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

• 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 sulphuric 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 [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.

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 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

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!)

• 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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