Chemistry Form 6 Sem 3 04
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Haloalkane...
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CHEMISTRY FORM 6 ORGANIC CHEMISTRY CHAPTER 4 HALOALKANE
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-chloropentane
2,3-dichloropent-2-ene 2-bromo-3-ethylpentane
2-chloro-4-methylhexane
1,3-dichlorocyclopentane
2-bromo-1-chlorobenzene 2-iodo-1-phenylpropane 1,1,1-trichloroethane
2,3-dibromo-3-methylbut-1-ene
4.1.1
Classification of Halogen
Primary haloalkane
Secondary haloalkane
Tertiary haloalkane
Example
Example
;
Example
20 10
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 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
1,2-dichloroethene
1,2-dichlorocyclopropane
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)
Equation
hydrogen chloride
1-chloropropane
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
4.4.2
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
1-bromopropane
Formation of alkene
NaOH / conc. ethanol under reflux
1-chloropropane
Formation of organometallic compound (Grignard reagent)
Mg / ether
Equation
1-chloropropane
sodium hydroxide
1-bromopropane potassium cyanide
conc. Ammonia
butanenitrile
propylamine
propene
CH3CH2CH2Br + Mg 1-bromopropane
propan-1-ol
magnesium
ether → CH3CH2CH2MgBr propylmagnesium bromide
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 Bond
C – Cl
C – Br
C–I
Bond energy (kJ / mol)
346
290
228
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–
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 : 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
Equation
2-methylbutylnitrile
2-methylbutylamine
2-methylbutylnitrile
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.
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.
Silver halide
AgCl
AgBr
AgI
Colour of silver halide
white
cream
yellow
Solubility in diluted ammonia solution
soluble
Solubility in concentrated ammonia solution
soluble
insoluble insoluble soluble
From the colour of precipitate formed, solubility in dilute and concentrated ammonia, Halogen in R–X can be determined
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
Formula
Systematic name
Commercial name
CF2Cl2
Dichlorodifluoromethane
Freon – 12
CFCl3
Trichlorofluoromethane
Freon – 11
CFCl2CF2Cl (C2F3Cl3)
Trichlorotrifluoroethane
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
Initiation
F2Cl–C–Cl F2Cl–C ● + ● Cl
Propagation
O3 + ● Cl O2 + ●OCl O3 + ●OCl 2 O2 + ● Cl
Termination
● 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
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
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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