Doc Brown's GCE Chemistry

Revising Advanced Level Organic Chemistry

GCE A Level Revision Notes PART 10 Summary of organic reaction mechanisms

A mechanistic introduction to organic chemistry and explanations of different types of organic reactions

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

See also Esterification of acid chlorides with alcohols to give an ester

Part 10.5 ALCOHOLS - 10.5.1 Introduction

  • The reaction mechanism described involve acid catalysis and the initial step in each case involves the protonation of the alcohol, this enables a subsequent nucleophilic substitution to take place.

  • IMPORTANT NOTE on structure classification

    • In the mechanism diagrams you will see part of the molecular structure shown as R3C

    • PLEASE do not assume this means a tertiary (tert) alcohol.

    • R3C– is used repeatedly to minimise the number of graphic images needed.

    • In general an aliphatic alcohol has the structure R3C–OH where R = H, alkyl or aryl.

    • A primary aliphatic alcohol can be shown as RCH2–OH where R = H, alkyl or aryl.

    • A secondary (sec) alcohol can shown as R2CH–OH where R = alkyl or aryl.

    • A tertiary (tert) alcohol can be shown as R3C–OH where R = alkyl or aryl.


10.5.2 The acid catalysed conversion of alcohol to haloalkane

The organic synthesis of halogenoalkanes from alcohols and aqueous hydrogen halides

  • Examples of alcohol to halogenoalkane conversions

    • (i) alcohols and ether structure and naming (c) doc b + HBr ==>(c) doc b + H2O

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

    • (ii) alcohols and ether structure and naming (c) doc b + HCl ==>(c) doc b + 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 sulphuric acid.

organic reaction mechanisms

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 SN1 nucleophilic substitution reaction [steps (2) and (3)]

    • 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.

organic reaction mechanisms

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

  • [mechanism 12 above] This is effectively an SN2 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).


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

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

        • The above conversion reaction of 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 – 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

keywords phrases: reaction conditions formula intermediates organic chemistry reaction mechanisms substitution reaction R3C–OH + HX ==> R3C–X + H2O R3C–OH + H3O+ + X– ==> R3C–X + 2H2O HX(g/aq) + H2O(l) => H3O+(aq) + X– (aq) (CH3)3C–OH + HCl (CH3)3C–Cl + H2O

APPENDIX -  COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)

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