Chemistry Form 6 Sem 3 04

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 C

H

X (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.

2-bromo-3-ethylpentane 1,1,1-trichloroethane 2,3-dibromo-3-methylbut-1-ene

2-chloropentane 2-chloro-4-methylhexane 1,3-dichlorocyclopentane

2,3-dichloropent-2-ene 2-bromo-1-chlorobenzene

4.1.1

Classification of Halogen

Primary haloalkane Secondary haloalkane Tertiary haloalkane

;

Example Example Example

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, C

H

Cl, 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

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

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 reaction Reagent used and

condition Equation 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)

4.4.2 Reaction of Haloalkane

Name of reaction Reagent used and

condition Equation

Hydrolysis of haloalkane

NaOH (aq)

under reflux 1-chloropropane sodium propan-1-ol hydroxide

Formation of nitrile KCN / 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 / ether CH₃CH₂CH₂Br + Mg CH₃CH₂CH₂MgBr

1-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

The class of haloalkane

For haloalkane with the same halogen atom, the rate of hydrolysis

increase in the order

30 _{haloalkane < 2}0 _{haloalkane < 1}0 _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. S_N1 mechanism and S_N2 mechanism

S_N1 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 :

S_N2 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

Name of reaction Reagent used and condition Equation Reduction of nitrile Lithium aluminium tetrahydride LiAlH₄ 2-methylbutylnitrile 2-methylbutylamine Hydrolysis of nitrile 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

If excess haloalkane is used, the reaction may further continue

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.

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

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.

Formation of secondary (2

_{) alcohol using Grignard reagent}

Reacting Grignard reagent with aldehyde (except methanal), it

form a secondary (2

_{) 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.

Formation of tertiary (3

_{) alcohol using Grignard reagent}

Reacting Grignard reagent with ketone will yield a tertiary (3

)

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

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

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 CH

CH

_{Cl + 4 Na + Pb  (CH}

CH

)

Pb + 4 NaCl

Tetraethyllead (IV) is used as an anti-block additive to increase the

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

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 S_N 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

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

Aerosol Propellant – Freon–12 (CF

Cl

) 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 (–30

C).

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 (CF

₎

group.

Plastic – the most well-known fluorine based polymer is known

as Teflon, where the monomer is CF

=CF

. 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

F

_{Cl–C–Cl  F}

Cl–C ● + ● Cl

O

_{+ ● Cl  O}

+ ●OCl

O

_{+ ●OCl  2 O}

+ ● Cl

● Cl + ● Cl  Cl

From the reaction above, the ozone molecule eventually

converted to become oxygen according to the general equation :

2 O

(g)

3 O

(g)

In order to reduce the depletion, an alternative source of HFC

(hydrofluoroalkane) such as CH

FCF

is used to replace

Freon-12.

CH₃CH=CHCH₃

Elimination reaction Reflux

CH₃CH₂OH + OH- _{ CH}

3CH2O- + 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.

I : sodium hydroxide under reflux

II : ethanolic sodium hydroxide under reflux

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

S_N2 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

CO_{2 + 2 NaOH  Na2}CO₃ + H₂O

RBr + NaOH  ROH + NaBr

Nucleophilic substitution reaction

a cream precipitate is formed Ag+ _{+ Br} - _{ AgBr}

Pale yellow solution turned brown 2 Br- + Cl_{2  2 Cl}– _{+ Br}