Chemistry Form 6 Sem 3 04

August 17, 2017 | Author: Ng Swee Loong Steven | Category: Alcohol, Chemical Reactions, Alkane, Alkene, Solubility
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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.


2,3-dichloropent-2-ene 2-bromo-3-ethylpentane



2-bromo-1-chlorobenzene 2-iodo-1-phenylpropane 1,1,1-trichloroethane



Classification of Halogen

Primary haloalkane

Secondary haloalkane

Tertiary haloalkane





20 10

20 20


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 Chain isomerism OR Positional isomerism

Not only it may exhibit structural isomerism, haloalkane sometimes exhibit stereoisomerism Geometrical isomerism may be exhibit when it involve haloalkene or halocycloalkane



Some haloalkane easily shows an optical isomerism, as such in the example above, chlorobutane.

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

Boiling point increase Explanation : 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.

b) Factors of the branched structure

Boiling point increase Explanation : 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

Boiling point increase

Explanation : When going down to halogen group, the molecular mass increase, causing a greater weak Van Der Waals forces which eventually resulting higher boiling point.

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. CCl4 Solubility trend :

Solubility decrease Explanation : When there’s more substituent group of Cl, molecule become less polar. As a result, polarity decrease and cause the solubility decrease.

4.4 4.4.1

Chemical Properties of Haloalkane Preparation of Haoalkane

Name of reaction

Reagent used and condition

Displacement of alcohol

Hydrogen halide (H – X) catalysed by zinc chloride, propan-1-ol ZnCl2 under reflux

Addition of hydrogen halide to alkene (see Chapter 2)


hydrogen chloride


Hydrogen halide (H–X) (X = Cl ; Br ; I)

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


Reaction of Haloalkane

Name of reaction

Reagent used and condition

Hydrolysis of haloalkane

NaOH (aq) under reflux

Formation of nitrile

KCN / ethanol under reflux

Formation of amine (alkylation)

concentrated NH3 / ethanol


Formation of alkene

NaOH / conc. ethanol under reflux


Formation of organometallic compound (Grignard reagent)

Mg / ether



sodium hydroxide

1-bromopropane potassium cyanide

conc. Ammonia




CH3CH2CH2Br + Mg 1-bromopropane



ether  → CH3CH2CH2MgBr propylmagnesium bromide


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 Bond

C – Cl

C – Br


Bond energy (kJ / mol)




So, when C – X bond is longer, lesser energy is required to break the bonding, thus the rate increase


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–


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 :  Occur at :  Process :

“substitution of nucleophile in 2nd order” Some 20 but mostly 10 haloalkane Occur in 1 steps

is the intermediate formed in reaction 

Rate equation : rate = k [CH3CH2CH2CH2Br][OH-]

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 reaction

Reagent used and condition

Reduction of nitrile

Lithium aluminium tetrahydride LiAlH4

Hydrolysis of nitrile





2-methylbutanoic acid

Diluted sulphuric acid H2SO4 under reflux

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

If excess haloalkane is used, the reaction may further continue until it forms a quaternary salt.


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 


1-methylbutoxide ion

Step 2 : Hydrolysis in acid. Alkoxide (strong base) react with acid to form alcohol + water.


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.

Silver halide




Colour of silver halide




Solubility in diluted ammonia solution


Solubility in concentrated ammonia solution


insoluble insoluble soluble

From the colour of precipitate formed, solubility in dilute and concentrated ammonia, Halogen in R–X can be determined


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



Systematic name

Commercial name



Freon – 12



Freon – 11

CFCl2CF2Cl (C2F3Cl3)


Freon – 113

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

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


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 Freon12. 

CH3CH=CHCH3 Elimination reaction Reflux Ethanolic sodium hydroxide

CH3CH2OH + OH-  CH3CH2O- + H2O


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


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

Reagent : sodium hydroxide Condition : ethanolic under reflux

CO2 + 2 NaOH  Na2CO3 + H2O

RBr + NaOH  ROH + NaBr Nucleophilic substitution reaction

a cream precipitate is formed Ag+ + Br -  AgBr

Pale yellow solution turned brown 2 Br- + Cl2  2 Cl– + Br2

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