoms or
groups attached to a C – X.
The presence of bulky alkyl groups will prevent the nucleophilic
attack and slow the reaction rate.
Relative reactivities of haloalkanes in an SN2 reaction :
R-X
< R-X
2o
< CH3-X
1o
increasing reactivity
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The rate of reaction depends on the concentration of the
haloalkane and the concentration of nucleophile.
∴rate = k [R-X] [Nu:-]
* SN2 is a second order reaction.
The mechanism of SN2 occurs in a single step.
General mechanism :
R
Nu:-
H
C X
H
slow
R
Nu C X
fast Nu C
H H
transition state
R
H
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H
+ X-
In SN2 reaction, the nucleophile attacks from the back
side of the electrophilic carbon, that is, from the side
directly opposite bonded to the halogen.
The transition state involves partial bonding between the
attacking nucleophile and the haloalkane.
Back-side attack causes the product formed has inverse
configuration from the original configuration.
*
turns the tetrahedron of the carbon atom inside
out, like umbrella caught by the wind.
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Example 3 :
Reaction of ethyl bromide with aqueous sodium hydroxide.
CH 3 CH 2 Br + NaOH (aq)
CH3
SN2 Mechanism
:
:OH-
H
C Br
H
slow
CH3
OH C Br
CH 3 CH 2 OH + NaBr
fast
H H
CH3
OH C
H
H
transition state
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+ Br-
COMPARISON OF S N1 AND S N2
REACTIONS
S N1
S N2
A two-step mechanism
A one-step mechanism
A unimolecular ratedetermining step
A bimolecular rate-determining
step
Second order :
rate = k [RX] [Nu]
First order :
rate = k [RX]
weak nucleophile
Strong nucleophile
Carbocation rearrangement
No carbocation rearrangement
Reactivity order :
3o > 2o > 1o
Reactivity order :
o
o
methyl >
29 1 > 2
IMPORTANCE OF HALOALKANES AS INERT
SUBSTANCE
Haloalkanes
Uses
CCl4
(carbon tetrachloride)
Solvent for dry cleaning,
spot removing
CHCl3
(chloroform)
Solvent for cleaning and
degreasing work
CF2Cl2 , Freon-12
(dichlorodifluoromethane)
CFC
(chloroflourocarbons)
DDT
(DichloroDiphenylTrichloroethane)
Propellants in aerosol
sprays
Refrigerant gas
Insecticide protects
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HALOALKANES AS
ANESTHETICS
Halothane (Fluothane)
F
F
C
Cl
C
Br
F
H
Fluothane is a haloalkane that is widely used as
an anesthetic, which is a compound that decreases
the ability of the nerve cells to conduct pain.
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OZONE LAYER
Ozone layer
Stratosphere
(10-30 miles
Above Earth)
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CHLOROFUOROCARBONS
(CFCS AND THE OZONE
LAYER
Ozone O3 layer absorbs most of the sun’s harmful
radiation.
CFCs - chlorofluorocarbons - are depleting that ozone
layer.
CFCs are used as Freons in refrigeration, air
conditioning, and foam insulation.
Their use in spray cans is no longer allowed.
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