Organic chemistry Part 10: Reactivity and reaction mechanisms

10.5 ALCOHOLS - introduction and the mechanism of the acid catalysed dehydration of alcohols to give alkenes

  All my advanced A level organic chemistry notes

10.5 ALCOHOLS - 10.5.1 Introduction - 10.5.3 Dehydration of alcohols

10.5.3 The acid catalysed reaction mechanism of the dehydration of an alcohol - elimination of water to give an alkene

The organic synthesis of alkenes by 'dehydration' of alcohols

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

  • (i) ==> + H₂O

    • propan-1-ol ==> propene + water

  • (ii)(a) ==> + H₂O

  • (ii)(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. R₂CH–C(OH)R₂ ==> R₂C=CR₂ + H₂O   [see mechanism 30 below]

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

mechanism 30 – acid catalysed mechanism of dehydration of an alcohol of water from an alcohol

• [mechanism 30 above] It is effectively an E1 mechanism of dehydration of an alcohol 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 C–O 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 C–H bonding pair of electrons 'left' as the proton is removed.

• FURTHER COMMENTS about the dehydration mechanism of alcohols

  • 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 (–1–ol) you can get isomeric alkenes formed.

    • e.g. butan–2–ol can form but–1–ene or but–2–ene.

    • 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.

APPENDIX

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