chemistry form 6 sem 3 chapter 4

7/24/2019 Chemistry Form 6 Sem 3 Chapter 4

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CHEMISTRY FORM 6

ORGANIC CHEMISTRY

CHAPTER 4

HALOALKANE


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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 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.


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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


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4.1.1 Classification of Halogen

Primary haloalkane Secondary haloalkane Tertiary haloalkane

;

Example Example Example

10

20 20

2030


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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


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Some haloalkane easily shows an optical isomerism, as such in

the example above, chlorobutane.

1,2-dichloroethene 1,2-dichlorocyclopropane


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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.


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.


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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 :


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.


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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)


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4.4.2 Reaction of Haloalkane


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


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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


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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


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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]


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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


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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.


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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


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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


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If excess haloalkane is used, the reaction may further continue

until it forms a quaternary salt.


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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


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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 (+)


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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)


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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)


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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




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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.




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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.


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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


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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


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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


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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


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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.


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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


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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


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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


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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.


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CH3CH=CHCH3

Elimination reaction

Reflux

Ethanolic sodium hydroxide


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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


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I : sodium hydroxide under refluxII : ethanolic sodium hydroxide under reflux

All 3 isomers react with Br2 via electrophilic additional reaction


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Easiness of haloalkane to dissociate increase from CH3CHFCH2CH3

chemistry form 6 sem 3 chapter 4

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