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

10.5.3 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) ==> + 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. sulphuric acid or conc. phosphoric(V) acid to produce an alkene.

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 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

  • 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.

keywords phrases: reaction conditions formula intermediates organic chemistry reaction mechanisms elimination reaction R2CH–C(OH)R2 ==> R2C=CR2 + H2O CH3CH2CH(OH)CH3 ==> {CH3CH2CH=CH2 or CH3CH=CHCH3} + H2O C4H10O isomers: (CH3)3COH > CH3CH2CHOHCH3 > CH3CH2CH2CH2OH

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APPENDIX

  COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)

• PART 10 An Introduction to organic chemical reaction mechanisms and technical terms explained

• Organic Synthesis - MECHANISMS INDEX

• Part 10.2 ALKANES - an introduction to their chemistry

  • Free radical chlorination/bromination to give halogenoalkanes (haloalkanes, alkyl halides)

  • Free radical mechanism for cracking hydrocarbons to give shorter alkanes and alkenes

  • Ionic mechanism for cracking hydrocarbons

• Part 10.3 ALKENES - introduction to their chemistry

  • Electrophilic addition of hydrogen bromide [HBr_{(conc. aq)} and HBr_{(g/non-polar solvent)}] to form halogenoalkanes

  • Electrophilic addition of bromine with pure bromine or in non-polar solvent (non-aqueous Br_2(l/solvent)) to give dibromoalkanes AND addition using bromine water [aqueous Br_2(aq)] to give bromo-alcohols

  • Electrophilic addition of sulphuric acid AND electrophilic addition of water [acid catalyst] to form alcohols

  • Free radical polymerisation to give poly(alkene) polymers e.g. ethene ==> poly(ethene)

  • Hydrogenation to give saturated alkanes

• Part 10.4 HALOGENOALKANES - introduction to the chemistry of haloalkanes/alkyl halides

  • Nucleophilic substitution by water/hydroxide ion [S_N1 or S_N2, hydrolysis to give alcohols]

    • with extra notes on kinetics, rds, molecularity, rate expression, activated complex etc.

  • Nucleophilic substitution by cyanide ion to give a nitrile [S_N1 or S_N2]

  • Nucleophilic substitution by ammonia/primary amine to give primary/secondary amines etc. [S_N1 or S_N2]

  • Elimination of hydrogen bromide to form alkenes [E1 and E2]

• Part 10.5 ALCOHOLS - introduction to their chemistry

  • Conversion of an alcohol to a halogenoalkane

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

• Part 10.6 Carbonyl compounds - ALDEHYDES and KETONES - introduction to their chemistry

  • Nucleophilic addition of hydrogen cyanide to form a hydroxy-nitrile

  • Addition of hydrogen - reduction with LiAlH₄ or NaBH₄ to give alcohols

  • The iodination of ketones e.g. a 2-one like propanone (a methyl ketone) to give iodo-ketones

• Part 10.7 Carboxylic Acids and ACID CHLORIDES - introduction - all nucleophilic addition-elimination reactions

  • Hydrolysis of acid chlorides with water to give a carboxylic acid

  • Esterification of acid chlorides with alcohols to give an ester

  • Amide formation from reaction of acid chlorides with ammonia or primary amines

• Part 10.8 AROMATIC HYDROCARBONS - introduction to arene electrophilic substitutions

  • Nitration to give nitro-aromatics like nitrobenzene

  • Chlorination to chloro-aromatics like chlorobenzene

  • Alkylation to give alkyl-aromatics like methylbenzene [Friedel-Crafts reaction]

  • Acylation to give aromatic ketones [Friedel-Crafts reaction]

  • Sulphonation/sulfonation to give a sulphonic/sulfonic acid like benzenesulphonic acid/benzenesulfonic acid

  • The orientation of products in aromatic substitution (2,4,6 or 3,5 positions, ortho, para & meta substitution products)

• Other important associated links

  • Isomerism - chain isomerism, positional isomerism, functional group isomerism & Tautomerism
  • Stereoisomerism introduction, E/Z isomerism ('ex' Geometric/Geometrical cis/trans Isomerism)
  • R/S Optical Isomerism and chiral auxiliary synthesis
  • All Advanced Organic Chemistry Notes
  • Summary of Organic Functional Groups
  • Quiz on Organic Structure Recognition
  • Summary of organic chemistry functional group tests
  • The shapes and bond angles of simple organic molecules
  • Aliphatic structure & nomenclature m/c BUMPER QUIZ
  • Type in name aliphatic structure and nomenclature BUMPER QUIZ

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