A-Level organic chemistry exam revision notes on alcohols

Part 4. The chemistry of ALCOHOLS - halogenation substitution of the OH group by hydrogen halides - halogenation of alcohols, via the substitution reaction ROH + HX ==> RX + H₂O

Part 4.7 Halogenation with hydrogen halides

Conversion of alcohols to halogenoalkanes (haloalkanes)

(0) Introduction to the reaction between an alcohol and a hydrogen halide

(1) The reaction between phosphorus(V) chloride and an alcohol

(2) The reaction between thionyl chloride and an alcohol

(3) The reaction between an alcohol and hydrogen chloride

(4) The reaction between an alcohol and potassium bromide - sulfuric acid reagent

(5) The reaction between phosphorus, iodine and an alcohol

(6) A summary of alcohol reactions (1) to (5) with hydrogen halides

 Index of GCSE level Oil - Useful Products Chemistry Revision Notes

(0) Introduction to the reaction between an alcohol and a hydrogen halide

These reactions involve substituting a halo functional group for a hydroxyl group, either directly or indirectly using the equivalent hydrogen halide.

i.e.  ROH  +  HX  ===>  RX  +  H₂O   (R = alkyl, X = Cl, Br or I)

The reaction is used as a method of synthesising halogenoalkanes.

Aliphatic alcohols

Reminders: You need to know the structures of the sub-classes of alcohols - primary, secondary and tertiary.

(1) The reaction between phosphorus(V) chloride and an alcohol

If a dry alcohol is treated with phosphorus(V) chloride (solid PCl₅) the hydroxyl 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) The reaction between thionyl chloride and an alcohol

If an alcohol is treated with thionyl chloride (liquid SOCl₂), again the hydroxyl 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) The reaction between an alcohol and hydrogen chloride

If conc. sulfuric is added to a mixture of the alcohol and potassium chloride which form potassium hydrogensulfate and hydrogen chloride.

The alcohol is protonated by the generated hydrogen chloride which converts the hydroxyl group of the alcohol to the chloro group of the haloalkane.

KCl  +  H₂SO₄  ===>  KHSO₄  +   HCl

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

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 via a hydrogen halide

(4) The reaction between 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) The reaction between 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