8/13/2019 Chemistry Form 6 Sem 3 04
1/44
CHEMISTRY FORM 6
ORGANIC CHEMISTRY
CHAPTER 4
HALOALKANE
8/13/2019 Chemistry Form 6 Sem 3 04
2/44
4.0 Haloalkane
~ derivatives of alkanes where one or more H is substitute with
halogen, X.
~ Homologous series of haloalkane is CnH2n+1X (where X may
represent Cl, Br and I)
~ compare to alkane, most haloalkanes are toxic and highlycarcinogenic
4.1 Nomenclature (Naming haloalkane)
The way of naming haloalkane is similar to the way of alkane. Find the longest possible carbon chain that contain halogen in the
chain
Find the branched alkyl and halogen attached. if there are more
than 1 branched substance, arrange them according to
alphabetical order.
Give the numbering of branched alkyl or halogen accordingly.
8/13/2019 Chemistry Form 6 Sem 3 04
3/44
2-bromo-3-ethylpentane 1,1,1-trichloroethane 2,3-dibromo-3-methylbut-1-ene
2-chloropentane2-chloro-4-methylhexane
1,3-dichlorocyclopentane
2,3-dichloropent-2-ene 2-bromo-1-chlorobenzene2-iodo-1-phenylpropane
8/13/2019 Chemistry Form 6 Sem 3 04
4/44
4.1.1 Classification of Halogen
Primary haloalkane Secondary haloalkane Tertiary haloalkane
;
Example Example Example
10
20 20
2030
8/13/2019 Chemistry Form 6 Sem 3 04
5/44
4.2 Isomerism in haloalkane
Haloalkane exhibit various types of structural and geometrical
isomerism
In structural isomerism, haloalkane may exhibit a chain isomerism
and positional isomerism
Example chlorobutane, C4H9Cl, exhibit chain and positional isomerism
Not only it may exhibit structural isomerism, haloalkane sometimes
exhibit stereoisomerism
Geometrical isomerism may be exhibit when it involve haloalkene or
halocycloalkane
Chain isomerism OR Positional isomerism
8/13/2019 Chemistry Form 6 Sem 3 04
6/44
Some haloalkane easily shows an optical isomerism, as such in
the example above, chlorobutane.
1,2-dichloroethene 1,2-dichlorocyclopropane
8/13/2019 Chemistry Form 6 Sem 3 04
7/44
4.3 Physical properties of haloalkane
1. Boiling point of haloalkane
The trend of the boiling points of haloalkane bay be caused by many factors
a) Factors of the number of carbon atom
b) Factors of the branched structure
Explanation :
Explanation :
Boiling point increase
When going down to homologous series, the boiling point increase. This
is due to the increase in relative molecular mass, which increase the weak Van Der
Waals forces causing boiling point increase.
Boiling point increase
Straight chain molecule has a larger total surface area compare to a
branched chain molecule. Hence, greater the total surface area exposed, greater the
Van Der Waals forces, higher the boiling point.
8/13/2019 Chemistry Form 6 Sem 3 04
8/44
c) Factors of different halogen used
2. Solubilities of haloalkane in water Even though CX is polar, haloalkane are
insoluble in water because they are not able to form hydrogen bond with
water. Though, it is soluble in organic solvent.
3. Density of haloalkane.
Explanation :
CCl4
Solubility trend :Explanation :
Boiling point increase
When going down to halogen group, the molecular mass increase,
causing a greater weak Van Der Waals forces which eventually resulting higher
boiling point.
Solubility decrease
When theres more substituent group of Cl, molecule become less polar.
As a result, polarity decrease and cause the solubility decrease.
8/13/2019 Chemistry Form 6 Sem 3 04
9/44
4.4 Chemical Properties of Haloalkane
4.4.1 Preparation of Haoalkane
Other than the 2 above, some of the reaction like halogenation of alkene
(under UV) [refer Chapter 2] and halogenation of alkene may produce a
dihaloalkane compound
Name of reactionReagent used and
conditionEquation
Displacement of
alcohol
Hydrogen halide
(H X) catalysed
by zinc chloride,ZnCl2 under reflux
propan-1-ol hydrogen 1-chloropropane
chloride
Addition of
hydrogen halide
to alkene (see
Chapter 2)
Hydrogen halide
( H X )
(X = Cl ; Br ; I)
8/13/2019 Chemistry Form 6 Sem 3 04
10/44
4.4.2 Reaction of Haloalkane
Name of reactionReagent used and
conditionEquation
Hydrolysis of
haloalkane
NaOH (aq)
under reflux 1-chloropropane sodium propan-1-ol
hydroxide
Formation of nitrileKCN / ethanol
under reflux 1-bromopropane potassium butanenitrile
cyanide
Formation of amine(alkylation) concentrated NH3 /ethanol 1-bromopropane conc. propylamine
Ammonia
Formation of
alkene
NaOH /
conc. ethanol
under reflux 1-chloropropane propene
Formation of
organometallic
compound(Grignard reagent)
Mg / etherCH3CH2CH2Br + Mg CH3CH2CH2MgBr1-bromopropane magnesium propylmagnesium
bromide
ether
8/13/2019 Chemistry Form 6 Sem 3 04
11/44
1) Hydrolysis of haloalkane
Haloalkane react moderately with sodium hydroxide, NaOH, under reflux
condition. OH-
act as nucleophile and attack the C that is bond to the halogen General equation for hydrolysis of haloalkane is
The rate of hydrolysis depend on the following factors The bonding of CX The class of haloalkane
The bonding of C X
For a given alkyl group, the rate of hydrolysis of haloalkane increase from R
Cl to RI. This is because, CX become longer going down to halogen
So, when C X bond is longer, lesser energy is required to break the bonding,
thus the rate increase
Bond C Cl C Br C I
Bond energy (kJ / mol) 346 290 228
8/13/2019 Chemistry Form 6 Sem 3 04
12/44
The class of haloalkane
For haloalkane with the same halogen atom, the rate of hydrolysis
increase in the order30 haloalkane < 20 haloalkane < 10 haloalkane
The extension of the reactivity of the class of haloalkane shall be discussed
in the mechanism.
The mechanism of the hydrolysis can be describe below The reactivity of haloalkane is due to the polarity of the C X bond as
+
C X
The partially positively charges carbon atom is susceptible to attack by
nucleophile. In this substitution reaction, there are 2 types of mechanism to
discuss. SN1 mechanism and SN2 mechanism
8/13/2019 Chemistry Form 6 Sem 3 04
13/44
SN1 mechanism
Meaning : substitution of nucleophile in 1st order
Occur at : Some 20 but mostly 30 haloalkane
Process : Occur in 2 steps
Step 1 : Formation of carbocation
Step 2 : Nucleophilic attack
Rate equation :
rate = k [C(CH3)3Br]
8/13/2019 Chemistry Form 6 Sem 3 04
14/44
SN2 mechanism
Meaning : substitution of nucleophile in 2nd order
Occur at : Some 20 but mostly 10 haloalkane
Process : Occur in 1 steps
Rate equation :
rate = k [CH3CH2CH2CH2Br][OH-]
is the
intermediateformed in
reaction
8/13/2019 Chemistry Form 6 Sem 3 04
15/44
2. Formation of nitrile method of increasing the number of carbon.
Haloalkane when react with alcoholic potassium cyanide causes halogen to be
substituted by cyanide ion to produce nitrile.
Haloalkane Alkylnitrile
Example, when 2-chlorobutane reacts with ethanolic potassium cyanide under
reflux
2-chlorobutane 2-methylbutylnitrile
The nitril formed will further react to form either an amine or carboxylic
acid.
8/13/2019 Chemistry Form 6 Sem 3 04
16/44
Name of reactionReagent used and
conditionEquation
Reduction of
nitrile
Lithium aluminium
tetrahydride
LiAlH42-methylbutylnitrile 2-methylbutylamine
Hydrolysis of
nitrile
Diluted sulphuric
acid H2SO4
under reflux
2-methylbutylnitrile 2-methylbutanoic acid
8/13/2019 Chemistry Form 6 Sem 3 04
17/44
3. Formation of amine : alkylation reaction
When haloalkane is dissolve using ethanolic concentrated ammonia (NH3)
solution, amine is formed.
Haloalkane Alkylamine
Unlike the reaction in the reduction of nitril, alkylation of haloalkane toconcentrated ammonia does not increase in number of carbon.
Example : Write out the chemical reaction when
1-chlorobutane react with ethanolic concentrated ammonia
2-bromopentane react with ethanolic concentrated ammonia
8/13/2019 Chemistry Form 6 Sem 3 04
18/44
If excess haloalkane is used, the reaction may further continue
until it forms a quaternary salt.
8/13/2019 Chemistry Form 6 Sem 3 04
19/44
4. Formation of alkene : An elimination reaction
When reacted with concentrated ethanolic sodium hydroxide,
elimination of HX occur and alkene is formed.
Unlike the formation of alcohol in (1), here, the hydroxide OH serve as
the base and remove H+ from haloalkane and at the same time, break
the CX bond and form alkene
Similar to the elimination learned earlier, according to Saytzeff rule, it
formed 2 products.
Example in the reaction below
8/13/2019 Chemistry Form 6 Sem 3 04
20/44
5. Formation of Organometallic Compounds : Grignard reagent
Grignard reagents are class of organometallic compound of magnesium
with the general formula of RMgX, where R is the alkyl group and X ishalogen
Grignard reagent is prepared by dissolving haloalkane to magnesium
metal in dry ether
Grignard reagent is useful in producing different class of alcohol, by
reacting with aldehyde and ketone. In CMg, since C is more
electronegative, so C carries a partial negative charge (). Thus, it act
as a strong nucleophile which attack the C which carries partial
positive charge (+)
8/13/2019 Chemistry Form 6 Sem 3 04
21/44
Formation of primary (1o) alcohol using Grignard reagent
When reacting Grignard reagent with methanal, it form a primary alcohol.
Reaction occur in 2 steps where
Step 1 : Addition of Grignard reagent. Grignard attack C atom of methanal to
form alkoxide ion
Propylmagnesium bromide butoxide ion Step 2 : Hydrolysis in acid. Alkoxide (strong base) react with acid to form
alcohol + water.
butoxide ion butan-1-ol (1o alcohol)
8/13/2019 Chemistry Form 6 Sem 3 04
22/44
Formation of secondary (20) alcohol using Grignard reagent
Reacting Grignard reagent with aldehyde (except methanal), itform a secondary (20) alcohol. Similar to the reaction in the formation of
primary alcohol, it occurs in 2 steps.
Step 1 : Addition of Grignard reagent. Grignard attack C atom of
propanal to form alkoxide ion
propylmagnesium bromide ethanal 1-methylbutoxide ion
Step 2 : Hydrolysis in acid. Alkoxide (strong base) react with acid to
form alcohol + water.
1-methylbutoxide pentan-2-ol (2o alcohol)
8/13/2019 Chemistry Form 6 Sem 3 04
23/44
Formation of tertiary (30) alcohol using Grignard reagent
Reacting Grignard reagent with ketone will yield a tertiary (30)
alcohol.
Step 1 : Addition of Grignard reagent. Grignard attack C atom of
butanone to form alkoxide ion
Step 2 : Hydrolysis in acid. Alkoxide (strong base) react with acid to
form alcohol + water.
8/13/2019 Chemistry Form 6 Sem 3 04
24/44
Formation of carboxylic acid using carbon dioxide
Reacting Grignard reagent with carbon dioxide will produce a carboxylic
acid. The steps of the formation of carboxylic acid from the reaction of
Grignard reagent with carbon dioxide are similar to those of the
formation of alcohol.
Step 1 : Addition of Grignard reagent. Grignard attack C atom ofbutanone and form a complex of magnesium salt.
Step 2 : Hydrolysis in acid. Alkoxide (strong base) react with acid to
form alcohol + water.
8/13/2019 Chemistry Form 6 Sem 3 04
25/44
4.4.3 Other organometallic compound
Organolithium can be prepared using the same way but required lowertemperature. Example, when 1-bromobutane react with lithium under
the presence of dry ether :
Tetraethyllead (IV) can be prepared by heating mixture of chloroethane
with alloy of sodiumlead (NaPb) according to the equation
4 CH3CH2Cl + 4 Na + Pb
(CH3CH2)4Pb + 4 NaCl
Tetraethyllead (IV) is used as an anti-block additive to increase the
octane number of petrol.
8/13/2019 Chemistry Form 6 Sem 3 04
26/44
4.5 Chemical Test for haloalkane
4.5.1 Reaction of haloalkane with solution of silver nitrate
The halogen which bond directly with C in haloalkane is readily to
dissociate with other substance. If an ethanolic silver nitrate is treated
to different halogen of haloalkane, different colour of precipitate will
formed. The results are described below.
From the colour of precipitate formed, solubility in dilute and
concentrated ammonia, Halogen in RX can be determined
Silver halide AgCl AgBr AgI
Colour of silver halide
Solubility in diluted ammonia solution
Solubility in concentrated ammonia solution
white cream yellow
soluble insoluble insoluble
soluble soluble insoluble
8/13/2019 Chemistry Form 6 Sem 3 04
27/44
4.5.2 Alkaline hydrolysis of haloalkanes
When haloalkane is hydrolysed (discussed in 4.4.2 (1) Alcohol can be formed
under such way.R X + NaOH ROH + NaCl
From the angle of alcohol, the class of haloalkane can be determined by using
different alcohol test.
4.6 Nucleophilic substitution of aryl halide Aryl halide ~ halogen attached to benzene ring directly.
Compare to alkyl halide, aryl halide react less readily in nucleophilic
substitution reaction. Neither does it go through SN mechanism as explained
earlier. Under high temperature and pressure
8/13/2019 Chemistry Form 6 Sem 3 04
28/44
The passiveness of the reaction of halo aryl is because
The inductive effect of C X bonding when unhybridise p-orbital in
chlorine interact with the p-orbital in benzene ring, will cause a drift of
electron toward C atom in benzene ring, to which it actually decrease the
polarity between CX. thus the bond become shorter and harder to remove.
The high charge density in alcohol ring repels the approaching negative
OH-.As a result, chlorobenzene react with NaOH (aq) with moderate speed
8/13/2019 Chemistry Form 6 Sem 3 04
29/44
4.7 Application of Haloalkane in our Daily Life
Chlorofluorocarbon (CFC) is alkane which all the hydrogen atoms are
substituted by other halogen atom. The commercial name of CFC iscalled as Freon
CFC has the following characteristics. They are volatile and odourless ;non-toxic and non-corrosive ; inert to chemical reaction and they are
non-flammable. Because of these properties, CFC is used as solvents
for cleaning and as inert substance use as
i) propellants in aerosol cans ii) refrigerant
iii) blowing agents in the plastic industries
iv) fire extinguishers
Formula Systematic name Commercial name
CF2Cl2 Dichlorodifluoromethane Freon 12
CFCl3 Trichlorofluoromethane Freon 11
CFCl2CF2Cl (C2F3Cl3) Trichlorotrifluoroethane Freon 113
8/13/2019 Chemistry Form 6 Sem 3 04
30/44
Aerosol Propellant Freon12 (CF2Cl2) is suitable for use as an aerosol
propellant. Under high pressure in an aerosol can the propellant is
liquid but when valve is open, some of the liquid become vapour andcarries with the active component, for example insecticide, paint or
hair lacquer.
Refrigerants also used Freon-12 as it has a low boiling point (30oC).
It is widely apply as refrigerant in refrigerator and air-conditioner.Freon-12 is liquefied by pressure in refrigerant. It is then vapourised
by sudden expansion and this give the cooling effect. Freon-12 is very
suitable for this purpose because it is unreactive and does not corrode
the machinery. Furthermore, Freon-12 is non-toxic and it is notdangerous if theres a leakage.
Insecticides well known by DDT (dichlorodiphenyldichloroethane).
The structure of DDT is shown as the diagram below. It is best known
of a number of highly chlorinated aromatic compound. Used widely asinsecticide in the early 40-50s to control mosquitoes from spreading
malaria.
8/13/2019 Chemistry Form 6 Sem 3 04
31/44
Since DDT is highly chlorinated, it is highly toxic. It also caused various kinds
of pollutions. DDT is very stable and does not decompose easily. This gives
an advantage as DDT stayed there and killed insects for weeks. Despite of
this property, it will stay permanently and accumulate in the soil.
Furthermore, DDT is fat-soluble and not water-soluble, when DDT is ingested
as a contaminant in food / water, it will concentrate in the fatty tissue of
living things and caused a toxic effect on the living things body, which will
results death. That is why, since 1972, many countries banned DDT. Fire Extinguishers organic compound obtained by replacing halogen with
hydrogen are called halons. Example : (CBrClF2) well known as BCF ;
(CBr2ClF) or (CBrF3). Halon is used extensively as fire extinguishers as they
are chemically inert and denser than air. When sprayed at fired object, haloneffectively covered with dense vapour. Furthermore, combustion will produce
radical reaction where bromine radical (Br) is produced. These radicals then
combined with the object burned and eventually stopped the combustion
8/13/2019 Chemistry Form 6 Sem 3 04
32/44
Solvent Freon-113 are used in industrially as solvent to
dissolve non-polar solutes. They are used to dissolve grease in
engineering equipment and electronic circuit. They are also usedin laundry for dry cleaning especially for textile materials
made of wool.
Anaesthetics diethyl ether as first general anaesthetic used insurgical practices, but due to its highly flammable and has side
effect of nausea, a modern fluorine base anaesthetics are used,
such as halothane, isoflurane and sevofkurane. They have
common features, which is contain a trifluoromethyl (CF3-
)group.
Plastic the most well-known fluorine based polymer is known
as Teflon, where the monomer is CF2=CF2. This polymer is
chemically inert toward most of reagent and it is an excellent
insulator. It has a slippery feel and is best known for its used
as a coating for non-stick pans
8/13/2019 Chemistry Form 6 Sem 3 04
33/44
E Effects of the haloalkane to the Environment.
CFC and ozone depletion CFC are unreactive, and this inert
nature allow then to persist in atmosphere. CFC diffuse into thestratosphere where they react with UV to form free radicals.
These highly reactive radicals react with ozone layer, therefore
deplete the ozone layer through these mechanisms
Initiation
Propagation
Termination
F2ClCCl F2ClC + Cl
O3 + Cl O2 + OCl
O3 + OCl 2 O2 + Cl
Cl + Cl Cl2
8/13/2019 Chemistry Form 6 Sem 3 04
34/44
From the reaction above, the ozone molecule eventually
converted to become oxygen according to the general equation :
2 O3 (g) 3 O2 (g)
In order to reduce the depletion, an alternative source of HFC
(hydrofluoroalkane) such as CH2FCF3 is used to replace Freon-12.
8/13/2019 Chemistry Form 6 Sem 3 04
35/44
CH3CH=CHCH3
Elimination reaction
Reflux
Ethanolic sodium hydroxide
8/13/2019 Chemistry Form 6 Sem 3 04
36/44
8/13/2019 Chemistry Form 6 Sem 3 04
37/44
CH3CH2OH + OH- CH3CH2O
- + H2O
G : C6H5CH2OH Type of reaction : nucleophilic substitution reaction
H, an ether, is formed when ethoxide ion react with G as CH3CH2O- is a strong
base, that react with G
C6H5Cl does not react with hot ethanolic KOH, while C6H5CH2CH2Cl react with
hot ethanolic KOH.
Equation : C6H5CH2CH2Cl C6H5CH=CH2
8/13/2019 Chemistry Form 6 Sem 3 04
38/44
8/13/2019 Chemistry Form 6 Sem 3 04
39/44
I : sodium hydroxide under refluxII : ethanolic sodium hydroxide under reflux
All 3 isomers react with Br2 via electrophilic additional reaction
8/13/2019 Chemistry Form 6 Sem 3 04
40/44
Easiness of haloalkane to dissociate increase from CH3CHFCH2CH3