Doc Brown's GCE Chemistry  Revising Advanced Level Organic Chemistry

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

Part 10.4 Halogenoalkanes - Reaction with cyanide ion

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Part 10.4 HALOGENOALKANES

Introduction to the mechanisms of halogenoalkanes (haloalkanes, alkyl halides).

Nucleophilic substitution by the nucleophile, the cyanide ion to form nitriles.

These revision notes include full diagrams and explanation of the nucleophilic substitution reaction mechanisms of halogenoalkanes (haloalkanes) and the 'molecular' equation and reaction conditions and other con-current reaction pathways and products are also explained when halogenoalkanes react with potassium cyanide to give nitriles.

• Halogenoalkanes owe their reactivity, especially compared to the unreactive alkanes, to two principal reasons.

• R₃C-X = halogenoalkane/haloalkane/alkyl halide/halogenated alkane etc. X = halogen e.g. Cl, Br or I

• The carbon-halogen bond is polar, C^δ+-X^δ- due to the difference in electronegativity between carbon and the halogen. The  C^δ+ carbon is then susceptible to nucleophilic attack by electron pair donor neutral molecules (e.g. the nucleophiles :NH₃, H₂O:) or ions (e.g. :OH^-, ^-:CN).

• The carbon-halogen bond is usually the weakest bond in the molecule and significantly weaker than the carbon-carbon or carbon-hydrogen bonds.

  • Average bond enthalpies/kJmol^-1: C-C 348, C-H 412, both relatively high requiring high activation energies for reaction.

  • Average bond enthalpies/kJmol^-1: C-Cl 338, C-Br 276, C-I 238, generally lower resulting in lower activation energies.

    • Even the lowering of the bond enthalpy by 10kJ from C-C to C-Cl, combined with the polarity of the C-Cl bond, makes all the difference when comparing alkane and halogenoalkane reactivity.

• 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) halogenoalkane (haloalkane).

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

  • In general a halogenoalkane (haloalkane) has the structure R₃C-X where R = H, alkyl or aryl.

  • A primary halogenoalkane (haloalkane) can be shown as RCH₂-X where R = H, alkyl or aryl.

  • A secondary (sec) halogenoalkane (haloalkane) can shown as R₂CH-X where R = alkyl or aryl.

  • A tertiary (tert) halogenoalkane (haloalkane) can be shown as R₃C-X where R = alkyl or aryl.

10.4.3 The nucleophilic substitution of halogenoalkane by cyanide ion

Organic synthesis of nitriles from halogenoalkanes (haloalkanes, alkyl halides) by reaction with potassium cyanide

• What is the reaction mechanism for the substitution of a halogen atom in a haloalkane/halogenoalkane with the cyanide ion?

• e.g. for the reaction

• R₃C-X + CN^- ==> R₃C-CN + X^-   [see mechanisms 36 and 8 below]

mechanism 36 - nucleophilic substitution of a halogenoalkane by cyanide ion (S_N1 'unimolecular' via carbocation)

• S_N1 unimolecular, a two step ionic mechanism via carbocation formation [mechanism 36 above]

  • In step (1) the polar C^δ+-Br^δ- bond of the halogenoalkane splits heterolytically to form a carbocation and a free bromide ion, and this is a reversible reaction.

  • In step (2) the negative cyanide ion rapidly adds to the positive carbocation to form the C-N bond and give the nitrile product. The cyanide ion donates a pair of electrons to form the new C-C bond, i.e. the cyanide ion is the nucleophile.

mechanism 8 - nucleophilic substitution of a halogenoalkane by cyanide ion (S_N2 'bimolecular')

• S_N2 'bimolecular', a one step mechanism [mechanisms 8 above]

  • The C^δ+-Br^δ- bond is polar, so the electron rich negative nucleophile, the cyanide ion, attacks the slightly positive carbon. The nucleophile (CN^-) acts is acting as an electron pair donor (Lewis base) to bond with the 'positive' carbon.

  • Simultaneously the bromine atom is ejected, taking with it the C-Br bond pair of electrons, so forming the nitrile and a bromide ion.

• FURTHER COMMENTS

  • The mechanisms are the same for chloroalkanes, but slower, due to the stronger C-Cl bond enthalpy.

  • In industry chloroalkanes are more likely to be used because they are cheaper.

  • Many of the comments for hydrolysis apply to this reaction.

keywords phrases: reaction conditions formula intermediates organic chemistry reaction mechanisms nucleophilic substitution R3C-X + CN- ==> R3C-CN + X-

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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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