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FJ / Chemistry Unit, KMPk / Mac 2006 1
CHAPTER
HALOALKANES
6
2
6.1 : Introduction
Haloalkanes or alkyl halides - compounds that contains halogen atom bonded to an sp3 hybridized carbon atom.
General formula : R-X or CnH2n+1X (acyclic)
or CnH2n-1X (cyclic)
where X : halogen atom (F, Cl, Br or I)
3
6.1.1 : Classification of Haloalkanes
Haloalkanes are classified according to the nature of carbon atom bonded to the halogen. General Formula Classification
methyl halide- halogen is bonded
to methyl group
Primary (10) halide- halogen is bonded
to 10 carbon atom
Secondary (20) halide- halogen is bonded
to 20 carbon atom
R CH2 X
R CH X
R
CH3X
4
R C X
R
R
General Formula Classification
Tertiary (30) halide- halogen is bonded to 30
carbon atom
Aryl halide- halogen is bonded to
aromatic ring
** Not a aryl halide
X
CH2X
5
Example :
Classify the following haloalkanes :
No.
Haloalkanes Classification
i. 10
ii. 20
iii. 30
iv. 30
CH3CH2Br
CH3CH(Cl)CH3
(CH3)3C(Br)
ClH3C
6
6.1.2 : 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.
7
Example :
i.
ii.
CH3CHCH2CH3
Br
BrCH2CH2CHCHCH2CH3
CH3
Cl
2-bromobutane
1-bromo-3-chloro-4-methylhexane
8
Example :
iii.
iv.
CH3CH2CH2CHCH2CH2CH3
CH2CH2F
4-(2-flouroethyl)heptane
H3C CH3Cl
2-chloro-1,1-dimethylcyclopentane
9
Example :
v.
vi. vii.
4-bromocyclohexene
(chloromethyl)benzene
CH2Cl
Br
ClCH3
2-chlorotoluene
10
6.1.3 : Structure of Haloalkane The carbon – halogen bond in haloalkene is polar
because halogens is more electronegative than carbon.
The polar C – X bond causes the carbon bearing the halogen is susceptible to nucleophilic attack.
Haloalkanes are reactive and undergo
nucleophilic substitution and elimination reaction.
δ+ δ- electrophilic siteC X
11
6.2 : Chemical Properties6.2.1 : 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 + X:
_ __ _
12
(a) : Hydrolysis of Haloalkane with Aqueous Solution of NaOH (H2O/NaOH)
Alkaline hydrolysis is carried out by boiling R-X with NaOH(aq) to form alcohol.
Example :
H2OR X + NaOH R OH + NaX__
H2OCH3
_CH3 C Br + NaOH_
CH3 CH3
_CH3 C OH + NaBr_CH3
13
(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.
Example :
refluxalcoholR X + CN R CN + X_ __ _
CH3CH2Br + KCN CH3CH2CN + KBralcoholreflux
14
(c) :Reaction of Haloalkane with Ammonia (NH3)
When R-X is heated with excess concentrated NH3, the halogen atom is replaced by the amino group, NH2
-.
Example :
(amine)
CH3CH2Cl + excess NH3 CH3CH2NH2 + NH4+Cl_
NH3R X RNH3+X_ R NH 2 + NH4+X
___ NH3
15
15.2.2 :Mechanisms of Nucleophilic Substitution Reaction
They are 2 important mechanisms for the substitution reaction:
(A). Unimolecular Nucleophilic Substitution Reaction (SN1)
(B). Bimolecular Nucleophilic Substitution Reaction (SN2)
16
(A) : Unimolecular Nucleophilic Substitution Reaction (SN1)
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
17
The rate of SN1 reaction does not depend on the concentration of nucleophile.
The rate depends only on the concentration of the substrate, alkyl halide.
rate = k [R3C-X]
* SN1 is a first order reaction
18
The mechanism of SN1 reaction involves 2 steps.
Step 1 : Formation of a carbocation (rate determining step)
3o alkyl halide carbocation halide ion
Step 2 : Nucleophilic attack on the carbocation
R
_R C X_R R
_R C + X_
R
+slow
R
_R C Nu_R
fast+
R
_R C + Nu:_
R
19
Example 1 : Reaction of 2-bromo-2-methylpropane with H2O.
SN1 mechanism :
Step 1 : Formation of a carbocation
CH3
_CH3 C Br + H2O_CH3 CH3
_CH3 C OH + HBr_
CH3
+slowCH3
_CH3 C Br_
CH3 CH3
_CH3 C + Br_
CH3
20
Step 2 : Nucleophilic attack on the carbocation
Loss of proton, H+ to solvent
fastCH3
_CH3 C + H2O
CH3
+
CH3
_CH3 C O_CH3 H
H+
CH3
_CH3 C OH + H3O+_CH3
H
HCH3
_CH3 C O + H2O_
CH3+
21
Example 2 :Write the mechanism for the following reaction.
SN1 Mechanism :
Step 1 : Formation of carbocation
CH3
_CH3 C CH2Br + NaOH(aq)_CH3 CH3
_CH3 C CH2CH3 + NaBr_
OH
slowCH3
_CH3 C CH2 Br_
CH3
_ _
CH3
_CH3 C CH2 + Br_CH3
+
22
Rearrangement :
Step 2 : Nucleophilic attack on the carbocation
1,2-methyl shift+
CH3_CH3 C CH2
_
CH3 CH3
_CH3 C CH2 _CH3
+
_+
CH3_CH3 C CH2 + OH
_
CH3
_CH3 C CH2CH3_CH3
OH
fast
23
Exercise 1 :
Write a reasonable structures of products formed when 1-iodobutane reacts with
i. KCNii. NaOH/H2Oiii. excess NH3
Write the mechanism for the reaction in (ii).
24
Exercise 2 :
The structure of compound A is as follows:
i. Give IUPAC name for A
ii.Compound A react with OH- forming an alcohol. Write the mechanism for the formation of this alcohol and name the reaction.
CH3 C Br
CH3
CH3
25
(B) :Bimolecular Nucleophilic Substitution Reaction (SN2)
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.
26
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 < CH3-X
2o 1o
increasing reactivity
27
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 :
slow fast
H
R
C X Nu C X Nu:-
H
H H
R
Nu C + X-R
H
transition state
H
28
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.
29
Example 3 :Reaction of ethyl bromide with aqueous sodium hydroxide.
SN2 Mechanism :
CH3CH2Br + NaOH(aq) CH3CH2OH + NaBr
slow fast
HH
:OH- C Br OH C Br
HH
CH3
OH C + Br-
CH3
HH
CH3
transition state
30
Comparison of SN1 and SN2 Reactions
SN1 SN2
A two-step mechanism
A one-step mechanism
A unimolecular rate-determining step
A bimolecular rate-determining step
Second order :
rate = k [RX] [Nu]
First order : rate = k [RX]
Strong nucleophile Weak nucleophile
Carbocation rearrangement
No carbocation rearrangement
Reactivity order :3o > 2o > 1o
Reactivity order : methyl > 1o > 2o
31
15.2.3 : Elimination Reaction (dehydrohalogenation of haloalkanes)
Halogen can be removed from one carbon of a haloalkane and hydrogen from an adjacent carbon to form a carbon-carbon double bond in the presence of a strong base.
General reaction :
alkene
basehaloalkane
H_ C C + :B C C + X_ _ _ __ _
X
32
Example :
i.
ii.
BrCH3CH2OH
CH3CH2ONa
CH3
CH3+
CH2
major minor
CH3CHCHCH3 CH3C CHCH3
Br
CH3 CH3CH3CH2OHCH3CH2ONa
+
CH3CHC CH2
CH3
major
minor
33
6.2.4 : Synthesis of Organomagnesium Compound ( Grignard Reagent ) Prepared by the reaction of haloalkanes
with magnesium metal in anhydrous ether as a solvent.
Example :i.
ii.
etherR-X + Mg R-MgXGrignard Reagent
( alkylmagnesium
halide)etherCH3CH2CH2Br + Mg CH3CH2CH2MgBr
etherCl + Mg MgCl
34
6.2.4.1 : Synthesis of Alkanes, Alcohols and Carboxylic Acids from Grignard Reagents.
The Grignard reagents undergo many reactions that make them useful as a starting materials in the synthesis of other organic compounds.
(i). Synthesis of alkane
The Grignard reagent is hydrolyzed to an alkane when warmed with H2O.
RMgX + H2O R-H + Mg(OH)XH+
35
Example :
i.
ii.
iii.
H+CH3CH2MgBr + H2O CH3CH3 + Mg(OH)Br
H2O/H+CH2MgCl CH3
+ Mg(OH)Cl
H+CH3CH-MgBr + H2O CH3CH2CH3 + Mg(OH)Br
CH3
36
(ii). Synthesis of 1o alcohol
Methanal reacts with the Grignard reagent, followed by the hydrolysis produces primary alcohol.
R-MgX + H-C-H R-C-OMgX
O H
H
H2O,H+
R-C-OH + Mg(OH)X
H
H
37
Example :
i.
ii.
CH3MgBr + H-C-H
OH3O+
CH3-C-OH + Mg(OH)Br
H
H
OH3O+MgBr
+ H-C-H + Mg(OH)BrCH2OH
38
(iii). Synthesis of 2o alcohol
Grignard reagent reacts with aldehydes to produce secondary alcohol.
R-MgX + H-C-R' R-C-OMgX
O R'
H
H2O,H+
R-C-OH + Mg(OH)X
R'
H
39
Example :
i.
ii.
OH2O/H+MgCl
+ CH3-C-H + Mg(OH)ClC-OH
CH3
H
O
H2O/H+
H
CH3CH2MgBr + CH3CH2-C-H
CH3CH2-C-CH2CH3 + Mg(OH)Br
OH
40
(iv). Synthesis of 3o alcohol
Grignard reagent reacts with ketons to produce the tertiary alcohol.
R-MgX + R'-C-R" R-C-OMgX
O R'
R"
H2O,H+
R-C-OH + Mg(OH)X
R'
R"
41
Example :
i.
ii.
O
H2O/H+
CH3
CH3CH2MgBr + CH3-C-CH3
CH3CH2-C-CH3 + Mg(OH)Br
OH
MgCl+ CH3-C-CH3 + Mg(OH)ClH3O+ C-OH
CH3
CH3
O
42
(v). Synthesis of carboxylic acid
Grignard reagent reacts with carbon dioxide (CO2) followed by hydrolysis to form carboxylic acid.
RMgX + O C O R-C-O-MgX
O
O
R-C-O-MgX + H2O R-C-OH + Mg(OH)X
OH+
43
Example :
CH3CH2MgI + CO2 CH3CH2-C-O-MgI
O
CH3CH2-C-O-MgI + H2O CH3CH2COH + Mg(OH)I
OH+ O
44
6.2.5 : Wurtz Reaction
Reaction of haloalkane (RX) with an alkali metal (usually Na) to synthesise longer alkane.
i. To prepare an even number of carbon atoms alkane
2RX + 2Na RR + 2NaX
Example:
2CH3CH2Br + 2Na CH3CH2CH2CH3 + 2NaBr
dry ether
45
ii. To prepare a odd number of carbon atoms alkane
RX + R’X + 6Na RR + RR’ + R’R’
+ 6NaX
Example:
CH3CH2Br + CH3Br + 6Na CH3CH2CH2CH3
+ CH3CH2CH3
+ CH3CH3 + 6NaBr
46
6.2.6 : 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)propellants in aerosol sprays
CFC(chloroflourocarbons)
refrigerant gas
DDT(DichloroDiphenylTrichloroet
hane)
insecticide protects