Advanced A Level Organic Chemistry: 3.2 Halogenoalkanes - synthesis & sources

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Part 3.2 The chemistry of HALOGENOALKANES - their synthesis

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Part 3.2 Laboratory methods for preparing halogenoalkanes (haloalkanes) and their industrial manufacture

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3.2.1 Laboratory methods for preparing halogenoalkanes (synthesising haloalkanes)

(1) Making chloroalkanes using phosphorus(V) chloride and an alcohol

(2) Making chloroalkanes using thionyl chloride and an alcohol

(3) Making a chloroalkane from an alcohol and hydrogen chloride

(4) Making bromoalkanes from an alcohol and potassium bromide - sulfuric acid reagent

(5) Making iodoalkanes from phosphorus, iodine and an alcohol

(6) A summary of the haloalkane synthesis reactions (1) to (5)

3.2.2 Industrial methods of manufacturing haloalkanes

(1) The free radical chlorination and bromination of alkanes

(2) Addition of chlorine or hydrogen chloride to alkenes

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3.2.1 Laboratory methods for preparing halogenoalkanes (synthesising haloalkanes)

(1) Making chloroalkanes using phosphorus(V) chloride and an alcohol

If an alcohol is treated with phosphorus(V) chloride (solid PCl5) the hydroxy functional group is replaced by the chloro functional group.

Beware of the nasty acidic fumes of hydrogen chloride gas evolved (in a fume cupboard) - though it can also convert the alcohol to the haloalkane! (see the next method)

The chloroalkane and phosphorus oxychloride are left behind in the reaction vessel.

e.g. hexan-1-ol is converted to 1-chlorohexane

CH3CH2CH2CH2CH2CH2OH  +  PCl5  ===>  CH3CH2CH2CH2CH2CH2Cl  + POCl3  +  HCl

 

(2) Making chloroalkanes using thionyl chloride and an alcohol

If an alcohol is treated with thionyl chloride (liquid SOCl2), again the hydroxy functional group is also replaced by the chloro functional group.

Again, beware of the nasty acidic fumes of hydrogen chloride and sulfur dioxide gases evolved - though this is also an advantages - the waste gases are automatically separated from the liquid product in a fume cupboard.

e.g. hexan-1-ol is converted to 1-chlorohexane (bpt 133oC)

CH3CH2CH2CH2CH2CH2OH  +  SOCl2  ===>  CH3CH2CH2CH2CH2CH2Cl  + SO2  +  HCl

(3) Making a chloroalkane from an alcohol and hydrogen chloride

If conc. sulfuric is added to a mixture of the alcohol and potassium chloride, the alcohol is protonated and the generated hydrogen chloride converts the hydroxy group of the alcohol to the chloro group of the haloalkane.

KCl  +  H2SO4  ===>  KHSO4  +   HCl

CH3CH2CH2CH2CH2CH2OH  +  HCl  ===>  CH3CH2CH2CH2CH2CH2Cl  +  H2O

This a less efficient method if a secondary or tertiary alcohol is used, because of dehydration converting the alcohol to an alkene.

The mechanism of converting an alcohol to a halogenoalkane

(4) Making bromoalkanes from an alcohol and potassium bromide - sulfuric acid reagent

You cannot use potassium bromide and potassium iodide to prepare bromoalkanes and iodoalkanes using the method described in (3) above because conc. sulfuric acid oxidises the bromide ion/hydrogen bromide molecule to bromine and the iodide ion/hydrogen iodide molecule to iodine - see method (5).

For more details on Halogen chemistry see Redox reaction between halide salts and conc. sulfuric acid

However, you can use potassium bromide by using ~50% sulfuric acid (~50% water).

The 50% sulfuric acid is added slowly to the alcohol, keeping the mixture cool in a beaker.

This mixture is then added to solid potassium bromide and the mixture gently heated under reflux in a suitable flask.

The bromoalkane can be distilled from the mixture.

e.g. making 1-bromopropane (bpt ~71oC) from propan-1-ol (bpt ~97oC).

KBr  +  H2SO4  ===>  KHSO4  +   HBr

CH3CH2CH2OH  +  HBr  ===>  CH3CH2CH2Br  +  H2O

(5) Making iodoalkanes from phosphorus, iodine and an alcohol

With reference to method (4), unfortunately, even 50% sulfuric acid will still oxidise hydrogen iodide/iodide ion, so another procedure must be adopted.

An iodoalkane can be made by gently warming a mixture of damp red phosphorus, iodine and the appropriate alcohol. The water helps facilitate the reaction between the phosphorus and iodine solids.

The iodine reacts with the phosphorus to form phosphorus(III) iodide, which in turn reacts with the alcohol to give iodoalkane e.g. to make 2-iodopropane from propan-2-ol.

2P  +  3I2  ===>  PI3

3CH3CH(OH)CH3  +  PI3  ===>  3CH3CHICH3  +  H3PO3

(6) A summary of the haloalkane product synthesis reactions (1) to (5)

R = alkyl

 		Chlorination Bromination Iodination
Reagent PCl5, SOCl2 or H2SO4/KCl KBr + 50% sulfuric acid Moist P(red) + I2
Conditions Room temperature Heat under reflux Gentle warming
Reaction type - substitution ROH + HCl ==> RCl + H2O

chloroalkane

 ROH + HBr ==> RBr + H2O

bromoalkane

 ROH == PI3 ==> RI

iodoalkane

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3.2.2 Industrial methods of manufacturing haloalkanes

(1) The free radical chlorination and bromination of alkanes

Using uv light or high temperature, chlorine and bromine with react with alkanes

e.g. halogenation of propane (X = Cl or Br)

CH3CH2CH3  +  X2  ===>  {CH3CH2CH2X  or  CH3CHXCH3}  +  HX

to give 1-chloroproapne and 2-chloropropane, or 1-bromopropane and 2-bromopropane

Note that you will always get a mixture of products (structural positional isomers in this case) as the free radicals will attack and abstract hydrogen atoms from anywhere in the carbon chain.

You will get other products like dihaloalkanes e.g. CH3CHXCH2X and lots of other possibilities from further halogenation, and all can be separated by fractional distillation.

The solvent 1,1,2-trichloroethane is made this way:

CH3CH3  +  3Cl2  ===> Cl2CHCH2Cl  +  3HCl

For more on the mechanism of this synthesis see the two pages

Free radical chlorination/bromination to give halogenoalkanes (haloalkanes, alkyl halides)

and Chlorination and bromination of alkanes, reaction mechanisms and uses of products

(2) Addition of chlorine or hydrogen chloride to alkenes

e.g.

(i) 1,2-dichloropropane from propene and chlorine (chlorine from the electrolysis of aqueous sodium chloride).

alkenes structure and naming (c) doc b  +  Cl2  ===>  (c) doc b  

 

(ii) 1-chlorobutane and 2-chlorobutane from but-1-ene and hydrogen chloride

alkene +  HCl  ===> (c) doc b  or  (c) doc b

2-chlorobutane is the major product - predicted from the Markownikoff Rule.

(iii) Chloroethene, the monomer for producing poly(chloroethene), PVC, is made in two stages from ethene, which originates from cracking oil fractions.

In this case the dihaloalkane is an intermediate compound.

ethene  +  chlorine  ===>  1,2-dichloroethane  == heat ==>  chloroethene  +  hydrogen chloride

H2C=CH2  +  Cl2  ===>  ClH2CCH2Cl  ===> H2C=CHCl  +  HCl

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