organic chemistry : haloalkanes
TRANSCRIPT
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 highly
carcinogenic
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.
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
� 4.1.1 Classification of Halogen
Primary haloalkane Secondary haloalkane Tertiary haloalkane
;
Example Example Example
10
20 20
20 30
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
� Some haloalkane easily shows an optical isomerism, as such in
the example above, chlorobutane.
1,2-dichloroethene 1,2-dichlorocyclopropane
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.
c) Factors of different halogen used
2. Solubilities of haloalkane in water – Even though C–X 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 there’s more substituent group of Cl, molecule become less polar.
As a result, polarity decrease and cause the solubility decrease.
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 refluxpropan-1-ol hydrogen 1-chloropropane
chloride
Addition of
hydrogen halide
to alkene (see
Chapter 2)
Hydrogen halide
( H – X )
(X = Cl ; Br ; I)
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
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 C–X � 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 R–I.
� This is because, C–X 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
� The class of haloalkane
� For haloalkane with the same halogen atom, the rate of hydrolysis
increase in the order
30 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
δ+ δ–
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
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]
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
intermediate
formed in
reaction
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.
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
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 to
concentrated 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
4. Formation of alkene : An elimination reaction
� When reacted with concentrated ethanolic sodium hydroxide,
elimination of H–X 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 C–X bond and form alkene
� Similar to the elimination learned earlier, according to Saytzeff rule, it
formed 2 products.
� Example in the reaction below
5. Formation of Organometallic Compounds : Grignard reagent
� Grignard reagents are class of organometallic compound of magnesium
with the general formula of R–MgX, where R is the alkyl group and X is
halogen
� 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 C–Mg, since C is more
electronegative, so C carries a partial negative charge (δ–). Thus, it act
as a strong nucleophile which attack the C which carries partial
positive charge (δ+)
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)
Formation of secondary (20) alcohol using Grignard reagent
� Reacting Grignard reagent with aldehyde (except methanal), it
form 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)
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.
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 of
butanone and form a complex of magnesium salt.
� Step 2 : Hydrolysis in acid. Alkoxide (strong base) react with acid to
form alcohol + water.
4.4.3 Other organometallic compound
� Organolithium can be prepared using the same way but required lower
temperature. 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 sodium–lead (Na–Pb) 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.
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 R–X 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
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 � R–OH + 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
� 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 C–X. 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
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 is
called 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
� Aerosol Propellant – Freon–12 (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 and
carries 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 not
dangerous if there’s 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 as
insecticide in the early 40-50’s to control mosquitoes from spreading
malaria.
� 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 thing’s 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, halon
effectively 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
� 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 used
in laundry for “dry cleaning” especially for textile materials
made of wool.
� Anaesthetics –diethyl ether as first general anaesthetic used in
surgical 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
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 the
stratosphere 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
F2Cl–C–Cl � F2Cl–C ● + ● Cl
O3 + ● Cl � O2 + ●OCl
O3 + ●OCl � 2 O2 + ● Cl
● Cl + ● Cl � Cl2
� 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.
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
I : sodium hydroxide under reflux
II : ethanolic sodium hydroxide under reflux
All 3 isomers react with Br2 via electrophilic additional reaction
� Easiness of haloalkane to dissociate increase from CH3CHFCH2CH3 <
CH3CHClCH2CH3 < CH3CHBrCH2CH3 < CH3CHICH2CH3
� This is due to bond length increase from C-F < C-Cl < C-Br < C-I
� As for C6H5Cl, no precipitate is formed since benzene is an electron withdrawing
group
� This will shortened C-Cl bond and caused no precipitate formed when AgNO3
SN2 mechanism
Rate of reaction increase with the bond length. Since C-Br has longer bond
length than C-Cl, so it has high
CO2 + 2 NaOH � Na2CO3 + H2O
RBr + NaOH � ROH + NaBr
Nucleophilic substitution reaction
a cream precipitate is formed
Ag+ + Br - � AgBr