Organic chemistry Part 10: Reactivity and reaction mechanisms

10.5.2 The substitution reaction mechanism of alcohols with hydrogen halides to give haloalkanes ROH + HX ==> RX + H2O

[Author ©  Dr Phil Brown PhD: Doc Brown's exam revision notes suitable for A level chemistry students of advanced pre-university/college advanced level organic chemistry courses:  mechanism of haloalkane formation from alcohols [mechanism updated April 14th 2026 *]

 All my revision notes on the chemistry of haloalkanes

10.5.2 Reaction of alcohols with hydrogen halides

Examples are explained of the organic chemistry mechanisms for alcohols - substitution mechanism for the conversion of an alcohol to a halogenoalkane and the mechanism of alcohol dehydration to form an alkene

• Examples of alcohol to halogenoalkane conversions

  • (i) + HBr ===> + H₂O

    • propan-1-ol + hydrobromic acid ==> 1-bromopropane

  • (ii) + HCl ===> + H₂O

    • butan-2-ol + hydrochloric acid ==> 2-chlorobutane + water

• What is the mechanism for the acid catalysis of making a halogenoalkane from an alcohol?

• e.g. R₃C–OH + HX ===> R₃C–X + H₂O    [see mechanisms 12 and 13 below]

• or more correctly: R₃C–OH + H₃O⁺ + X^– ===> R₃C–X + 2H₂O

  • because hydrogen halides are fully ionised in water,

  • HX_(g/aq) + H₂O_(l) ===> H₃O⁺_(aq) + X^–_(aq) 

  • For tertiary halogenoalkanes the reaction is reversible.

  • You can use conc. hydrochloric acid to make a chloroalkane or conc. hydrobromic acid to make a bromoalkane.

    • In fact both HX acids are often made in situ using sodium chloride/potassium bromide/potassium iodide with moderately concentrated sulfuric acid.

mechanism 13 – substitution of the OH group of an alcohol by a halide ion ('unimolecular' via carbocation)

• [mechanism 13 above] This is effectively, overall an S_N1 nucleophilic substitution reaction.

  • In step (1) the alcohol is initially protonated by an oxonium ion from the acid.

  • Heterolytic C–O bond fission occurs in step (2) to give a carbocation and water and this is the 'unimolecular' rate determining step.

  • In step (3) the halide ion (chloride Cl^–, bromide Br^– or iodide I^–) rapidly adds to give chloro/bromo/iodo–alkane product.

mechanism 12 – substitution of the OH group of an alcohol by a halide ion (bimolecular)

• [mechanism 12 above] This is effectively an S_N2 nucleophilic substitution reaction via step (2).

  • In step (1) the alcohol is initially protonated by the oxonium ion from the acid solution.

  • Step (2) involves nucleophilic attack by the halide ion on the carbon of the C–O bond.

    • Step (2) is the 'bimolecular' rate determining step (rds).

• FURTHER COMMENTS

  • The general order of reactivity is tert > sec > prim alcohol

    • (CH₃)₃C–OH + HCl (CH₃)₃C–Cl + H₂O 

    • The above conversion reaction of a tertiary alcohol, 2-methylpropan-2-ol to form 2-chloro-2-methylpropane, occurs with conc. hydrochloric acid at room temperature and is readily reversed on dilution of the product with water.

10.5.3 The acid catalysed elimination of water from an alcohol on separate page now

10.6 Carbonyl compounds – nucleophilic addition to ALDEHYDES and KETONES on separate page now

10.6.3 Nucleophilic addition of a hydride ion in the reduction of aldehydes/ketones to primary/secondary alcohols on separate page now

10.6.4 The iodination of ketones (substitution reaction, not a nucleophilic addition) on separate page now

10.7 CARBOXYLIC ACIDS and DERIVATIVES – Acyl/acid chlorides - hydrolysis on separate page now

10.7.3 Acyl chloride esterification by nucleophilic addition–elimination on separate page now

10.7.4 Amide formation via acyl chloride by nucleophilic addition–elimination on separate page now

APPENDIX

  COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)