Part 3.2 The chemistry of HALOGENOALKANES

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Part 3.2 The Laboratory Synthesis of halogenoalkanes (haloalkanes) and their industrial manufacture

Sub-index for this page

3.2.1 Laboratory methods for preparing halogenoalkanes (synthesising haloalkanes)

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

(2) Making chlorohalogenoalkanes using thionyl chloride and an alcohol

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

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

(5) Making iodohalogenoalkanes 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

3.2.1 Laboratory methods for preparing halogenoalkanes (synthesising haloalkanes)

(1) Making chlorohalogenoalkanes 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 chlorohalogenoalkanes 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 133^oC)

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 bromohalogenoalkanes 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 ~71^oC) from propan-1-ol (bpt ~97^oC).

KBr  +  H₂SO₄  ===>  KHSO₄  +   HBr

CH₃CH₂CH₂OH  +  HBr  ===>  CH₃CH₂CH₂Br  +  H₂O

(5) Making iodohalogenoalkanes 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

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

	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 Product - haloalkane R = alkyl	ROH + HCl ==> RCl + H2O chloroalkane	ROH + HBr ==> RBr + H2O bromoalkane	ROH == PI3 ==> RI iodoalkane

TOP OF PAGE and sub-index

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3.2.2 Industrial methods of manufacturing haloalkanes

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(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).

  +  Cl₂  ===>    

 

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

+  HCl  ===>   or 

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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