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A Level Organic Chemistry: Reaction mechanisms - acid catalysed dehydration of alcohols

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

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10.5 ALCOHOLS - 10.5.1 Introduction

1.5.3 Dehydration of alcohols

  • Elimination of water from an alcohol to give an alkene [acid catalysed, E1 and E2] elimination reaction mechanisms

  • The reaction mechanism described involve acid catalysis and the initial step in each case involves the protonation of the alcohol, this enables a subsequent elimination reaction 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 R3COH where R = H, alkyl or aryl.

    • A primary aliphatic alcohol can be shown as RCH2OH where R = H, alkyl or aryl.

    • A secondary (sec) alcohol can shown as R2CHOH where R = alkyl or aryl.

    • A tertiary (tert) alcohol can be shown as R3COH where R = alkyl or aryl.



10.5.3 The acid catalysed elimination of water from an alcohol

The organic synthesis of alkenes by 'dehydration' of alcohols

  • Examples of elimination of water from an alcohol, its conversion to an alkene

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

      • propan-1-ol ==> propene + water

    • (ii)(a) alcohols and ether structure and naming (c) doc b ==> alkenes structure and naming (c) doc b + H2O

    • (ii)(b) alcohols and ether structure and naming (c) doc b ==> alkenes structure and naming (c) doc b + H2O

      • butan-2-ol ==> but-1-ene/but-2-ene + water

      • Note in (ii) the possibility of isomeric products, two possible structural isomers

  • What is the mechanism for the acid catalysis of dehydrating an alcohol?

  • e.g. R2CHC(OH)R2 ==> R2C=CR2 + H2O   [see mechanism 30 below]

  • The alcohol can be heated with mod. conc. sulphuric acid or conc. phosphoric(V) acid to produce an alkene.

organic reaction mechanisms

mechanism 30 acid catalysed elimination of water from an alcohol

  • [mechanism 30 above] It is effectively an E1 elimination reaction since step (2) is the rate determining step.

    • Step (1) The alcohol is protonated by the oxonium ion from the acid solution.

    • Step (2) Heterolytic CO bond fission occurs to give the carbocation and water.

    • In Step (3) the carbocation is 'deprotonated' to give the alkene and oxonium ion.

    • The double bond is completed from the CH bonding pair of electrons 'left' as the proton is removed.

  • FURTHER COMMENTS

    • This mechanism is the complete reverse of [mechanism 29], the acid catalysed hydration of an alkene.

    • If the C chain >C3, and not a primary alcohol (1ol) you can get isomeric alkenes formed.

      • e.g. butan2ol can form but1ene or but2ene.

        • CH3CH2CH(OH)CH3 ==> {CH3CH2CH=CH2 or CH3CH=CHCH3} + H2O

    • The ease of dehydration of an alcohols is tert > sec > prim e.g. for

      • C4H10O isomers: (CH3)3COH > CH3CH2CHOHCH3 > CH3CH2CH2CH2OH

      • due to the ease of formation and stability of tert > sec > prim carbocations.

      • The reason for this carbocation stability trend is often referred to as an increasing +I or inductive effect of an increasing number of alkyl groups.

        • The alkyl group is considered to 'donate' electron charge to the positive carbon atom to stabilise the carbocation.

        • However, it is better considered that the positive charge on the positive carbon attracts the greater electron clouds of the alkyl groups compared to hydrogen atoms and so spreads the charge more widely stabilising the carbocation.

        • Hence the more alkyl groups on the functional group carbon, the more stabilisation can occur.


keywords phrases: reaction conditions formula intermediates organic chemistry reaction mechanisms elimination reaction R2CHC(OH)R2 ==> R2C=CR2 + H2O CH3CH2CH(OH)CH3 ==> {CH3CH2CH=CH2 or CH3CH=CHCH3} + H2O C4H10O isomers: (CH3)3COH > CH3CH2CHOHCH3 > CH3CH2CH2CH2OH

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Doc Brown's Advanced Level Chemistry Revision Notes

APPENDIX

See also INDEX of all revision notes on ALCOHOLS (and mention of ethers)

  COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)

See also INDEX of all revision notes on ALCOHOLS (and mention of ethers)

Doc Brown's Advanced Level Chemistry Revision Notes

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